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Introduction
************

The C language provides no built-in facilities for performing such common operations as input/output, memory management, string manipulation and the like.  Instead, these facilities are defined in a standard library, which you compile and link with your programs.

The GNU C Library, described in this document, defines all of the library functions that are specified by the ISO C standard, as well as additional features specific to POSIX and other derivatives of the Unix operating system, and extensions specific to the GNU system.

The purpose of this manual is to tell you how to use the facilities of the GNU library.  We have mentioned which features belong to which standards to help you identify things that are potentially nonportable. But the emphasis in this manual is not on strict portability.

Getting Started
===============

This manual is written with the assumption that you are at least somewhat familiar with the C programming language and basic programming concepts.  Specifically, familiarity with ISO standard C (see ISO C), rather than ``traditional'' pre-ISO C dialects, is assumed.

The GNU C Library includes several header files, each of which provides definitions and declarations for a group of related facilities; this information is used by the C compiler when processing your program. For example, the header file stdio.h declares facilities for performing input and output, and the header file string.h declares string-processing utilities.  The organization of this manual generally follows the same division as the header files.

If you are reading this manual for the first time, you should read all of the introductory material and skim the remaining chapters.  There are a lot of functions in the GNU C Library and it is not realistic to expect that you will be able to remember exactly how to use each and every one of them.  It is more important to become generally familiar with the kinds of facilities that the library provides, so that when you are writing your programs you can recognize when to make use of library functions, and where in this manual you can find more specific information about them.

Standards and Portability
=========================

This section discusses the various standards and other sources that the GNU C Library is based upon.  These sources include the ISO C and POSIX standards, and the System V and Berkeley Unix implementations.

The primary focus of this manual is to tell you how to make effective use of the GNU library facilities.  But if you are concerned about making your programs compatible with these standards, or portable to operating systems other than GNU, this can affect how you use the library.  This section gives you an overview of these standards, so that you will know what they are when they are mentioned in other parts of the manual.

See Library Summary, for an alphabetical list of the functions and other symbols provided by the library.  This list also states which standards each function or symbol comes from.

ISO C
-----

The GNU C Library is compatible with the C standard adopted by the American National Standards Institute (ANSI)as American National Standard X3.159-1989---"ANSI C" and later by the International Standardization Organization (ISO) as ISO/IEC 9899:1990, "Programming languages---C".  In this manual, we refer to the standard as ISO C since this is the more general standard with respect to ratification.  The header files and library facilities that make up the GNU library are a superset of those specified by the ISO C standard.

If you are concerned about strict adherence to the ISO C standard, you should use the -ansi option when you compile your programs with the GNU C Compiler.  This tells the compiler to define only ISO standard features from the library header files, unless you explicitly ask for additional features. See Feature-Test Macros, for information on how to do this.

Being able to restrict the library to include only ISO C features is important because ISO C puts limitations on what names can be defined by the library implementation, and the GNU extensions don't fit these limitations.  See Reserved Names, for more information about these restrictions.

This manual does not attempt to give you complete details on the differences between ISO C and older dialects.  It gives advice on how to write programs to work portably under multiple C dialects, but does not aim for completeness.

POSIX (The Portable Operating System Interface)
-----------------------------------------------

The GNU library is also compatible with the ISO POSIX family of standards, known more formally as the Portable Operating System Interface for Computer Environments (ISO/IEC 9945).  They were also published as ANSI/IEEE Std 1003.  POSIX is derived mostly from various versions of the Unix operating system.

The library facilities specified by the POSIX standards are a superset of those required by ISO C; POSIX specifies additional features for ISO C functions, as well as specifying new additional functions.  In general, the additional requirements and functionality defined by the POSIX standards are aimed at providing lower-level support for a particular kind of operating system environment, rather than general programming language support that can run in many diverse operating system environments.

The GNU C Library implements all of the functions specified in ISO/IEC 9945-1:1996, the POSIX System Application Program Interface, commonly referred to as POSIX.1.  The primary extensions to the ISO C facilities specified by this standard include file-system interface primitives (See Sandra Loosemore et al., ``File-System Interface'', GNU C Library: Systems & Network Applications (Boston: GNU Press, 2004), available online at http:// www.gnu.org/ manual/ manual.html.), device-specific terminal control functions (Ibid., ``Low-Level Terminal Interface''.) and process control functions. (Ibid., ``Processes''.)

Some facilities from ISO/IEC 9945-2:1993, the POSIX Shell and Utilities standard (POSIX.2) are also implemented in the GNU library.  These include utilities for dealing with regular expressions and other pattern-matching facilities (see Pattern Matching).

Berkeley Unix
-------------

The GNU C Library defines facilities from some versions of Unix that are not formally standardized, specifically from the 4.2 BSD, 4.3 BSD and 4.4 BSD Unix systems (also known as Berkeley Unix) and from SunOS (a popular 4.2 BSD derivative that includes some Unix System V functionality).  These systems support most of the ISO C and POSIX facilities, and 4.4 BSD and newer releases of SunOS in fact support them all.

The BSD facilities include symbolic links (Ibid., ``Symbolic Links''.), the select function (Ibid., ``Waiting for Input or Output''.), the BSD signal functions (Ibid., ``BSD Signal Handling''.) and sockets. (Ibid., ``Sockets''.)

SVID (The System V Interface Description)
-----------------------------------------

The System V Interface Description (SVID) is a document describing the AT&T Unix System V operating system.  It is to some extent a superset of the POSIX standard.

The GNU C Library defines most of the facilities required by the SVID that are not also required by the ISO C or POSIX standards, for compatibility with System V Unix and other Unix systems (such as SunOS) that include these facilities.  However, many of the more obscure and less generally useful facilities required by the SVID are not included.  (In fact, Unix System V itself does not provide them all.)

The supported facilities from System V include the methods for inter-process communication and shared memory, the hsearch and drand48 families of functions, fmtmsg and several of the mathematical functions.

XPG (The X/Open Portability Guide)
----------------------------------

The X/Open Portability Guide (X/Open Company, X/Open Portability Guide, Issue 4 (Reading, UK: X/Open Company, Ltd., 1992).) is a more general standard than POSIX.  X/Open owns the Unix copyright and the XPG specifies the requirements for systems that are intended to be Unix systems.

The GNU C Library complies with the X/Open Portability Guide, Issue 4.2, with all extensions common to XSI (X/Open System Interface) compliant systems and also all X/Open Unix extensions.

The additions on top of POSIX are mainly derived from functionality available in System V and BSD systems, though some of the really bad mistakes in System V systems were corrected.  Since fulfilling the XPG standard with the Unix extensions is a precondition for getting the Unix brand, chances are good that the functionality is available on commercial systems.

Using the Library
=================

This section describes some of the practical issues involved in using the GNU C Library.

Header Files
------------

Libraries for use by C programs really consist of two parts: header files that define types and macros and declare variables and functions, and the actual library or archive that contains the definitions of the variables and functions.

(Recall that in C, a declaration merely provides information that a function or variable exists and gives its type.  For a function declaration, information about the types of its arguments might be provided as well.  The purpose of declarations is to allow the compiler to correctly process references to the declared variables and functions. A definition, on the other hand, actually allocates storage for a variable or says what a function does.)

In order to use the facilities in the GNU C Library, you should be sure that your program source files include the appropriate header files. This is so that the compiler has declarations of these facilities available and can correctly process references to them.  Once your program has been compiled, the linker resolves these references to the actual definitions provided in the archive file.

Header files are included into a program source file by the #include preprocessor directive.  The C language supports two forms of this directive; the first,

     #include "HEADER"

is typically used to include a header file HEADER that you write yourself; this would contain definitions and declarations describing the interfaces between the different parts of your particular application. By contrast,

     #include <file.h>

is typically used to include a header file file.h that contains definitions and declarations for a standard library.  This file would normally be installed in a standard place by your system administrator. You should use this second form for the C library header files.

Typically, #include directives are placed at the top of the C source file, before any other code. (For more information about the use of header files and #include directives, see Richard M. Stallman and the GCC Developer Community, ``Header Files'' in The GNU C Preprocessor Manual (2003), http:// gcc.gnu.org/ onlinedocs/ gcc-3.3.2/ cpp/.)  If you begin your source files with some comments explaining what the code in the file does (a good idea), put the #include directives immediately afterward, following the feature-test macro definition (see Feature-Test Macros).

The GNU C Library provides several header files, each of which contains the type and macro definitions and variable and function declarations for a group of related facilities.  This means that your programs may need to include several header files, depending on exactly which facilities you are using.

Some library header files include other library header files automatically.  However, as a matter of programming style, you should not rely on this; it is better to explicitly include all the header files required for the library facilities you are using.  The GNU C Library header files have been written in such a way that it doesn't matter if a header file is accidentally included more than once; including a header file a second time has no effect.  Likewise, if your program needs to include multiple header files, the order in which they are included doesn't matter.

Compatibility Note: Inclusion of standard header files in any order and any number of times works in any ISO C implementation. However, this has traditionally not been the case in many older C implementations.

Strictly speaking, you don't have to include a header file to use a function it declares; you could declare the function explicitly yourself, according to the specifications in this manual.  But it is usually better to include the header file because it may define types and macros that are not otherwise available and because it may define more efficient macro replacements for some functions.  It is also a sure way to have the correct declaration.

Macro Definitions of Functions
------------------------------

If we describe something as a function in this manual, it may have a macro definition as well.  This normally has no effect on how your program runs---the macro definition does the same thing as the function would.  In particular, macro equivalents for library functions evaluate arguments exactly once, in the same way that a function call would.  The main reason for these macro definitions is that sometimes they can produce an in-line expansion that is considerably faster than an actual function call.

Taking the address of a library function works even if it is also defined as a macro.  This is because, in this context, the name of the function isn't followed by the left parenthesis that is syntactically necessary to recognize a macro call.

You might occasionally want to avoid using the macro definition of a function---perhaps to make your program easier to debug.  There are two ways you can do this:

 1. You can avoid a macro definition in a specific use by enclosing the name of the function in parentheses.  This works because the name of the function does not appear in a syntactic context where it is recognizable as a macro call.

 2. You can suppress any macro definition for a whole source file by using the #undef preprocessor directive, unless otherwise stated explicitly in the description of that facility.

For example, suppose the header file stdlib.h declares a function named abs with:

     extern int abs (int);

and also provides a macro definition for abs.  Then, in:

     #include <stdlib.h>
     int f (int *i) { return abs (++*i); }

the reference to abs might refer to either a macro or a function. On the other hand, in each of the following examples, the reference is to a function and not a macro:

     #include <stdlib.h>
     int g (int *i) { return (abs) (++*i); }

     #undef abs
     int h (int *i) { return abs (++*i); }

Since macro definitions that double for a function behave in exactly the same way as the actual function version, there is usually no need for any of these methods.  In fact, removing macro definitions usually just makes your program slower.

Reserved Names
--------------

The names of all library types, macros, variables and functions that come from the ISO C standard are reserved unconditionally; your program may not redefine these names.  All other library names are reserved if your program explicitly includes the header file that defines or declares them.  There are several reasons for these restrictions:

  * Other people reading your code could get very confused if, for example, you were using a function named exit to do something completely different from what the standard exit function does. Preventing this situation helps to make your programs easier to understand and contributes to modularity and maintainability.

  * It avoids the possibility of a user accidentally redefining a library function that is called by other library functions.  If redefinition were allowed, those other functions would not work properly.

  * It allows the compiler to do whatever special optimizations it pleases on calls to these functions, without the possibility that they may have been redefined by the user.  Some library facilities, such as those for dealing with variadic arguments (See Loosemore et al., ``Variadic Functions''.) and nonlocal exits (Ibid., ``Nonlocal Exits''.), actually require a considerable amount of cooperation on the part of the C compiler, and with respect to the implementation, it might be easier for the compiler to treat these as built-in parts of the language.

In addition to the names documented in this manual, reserved names include all external identifiers (global functions and variables) that begin with an underscore (_) and all identifiers regardless of use that begin with either two underscores or an underscore followed by a capital letter.  This is so that the library and header files can define functions, variables, and macros for internal purposes without risk of conflict with names in user programs.

Some additional classes of identifier names are reserved for future extensions to the C language or the POSIX.1 environment.  While using these names for your own purposes right now might not cause a problem, there is the possibility of conflict with future versions of the C or POSIX standards, so you should avoid using them:

  * Names beginning with a capital E followed by a digit or uppercase letter may be used for additional error-code names (see Error Reporting).

  * Names that begin with either is or to followed by a lowercase letter may be used for additional character testing and conversion functions (see Character Handling).

  * Names that begin with LC_ followed by an uppercase letter may be used for additional macros specifying locale attributes (see Locales).

  * Names of all existing mathematics functions (see Mathematics) suffixed with f or l are reserved for corresponding functions that operate on float and long double arguments, respectively.

  * Names that begin with SIG followed by an uppercase letter are reserved for additional signal names. (Ibid., ``Standard Signals''.)

  * Names that begin with SIG_ followed by an uppercase letter are reserved for additional signal actions. (Ibid., ``Basic Signal Handling''.)

  * Names beginning with str, mem, or wcs followed by a lowercase letter are reserved for additional string and array functions (see String and Array Utilities).

  * Names that end with _t are reserved for additional type names.

In addition, some individual header files reserve names beyond those that they actually define.  You only need to worry about these restrictions if your program includes that particular header file.

  * The header file dirent.h reserves names prefixed with d_.

  * The header file fcntl.h reserves names prefixed with l_, F_, O_, and S_.

  * The header file grp.h reserves names prefixed with gr_.

  * The header file limits.h reserves names suffixed with _MAX.

  * The header file pwd.h reserves names prefixed with pw_.

  * The header file signal.h reserves names prefixed with sa_ and SA_.

  * The header file sys/stat.h reserves names prefixed with st_ and S_.

  * The header file sys/times.h reserves names prefixed with tms_.

  * The header file termios.h reserves names prefixed with c_, V, I, O, and TC; and names prefixed with B followed by a digit.

Feature-Test Macros
-------------------

The exact set of features available when you compile a source file is controlled by which feature-test macros you define.

If you compile your programs using gcc -ansi, you get only the ISO C library features, unless you explicitly request additional features by defining one or more of the feature macros. (See Richard M. Stallman and the GCC Developer Community, ``Invoking GCC'' in Using GCC: The GNU Compiler Collection Reference Manual (Boston, MA: GNU Press, October 2003), http:// gcc.gnu.org/ onlinedocs/ gcc-3.3.2/ gcc/, for more information about GCC options.)

You should define these macros by using #define preprocessor directives at the top of your source code files.  These directives must come before any #include of a system header file.  It is best to make them the very first thing in the file, preceded only by comments.  You could also use the -D option to GCC, but it is better if you make the source files indicate their own meaning in a self-contained way.

This system exists to allow the library to conform to multiple standards. Although the different standards are often described as supersets of each other, they are usually incompatible because larger standards require functions with names that smaller ones reserve to the user program.  This is not mere pedantry---it has been a problem in practice.  For instance, some non-GNU programs define functions named getline that have nothing to do with this library's getline.  They would not be compilable if all features were enabled indiscriminately.

This should not be used to verify that a program conforms to a limited standard.  It is insufficient for this purpose, as it will not protect you from including header files outside the standard, or relying on semantics undefined within the standard.

 -- Macro: _POSIX_SOURCE
     If you define this macro, then the functionality from the POSIX.1 standard (IEEE Standard 1003.1) is available, as well as all of the ISO C facilities.

     The state of _POSIX_SOURCE is irrelevant if you define the macro _POSIX_C_SOURCE to a positive integer.

 -- Macro: _POSIX_C_SOURCE
     Define this macro to a positive integer to control which POSIX functionality is made available.  The greater the value of this macro, the more functionality is made available.

     If you define this macro to a value greater than or equal to 1, then the functionality from the 1990 edition of the POSIX.1 standard (IEEE Standard 1003.1-1990) is made available.

     If you define this macro to a value greater than or equal to 2, then the functionality from the 1992 edition of the POSIX.2 standard (IEEE Standard 1003.2-1992) is made available.

     If you define this macro to a value greater than or equal to 199309L, then the functionality from the 1993 edition of the POSIX.1b standard (IEEE Standard 1003.1b-1993) is made available.

     Greater values for _POSIX_C_SOURCE will enable future extensions. The POSIX standards process will define these values as necessary, and the GNU C Library should support them some time after they become standardized. The 1996 edition of POSIX.1 (ISO/IEC 9945-1: 1996) states that if you define _POSIX_C_SOURCE to a value greater than or equal to 199506L, then the functionality from the 1996 edition is made available.

 -- Macro: _BSD_SOURCE
     If you define this macro, functionality derived from 4.3 BSD Unix is included as well as the ISO C, POSIX.1, and POSIX.2 material.

     Some of the features derived from 4.3 BSD Unix conflict with the corresponding features specified by the POSIX.1 standard.  If this macro is defined, the 4.3 BSD definitions take precedence over the POSIX definitions.

     Due to the nature of some of the conflicts between 4.3 BSD and POSIX.1, you need to use a special BSD compatibility library when linking programs compiled for BSD compatibility.  This is because some functions must be defined in two different ways, one in the normal C library, and one in the compatibility library.  If your program defines _BSD_SOURCE, you must give the option -lbsd-compat to the compiler or linker when linking the program, to tell it to find functions in this special compatibility library before looking for them in the normal C library.

 -- Macro: _SVID_SOURCE
     If you define this macro, functionality derived from SVID is included as well as the ISO C, POSIX.1, POSIX.2 and X/Open material.

 -- Macro: _XOPEN_SOURCE
 -- Macro: _XOPEN_SOURCE_EXTENDED
     If you define this macro, functionality described in the X/Open Portability Guide (X/Open Company, X/Open Portability Guide, Issue 4, Version 2 (Reading, UK: X/Open Company, Ltd., 1994).) is included.  This is a superset of the POSIX.1 and POSIX.2 functionality and in fact _POSIX_SOURCE and _POSIX_C_SOURCE are automatically defined.

     As the unification of all Unices, functionality only available in BSD and SVID is also included.

     If the macro _XOPEN_SOURCE_EXTENDED is also defined, even more functionality is available.  The extra functions will make all functions available that are necessary for the X/Open Unix brand.

     If the macro _XOPEN_SOURCE has the value 500, this includes all functionality described so far plus some new definitions from the Single Unix Specification, version 2.

 -- Macro: _LARGEFILE_SOURCE
     If this macro is defined, some extra functions are available that rectify a few shortcomings in all previous standards.  Specifically, the functions fseeko and ftello are available.  Without these functions, the difference between the ISO C interface (fseek, ftell) and the low-level POSIX interface (lseek) would lead to problems.

     This macro was introduced as part of the Large File Support extension (LFS).

 -- Macro: _LARGEFILE64_SOURCE
     If you define this macro, an additional set of functions is made available that enables 32-bit systems to use files of sizes beyond the usual limit of 2GB.  This interface is not available if the system does not support files that large.  On systems where the natural file size limit is greater than 2GB (i.e., on 64-bit systems), the new functions are identical to the replaced functions.

     The new functionality is made available by a new set of types and functions that replace the existing ones.  The names of these new objects contain 64 to indicate the intention, e.g., off_t vs. off64_t and fseeko vs. fseeko64.

     This macro was introduced as part of the Large File Support extension (LFS).  It is a transition interface for the period when 64-bit offsets are not generally used (see _FILE_OFFSET_BITS).

 -- Macro: _FILE_OFFSET_BITS
     This macro determines which file-system interface will be used, one replacing the other.  Whereas _LARGEFILE64_SOURCE makes the 64-bit interface available as an additional interface, _FILE_OFFSET_BITS allows the 64-bit interface to replace the old interface.

     If _FILE_OFFSET_BITS is undefined, or if it is defined to the value 32, nothing changes.  The 32-bit interface is used and types like off_t have a size of 32 bits on 32-bit systems.

     If the macro is defined to the value 64, the large file interface replaces the old interface.  The functions are not made available under different names (as they are with _LARGEFILE64_SOURCE); instead, the old function names now reference the new functions, e.g., a call to fseeko now indeed calls fseeko64.

     This macro should only be selected if the system provides mechanisms for handling large files.  On 64-bit systems this macro has no effect since the *64 functions are identical to the normal functions.

     This macro was introduced as part of the Large File Support extension (LFS).

 -- Macro: _ISOC99_SOURCE
     Until the revised ISO C standard is widely adopted the new features are not automatically enabled.  The GNU libc nevertheless has a complete implementation of the new standard.  To enable the new features the macro _ISOC99_SOURCE should be defined.

 -- Macro: _GNU_SOURCE
     If you define this macro, everything is included: ISO C89, ISO C99, POSIX.1, POSIX.2, BSD, SVID, X/Open, LFS, and GNU extensions.  In the cases where POSIX.1 conflicts with BSD, the POSIX definitions take precedence.

     If you want to get the full effect of _GNU_SOURCE but make the BSD definitions take precedence over the POSIX definitions, use this sequence of definitions:

          #define _GNU_SOURCE
          #define _BSD_SOURCE
          #define _SVID_SOURCE

     If you do this, you must link your program with the BSD compatibility library by passing the -lbsd-compat option to the compiler or linker.  If you forget, you may get very strange errors at run time.

 -- Macro: _REENTRANT
 -- Macro: _THREAD_SAFE
     If you define one of these macros, reentrant versions of several functions get declared.  Some of the functions are specified in POSIX.1c, but many others are only available on a few other systems or are unique to GNU libc. The problem is the delay in the standardization of the thread safe C library interface.

     Unlike on some other systems, no special version of the C library must be used for linking.  There is only one version---but while compiling this, it must have been specified to compile as thread safe.

We recommend you use _GNU_SOURCE in new programs.  If you don't specify the -ansi option to GCC and do not define any of these macros explicitly, the effect is the same as defining _POSIX_C_SOURCE to 2 and _POSIX_SOURCE, _SVID_SOURCE and _BSD_SOURCE to 1.

When you define a feature-test macro to request a larger class of features, it is harmless to define, in addition, a feature-test macro for a subset of those features.  For example, if you define _POSIX_C_SOURCE, then defining _POSIX_SOURCE as well has no effect.  Likewise, if you define _GNU_SOURCE, defining either _POSIX_SOURCE, _POSIX_C_SOURCE, or _SVID_SOURCE as well has no effect.

Note, however, that the features of _BSD_SOURCE are not a subset of any of the other feature-test macros supported.  This is because it defines BSD features that take precedence over the POSIX features that are requested by the other macros.  For this reason, defining _BSD_SOURCE in addition to the other feature-test macros does have an effect---it causes the BSD features to take priority over the conflicting POSIX features.

Road Map to the Manual
======================

Here is an overview of the contents of the remaining chapters of this manual.

The following chapters are found in the first volume, Sandra Loosemore et al., GNU C Library: Application Fundamentals (Boston: GNU Press, 2004), available online at http:// www.gnu.org/ manual/ manual.html.

  * ``Error Reporting'' describes how errors detected by the library are reported.

  * ``Virtual Memory Allocation and Paging'' describes the GNU library's facilities for managing and using virtual and real memory, including dynamic allocation of virtual memory.  If you do not know in advance how much memory your program needs, you can allocate it dynamically instead, and manipulate it via pointers.

  * ``Character Handling'' contains information about character-classification functions (such as isspace) and functions for performing case conversion.

  * ``String and Array Utilities'' has descriptions of functions for manipulating strings (null-terminated character arrays) and general byte arrays, including operations such as copying and comparison.

  * ``Character-Set Handling'' contains information about manipulating characters and strings using character sets larger than will fit in the usual char data type.

  * ``Locales and Internationalization'' describes how selecting a particular country or language affects the behavior of the library. For example, the locale affects collation sequences for strings and how monetary values are formatted.

  * ``Mathematics'' contains information about the math library functions. These include things like random-number generators and remainder functions on integers as well as the usual trigonometric and exponential functions on floating-point numbers.

  * ``Arithmetic Functions'' describes functions for simple arithmetic, analysis of floating-point values, and reading numbers from strings.

  * ``Date and Time'' describes functions for measuring both calendar time and CPU time, as well as functions for setting alarms and timers.

  * ``Message Translation'' describes how to write programs that are capable of delivering messages in whatever language the user selects without filling the source code with sets of translations.

  * ``Searching and Sorting'' contains information about functions for searching and sorting arrays.  You can use these functions on any kind of array by providing an appropriate comparison function.

  * ``Pattern Matching'' presents functions for matching regular expressions and shell file-name patterns, and for expanding words as the shell does.

  * ``The Basic Program/System Interface'' tells how your programs can access their command-line arguments and environment variables.

  * ``Input/Output Overview'' gives an overall look at the input and output facilities in the library, and contains information about basic concepts such as file names.

  * ``Debugging Support'' describes functions provided by the library to make the debugging process easier, whether or not a dedicated debugger program is being used.

  * ``Input/Output on Streams'' describes I/O operations involving streams (or FILE * objects). These are the normal C library functions from stdio.h.

  * ``Summary of Library Facilities'' gives a summary of all the functions, variables, and macros in the library, with complete data types and function prototypes, and says what standard or system each is derived from. This section is also found in the second volume, for convenient reference.

The following chapters are found in the second volume, Sandra Loosemore et al., GNU C Library: System & Network Applications (Boston: GNU Press, 2004), available online at http:// www.gnu.org/ manual/ manual.html.

  * ``Low-Level Input/Output'' contains information about I/O operations on file descriptors.  File descriptors are a lower-level mechanism specific to the Unix family of operating systems.

  * ``File-System Interface'' has descriptions of operations on entire files, such as functions for deleting and renaming them and for creating new directories.  This chapter also contains information about how you can access the attributes of a file, such as its owner and file-protection modes.

  * ``Pipes and FIFOs'' contains information about simple interprocess-communication mechanisms.  Pipes allow communication between two related processes (such as between a parent and child), while FIFOs allow communication between processes sharing a common file-system on the same machine.

  * ``Sockets'' describes a more complicated interprocess-communication mechanism that allows processes running on different machines to communicate over a network.  This chapter also contains information about Internet host-addressing and how to use the system network databases.

  * ``Low-Level Terminal Interface'' describes how you can change the attributes of a terminal device.  If you want to disable echo of characters typed by the user, for example, read this chapter.

  * ``Processes'' contains information about how to start new processes and run programs.

  * ``Job Control'' describes functions for manipulating process groups and the controlling terminal.  This material is probably only of interest if you are writing a shell or other program that handles job control specially.

  * ``System Databases and Name-Service Switch'' describes the services that are available for looking up names in the system databases, how to determine which service is used for which database, and how these services are implemented so that contributors can design their own services.

  * ``Users and Groups'' tells you how to access the system user- and group-databases.

  * ``System Management'' describes functions for controlling and getting information about the hardware and software configuration your program is executing under.

  * ``System Configuration Parameters'' tells you how you can get information about various operating system limits.  Most of these parameters are provided for compatibility with POSIX.

  * ``DES Encryption and Password Handling'' discusses the legal and technical issues related to password encryption and security, as well as the functions necessary to implement effective encryption.

  * ``Resource Usage and Limitation'' tells you how to monitor the memory and other resource usage totals of processes, and how to regulate this usage. It also covers prioritization and scheduling.

  * ``Syslog'' describes facilities for issuing and logging messages of system administration interest.

  * ``Nonlocal Exits'' contains descriptions of the setjmp and longjmp functions.  These functions provide a facility for goto-like jumps that can jump from one function to another.

  * ``Signal Handling'' tells you all about signals---what they are, how to establish a handler that is called when a particular kind of signal is delivered, and how to prevent signals from arriving during critical sections of your program.

  * ``POSIX Threads'' describes the pthreads (POSIX threads) library.  This library provides support functions for multithreaded programs: thread primitives, synchronization objects, etc.  It also implements POSIX 1003.1b semaphores.

  * ``C Language Facilities in the Library'' contains information about library support for standard parts of the C language, including things like the sizeof operator and the symbolic constant NULL, how to write functions accepting variable numbers of arguments, and constants describing the ranges and other properties of the numerical types.  There is also a simple debugging mechanism that allows you to put assertions in your code and have diagnostic messages printed if the tests fail.

  * ``Installing the GNU C Library'' provides a detailed reference for installing, compiling and configuring the GNU C Library. Configuration and optimization command-line options are covered here.

  * ``Library Maintenance'' explains how to port and enhance the GNU C Library and how to report any bugs you might find.

If you already know the name of the facility you are interested in, you can look it up in Library Summary.  This gives you a summary of its syntax and a pointer to where you can find a more detailed description.  This appendix is particularly useful if you just want to verify the order and type of arguments to a function, for example.  It also tells you what standard or system each function, variable, or macro is derived from.

Error Reporting
***************

Many functions in the GNU C Library detect and report error conditions, and sometimes your programs need to check for these error conditions. For example, when you open an input file, you should verify that the file was actually opened correctly, and print an error message or take other appropriate action if the call to the library function failed.

This chapter describes how the error-reporting facility works.  Your program should include the header file errno.h to use this facility.

Checking for Errors
===================

Most library functions return a special value to indicate that they have failed.  The special value is typically -1, a null pointer, or a constant such as EOF that is defined for that purpose.  But this return value tells you only that an error has occurred.  To find out what kind of error it was, you need to look at the error code stored in the variable errno.  This variable is declared in the header file errno.h.

 -- Variable: volatile int errno
     The variable errno contains the system error number.  You can change the value of errno.

     Since errno is declared volatile, it might be changed asynchronously by a signal handler. (See Loosemore et al., ``Defining Handlers'' (see chap. 1, n. 1).)  However, a properly written signal handler saves and restores the value of errno, so you generally do not need to worry about this possibility except when writing signal handlers.

     The initial value of errno at program start-up is zero.  Many library functions are guaranteed to set it to certain nonzero values when they encounter certain kinds of errors.  These error conditions are listed for each function.  These functions do not change errno when they succeed; thus, the value of errno after a successful call is not necessarily zero, and you should not use errno to determine whether a call failed.  The proper way to do that is documented for each function.  If the call failed, you can examine errno.

     Many library functions can set errno to a nonzero value as a result of calling other library functions that might fail.  You should assume that any library function might alter errno when the function returns an error.

     Portability Note: ISO C specifies errno as a "modifiable lvalue" rather than as a variable, permitting it to be implemented as a macro.  For example, its expansion might involve a function call, like *_errno ().  In fact, that is what it is on the GNU system itself.  The GNU library, on non-GNU systems, does whatever is right for the particular system.

     There are a few library functions, like sqrt and atan, that return a perfectly legitimate value in case of an error, but also set errno.  For these functions, if you want to check to see whether an error occurred, the recommended method is to set errno to zero before calling the function, and then check its value afterward.

All the error codes have symbolic names; they are macros defined in errno.h.  The names start with E and an uppercase letter or digit; you should consider names of this form to be reserved names (see Reserved Names).

The error code values are all positive integers and are all distinct, with one exception: EWOULDBLOCK and EAGAIN are the same. Since the values are distinct, you can use them as labels in a switch statement; just do not use both EWOULDBLOCK and EAGAIN.  Your program should not make any other assumptions about the specific values of these symbolic constants.

The value of errno doesn't necessarily have to correspond to any of these macros, since some library functions might return other error codes of their own for other situations.  The only values that are guaranteed to be meaningful for a particular library function are the ones that this manual lists for that function.

On non-GNU systems, almost any system call can return EFAULT if it is given an invalid pointer as an argument.  Since this could only happen as a result of a bug in your program, and since it will not happen on the GNU system, we have saved space by not mentioning EFAULT in the descriptions of individual functions.

In some Unix systems, many system calls can also return EFAULT if given as an argument a pointer into the stack, and the kernel for some obscure reason fails in its attempt to extend the stack.  If this ever happens, you should probably try using statically or dynamically allocated memory instead of stack memory on that system.

Error Codes
===========

The error code macros are defined in the header file errno.h. All of them expand into integer constant values.  Some of these error codes cannot occur on the GNU system, but they can occur using the GNU library on other systems.

 -- Macro: int EPERM
     This means the operation is not permitted; only the owner of the file (or other resource) or processes with special privileges can perform the operation.

 -- Macro: int ENOENT
     This means there is no such file or directory.  This is a file does not exist error for ordinary files that are referenced in contexts where they are expected to already exist.

 -- Macro: int ESRCH
     This means no process matches the specified process ID.

 -- Macro: int EINTR
     This indicates an interrupted function call; an asynchronous signal occurred and prevented completion of the call.  When this happens, you should try the call again.

     You can choose to have functions resume after a signal that is handled, rather than failing with EINTR. (Ibid., ``Primitives Interrupted by Signals''.)

 -- Macro: int EIO
     This indicates an input/output error; usually used for physical read or write errors.

 -- Macro: int ENXIO
     This means there is no such device or address.  The system tried to use the device represented by a file you specified, and it could not find the device.  This can mean that the device file was installed incorrectly, or that the physical device is missing or not correctly attached to the computer.

 -- Macro: int E2BIG
     This means that the argument list is too long; it is used when the arguments passed to a new program being executed with one of the exec functions (Ibid., ``Executing a File''.) occupy too much memory space.  This condition never arises in the GNU system.

 -- Macro: int ENOEXEC
     This indicates an invalid executable file format.  This condition is detected by the exec functions. (Ibid.)

 -- Macro: int EBADF
     This indicates a bad file descriptor; for example, I/O on a descriptor that has been closed or reading from a descriptor open only for writing (or vice versa).

 -- Macro: int ECHILD
     There are no child processes.  This error happens on operations that are supposed to manipulate child processes when there are not any processes to manipulate.

 -- Macro: int EDEADLK
     This means a deadlock was avoided; allocating a system resource would have resulted in a deadlock situation.  The system does not guarantee that it will notice all such situations.  This error means you got lucky and the system noticed; it might just hang. (For an example, see Loosemore et al., ``File Locks''.)

 -- Macro: int ENOMEM
     This means no memory is available.  The system cannot allocate more virtual memory because its capacity is full.

 -- Macro: int EACCES
     This means that permission is denied; the file permissions do not allow the attempted operation.

 -- Macro: int EFAULT
     This indicates a bad address; an invalid pointer was detected. In the GNU system, this error never happens; you get a signal instead.

 -- Macro: int ENOTBLK
     A file that is not a block special file was given in a situation that requires one.  For example, trying to mount an ordinary file as a file system in Unix gives this error.

 -- Macro: int EBUSY
     This means that a resource is busy; a system resource that cannot be shared is already in use.  For example, if you try to delete a file that is the root of a currently mounted file system, you get this error.

 -- Macro: int EEXIST
     This means a file exists; an existing file was specified in a context where it only makes sense to specify a new file.

 -- Macro: int EXDEV
     An attempt to make an improper link across file systems was detected. This happens not only when you use link, (Ibid., ``Hard Links''.) but also when you rename a file with rename. (Ibid., ``Renaming Files''.)

 -- Macro: int ENODEV
     The wrong type of device was given to a function that expects a particular sort of device.

 -- Macro: int ENOTDIR
     A file that is not a directory was specified when a directory is required.

 -- Macro: int EISDIR
     A file is a directory; you cannot open a directory for writing, or create or remove hard links to it.

 -- Macro: int EINVAL
     This indicates an invalid argument.  This is used to indicate various kinds of problems with passing the wrong argument to a library function.

 -- Macro: int EMFILE
     The current process has too many files open and cannot open any more. Duplicate descriptors do count toward this limit.

     In BSD and GNU, the number of open files is controlled by a resource limit that can usually be increased.  If you get this error, you might want to increase the RLIMIT_NOFILE limit or make it unlimited. (Ibid., ``Limiting Resource Usage''.)

 -- Macro: int ENFILE
     There are too many distinct file openings in the entire system.  Note that any number of linked channels count as just one file opening. (Ibid., ``Linked Channels''.)  This error never occurs in the GNU system.

 -- Macro: int ENOTTY
     This indicates an inappropriate I/O control operation, such as trying to set terminal modes on an ordinary file.

 -- Macro: int ETXTBSY
     This indicates an attempt to execute a file that is currently open for writing, or write to a file that is currently being executed.  Often using a debugger to run a program is considered having it open for writing and will cause this error.  (The name stands for text file busy.)  This is not an error in the GNU system; the text is copied as necessary.

 -- Macro: int EFBIG
     A file is too big; the size of a file would be larger than allowed by the system.

 -- Macro: int ENOSPC
     No space is left on device; a write operation on a file failed because the disk is full.

 -- Macro: int ESPIPE
     This indicates an invalid seek operation (such as on a pipe).

 -- Macro: int EROFS
     An attempt was made to modify something on a read-only file system.

 -- Macro: int EMLINK
     There are too many links; the link count of a single file would become too large.  rename can cause this error if the file being renamed already has as many links as it can take. (Ibid., ``Renaming Files''.)

 -- Macro: int EPIPE
     This indicates a broken pipe; there is no process reading from the other end of a pipe.  Every library function that returns this error code also generates a SIGPIPE signal; this signal terminates the program if not handled or blocked.  Thus, your program will never actually see EPIPE unless it has handled or blocked SIGPIPE.

 -- Macro: int EDOM
     This indicates a domain error; used by mathematical functions when an argument value does not fall into the domain over which the function is defined.

 -- Macro: int ERANGE
     This indicates a range error; used by mathematical functions when the result value is not representable because of overflow or underflow.

 -- Macro: int EAGAIN
     Resource temporarily unavailable; the call might work if you try again later.  The macro EWOULDBLOCK is another name for EAGAIN; they are always the same in the GNU C Library.

     This error can happen in a few different situations:

       * An operation that would block was attempted on an object that has nonblocking mode selected.  Trying the same operation again will block until some external condition makes it possible to read, write, or connect (whatever the operation).  You can use select to find out when the operation will be possible. (Ibid., ``Waiting for Input or Output''.)

          Portability Note: In many older Unix systems, this condition was indicated by EWOULDBLOCK, which was a distinct error code different from EAGAIN.  To make your program portable, you should check for both codes and treat them the same.

       * A temporary resource shortage made an operation impossible.  fork can return this error.  It indicates that the shortage is expected to pass, so your program can try the call again later and it may succeed. It is probably a good idea to delay for a few seconds before trying it again, to allow time for other processes to release scarce resources. Such shortages are usually fairly serious and affect the whole system, so usually an interactive program should report the error to the user and return to its command loop.

 -- Macro: int EWOULDBLOCK
     In the GNU C Library, this is another name for EAGAIN.  The values are always the same, on every operating system.

     C libraries in many older Unix systems have EWOULDBLOCK as a separate error code.

 -- Macro: int EINPROGRESS
     An operation that cannot complete immediately was initiated on an object that has nonblocking mode selected.  Some functions that must always block (such as connect (Ibid., ``Connecting''.)) never return EAGAIN.  Instead, they return EINPROGRESS to indicate that the operation has begun and will take some time.  Attempts to manipulate the object before the call completes return EALREADY.  You can use the select function to find out when the pending operation has completed. (Ibid., ``Waiting for Input or Output''.)

 -- Macro: int EALREADY
     An operation is already in progress on an object that has nonblocking mode selected.

 -- Macro: int ENOTSOCK
     A file that isn't a socket was specified when a socket is required.

 -- Macro: int EMSGSIZE
     The size of a message sent on a socket was larger than the supported maximum size.

 -- Macro: int EPROTOTYPE
     The socket type does not support the requested communications protocol.

 -- Macro: int ENOPROTOOPT
     You specified a socket option that doesn't make sense for the particular protocol being used by the socket. (Ibid., ``Socket Options''.)

 -- Macro: int EPROTONOSUPPORT
     The socket domain does not support the requested communications protocol (perhaps because the requested protocol is completely invalid). (Ibid., ``Creating a Socket''.)

 -- Macro: int ESOCKTNOSUPPORT
     The socket type is not supported.

 -- Macro: int EOPNOTSUPP
     The operation you requested is not supported.  Some socket functions don't make sense for all types of sockets, and others may not be implemented for all communications protocols.  In the GNU system, this error can happen for many calls when the object does not support the particular operation; it is a generic indication that the server knows nothing to do for that call.

 -- Macro: int EPFNOSUPPORT
     The socket communications protocol family you requested is not supported.

 -- Macro: int EAFNOSUPPORT
     The address family specified for a socket is not supported; it is inconsistent with the protocol being used on the socket. (Ibid., ``Sockets''.)

 -- Macro: int EADDRINUSE
     The requested socket address is already in use. (Ibid., ``Socket Addresses''.)

 -- Macro: int EADDRNOTAVAIL
     The requested socket address is not available; for example, you tried to give a socket a name that doesn't match the local host name. (Ibid.)

 -- Macro: int ENETDOWN
     A socket operation failed because the network was down.

 -- Macro: int ENETUNREACH
     A socket operation failed because the subnet containing the remote host was unreachable.

 -- Macro: int ENETRESET
     A network connection was reset because the remote host crashed.

 -- Macro: int ECONNABORTED
     A network connection was aborted locally.

 -- Macro: int ECONNRESET
     A network connection was closed for reasons outside the control of the local host, such as by the remote machine rebooting or an unrecoverable protocol violation.

 -- Macro: int ENOBUFS
     The kernel's buffers for I/O operations are all in use.  In GNU, this error is always synonymous with ENOMEM; you may get one or the other from network operations.

 -- Macro: int EISCONN
     You tried to connect a socket that is already connected. (Ibid., ``Making a Connection''.)

 -- Macro: int ENOTCONN
     The socket is not connected to anything.  You get this error when you try to transmit data over a socket, without first specifying a destination for the data.  For a connectionless socket (for datagram protocols, such as UDP), you get EDESTADDRREQ instead.

 -- Macro: int EDESTADDRREQ
     No default destination address was set for the socket.  You get this error when you try to transmit data over a connectionless socket, without first specifying a destination for the data with connect.

 -- Macro: int ESHUTDOWN
     The socket has already been shut down.

 -- Macro: int ETOOMANYREFS
     There are too many references; cannot splice.

 -- Macro: int ETIMEDOUT
     A socket operation with a specified time-out received no response during the time-out period.

 -- Macro: int ECONNREFUSED
     A remote host refused to allow the network connection (typically because it is not running the requested service).

 -- Macro: int ELOOP
     Too many levels of symbolic links were encountered in looking up a file name. This often indicates a cycle of symbolic links.

 -- Macro: int ENAMETOOLONG
     The file name is too long (longer than PATH_MAX (Ibid., ``Limits on File-System Capacity''.)) or the host name is too long (in gethostname or sethostname (Ibid., ``Host Identification''.)).

 -- Macro: int EHOSTDOWN
     The remote host for a requested network connection is down.

 -- Macro: int EHOSTUNREACH
     The remote host for a requested network connection is not reachable.

 -- Macro: int ENOTEMPTY
     A directory was not empty, where an empty directory was expected. Typically, this error occurs when you are trying to delete a directory.

 -- Macro: int EPROCLIM
     This means that the per-user limit on new processes would be exceeded by an attempted fork. (For details on the RLIMIT_NPROC limit, see Loosemore et al., ``Limiting Resource Usage''.)

 -- Macro: int EUSERS
     The file quota system is confused because there are too many users.

 -- Macro: int EDQUOT
     The user's disk quota was exceeded.

 -- Macro: int ESTALE
     There is a stale NFS file handle.  This indicates an internal confusion in the NFS system which is due to file system rearrangements on the server host.  Repairing this condition usually requires unmounting and remounting the NFS file system on the local host.

 -- Macro: int EREMOTE
     An attempt was made to NFS-mount a remote file system with a file name that already specifies an NFS-mounted file. (This is an error on some operating systems, but we expect it to work properly on the GNU system, making this error code impossible.)

 -- Macro: int EBADRPC
     RPC struct is bad.

 -- Macro: int ERPCMISMATCH
     RPC version is wrong.

 -- Macro: int EPROGUNAVAIL
     RPC program is not available.

 -- Macro: int EPROGMISMATCH
     RPC program version is wrong.

 -- Macro: int EPROCUNAVAIL
     RPC procedure for program is bad.

 -- Macro: int ENOLCK
     No locks are available.  This is used by the file-locking facilities. (Ibid., ``File Locks''.)  This error is never generated by the GNU system, but it can result from an operation to an NFS server running another operating system.

 -- Macro: int EFTYPE
     This indicates an inappropriate file type or format.  The file was the wrong type for the operation, or a data file had the wrong format.

     On some systems chmod returns this error if you try to set the sticky bit on a nondirectory file. (Ibid., ``Assigning File Permissions''.)

 -- Macro: int EAUTH
     There was an authentication error.

 -- Macro: int ENEEDAUTH
     An authenticator is needed.

 -- Macro: int ENOSYS
     Function not implemented.  This indicates that the function called is not implemented at all, either in the C library itself or in the operating system.  When you get this error, you can be sure that this particular function will always fail with ENOSYS unless you install a new version of the C library or the operating system.

 -- Macro: int ENOTSUP
     Not supported.  A function returns this error when certain parameter values are valid, but the functionality they request is not available. This can mean that the function does not implement a particular command or option value or flag bit at all.  For functions that operate on some object given in a parameter, such as a file descriptor or a port, it might instead mean that only that specific object (file descriptor, port, etc.) is unable to support the other parameters given; different file descriptors might support different ranges of parameter values.

     If the entire function is not available at all in the implementation, it returns ENOSYS instead.

 -- Macro: int EILSEQ
     While decoding a multibyte character, the function came to an invalid or incomplete sequence of bytes, or the given wide character is invalid.

 -- Macro: int EBACKGROUND
     In the GNU system, servers supporting the term protocol return this error for certain operations when the caller is not in the foreground process group of the terminal.  Users do not usually see this error because functions such as read and write translate it into a SIGTTIN or SIGTTOU signal. (For information on process groups and these signals, see Loosemore et al., ``Job Control''.)

 -- Macro: int EDIED
     In the GNU system, opening a file returns this error when the file is translated by a program and the translator program dies while starting up, before it has connected to the file.

 -- Macro: int ED
     The experienced user will know what is wrong.

 -- Macro: int EGREGIOUS
     You did what?

 -- Macro: int EIEIO
     Go home and have a glass of warm, dairy-fresh milk.

 -- Macro: int EGRATUITOUS
     This error code has no purpose.

 -- Macro: int EBADMSG
     Message was bad.

 -- Macro: int EIDRM
     An identifier was removed.

 -- Macro: int EMULTIHOP
     A multihop was attempted.

 -- Macro: int ENODATA
     No data was available.

 -- Macro: int ENOLINK
     A link has been severed.

 -- Macro: int ENOMSG
     There is no message of the desired type.

 -- Macro: int ENOSR
     There are no more streams resources.

 -- Macro: int ENOSTR
     The device is not a stream.

 -- Macro: int EOVERFLOW
     The value is too large for the defined data type.

 -- Macro: int EPROTO
     There was a protocol error.

 -- Macro: int ETIME
     The timer expired.

 -- Macro: int ECANCELED
     Operation canceled; an asynchronous operation was canceled before it completed. (Ibid., ``Perform I/O Operations in Parallel''.)  When you call aio_cancel, the normal result is for the operations affected to complete with this error. (Ibid., ``Cancellation of AIO Operations''.)

The following error codes are defined by the Linux/i386 kernel. They are not yet documented.

 -- Macro: int ERESTART

 -- Macro: int ECHRNG

 -- Macro: int EL2NSYNC

 -- Macro: int EL3HLT

 -- Macro: int EL3RST

 -- Macro: int ELNRNG

 -- Macro: int EUNATCH

 -- Macro: int ENOCSI

 -- Macro: int EL2HLT

 -- Macro: int EBADE

 -- Macro: int EBADR

 -- Macro: int EXFULL

 -- Macro: int ENOANO

 -- Macro: int EBADRQC

 -- Macro: int EBADSLT

 -- Macro: int EDEADLOCK

 -- Macro: int EBFONT

 -- Macro: int ENONET

 -- Macro: int ENOPKG

 -- Macro: int EADV

 -- Macro: int ESRMNT

 -- Macro: int ECOMM

 -- Macro: int EDOTDOT

 -- Macro: int ENOTUNIQ

 -- Macro: int EBADFD

 -- Macro: int EREMCHG

 -- Macro: int ELIBACC

 -- Macro: int ELIBBAD

 -- Macro: int ELIBSCN

 -- Macro: int ELIBMAX

 -- Macro: int ELIBEXEC

 -- Macro: int ESTRPIPE

 -- Macro: int EUCLEAN

 -- Macro: int ENOTNAM

 -- Macro: int ENAVAIL

 -- Macro: int EISNAM

 -- Macro: int EREMOTEIO

 -- Macro: int ENOMEDIUM

 -- Macro: int EMEDIUMTYPE

Error Messages
==============

The library has functions and variables designed to make it easy for your program to report informative error messages in the customary format about the failure of a library call.  The functions strerror and perror give you the standard error message for a given error code; the variable program_invocation_short_name gives you convenient access to the name of the program that encountered the error.

 -- Function: char * strerror (int ERRNUM)
     The strerror function maps the error code (see Checking for Errors) specified by the ERRNUM argument to a descriptive error message string.  The return value is a pointer to this string.

     The value ERRNUM normally comes from the variable errno.

     You should not modify the string returned by strerror.  Also, if you make subsequent calls to strerror, the string might be overwritten.  (But it's guaranteed that no library function ever calls strerror behind your back.)

     The function strerror is declared in string.h.

 -- Function: char * strerror_r (int ERRNUM, char *BUF, size_t N)
     The strerror_r function works like strerror but instead of returning the error message in a statically allocated buffer shared by all threads in the process, it returns a private copy for the thread. This might be either some permanent global data or a message string in the user-supplied buffer starting at BUF with the length of N bytes.

     At most N characters are written (including the NUL byte), so it is up to the user to select the buffer large enough.

     This function should always be used in multithreaded programs since there is no way to guarantee that the string returned by strerror really belongs to the last call of the current thread.

     This function strerror_r is a GNU extension and it is declared in string.h.

 -- Function: void perror (const char *MESSAGE)
     This function prints an error message to the stream stderr (see Standard Streams).  The orientation of stderr is not changed.

     If you call perror with a MESSAGE that is either a null pointer or an empty string, perror just prints the error message corresponding to errno, adding a trailing newline.

     If you supply a nonnull MESSAGE argument, then perror prefixes its output with this string.  It adds a colon and a space character to separate the MESSAGE from the error string corresponding to errno.

     The function perror is declared in stdio.h.

strerror and perror produce the exact same message for any given error code; the precise text varies from system to system.  On the GNU system, the messages are fairly short; there are no multiline messages or embedded newlines.  Each error message begins with a capital letter and does not include any terminating punctuation.

Compatibility Note: The strerror function was introduced in ISO C89.  Many older C systems do not support this function yet.

Many programs that don't read input from the terminal are designed to exit if any system call fails.  By convention, the error message from such a program should start with the program's name, sans directories. You can find that name in the variable program_invocation_short_name; the full file name is stored the variable program_invocation_name.

 -- Variable: char * program_invocation_name
     This variable's value is the name that was used to invoke the program running in the current process.  It is the same as argv[0].  This is not necessarily a useful file name; often it contains no directory names (see Program Arguments).

 -- Variable: char * program_invocation_short_name
     This variable's value is the name that was used to invoke the program running in the current process, with directory names removed---it is the same as program_invocation_name minus everything up to the last slash, if any.

The library initialization code sets up both of these variables before calling main.

Portability Note: These two variables are GNU extensions.  If you want your program to work with non-GNU libraries, you must save the value of argv[0] in main, and then strip off the directory names yourself.  We added these extensions to make it possible to write self-contained error-reporting subroutines that require no explicit cooperation from main.

Here is an example showing how to handle a failure to open a file correctly.  The function open_sesame tries to open the named file for reading and returns a stream if successful.  The fopen library function returns a null pointer if it couldn't open the file for some reason.  In that situation, open_sesame constructs an appropriate error message using the strerror function, and terminates the program.  If we were going to make some other library calls before passing the error code to strerror, we would have to save it in a local variable instead, because those other library functions might overwrite errno in the meantime.

     #include <errno.h>
     #include <stdio.h>
     #include <stdlib.h>
     #include <string.h>

     FILE *
     open_sesame (char *name)
     {
       FILE *stream;

       errno = 0;
       stream = fopen (name, "r");
       if (stream == NULL)
         {
           fprintf (stderr, "%s: Couldn't open file %s; %s\n",
                    program_invocation_short_name, name, strerror (errno));
           exit (EXIT_FAILURE);
         }
       else
         return stream;
     }

Using perror has the advantage that the function is portable and available on all systems implementing ISO C.  But often the text perror generates is not what is wanted, and there is no way to extend or change what perror does.  The GNU coding standard, for instance, requires error messages to be preceded by the program name, and programs that read some input files should provide information about the input file name and the line number in case an error is encountered while reading the file.  For these occasions there are two functions available which are widely used throughout the GNU project.  These functions are declared in error.h.

 -- Function: void error (int STATUS, int ERRNUM, const char *FORMAT, ...)
     The error function can be used to report general problems during program execution.  The FORMAT argument is a format string just like those given to the printf family of functions.  The arguments required for the format can follow the FORMAT parameter. Just like perror, error also can report an error code in textual form.  But unlike perror the error value is explicitly passed to the function in the ERRNUM parameter.  This eliminates the problem mentioned above that the error-reporting function must be called immediately after the function causing the error, since otherwise errno might have a different value.

     The error prints first the program name.  If the application defined a global variable error_print_progname and points it to a function, this function will be called to print the program name. Otherwise, the string from the global variable program_name is used.  The program name is followed by a colon and a space, which in turn is followed by the output produced by the format string.  If the ERRNUM parameter is nonzero, the format string output is followed by a colon and a space, followed by the error message for the error code ERRNUM.  In any case, the output is terminated with a newline.

     The output is directed to the stderr stream.  If the stderr wasn't oriented before the call, it will be narrow-oriented afterward.

     The function will return unless the STATUS parameter has a nonzero value.  In this case, the function will call exit with the STATUS value for its parameter and therefore never return.  If error returns, the global variable error_message_count is incremented by one to keep track of the number of errors reported.

 -- Function: void error_at_line (int STATUS, int ERRNUM, const char *FNAME, unsigned int LINENO, const char *FORMAT, ...)

     The error_at_line function is very similar to the error function.  The only difference are the additional parameters FNAME and LINENO.  The handling of the other parameters is identical to that of error except that between the program name and the string generated by the format string additional text is inserted.

     Directly following the program name, a colon followed by the file name pointed to by FNAME, another colon, and a value of LINENO are printed.

     This additional output of course is meant to be used to locate an error in an input file (like a programming language source code file, for example).

     If the global variable error_one_per_line is set to a nonzero value error_at_line will avoid printing consecutive messages for the same file anem line.  Repetitions that are not directly following each other are not caught.

     Just like error, this function only returns if STATUS is zero.  Otherwise, exit is called with the nonzero value.  If error returns, the global variable error_message_count is incremented by one to keep track of the number of errors reported.

As mentioned above the error and error_at_line functions can be customized by defining a variable named error_print_progname.

 -- Variable: void (* error_print_progname ) (void)
     If the error_print_progname variable is defined to a nonzero value, the function pointed to is called by error or error_at_line.  It is expected to print the program name or do something similarly useful.

     The function is expected to be printed to the stderr stream, and must be able to handle whatever orientation the stream has.

     The variable is global and shared by all threads.

 -- Variable: unsigned int error_message_count
     The error_message_count variable is incremented whenever one of the functions error or error_at_line returns.  The variable is global and shared by all threads.

 -- Variable: int error_one_per_line
     The error_one_per_line variable influences only error_at_line.  Normally the error_at_line function creates output for every invocation.  If error_one_per_line is set to a nonzero value, error_at_line keeps track of the last file name and line number for which an error was reported and avoids directly following messages for the same file and line.  This variable is global and shared by all threads.

A program that read an input file and reports errors in it could look like this:

     {
       char *line = NULL;
       size_t len = 0;
       unsigned int lineno = 0;

       error_message_count = 0;
       while (! feof_unlocked (fp))
         {
           ssize_t n = getline (&line, &len, fp);
           if (n <= 0)
             /* End of file or error.  */
             break;
           ++lineno;

           /* Process the line.  */
           ...

           if (Detect error in line)
             error_at_line (0, errval, filename, lineno,
                            "some error text %s", some_variable);
         }

       if (error_message_count != 0)
         error (EXIT_FAILURE, 0, "%u errors found", error_message_count);
     }

error and error_at_line are clearly the functions of choice and enable the programmer to write applications that follow the GNU coding standard.  The GNU libc also contains functions that are used in BSD for the same purpose.  These functions are declared in err.h.  It is generally advised to not use these functions.  They are included only for compatibility.

 -- Function: void warn (const char *FORMAT, ...)
     The warn function is roughly equivalent to a call like:

            error (0, errno, format, the parameters)

     except the global variables that error respects and modifies are not used.

 -- Function: void vwarn (const char *FORMAT, va_list)
     The vwarn function is just like warn except that the parameters for the handling of the format string FORMAT are passed in as a value of type va_list.

 -- Function: void warnx (const char *FORMAT, ...)
     The warnx function is roughly equivalent to a call like:

            error (0, 0, format, the parameters)

     except the global variables that error respects and modifies are not used.  The difference with warn is that no error number string is printed.

 -- Function: void vwarnx (const char *FORMAT, va_list)
     The vwarnx function is just like warnx except that the parameters for the handling of the format string FORMAT are passed in as a value of type va_list.

 -- Function: void err (int STATUS, const char *FORMAT, ...)
     The err function is roughly equivalent to a call like:

            error (status, errno, format, the parameters)

     except that the global variables error respects and modifies are not used and that the program is exited even if STATUS is zero.

 -- Function: void verr (int STATUS, const char *FORMAT, va_list)
     The verr function is just like err except that the parameters for the handling of the format string FORMAT are passed in as a value of type va_list.

 -- Function: void errx (int STATUS, const char *FORMAT, ...)
     The errx function is roughly equivalent to a call like:

            error (status, 0, format, the parameters)

     except that the global variables error respects and modifies are not used and that the program is exited even if STATUS is zero.  The difference with err is that no error number string is printed.

 -- Function: void verrx (int STATUS, const char *FORMAT, va_list)
     The verrx function is just like errx except that the parameters for the handling of the format string FORMAT are passed in as a value of type va_list.

Virtual Memory Allocation and Paging
************************************

This chapter describes how processes manage and use memory in a system that uses the GNU C Library.

The GNU C Library has several functions for dynamically allocating virtual memory in various ways.  They vary in generality and in efficiency.  The library also provides functions for controlling paging and allocation of real memory.

Memory-mapped I/O is not discussed in this chapter. (See Loosemore et al., ``Memory-Mapped I/O'' (see chap. 1, n. 1).)

Process Memory Concepts
=======================

One of the most basic resources a process has available to it is memory. There are many different ways systems organize memory, but in a typical one, each process has one linear virtual address space, with addresses running from zero to some huge maximum.  It need not be contiguous; i.e.  not all of these addresses actually can be used to store data.

The virtual memory is divided into pages (4 kilobytes is typical). Backing each page of virtual memory is a page of real memory (called a frame) or some secondary storage, usually disk space.  The disk space might be swap space or just some ordinary disk file.  Actually, a page of all zeros sometimes has nothing at all backing it---there's just a flag saying it is all zeros.

The same frame of real memory or backing store can back multiple virtual pages belonging to multiple processes.  This is normally the case, for example, with virtual memory occupied by GNU C Library code.  The same real memory frame containing the printf function backs a virtual memory page in each of the existing processes that has a printf call in its program.

In order for a program to access any part of a virtual page, the page must at that moment be backed by (connected to) a real frame.  But because there is usually a lot more virtual memory than real memory, the pages must move back and forth between real memory and backing store regularly, coming into real memory when a process needs to access them and then retreating to backing store when not needed anymore.  This movement is called paging.

When a program attempts to access a page which is not at that moment backed by real memory, this is known as a page fault.  When a page fault occurs, the kernel suspends the process, places the page into a real page frame (this is called "paging in" or "faulting in"), then resumes the process so that from the process' point of view, the page was in real memory all along.  In fact, to the process, all pages always seem to be in real memory.  Except for one thing: the elapsed execution time of an instruction that would normally be a few nanoseconds is suddenly much, much, longer (because the kernel normally has to do I/O to complete the page-in).  For programs sensitive to that, the functions described in Locking Pages can control it.

Within each virtual address space, a process has to keep track of what is at which addresses, and that process is called memory allocation. Allocation usually brings to mind meting out scarce resources, but in the case of virtual memory, that's not a major goal, because there is generally much more of it than anyone needs.  Memory allocation within a process is mainly just a matter of making sure that the same byte of memory isn't used to store two different things.

Processes allocate memory in two major ways: by exec and programmatically.  Actually, forking is a third way, but it's not very interesting. (Ibid., ``Creating a Process''.)

Exec is the operation of creating a virtual address space for a process, loading its basic program into it, and executing the program.  It is done by the exec family of functions (execl, for example). The operation takes a program file (an executable), allocates space to load all the data in the executable, loads it, and transfers control to it.  The most notable data are the instructions of the program (the text), but also literals and constants in the program and even some variables---C variables with the static storage class (see Memory Allocation and C).

Once that program begins to execute, it uses programmatic allocation to gain additional memory.  In a C program with the GNU C Library, there are two kinds of programmatic allocation: automatic and dynamic (see Memory Allocation and C).

Memory-mapped I/O is another form of dynamic virtual memory allocation. Mapping memory to a file means declaring that the contents of certain range of a process' addresses shall be identical to the contents of a specified regular file.  The system makes the virtual memory initially contain the contents of the file, and if you modify the memory, the system writes the same modification to the file.  Note that due to the magic of virtual memory and page faults, there is no reason for the system to do I/O to read the file, or allocate real memory for its contents, until the program accesses the virtual memory. (Ibid., ``Memory-Mapped I/O''.)

Just as it programmatically allocates memory, the program can programmatically deallocate (free) it.  You can't free the memory that was allocated by exec.  When the program exits or execs, you might say that all its memory gets freed, but since in both cases the address space ceases to exist, the point is really moot (see Program Termination).

A process' virtual address space is divided into segments.  A segment is a contiguous range of virtual addresses.  Three important segments are

  * The text segment contains a program's instructions and literals and static constants.  It is allocated by exec and stays the same size for the life of the virtual address space.

  * The data segment is working storage for the program.  It can be preallocated and preloaded by exec and the process can extend or shrink it by calling functions as described in Resizing the Data Segment.  Its lower end is fixed.

  * The stack segment contains a program stack.  It grows as the stack grows, but doesn't shrink when the stack shrinks.

Allocating Storage for Program Data
===================================

This section covers how ordinary programs manage storage for their data, including the famous malloc function and some fancier facilities that are special to the GNU C Library and GNU Compiler.

Memory Allocation in C Programs
-------------------------------

The C language supports two kinds of memory allocation through the variables in C programs:

  * Static allocation is what happens when you declare a static or global variable.  Each static or global variable defines one block of space, of a fixed size.  The space is allocated once, when your program is started (part of the exec operation), and is never freed.

  * Automatic allocation happens when you declare an automatic variable, such as a function argument or a local variable.  The space for an automatic variable is allocated when the compound statement containing the declaration is entered, and is freed when that compound statement is exited.

     In GNU C, the size of the automatic storage can be an expression that varies.  In other C implementations, it must be a constant.

A third important kind of memory allocation, dynamic allocation, is not supported by C variables but is available via GNU C Library functions.

Dynamic Memory Allocation
.........................

Dynamic memory allocation is a technique in which programs determine as they are running where to store some information.  You need dynamic allocation when the amount of memory you need, or how long you continue to need it, depends on factors that are not known before the program runs.

For example, you may need a block to store a line read from an input file; since there is no limit to how long a line can be, you must allocate the memory dynamically and make it dynamically larger as you read more of the line.

Or, you may need a block for each record or each definition in the input data; since you can't know in advance how many there will be, you must allocate a new block for each record or definition as you read it.

When you use dynamic allocation, the allocation of a block of memory is an action that the program requests explicitly.  You call a function or macro when you want to allocate space, and specify the size with an argument.  If you want to free the space, you do so by calling another function or macro.  You can do these things whenever you want, as often as you want.

Dynamic allocation is not supported by C variables; there is no storage class dynamic, and there can never be a C variable whose value is stored in dynamically allocated space.  The only way to get dynamically allocated memory is via a system call (which is generally via a GNU C Library function call), and the only way to refer to dynamically allocated space is through a pointer.  Because it is less convenient, and because the actual process of dynamic allocation requires more computation time, programmers generally use dynamic allocation only when neither static nor automatic allocation will serve.

For example, if you want to allocate dynamically some space to hold a struct foobar, you cannot declare a variable of type struct foobar whose contents are the dynamically allocated space.  But you can declare a variable of pointer type struct foobar * and assign it the address of the space.  Then you can use the operators * and -> on this pointer variable to refer to the contents of the space:

     {
       struct foobar *ptr
          = (struct foobar *) malloc (sizeof (struct foobar));
       ptr->name = x;
       ptr->next = current_foobar;
       current_foobar = ptr;
     }

Unconstrained Allocation
------------------------

The most general dynamic allocation facility is malloc.  It allows you to allocate blocks of memory of any size at any time, make them bigger or smaller at any time, and free the blocks individually at any time (or never).

Basic Memory Allocation
.......................

To allocate a block of memory, call malloc.  The prototype for this function is in stdlib.h.

 -- Function: void * malloc (size_t SIZE)
     This function returns a pointer to a newly allocated block SIZE bytes long, or a null pointer if the block could not be allocated.

The contents of the block are undefined; you must initialize it yourself (or use calloc instead; see Allocating Cleared Space). Normally you would cast the value as a pointer to the kind of object that you want to store in the block.  Here we show an example of doing so, and of initializing the space with zeros using the library function memset (see Copying and Concatenation):

     struct foo *ptr;
     ...
     ptr = (struct foo *) malloc (sizeof (struct foo));
     if (ptr == 0) abort ();
     memset (ptr, 0, sizeof (struct foo));

You can store the result of malloc into any pointer variable without a cast, because ISO C automatically converts the type void * to another type of pointer when necessary.  But the cast is necessary in contexts other than assignment operators or if you might want your code to run in traditional C.

Remember that when allocating space for a string, the argument to malloc must be one plus the length of the string.  This is because a string is terminated with a null character that doesn't count in the "length" of the string but does need space.  For example:

     char *ptr;
     ...
     ptr = (char *) malloc (length + 1);

See Representation of Strings, for more information about this.

Examples of malloc
..................

If no more space is available, malloc returns a null pointer. You should check the value of every call to malloc.  It is useful to write a subroutine that calls malloc and reports an error if the value is a null pointer, returning only if the value is nonzero.  This function is conventionally called xmalloc.  Here it is:

     void *
     xmalloc (size_t size)
     {
       register void *value = malloc (size);
       if (value == 0)
         fatal ("virtual memory exhausted");
       return value;
     }

Here is a real example of using malloc (by way of xmalloc). The function savestring will copy a sequence of characters into a newly allocated null-terminated string:

     char *
     savestring (const char *ptr, size_t len)
     {
       register char *value = (char *) xmalloc (len + 1);
       value[len] = '\0';
       return (char *) memcpy (value, ptr, len);
     }

The block that malloc gives you is guaranteed to be aligned so that it can hold any type of data.  In the GNU system, the address is always a multiple of eight on most systems, and a multiple of sixteen on 64-bit systems.  Only rarely is any higher boundary (such as a page boundary) necessary; for those cases, use memalign, posix_memalign or valloc (see Aligned Memory Blocks).

Note that the memory located after the end of the block is likely to be in use for something else; perhaps a block already allocated by another call to malloc.  If you attempt to treat the block as longer than you asked for it to be, you are liable to destroy the data that malloc uses to keep track of its blocks, or you may destroy the contents of another block.  If you have already allocated a block and discover you want it to be bigger, use realloc (see Changing Block Size).

Freeing Memory Allocated with malloc
....................................

When you no longer need a block that you got with malloc, use the function free to make the block available to be allocated again. The prototype for this function is in stdlib.h.

 -- Function: void free (void *PTR)
     The free function deallocates the block of memory pointed at by PTR.

 -- Function: void cfree (void *PTR)
     This function does the same thing as free.  It's provided for backward compatibility with SunOS; you should use free instead.

Freeing a block alters the contents of the block.  Do not expect to find any data (such as a pointer to the next block in a chain of blocks) in the block after freeing it.  Copy whatever you need out of the block before freeing it!  Here is an example of the proper way to free all the blocks in a chain, and the strings that they point to:

     struct chain
       {
         struct chain *next;
         char *name;
       }

     void
     free_chain (struct chain *chain)
     {
       while (chain != 0)
         {
           struct chain *next = chain->next;
           free (chain->name);
           free (chain);
           chain = next;
         }
     }

Occasionally, free can actually return memory to the operating system and make the process smaller.  Usually, all it can do is allow a later call to malloc to reuse the space.  In the meantime, the space remains in your program as part of a free-list used internally by malloc.

There is no point in freeing blocks at the end of a program, because all of the program's space is given back to the system when the process terminates.

Changing the Size of a Block
............................

Often you do not know for certain how big a block you will ultimately need at the time you must begin to use the block.  For example, the block might be a buffer that you use to hold a line being read from a file; no matter how long you make the buffer initially, you may encounter a line that is longer.

You can make the block longer by calling realloc.  This function is declared in stdlib.h.

 -- Function: void * realloc (void *PTR, size_t NEWSIZE)
     The realloc function changes the size of the block whose address is PTR to be NEWSIZE.

     Since the space after the end of the block may be in use, realloc may find it necessary to copy the block to a new address where more free space is available.  The value of realloc is the new address of the block.  If the block needs to be moved, realloc copies the old contents.

     If you pass a null pointer for PTR, realloc behaves just like malloc (NEWSIZE).  This can be convenient, but be aware that older implementations (before ISO C) may not support this behavior, and will probably crash when realloc is passed a null pointer.

Like malloc, realloc may return a null pointer if no memory space is available to make the block bigger.  When this happens, the original block is untouched; it has not been modified or relocated.

In most cases it makes no difference what happens to the original block when realloc fails, because the application program cannot continue when it is out of memory, and the only thing to do is to give a fatal error message.  Often it is convenient to write and use a subroutine, conventionally called xrealloc, that takes care of the error message as xmalloc does for malloc:

     void *
     xrealloc (void *ptr, size_t size)
     {
       register void *value = realloc (ptr, size);
       if (value == 0)
         fatal ("Virtual memory exhausted");
       return value;
     }

You can also use realloc to make a block smaller.  The reason you would do this is to avoid tying up a lot of memory space when only a little is needed. In several allocation implementations, making a block smaller sometimes necessitates copying it, so it can fail if no other space is available.

If the new size you specify is the same as the old size, realloc is guaranteed to change nothing and return the same address that you gave.

Allocating Cleared Space
........................

The function calloc allocates memory and clears it to zero.  It is declared in stdlib.h.

 -- Function: void * calloc (size_t COUNT, size_t ELTSIZE)
     This function allocates a block long enough to contain a vector of COUNT elements, each of size ELTSIZE.  Its contents are cleared to zero before calloc returns.

You could define calloc as follows:

     void *
     calloc (size_t count, size_t eltsize)
     {
       size_t size = count * eltsize;
       void *value = malloc (size);
       if (value != 0)
         memset (value, 0, size);
       return value;
     }

But in general, it is not guaranteed that calloc calls malloc internally.  Therefore, if an application provides its own malloc/realloc/free outside the C library, it should always define calloc, too.

Efficiency Considerations for malloc
....................................

As opposed to other versions, the malloc in the GNU C Library does not round up block sizes to powers of two, either for large or small sizes.  Neighboring chunks can be coalesced on a free no matter what their size is.  This makes the implementation suitable for all kinds of allocation patterns, without generally incurring high memory waste through fragmentation.

Very large blocks (much larger than a page) are allocated with mmap (anonymous or via /dev/zero) by this implementation. This has the great advantage that these chunks are returned to the system immediately when they are freed.  Therefore, it cannot happen that a large chunk becomes locked between smaller ones and, even after calling free, wastes memory.  The size threshold for mmap to be used can be adjusted with mallopt.  The use of mmap can also be disabled completely.

Allocating Aligned Memory Blocks
................................

The address of a block returned by malloc or realloc in the GNU system is always a multiple of eight (or sixteen on 64-bit systems).  If you need a block whose address is a multiple of a higher power of two than that, use memalign, posix_memalign, or valloc.  memalign is declared in malloc.h and posix_memalign is declared in stdlib.h.

With the GNU library, you can use free to free the blocks that memalign, posix_memalign, and valloc return.  That does not work in BSD, however---BSD does not provide any way to free such blocks.

 -- Function: void * memalign (size_t BOUNDARY, size_t SIZE)
     The memalign function allocates a block of SIZE bytes whose address is a multiple of BOUNDARY.  The BOUNDARY must be a power of two!  The function memalign works by allocating a somewhat larger block, and then returning an address within the block that is on the specified boundary.

 -- Function: int posix_memalign (void **MEMPTR, size_t ALIGNMENT, size_t SIZE)
     The posix_memalign function is similar to the memalign function in that it returns a buffer of SIZE bytes aligned to a multiple of ALIGNMENT.  But it adds one requirement to the parameter ALIGNMENT: the value must be a power-of-two multiple of sizeof (void *).

     If the function succeeds in allocating memory, a pointer to the allocated memory is returned in *MEMPTR and the return value is zero. Otherwise, the function returns an error value indicating the problem.

     This function was introduced in POSIX 1003.1d.

 -- Function: void * valloc (size_t SIZE)
     Using valloc is like using memalign and passing the page size as the value of the second argument.  It is implemented like this: (See Loosemore et al., ``Query Memory Parameters'', for more information about the memory subsystem.)

          void *
          valloc (size_t size)
          {
            return memalign (getpagesize (), size);
          }

malloc Tunable Parameters
.........................

You can adjust some parameters for dynamic memory allocation with the mallopt function.  This function is the general SVID/XPG interface, defined in malloc.h.

 -- Function: int mallopt (int PARAM, int VALUE)
     When calling mallopt, the PARAM argument specifies the parameter to be set, and VALUE the new value to be set.  Possible choices for PARAM, as defined in malloc.h, are

     M_TRIM_THRESHOLD
          This is the minimum size (in bytes) of the topmost, releasable chunk that will cause sbrk to be called with a negative argument in order to return memory to the system.

     M_TOP_PAD
          This parameter determines the amount of extra memory to obtain from the system when a call to sbrk is required.  It also specifies the number of bytes to retain when shrinking the heap by calling sbrk with a negative argument.  This provides the necessary hysteresis in heap size such that excessive amounts of system calls can be avoided.

     M_MMAP_THRESHOLD
          All chunks larger than this value are allocated outside the normal heap, using the mmap system call.  This way it is guaranteed that the memory for these chunks can be returned to the system on free.  Requests smaller than this threshold might still be allocated via mmap.

     M_MMAP_MAX
          The maximum number of chunks to allocate with mmap.  Setting this to zero disables all use of mmap.

Heap Consistency Checking
.........................

You can ask malloc to check the consistency of dynamic memory by using the mcheck function.  This function is a GNU extension, declared in mcheck.h.

 -- Function: int mcheck (void (*ABORTFN) (enum mcheck_status STATUS))
     Calling mcheck tells malloc to perform occasional consistency checks.  These will catch things such as writing past the end of a block that was allocated with malloc.

     The ABORTFN argument is the function to call when an inconsistency is found.  If you supply a null pointer, then mcheck uses a default function that prints a message and calls abort (see Aborting a Program).  The function you supply is called with one argument, which says what sort of inconsistency was detected; its type is described below.

     It is too late to begin allocation checking once you have allocated anything with malloc,  so mcheck does nothing in that case.  The function returns -1 if you call it too late, and 0 otherwise (when it is successful).

     The easiest way to arrange to call mcheck early enough is to use the option -lmcheck when you link your program; then you don't need to modify your program source at all.  Alternatively, you might use a debugger to insert a call to mcheck whenever the program is started. For example, these gdb commands will automatically call mcheck whenever the program starts:

          (gdb) break main
          Breakpoint 1, main (argc=2, argv=0xbffff964) at whatever.c:10
          (gdb) command 1
          Type commands for when breakpoint 1 is hit, one per line.
          End with a line saying just "end".
          >call mcheck(0)
          >continue
          >end
          (gdb) ...

     However, this will only work if no initialization function of any object involved calls any of the malloc functions, since mcheck must be called before the first such function.

 -- Function: enum mcheck_status mprobe (void *POINTER)
     The mprobe function lets you explicitly check for inconsistencies in a particular allocated block.  You must have already called mcheck at the beginning of the program to do its occasional checks; calling mprobe requests an additional consistency check to be done at the time of the call.

     The argument POINTER must be a pointer returned by malloc or realloc.  mprobe returns a value that says what inconsistency, if any, was found.  The values are described below.

 -- Data Type: enum mcheck_status
     This enumerated type describes what kind of inconsistency was detected in an allocated block, if any.  Here are the possible values:

     MCHECK_DISABLED
          mcheck was not called before the first allocation. No consistency checking can be done.

     MCHECK_OK
          No inconsistency was detected.

     MCHECK_HEAD
          The data immediately before the block were modified.  This commonly happens when an array index or pointer is decremented too far.

     MCHECK_TAIL
          The data immediately after the block were modified.  This commonly happens when an array index or pointer is incremented too far.

     MCHECK_FREE
          The block was already freed.

Another way to check for and guard against bugs in the use of malloc, realloc and free is to set the environment variable MALLOC_CHECK_.  When MALLOC_CHECK_ is set, a special (less efficient) implementation is used, which is designed to be tolerant against simple errors, such as double calls of free with the same argument, or overruns of a single byte (off-by-one bugs).  Not all such errors can be protected against, however, and memory leaks can result.  If MALLOC_CHECK_ is set to 0, any detected heap corruption is silently ignored; if set to 1, a diagnostic is printed on stderr; if set to 2, abort is called immediately.  This can be useful because otherwise a crash may happen much later, and the true cause for the problem is then very hard to track down.

There is one problem with MALLOC_CHECK_; in SUID or SGID binaries, it could possibly be exploited since, diverging from the normal program behavior, it now writes something to the standard error descriptor. Therefore, the use of MALLOC_CHECK_ is disabled by default for SUID and SGID binaries.  It can be enabled again by the system administrator by adding a file /etc/suid-debug (the content is not important---it could be empty).

So, what is the difference between using MALLOC_CHECK_ and linking with -lmcheck?  MALLOC_CHECK_ is orthogonal with respect to -lmcheck.  -lmcheck has been added for backward compatibility.  Both MALLOC_CHECK_ and -lmcheck should uncover the same bugs---but using MALLOC_CHECK_, you don't need to recompile your application.

Memory Allocation Hooks
.......................

The GNU C Library lets you modify the behavior of malloc, realloc, and free by specifying appropriate hook functions.  You can use these hooks to help you debug programs that use dynamic memory allocation, for example.

The hook variables are declared in malloc.h.

 -- Variable: __malloc_hook
     The value of this variable is a pointer to the function that malloc uses whenever it is called.  You should define this function to look like malloc:

          void *FUNCTION (size_t SIZE, const void *CALLER)

     The value of CALLER is the return address found on the stack when the malloc function was called.  This value allows you to trace the memory consumption of the program.

 -- Variable: __realloc_hook
     The value of this variable is a pointer to the function that realloc uses whenever it is called.  You should define this function to look like realloc:

          void *FUNCTION (void *PTR, size_t SIZE, const void *CALLER)

     The value of CALLER is the return address found on the stack when the realloc function was called.  This value allows you to trace the memory consumption of the program.

 -- Variable: __free_hook
     The value of this variable is a pointer to the function that free uses whenever it is called.  You should define this function to look like free:

          void FUNCTION (void *PTR, const void *CALLER)

     The value of CALLER is the return address found on the stack when the free function was called.  This value allows you to trace the memory consumption of the program.

 -- Variable: __memalign_hook
     The value of this variable is a pointer to the function that memalign uses whenever it is called.  You should define this function to look like memalign:

          void *FUNCTION (size_t ALIGNMENT, size_t SIZE, const void *CALLER)

     The value of CALLER is the return address found on the stack when the memalign function was called.  This value allows you to trace the memory consumption of the program.

You must make sure that the function you install as a hook for one of these functions does not call that function recursively without restoring the old value of the hook first!  Otherwise, your program will get stuck in an infinite recursion.  Before calling the function recursively, you should make sure to restore all the hooks to their previous value.  When coming back from the recursive call, all the hooks should be resaved, since a hook might modify itself.

 -- Variable: __malloc_initialize_hook
     The value of this variable is a pointer to a function that is called once when the malloc implementation is initialized.  This is a weak variable, so it can be overridden in the application with a definition like the following:

          void (*__MALLOC_INITIALIZE_HOOK) (void) = my_init_hook;

An issue to look out for is the time at which the malloc hook functions can be safely installed.  If the hook functions call the malloc-related functions recursively, it is necessary that malloc already have properly initialized itself when the function (__malloc_hook, for example) is assigned to.  On the other hand, if the hook functions provide a complete malloc implementation of their own, it is vital that the hooks are assigned to before the very first malloc call has completed, because otherwise a chunk obtained from the ordinary, unhooked malloc may later be handed to, for example, __free_hook.

In both cases, the problem can be solved by setting up the hooks from within a user-defined function pointed to by __malloc_initialize_hook---then the hooks will be set up safely at the right time.

Here is an example showing how to use __malloc_hook and __free_hook properly.  It installs a function that prints out information every time malloc or free is called.  We just assume here that realloc and memalign are not used in our program.

     /* Prototypes for __malloc_hook, __free_hook */
     #include <malloc.h>

     /* Prototypes for our hooks.  */
     static void *my_init_hook (void);
     static void *my_malloc_hook (size_t, const void *);
     static void my_free_hook (void*, const void *);

     /* Override initializing hook from the C library. */
     void (*__malloc_initialize_hook) (void) = my_init_hook;

     static void
     my_init_hook (void)
     {
       old_malloc_hook = __malloc_hook;
       old_free_hook = __free_hook;
       __malloc_hook = my_malloc_hook;
       __free_hook = my_free_hook;
     }

     static void *
     my_malloc_hook (size_t size, const void *caller)
     {
       void *result;
       /* Restore all old hooks */
       __malloc_hook = old_malloc_hook;
       __free_hook = old_free_hook;
       /* Call recursively */
       result = malloc (size);
       /* Save underlying hooks */
       old_malloc_hook = __malloc_hook;
       old_free_hook = __free_hook;
       /* printf might call malloc, so protect it too. */
       printf ("malloc (%u) returns %p\n", (unsigned int) size, result);
       /* Restore our own hooks */
       __malloc_hook = my_malloc_hook;
       __free_hook = my_free_hook;
       return result;
     }

     static void *
     my_free_hook (void *ptr, const void *caller)
     {
       /* Restore all old hooks */
       __malloc_hook = old_malloc_hook;
       __free_hook = old_free_hook;
       /* Call recursively */
       free (ptr);
       /* Save underlying hooks */
       old_malloc_hook = __malloc_hook;
       old_free_hook = __free_hook;
       /* printf might call free, so protect it too. */
       printf ("freed pointer %p\n", ptr);
       /* Restore our own hooks */
       __malloc_hook = my_malloc_hook;
       __free_hook = my_free_hook;
     }

     main ()
     {
       ...
     }

The mcheck function (see Heap Consistency Checking) works by installing such hooks.

Statistics for Memory Allocation with malloc
............................................

You can get information about dynamic memory allocation by calling the mallinfo function.  This function and its associated data type are declared in malloc.h; they are an extension of the standard SVID/XPG version.

 -- Data Type: struct mallinfo
     This structure type is used to return information about the dynamic memory allocator.  It contains the following members:

     int arena
          This is the total size of memory allocated with sbrk by malloc, in bytes.

     int ordblks
          This is the number of chunks not in use.  (The memory allocator internally gets chunks of memory from the operating system, and then carves them up to satisfy individual malloc requests; see Efficiency and Malloc.)

     int smblks
          This field is unused.

     int hblks
          This is the total number of chunks allocated with mmap.

     int hblkhd
          This is the total size of memory allocated with mmap, in bytes.

     int usmblks
          This field is unused.

     int fsmblks
          This field is unused.

     int uordblks
          This is the total size of memory occupied by chunks handed out by malloc.

     int fordblks
          This is the total size of memory occupied by free (not in use) chunks.

     int keepcost
          This is the size of the topmost releasable chunk that normally borders the end of the heap (i.e., the high end of the virtual address space's data segment).

 -- Function: struct mallinfo mallinfo (void)
     This function returns information about the current dynamic memory usage in a structure of type struct mallinfo.

Summary of malloc-Related Functions
...................................

Here is a summary of the functions that work with malloc:

void *malloc (size_t SIZE)
     Allocate a block of SIZE bytes (see Basic Allocation).

void free (void *ADDR)
     Free a block previously allocated by malloc (see Freeing after Malloc).

void *realloc (void *ADDR, size_t SIZE)
     Make a block previously allocated by malloc larger or smaller, possibly by copying it to a new location (see Changing Block Size).

void *calloc (size_t COUNT, size_t ELTSIZE)
     Allocate a block of COUNT * ELTSIZE bytes using malloc, and set its contents to zero (see Allocating Cleared Space).

void *valloc (size_t SIZE)
     Allocate a block of SIZE bytes, starting on a page boundary (see Aligned Memory Blocks).

void *memalign (size_t SIZE, size_t BOUNDARY)
     Allocate a block of SIZE bytes, starting on an address that is a multiple of BOUNDARY (see Aligned Memory Blocks).

int mallopt (int PARAM, int VALUE)
     Adjust a tunable parameter (see Malloc Tunable Parameters).

int mcheck (void (*ABORTFN) (void))
     Tell malloc to perform occasional consistency checks on dynamically allocated memory, and to call ABORTFN when an inconsistency is found (see Heap Consistency Checking).

void *(*__malloc_hook) (size_t SIZE, const void *CALLER)
     This is a pointer to a function that malloc uses whenever it is called.

void *(*__realloc_hook) (void *PTR, size_t SIZE, const void *CALLER)
     This is a pointer to a function that realloc uses whenever it is called.

void (*__free_hook) (void *PTR, const void *CALLER)
     This is a pointer to a function that free uses whenever it is called.

void (*__memalign_hook) (size_t SIZE, size_t ALIGNMENT, const void *CALLER)
     This is a pointer to a function that memalign uses whenever it is called.

struct mallinfo mallinfo (void)
     Return information about the current dynamic memory usage (see Statistics of Malloc).

Allocation Debugging
--------------------

A complicated task when programming with languages that do not use garbage-collected dynamic memory allocation is to find memory leaks. Long running programs must assure that dynamically allocated objects are freed at the end of their lifetime.  If this does not happen, the system runs out of memory, sooner or later.

The malloc implementation in the GNU C Library provides some simple means to detect such leaks and obtain some information on their location.  To do this the application must be started in a special mode, which is enabled by an environment variable.  There are no speed penalties for the program if the debugging mode is not enabled.

How to Install the Tracing Functionality
........................................

 -- Function: void mtrace (void)
     When the mtrace function is called, it looks for an environment variable named MALLOC_TRACE.  This variable is supposed to contain a valid file name.  The user must have write access.  If the file already exists, it is truncated.  If the environment variable is not set or it does not name a valid file that can be opened for writing, nothing is done.  The behavior of, for example, malloc, is not changed. For obvious reasons, this also happens if the application is installed with the SUID or SGID bit set.

     If the named file is successfully opened, mtrace installs special handlers for the functions malloc, realloc and free (see Hooks for Malloc).  From then on, all uses of these functions are traced and protocolled into the file.  There is now of course a speed penalty for all calls to the traced functions, so tracing should not be enabled during normal use.

     This function is a GNU extension and generally not available on other systems.  The prototype can be found in mcheck.h.

 -- Function: void muntrace (void)
     The muntrace function can be called after mtrace was used to enable tracing the malloc calls.  If no (successful) call of mtrace was made, muntrace does nothing.

     Otherwise, it deinstalls the handlers for malloc, realloc and free, then closes the protocol file.  No calls are protocolled anymore and the program runs again at full speed.

     This function is a GNU extension and generally not available on other systems.  The prototype can be found in mcheck.h.

Example Program Excerpts
........................

Even though the tracing functionality does not influence the run-time behavior of the program, it is not a good idea to call mtrace in all programs.  Just imagine that you debug a program using mtrace and all other programs used in the debugging session also trace their malloc calls.  The output file would be the same for all programs and thus unusable.  Therefore, one should call mtrace only if compiled for debugging.  A program could therefore start like this:

     #include <mcheck.h>

     int
     main (int argc, char *argv[])
     {
     #ifdef DEBUGGING
       mtrace ();
     #endif
       ...
     }

This is all that is needed if you want to trace the calls during the whole run-time of the program.  Alternatively, you can stop the tracing at any time with a call to muntrace.  It is even possible to restart the tracing again with a new call to mtrace.  But this can cause unreliable results, since there may be calls of the functions that are not called.  Please note that not only the application uses the traced functions---libraries (including the C library itself) also use these functions.

This last point is also why it is not a good idea to call muntrace before the program terminates.  The libraries are informed about the termination of the program only after the program returns from main or calls exit and so cannot free the memory they use before this time.

So the best thing you can do is to call mtrace as the very first function in the program and never call muntrace.  So the program traces almost all uses of the malloc functions (except those calls that are executed by constructors of the program or used libraries).

Some More or Less Clever Ideas
..............................

You know the situation.  The program is prepared for debugging and in all debugging sessions it runs well.  But once it is started without debugging the error shows up.  A typical example is a memory leak that becomes visible only when we turn off the debugging.  If you foresee such situations you can still win.  Simply use something equivalent to the following little program:

     #include <mcheck.h>
     #include <signal.h>

     static void
     enable (int sig)
     {
       mtrace ();
       signal (SIGUSR1, enable);
     }

     static void
     disable (int sig)
     {
       muntrace ();
       signal (SIGUSR2, disable);
     }

     int
     main (int argc, char *argv[])
     {
       ...

       signal (SIGUSR1, enable);
       signal (SIGUSR2, disable);

       ...
     }

The user can start the memory debugger any time she wants if the program was started with MALLOC_TRACE set in the environment.  The output will of course not show the allocations that happened before the first signal, but if there is a memory leak, this will nevertheless show up.

Interpreting the Traces
.......................

If you take a look at the output it will look similar to this:

     = Start
      [0x8048209] - 0x8064cc8
      [0x8048209] - 0x8064ce0
      [0x8048209] - 0x8064cf8
      [0x80481eb] + 0x8064c48 0x14
      [0x80481eb] + 0x8064c60 0x14
      [0x80481eb] + 0x8064c78 0x14
      [0x80481eb] + 0x8064c90 0x14
     = End

What this all means is not really important since the trace file is not meant to be read by a human.  Therefore no attention is given to readability.  Instead there is a program that comes with the GNU C library that interprets the traces and outputs a summary in a user-friendly way.  The program is called mtrace (it is in fact a Perl script) and it takes one or two arguments.  In any case, the name of the file with the trace output must be specified.  If an optional argument precedes the name of the trace file, this must be the name of the program that generated the trace.

     drepper$ mtrace tst-mtrace log
     No memory leaks.

In this case, the program tst-mtrace was run and it produced a trace file log.  The message printed by mtrace shows there are no problems with the code---all allocated memory was freed afterwards.

If we call mtrace on the example trace given above, we would get a different output:

     drepper$ mtrace errlog
     - 0x08064cc8 Free 2 was never alloc'd 0x8048209
     - 0x08064ce0 Free 3 was never alloc'd 0x8048209
     - 0x08064cf8 Free 4 was never alloc'd 0x8048209

     Memory not freed:
     -----------------
        Address     Size     Caller
     0x08064c48     0x14  at 0x80481eb
     0x08064c60     0x14  at 0x80481eb
     0x08064c78     0x14  at 0x80481eb
     0x08064c90     0x14  at 0x80481eb

We have called mtrace with only one argument, so the script has no chance to find out what is meant with the addresses given in the trace.  We can do better:

     drepper$ mtrace tst errlog
     - 0x08064cc8 Free 2 was never alloc'd /home/drepper/tst.c:39
     - 0x08064ce0 Free 3 was never alloc'd /home/drepper/tst.c:39
     - 0x08064cf8 Free 4 was never alloc'd /home/drepper/tst.c:39

     Memory not freed:
     -----------------
        Address     Size     Caller
     0x08064c48     0x14  at /home/drepper/tst.c:33
     0x08064c60     0x14  at /home/drepper/tst.c:33
     0x08064c78     0x14  at /home/drepper/tst.c:33
     0x08064c90     0x14  at /home/drepper/tst.c:33

Suddenly the output makes much more sense and the user can see immediately where the function calls causing the trouble can be found.

Interpreting this output is not complicated.  There are at most two different situations being detected.  First, free was called for pointers that were never returned by one of the allocation functions. This is usually a very bad problem and what this looks like is shown in the first three lines of the output.  Situations like this are quite rare and if they appear show up drastically---the program normally crashes.

The other situation, which is much harder to detect, is memory leaks.  As you can see in the output, the mtrace function collects all this information and so can say that the program calls an allocation function from line 33 in the source file /home/drepper/tst-mtrace.c four times without freeing this memory before the program terminates. Whether this is a real problem remains to be investigated.

Obstacks
--------

An obstack is a pool of memory containing a stack of objects.  You can create any number of separate obstacks, and then allocate objects in specified obstacks.  Within each obstack, the last object allocated must always be the first one freed, but distinct obstacks are independent of each other.

Aside from this one constraint on the order of freeing, obstacks are totally general---an obstack can contain any number of objects of any size.  They are implemented with macros, so allocation is usually very fast as long as the objects are usually small.  And the only space overhead per object is the padding needed to start each object on a suitable boundary.

Creating Obstacks
.................

The utilities for manipulating obstacks are declared in the header file obstack.h.

 -- Data Type: struct obstack
     An obstack is represented by a data structure of type struct obstack.  This structure has a small fixed size; it records the status of the obstack and how to find the space in which objects are allocated. It does not contain any of the objects themselves.  You should not try to access the contents of the structure directly; use only the functions described in this chapter.

You can declare variables of type struct obstack and use them as obstacks, or you can allocate obstacks dynamically like any other kind of object.  Dynamic allocation of obstacks allows your program to have a variable number of different stacks.  (You can even allocate an obstack structure in another obstack, but this is rarely useful.)

All the functions that work with obstacks require you to specify which obstack to use.  You do this with a pointer of type struct obstack *.  In the following, we often say "an obstack" when, strictly speaking, the object at hand is such a pointer.

The objects in the obstack are packed into large blocks called chunks.  The struct obstack structure points to a chain of the chunks currently in use.

The obstack library obtains a new chunk whenever you allocate an object that won't fit in the previous chunk.  Since the obstack library manages chunks automatically, you don't need to pay much attention to them, but you do need to supply a function that the obstack library should use to get a chunk.  Usually you supply a function that uses malloc directly or indirectly.  You must also supply a function to free a chunk. These matters are described in the following section.

Preparing for Using Obstacks
............................

Each source file in which you plan to use the obstack functions must include the header file obstack.h, like this:

     #include <obstack.h>

Also, if the source file uses the macro obstack_init, it must declare or define two functions or macros that will be called by the obstack library.  One, obstack_chunk_alloc, is used to allocate the chunks of memory into which objects are packed.  The other, obstack_chunk_free, is used to return chunks when the objects in them are freed.  These macros should appear before any use of obstacks in the source file.

Usually these are defined to use malloc via the intermediary xmalloc (see Unconstrained Allocation).  This is done with the following pair of macro definitions:

     #define obstack_chunk_alloc xmalloc
     #define obstack_chunk_free free

Though the memory you get using obstacks really comes from malloc, using obstacks is faster because malloc is called less often, for larger blocks of memory (see Obstack Chunks, for full details).

At run time, before the program can use a struct obstack object as an obstack, it must initialize the obstack by calling obstack_init.

 -- Function: int obstack_init (struct obstack *OBSTACK-PTR)
     Initialize obstack OBSTACK-PTR for allocation of objects.  This function calls the obstack's obstack_chunk_alloc function.  If allocation of memory fails, the function pointed to by obstack_alloc_failed_handler is called.  The obstack_init function always returns 1. Compatibility Note: Former versions of obstack returned 0 if allocation failed.)

Here are two examples of how to allocate the space for an obstack and initialize it; first, an obstack that is a static variable:

     static struct obstack myobstack;
     ...
     obstack_init (&myobstack);

second, an obstack that is itself dynamically allocated:

     struct obstack *myobstack_ptr
       = (struct obstack *) xmalloc (sizeof (struct obstack));

     obstack_init (myobstack_ptr);

 -- Variable: obstack_alloc_failed_handler
     The value of this variable is a pointer to a function that obstack uses when obstack_chunk_alloc fails to allocate memory.  The default action is to print a message and abort.  You should supply a function that either calls exit (see Program Termination) or longjmp (Ibid., ``Nonlocal Exits''.) and doesn't return.

          void my_obstack_alloc_failed (void)
          ...
          obstack_alloc_failed_handler = &my_obstack_alloc_failed;

Allocation in an Obstack
........................

The most direct way to allocate an object in an obstack is with obstack_alloc, which is invoked almost like malloc.

 -- Function: void * obstack_alloc (struct obstack *OBSTACK-PTR, int SIZE)
     This allocates an uninitialized block of SIZE bytes in an obstack and returns its address.  Here OBSTACK-PTR specifies which obstack to allocate the block in; it is the address of the struct obstack object that represents the obstack.  Each obstack function or macro requires you to specify an OBSTACK-PTR as the first argument.

     This function calls the obstack's obstack_chunk_alloc function if it needs to allocate a new chunk of memory; it calls obstack_alloc_failed_handler if allocation of memory by obstack_chunk_alloc failed.

For example, here is a function that allocates a copy of a string STR in a specific obstack, which is in the variable string_obstack:

     struct obstack string_obstack;

     char *
     copystring (char *string)
     {
       size_t len = strlen (string) + 1;
       char *s = (char *) obstack_alloc (&string_obstack, len);
       memcpy (s, string, len);
       return s;
     }

To allocate a block with specified contents, use the function obstack_copy, declared like this:

 -- Function: void * obstack_copy (struct obstack *OBSTACK-PTR, void *ADDRESS, int SIZE)
     This allocates a block and initializes it by copying SIZE bytes of data starting at ADDRESS.  It calls obstack_alloc_failed_handler if allocation of memory by obstack_chunk_alloc failed.

 -- Function: void * obstack_copy0 (struct obstack *OBSTACK-PTR, void *ADDRESS, int SIZE)
     Like obstack_copy, but appends an extra byte containing a null character.  This extra byte is not counted in the argument SIZE.

The obstack_copy0 function is convenient for copying a sequence of characters into an obstack as a null-terminated string.  Here is an example of its use:

     char *
     obstack_savestring (char *addr, int size)
     {
       return obstack_copy0 (&myobstack, addr, size);
     }

Contrast this with the previous example of savestring using malloc (see Basic Allocation).

Freeing Objects in an Obstack
.............................

To free an object allocated in an obstack, use the function obstack_free.  Since the obstack is a stack of objects, freeing one object automatically frees all other objects allocated more recently in the same obstack.

 -- Function: void obstack_free (struct obstack *OBSTACK-PTR, void *OBJECT)
     If OBJECT is a null pointer, everything allocated in the obstack is freed.  Otherwise, OBJECT must be the address of an object allocated in the obstack.  Then OBJECT is freed, along with everything allocated in OBSTACK since OBJECT.

Note that if OBJECT is a null pointer, the result is an uninitialized obstack.  To free all memory in an obstack but leave it valid for further allocation, call obstack_free with the address of the first object allocated on the obstack:

     obstack_free (obstack_ptr, first_object_allocated_ptr);

Recall that the objects in an obstack are grouped into chunks.  When all the objects in a chunk become free, the obstack library automatically frees the chunk (see Preparing for Obstacks).  Then other obstacks, or non-obstack allocation, can reuse the space of the chunk.

Obstack Functions and Macros
............................

The interfaces for using obstacks may be defined either as functions or as macros, depending on the compiler.  The obstack facility works with all C compilers, including both ISO C and traditional C, but there are precautions you must take if you plan to use compilers other than GNU C.

If you are using an old-fashioned non-ISO C compiler, all the obstack "functions" are actually defined only as macros.  You can call these macros like functions, but you cannot use them in any other way (for example, you cannot take their address).

Calling the macros requires a special precaution---the first operand (the obstack pointer) may not contain any side effects, because it may be computed more than once.  For example, if you write this:

     obstack_alloc (get_obstack (), 4);

you will find that get_obstack may be called several times. If you use *obstack_list_ptr++ as the obstack pointer argument, you will get very strange results, since the incrementation may occur several times.

In ISO C, each function has both a macro definition and a function definition.  The function definition is used if you take the address of the function without calling it.  An ordinary call uses the macro definition by default, but you can request the function definition instead by writing the function name in parentheses, as shown here:

     char *x;
     void *(*funcp) ();
     /* Use the macro.  */
     x = (char *) obstack_alloc (obptr, size);
     /* Call the function.  */
     x = (char *) (obstack_alloc) (obptr, size);
     /* Take the address of the function.  */
     funcp = obstack_alloc;

This is the same situation that exists in ISO C for the standard library functions (see Macro Definitions).

Warning: When you do use the macros, you must observe the precaution of avoiding side effects in the first operand, even in ISO C.

If you use the GNU C Compiler, this precaution is not necessary, because various language extensions in GNU C permit defining the macros so as to compute each argument only once.

Growing Objects
...............

Because memory in obstack chunks is used sequentially, it is possible to build up an object step-by-step, adding one or more bytes at a time to the end of the object.  With this technique, you do not need to know how much data you will put in the object until you come to the end of it.  We call this the technique of growing objects.  The special functions for adding data to the growing object are described in this section.

You don't need to do anything special when you start to grow an object. Using one of the functions to add data to the object automatically starts it.  However, it is necessary to say explicitly when the object is finished.  This is done with the function obstack_finish.

The actual address of the object thus built up is not known until the object is finished.  Until then, it always remains possible that you will add so much data that the object must be copied into a new chunk.

While the obstack is in use for a growing object, you cannot use it for ordinary allocation of another object.  If you try to do so, the space already added to the growing object will become part of the other object.

 -- Function: void obstack_blank (struct obstack *OBSTACK-PTR, int SIZE)
     The most basic function for adding to a growing object is obstack_blank, which adds space without initializing it.

 -- Function: void obstack_grow (struct obstack *OBSTACK-PTR, void *DATA, int SIZE)
     To add a block of initialized space, use obstack_grow, which is the growing-object analogue of obstack_copy.  It adds SIZE bytes of data to the growing object, copying the contents from DATA.

 -- Function: void obstack_grow0 (struct obstack *OBSTACK-PTR, void *DATA, int SIZE)
     This is the growing-object analogue of obstack_copy0.  It adds SIZE bytes copied from DATA, followed by an additional null character.

 -- Function: void obstack_1grow (struct obstack *OBSTACK-PTR, char C)
     To add one character at a time, use the function obstack_1grow. It adds a single byte containing C to the growing object.

 -- Function: void obstack_ptr_grow (struct obstack *OBSTACK-PTR, void *DATA)
     Adding the value of a pointer one can use the function obstack_ptr_grow.  It adds sizeof (void *) bytes containing the value of DATA.

 -- Function: void obstack_int_grow (struct obstack *OBSTACK-PTR, int DATA)
     A single value of type int can be added by using the obstack_int_grow function.  It adds sizeof (int) bytes to the growing object and initializes them with the value of DATA.

 -- Function: void * obstack_finish (struct obstack *OBSTACK-PTR)
     When you are finished growing the object, use the function obstack_finish to close it off and return its final address.

     Once you have finished the object, the obstack is available for ordinary allocation or for growing another object.

     This function can return a null pointer under the same conditions as obstack_alloc (see Allocation in an Obstack).

When you build an object by growing it, you will probably need to know afterward how long it became.  You need not keep track of this as you grow the object, because you can find out the length from the obstack just before finishing the object with the function obstack_object_size, declared as follows:

 -- Function: int obstack_object_size (struct obstack *OBSTACK-PTR)
     This function returns the current size of the growing object, in bytes. Remember to call this function before finishing the object. After it is finished, obstack_object_size will return zero.

If you have started growing an object and wish to cancel it, you should finish it and then free it, like this:

     obstack_free (obstack_ptr, obstack_finish (obstack_ptr));

This has no effect if no object was growing.

You can use obstack_blank with a negative size argument to make the current object smaller.  Just don't try to shrink it beyond zero length---there's no telling what will happen if you do that.

Extra-Fast Growing Objects
..........................

The usual functions for growing objects incur overhead for checking whether there is room for the new growth in the current chunk.  If you are frequently constructing objects in small steps of growth, this overhead can be significant.

You can reduce the overhead by using special "fast growth" functions that grow the object without checking.  In order to have a robust program, you must do the checking yourself.  If you do this checking in the simplest way each time you are about to add data to the object, you have not saved anything, because that is what the ordinary growth functions do.  But if you can arrange to check less often, or check more efficiently, then you make the program faster.

The function obstack_room returns the amount of room available in the current chunk.  It is declared as follows:

 -- Function: int obstack_room (struct obstack *OBSTACK-PTR)
     This returns the number of bytes that can be added safely to the current growing object (or to an object about to be started) in obstack OBSTACK using the fast-growth functions.

While you know there is room, you can use these fast-growth functions for adding data to a growing object:

 -- Function: void obstack_1grow_fast (struct obstack *OBSTACK-PTR, char C)
     The function obstack_1grow_fast adds one byte containing the character C to the growing object in obstack OBSTACK-PTR.

 -- Function: void obstack_ptr_grow_fast (struct obstack *OBSTACK-PTR, void *DATA)
     The function obstack_ptr_grow_fast adds sizeof (void *) bytes containing the value of DATA to the growing object in obstack OBSTACK-PTR.

 -- Function: void obstack_int_grow_fast (struct obstack *OBSTACK-PTR, int DATA)
     The function obstack_int_grow_fast adds sizeof (int) bytes containing the value of DATA to the growing object in obstack OBSTACK-PTR.

 -- Function: void obstack_blank_fast (struct obstack *OBSTACK-PTR, int SIZE)
     The function obstack_blank_fast adds SIZE bytes to the growing object in obstack OBSTACK-PTR without initializing them.

When you check for space using obstack_room and there is not enough room for what you want to add, the fast-growth functions are not safe.  In this case, simply use the corresponding ordinary growth function instead.  Very soon this will copy the object to a new chunk; then there will be lots of room available again.

So, each time you use an ordinary growth function, check afterward for sufficient space using obstack_room.  Once the object is copied to a new chunk, there will be plenty of space again, so the program will start using the fast-growth functions again.

Here is an example:

     void
     add_string (struct obstack *obstack, const char *ptr, int len)
     {
       while (len > 0)
         {
           int room = obstack_room (obstack);
           if (room == 0)
             {
               /* Not enough room. Add one character slowly,
                  which may copy to a new chunk and make room.  */
               obstack_1grow (obstack, *ptr++);
               len--;
             }
           else
             {
               if (room > len)
                 room = len;
               /* Add fast as much as we have room for. */
               len -= room;
               while (room-- > 0)
                 obstack_1grow_fast (obstack, *ptr++);
             }
         }
     }

Status of an Obstack
....................

Here are functions that provide information on the current status of allocation in an obstack.  You can use them to learn about an object while still growing it.

 -- Function: void * obstack_base (struct obstack *OBSTACK-PTR)
     This function returns the tentative address of the beginning of the currently growing object in OBSTACK-PTR.  If you finish the object immediately, it will have that address.  If you make it larger first, it may outgrow the current chunk---then its address will change!

     If no object is growing, this value says where the next object you allocate will start (once again assuming it fits in the current chunk).

 -- Function: void * obstack_next_free (struct obstack *OBSTACK-PTR)
     This function returns the address of the first free byte in the current chunk of obstack OBSTACK-PTR.  This is the end of the currently growing object.  If no object is growing, obstack_next_free returns the same value as obstack_base.

 -- Function: int obstack_object_size (struct obstack *OBSTACK-PTR)
     This function returns the size in bytes of the currently growing object. This is equivalent to:

          obstack_next_free (OBSTACK-PTR) - obstack_base (OBSTACK-PTR)

Alignment of Data in Obstacks
.............................

Each obstack has an alignment boundary; each object allocated in the obstack automatically starts on an address that is a multiple of the specified boundary.  By default, this boundary is 4 bytes.

To access an obstack's alignment boundary, use the macro obstack_alignment_mask, whose function prototype looks like this:

 -- Macro: int obstack_alignment_mask (struct obstack *OBSTACK-PTR)
     The value is a bit mask; a bit that is 1 indicates that the corresponding bit in the address of an object should be 0.  The mask value should be one less than a power of 2; the effect is that all object addresses are multiples of that power of 2.  The default value of the mask is 3, so that addresses are multiples of 4.  A mask value of 0 means an object can start on any multiple of 1 (that is, no alignment is required).

     The expansion of the macro obstack_alignment_mask is an lvalue, so you can alter the mask by assignment.  For example, this statement:

          obstack_alignment_mask (obstack_ptr) = 0;

     has the effect of turning off alignment processing in the specified obstack.

Note that a change in alignment mask does not take effect until after the next time an object is allocated or finished in the obstack.  If you are not growing an object, you can make the new alignment mask take effect immediately by calling obstack_finish. This will finish a zero-length object and then do proper alignment for the next object.

Obstack Chunks
..............

Obstacks work by allocating space for themselves in large chunks, and then parceling out space in the chunks to satisfy your requests.  Chunks are normally 4096 bytes long unless you specify a different chunk size. The chunk size includes 8 bytes of overhead that are not actually used for storing objects.  Regardless of the specified size, longer chunks will be allocated when necessary for long objects.

The obstack library allocates chunks by calling the function obstack_chunk_alloc, which you must define.  When a chunk is no longer needed because you have freed all the objects in it, the obstack library frees the chunk by calling obstack_chunk_free, which you must also define.

These two must be defined (as macros) or declared (as functions) in each source file that uses obstack_init (see Creating Obstacks). Most often they are defined as macros like this:

     #define obstack_chunk_alloc malloc
     #define obstack_chunk_free free

Note that these are simple macros (no arguments).  Macro definitions with arguments will not work!  It is necessary that obstack_chunk_alloc or obstack_chunk_free, alone, expand into a function name if it is not itself a function name.

If you allocate chunks with malloc, the chunk size should be a power of 2.  The default chunk size, 4096, was chosen because it is long enough to satisfy many typical requests on the obstack yet short enough not to waste too much memory in the portion of the last chunk not yet used.

 -- Macro: int obstack_chunk_size (struct obstack *OBSTACK-PTR)
     This returns the chunk size of the given obstack.

Since this macro expands to an lvalue, you can specify a new chunk size by assigning it a new value.  Doing so does not affect the chunks already allocated, but will change the size of chunks allocated for that particular obstack in the future.  It is unlikely to be useful to make the chunk size smaller, but making it larger might improve efficiency if you are allocating many objects whose size is comparable to the chunk size.  Here is how to do so cleanly:

     if (obstack_chunk_size (obstack_ptr) < NEW-CHUNK-SIZE)
       obstack_chunk_size (obstack_ptr) = NEW-CHUNK-SIZE;

Summary of Obstack Functions
............................

Here is a summary of all the functions associated with obstacks.  Each takes the address of an obstack (struct obstack *) as its first argument.

void obstack_init (struct obstack *OBSTACK-PTR)
     Initialize use of an obstack (see Creating Obstacks).

void *obstack_alloc (struct obstack *OBSTACK-PTR, int SIZE)
     Allocate an object of SIZE uninitialized bytes (see Allocation in an Obstack).

void *obstack_copy (struct obstack *OBSTACK-PTR, void *ADDRESS, int SIZE)
     Allocate an object of SIZE bytes, with contents copied from ADDRESS (see Allocation in an Obstack).

void *obstack_copy0 (struct obstack *OBSTACK-PTR, void *ADDRESS, int SIZE)
     Allocate an object of SIZE+1 bytes, with SIZE of them copied from ADDRESS, followed by a null character at the end (see Allocation in an Obstack).

void obstack_free (struct obstack *OBSTACK-PTR, void *OBJECT)
     Free OBJECT and everything allocated in the specified obstack more recently than OBJECT (see Freeing Obstack Objects).

void obstack_blank (struct obstack *OBSTACK-PTR, int SIZE)
     Add SIZE uninitialized bytes to a growing object (see Growing Objects).

void obstack_grow (struct obstack *OBSTACK-PTR, void *ADDRESS, int SIZE)
     Add SIZE bytes, copied from ADDRESS, to a growing object (see Growing Objects).

void obstack_grow0 (struct obstack *OBSTACK-PTR, void *ADDRESS, int SIZE)
     Add SIZE bytes, copied from ADDRESS, to a growing object, and then add another byte containing a null character (see Growing Objects).

void obstack_1grow (struct obstack *OBSTACK-PTR, char DATA-CHAR)
     Add one byte containing DATA-CHAR to a growing object (see Growing Objects).

void *obstack_finish (struct obstack *OBSTACK-PTR)
     Finalize the object that is growing and return its permanent address (see Growing Objects).

int obstack_object_size (struct obstack *OBSTACK-PTR)
     Get the current size of the currently growing object (see Growing Objects).

void obstack_blank_fast (struct obstack *OBSTACK-PTR, int SIZE)
     Add SIZE uninitialized bytes to a growing object without checking that there is enough room (see Extra-Fast Growing).

void obstack_1grow_fast (struct obstack *OBSTACK-PTR, char DATA-CHAR)
     Add one byte containing DATA-CHAR to a growing object without checking that there is enough room (see Extra-Fast Growing).

int obstack_room (struct obstack *OBSTACK-PTR)
     Get the amount of room now available for growing the current object (see Extra-Fast Growing).

int obstack_alignment_mask (struct obstack *OBSTACK-PTR)
     The mask used for aligning the beginning of an object.  This is an lvalue (see Obstacks Data Alignment).

int obstack_chunk_size (struct obstack *OBSTACK-PTR)
     The size for allocating chunks.  This is an lvalue (see Obstack Chunks).

void *obstack_base (struct obstack *OBSTACK-PTR)
     Tentative starting address of the currently growing object (see Status of an Obstack).

void *obstack_next_free (struct obstack *OBSTACK-PTR)
     Address just after the end of the currently growing object (see Status of an Obstack).

Automatic Storage with Variable Size
------------------------------------

The function alloca supports a kind of half-dynamic allocation in which blocks are allocated dynamically but freed automatically.

Allocating a block with alloca is an explicit action; you can allocate as many blocks as you wish, and compute the size at run time.  But all the blocks are freed when you exit the function that alloca was called from, just as if they were automatic variables declared in that function.  There is no way to free the space explicitly.

The prototype for alloca is in stdlib.h.  This function is a BSD extension.

 -- Function: void * alloca (size_t SIZE);
     The return value of alloca is the address of a block of SIZE bytes of memory, allocated in the stack frame of the calling function.

Do not use alloca inside the arguments of a function call---you will get unpredictable results, because the stack space for the alloca would appear on the stack in the middle of the space for the function arguments.  An example of what to avoid is foo (x, alloca (4), y).

alloca Example
..............

As an example of the use of alloca, here is a function that opens a file name made from concatenating two argument strings, and returns a file descriptor or minus one, signifying failure:

     int
     open2 (char *str1, char *str2, int flags, int mode)
     {
       char *name = (char *) alloca (strlen (str1) + strlen (str2) + 1);
       stpcpy (stpcpy (name, str1), str2);
       return open (name, flags, mode);
     }

Here is how you would get the same results with malloc and free:

     int
     open2 (char *str1, char *str2, int flags, int mode)
     {
       char *name = (char *) malloc (strlen (str1) + strlen (str2) + 1);
       int desc;
       if (name == 0)
         fatal ("virtual memory exceeded");
       stpcpy (stpcpy (name, str1), str2);
       desc = open (name, flags, mode);
       free (name);
       return desc;
     }

As you can see, it is simpler with alloca.  But alloca has other, more important advantages, and some disadvantages.

Advantages of alloca
....................

Here are the reasons why alloca may be preferable to malloc:

  * Using alloca wastes very little space and is very fast.  (It is open coded by the GNU C Compiler.)

  * Since alloca does not have separate pools for different sizes of block, space used for any size block can be reused for any other size. alloca does not cause memory fragmentation.

  * Nonlocal exits done with longjmp (Ibid., ``Nonlocal Exits''.) automatically free the space allocated with alloca when they exit through the function that called alloca.  This is the most important reason to use alloca.

     To illustrate this, suppose you have a function open_or_report_error that returns a descriptor, like open, if it succeeds, but does not return to its caller if it fails.  If the file cannot be opened, it prints an error message and jumps out to the command level of your program using longjmp. Let's change open2 (see Alloca Example) to use this subroutine:

          int
          open2 (char *str1, char *str2, int flags, int mode)
          {
            char *name = (char *) alloca (strlen (str1) + strlen (str2) + 1);
            stpcpy (stpcpy (name, str1), str2);
            return open_or_report_error (name, flags, mode);
          }

     Because of the way alloca works, the memory it allocates is freed even when an error occurs, with no special effort required.

     By contrast, the previous definition of open2 (which uses malloc and free) would develop a memory leak if it were changed in this way.  Even if you are willing to make more changes to fix it, there is no easy way to do so.

Disadvantages of alloca
.......................

These are the disadvantages of alloca in comparison with malloc:

  * If you try to allocate more memory than the machine can provide, you don't get a clean error message.  Instead you get a fatal signal like the one you would get from an infinite recursion; probably a segmentation violation. (Ibid., ``Program-Error Signals''.)

  * Some non-GNU systems fail to support alloca, so it is less portable.  However, a slower emulation of alloca written in C is available for use on systems with this deficiency.

GNU C Variable-Size Arrays
..........................

In GNU C, you can replace most uses of alloca with an array of variable size.  Here is how open2 would look then:

     int open2 (char *str1, char *str2, int flags, int mode)
     {
       char name[strlen (str1) + strlen (str2) + 1];
       stpcpy (stpcpy (name, str1), str2);
       return open (name, flags, mode);
     }

But alloca is not always equivalent to a variable-size array, for several reasons:

  * A variable-size array's space is freed at the end of the scope of the name of the array.  The space allocated with alloca remains until the end of the function.

  * It is possible to use alloca within a loop, allocating an additional block on each iteration.  This is impossible with variable-size arrays.

Note: If you mix use of alloca and variable-size arrays within one function, exiting a scope in which a variable-size array was declared frees all blocks allocated with alloca during the execution of that scope.

Resizing the Data Segment
=========================

The symbols in this section are declared in unistd.h.

You will not normally use the functions in this section, because the functions described in Memory Allocation are easier to use.  Those are interfaces to a GNU C Library memory allocator that uses the functions below itself.  The functions below are simple interfaces to system calls.

 -- Function: int brk (void *ADDR)

     brk sets the high end of the calling process' data segment to ADDR.

     The address of the end of a segment is defined to be the address of the last byte in the segment plus 1.

     The function has no effect if ADDR is lower than the low end of the data segment.  (This is considered success.)

     The function fails if it would cause the data segment to overlap another segment or exceed the process' data storage limit. (Ibid., ``Limiting Resource Usage''.)

     The function is named for a common historical case where data storage and the stack are in the same segment.  Data storage allocation grows upward from the bottom of the segment while the stack grows downward toward it from the top of the segment and the curtain between them is called the break.

     The return value is zero on success.  On failure, the return value is -1 and errno is set accordingly.  The following errno values are specific to this function:

     ENOMEM
          The request would cause the data segment to overlap another segment or exceed the process' data storage limit.

 -- Function: int sbrk (ptrdiff_t DELTA)
     This function is the same as brk except that you specify the new end of the data segment as an offset DELTA from the current end and on success the return value is the address of the resulting end of the data segment instead of zero.

     This means you can use sbrk(0) to find out what the current end of the data segment is.

Locking Pages
=============

You can tell the system to associate a particular virtual memory page with a real page frame and keep it that way---i.e., cause the page to be paged in if it isn't already and mark it so it will never be paged out and consequently will never cause a page fault.  This is called locking a page.

The functions in this chapter lock and unlock the calling process' pages.

Why Lock Pages?
---------------

Because page faults cause paged out pages to be paged in transparently, a process rarely needs to be concerned about locking pages.  However, there are two reasons people sometimes are:

  * Speed.  A page fault is transparent only insofar as the process is not sensitive to how long it takes to do a simple memory access.  Time-critical processes, especially real-time processes, may not be able to wait or may not be able to tolerate variance in execution speed.

     A process that needs to lock pages for this reason probably also needs priority among other processes for use of the CPU. (Ibid., ``Process CPU Priority and Scheduling''.)

     In some cases, the programmer knows better than the system's demand paging allocator which pages should remain in real memory to optimize system performance.  In this case, locking pages can help.

  * Privacy.  If you keep secrets in virtual memory and that virtual memory gets paged out, that increases the chance that the secrets will get out. If a password gets written out to disk swap space, for example, it might still be there long after virtual and real memory have been wiped clean.

Be aware that when you lock a page, that is one fewer page frame that can be used to back other virtual memory (by the same or other processes), which can mean more page faults, which means the system runs more slowly.  In fact, if you lock enough memory, some programs may not be able to run at all for lack of real memory.

Locked-Memory Details
---------------------

A memory lock is associated with a virtual page, not a real frame.  The paging rule is: If a frame backs at least one locked page, don't page it out.

Memory locks do not stack---you can't lock a particular page twice so that it has to be unlocked twice before it is truly unlocked.  It is either locked or it isn't.

A memory lock persists until the process that owns the memory explicitly unlocks it.  (But process termination and exec cause the virtual memory to cease to exist, which you might say means it isn't locked any more.)

Memory locks are not inherited by child processes  (but note that on a modern Unix system, immediately after a fork, the parent's and the child's virtual address space are backed by the same real page frames, so the child enjoys the parent's locks). (Ibid., ``Creating a Process''.)

Because of its ability to impact other processes, only the superuser can lock a page.  Any process can unlock its own page.

The system sets limits on the amount of memory a process can have locked and the amount of real memory it can have dedicated to it. (Ibid., ``Limiting Resource Usage''.)

In Linux, locked pages aren't as locked as you might think. Two virtual pages that are not shared memory can nonetheless be backed by the same real frame.  The kernel does this in the name of efficiency when it knows both virtual pages contain identical data, and does it even if one or both of the virtual pages are locked.

But when a process modifies one of those pages, the kernel must get it a separate frame and fill it with the page's data.  This is known as a copy-on-write page fault.  It takes a small amount of time and in a pathological case, getting that frame may require I/O.

To make sure this doesn't happen to your program, don't just lock the pages.  Write to them as well, unless you know you won't ever write to them ever.  And to make sure you have preallocated frames for your stack, enter a scope that declares a C automatic variable larger than the maximum stack size you will need, set it to something, then return from its scope.

Functions to Lock and Unlock Pages
----------------------------------

The symbols in this section are declared in sys/mman.h.  These functions are defined by POSIX.1b, but their availability depends on your kernel.  If your kernel doesn't allow these functions, they exist but always fail.  They are available with a Linux kernel.

Portability Note: POSIX.1b requires that when the mlock and munlock functions are available, the file unistd.h define the macro _POSIX_MEMLOCK_RANGE and the file limits.h define the macro PAGESIZE to be the size of a memory page in bytes.  It requires that when the mlockall and munlockall functions are available, the unistd.h file define the macro _POSIX_MEMLOCK.  The GNU C Library conforms to this requirement.

 -- Function: int mlock (const void *ADDR, size_t LEN)

     mlock locks a range of the calling process' virtual pages.

     The range of memory starts at address ADDR and is LEN bytes long.  Actually, since you must lock whole pages, it is the range of pages that include any part of the specified range.

     When the function returns successfully, each of those pages is backed by (connected to) a real frame (is resident) and is marked to stay that way.  This means the function may cause page-ins and have to wait for them.

     When the function fails, it does not affect the lock status of any pages.

     The return value is zero if the function succeeds.  Otherwise, it is -1 and errno is set accordingly.  errno values specific to this function are

     ENOMEM

            * At least some of the specified address range does not exist in the calling process' virtual-address space.

            * The locking would cause the process to exceed its locked page limit.

     EPERM
          The calling process is not superuser.

     EINVAL
          LEN is not positive.

     ENOSYS
          The kernel does not provide mlock capability.

     You can lock all a process' memory with mlockall.  You unlock memory with munlock or munlockall.

     To avoid all page faults in a C program, you have to use mlockall, because some of the memory a program uses is hidden from the C code, e.g., the stack and automatic variables, and you wouldn't know what address to tell mlock.

 -- Function: int munlock (const void *ADDR, size_t LEN)

     mlock unlocks a range of the calling process' virtual pages.

     munlock is the inverse of mlock and functions completely analogously to mlock, except that there is no EPERM failure.

 -- Function: int mlockall (int FLAGS)

     mlockall locks all the pages in a process' virtual-memory address space, and/or any that are added to it in the future.  This includes the pages of the code, data and stack segment, as well as shared libraries, user-space kernel data, shared memory, and memory-mapped files.

     FLAGS is a string of single-bit flags represented by the following macros.  They tell mlockall which of its functions you want.  All other bits must be zero.

     MCL_CURRENT
          Lock all pages that currently exist in the calling process' virtual-address space.

     MCL_FUTURE
          Set a mode such that any pages added to the process' virtual-address space in the future will be locked from birth.  This mode does not affect future address spaces owned by the same process, so exec, which replaces a process' address space, wipes out MCL_FUTURE. (Ibid., ``Executing a File''.)

     When the function returns successfully, and you specified MCL_CURRENT, all of the process' pages are backed by (connected to) real frames (they are resident) and are marked to stay that way. This means the function may cause page-ins and have to wait for them.

     When the process is in MCL_FUTURE mode because it successfully executed this function and specified MCL_CURRENT, any system call by the process that requires space be added to its virtual address space fails with errno = ENOMEM if locking the additional space would cause the process to exceed its locked page limit.  In the case that the address space addition that can't be accommodated is stack expansion, the stack expansion fails and the kernel sends a SIGSEGV signal to the process.

     When the function fails, it does not affect the lock status of any pages or the future locking mode.

     The return value is zero if the function succeeds.  Otherwise, it is -1 and errno is set accordingly.  errno values specific to this function are

     ENOMEM

            * At least some of the specified address range does not exist in the calling process' virtual-address space.

            * The locking would cause the process to exceed its locked page limit.

     EPERM
          The calling process is not superuser.

     EINVAL
          Undefined bits in FLAGS are not zero.

     ENOSYS
          The kernel does not provide mlockall capability.

     You can lock just specific pages with mlock.  You unlock pages with munlockall and munlock.

 -- Function: int munlockall (void)

     munlockall unlocks every page in the calling process' virtual-address space and turn off MCL_FUTURE future locking mode.

     The return value is zero if the function succeeds.  Otherwise, it is -1 and errno is set accordingly.  The only way this function can fail is for generic reasons that all functions and system calls can fail, so there are no specific errno values.

Character Handling
******************

Programs that work with characters and strings often need to classify a character as alphabetic, digit, white space, etc., and perform case conversion operations on characters.  The functions in the header file ctype.h are provided for this purpose.

Since the choice of locale and character set can alter the classifications of particular character codes, all of these functions are affected by the current locale.  (More precisely, they are affected by the locale currently selected for character classification---the LC_CTYPE category; see Locale Categories.)

The ISO C standard specifies two different sets of functions.  The one set works on char type characters, the other one on wchar_t wide characters (see Extended Char Intro).

Classification of Characters
============================

This section explains the library functions for classifying characters. For example, isalpha is the function to test for an alphabetic character.  It takes one argument, the character to test, and returns a nonzero integer if the character is alphabetic, and zero otherwise.  You would use it like this:

     if (isalpha (c))
       printf ("The character `%c' is alphabetic.\n", c);

Each of the functions in this section tests for membership in a particular class of characters; each has a name starting with is. Each of them takes one argument, which is a character to test, and returns an int which is treated as a Boolean value.  The character argument is passed as an int, and it may be the constant value EOF instead of a real character.

The attributes of any given character can vary between locales. See Locales, for more information on locales.

These functions are declared in the header file ctype.h.

 -- Function: int islower (int C)
     Returns true if C is a lowercase letter.  The letter need not be from the Latin alphabet---any representable alphabet is valid.

 -- Function: int isupper (int C)
     Returns true if C is an uppercase letter.  The letter need not be from the Latin alphabet---any representable alphabet is valid.

 -- Function: int isalpha (int C)
     Returns true if C is an alphabetic character (a letter).  If islower or isupper is true of a character, then isalpha is also true.

     In some locales, there may be additional characters for which isalpha is true---letters that are neither uppercase nor lowercase.  But in the standard "C" locale, there are no such additional characters.

 -- Function: int isdigit (int C)
     Returns true if C is a decimal digit (0 through 9).

 -- Function: int isalnum (int C)
     Returns true if C is an alphanumeric character (a letter or number); in other words, if either isalpha or isdigit is true of a character, then isalnum is also true.

 -- Function: int isxdigit (int C)
     Returns true if C is a hexadecimal digit. Hexadecimal digits include the normal decimal digits 0 through 9 and the letters A through F and a through f.

 -- Function: int ispunct (int C)
     Returns true if C is a punctuation character. This means any printing character that is not alphanumeric or a space character.

 -- Function: int isspace (int C)
     Returns true if C is a white-space character.  In the standard "C" locale, isspace returns true for only the standard white-space characters:

     ' '
          space

     '\f'
          formfeed

     '\n'
          newline

     '\r'
          carriage return

     '\t'
          horizontal tab

     '\v'
          vertical tab

 -- Function: int isblank (int C)
     Returns true if C is a blank character; that is, a space or a tab. This function is a GNU extension.

 -- Function: int isgraph (int C)
     Returns true if C is a graphic character; that is, a character that has a glyph associated with it.  The white-space characters are not considered graphic.

 -- Function: int isprint (int C)
     Returns true if C is a printing character.  Printing characters include all the graphic characters, plus the space ( ) character.

 -- Function: int iscntrl (int C)
     Returns true if C is a control character; that is, a character that is not a printing character.

 -- Function: int isascii (int C)
     Returns true if C is a 7-bit unsigned char value that fits into the US/UK ASCII character set.  This function is a BSD extension and is also an SVID extension.

Case Conversion
===============

This section explains the library functions for performing conversions such as case mappings on characters.  For example, toupper converts any character to upper case if possible.  If the character can't be converted, toupper returns it unchanged.

These functions take one argument of type int, which is the character to convert, and return the converted character as an int.  If the conversion is not applicable to the argument given, the argument is returned unchanged.

Compatibility Note: In pre-ISO C dialects, instead of returning the argument unchanged, these functions may fail when the argument is not suitable for the conversion.  Thus for portability, you may need to write islower(c) ? toupper(c) : c rather than just toupper(c).

These functions are declared in the header file ctype.h.

 -- Function: int tolower (int C)
     If C is an uppercase letter, tolower returns the corresponding lowercase letter.  If C is not an uppercase letter, C is returned unchanged.

 -- Function: int toupper (int C)
     If C is a lowercase letter, toupper returns the corresponding uppercase letter.  Otherwise C is returned unchanged.

 -- Function: int toascii (int C)
     This function converts C to a 7-bit unsigned char value that fits into the US/UK ASCII character set, by clearing the high-order bits.  This function is a BSD extension and is also an SVID extension.

 -- Function: int _tolower (int C)
     This is identical to tolower, and is provided for compatibility with the SVID (see SVID).

 -- Function: int _toupper (int C)
     This is identical to toupper, and is provided for compatibility with the SVID.

Character Class Determination for Wide Characters
=================================================

Amendment 1 to ISO C90 defines functions to classify wide characters.  Although the original ISO C90 standard already defined the type wchar_t, no functions operating on them were defined.

The general design of the classification functions for wide characters is more general.  It allows extensions to the set of available classifications, beyond those that are always available.  The POSIX standard specifies how extensions can be made, and this is already implemented in the GNU C Library implementation of the localedef program.

The character class functions are normally implemented with bitsets, with a bitset per character.  For a given character, the appropriate bitset is read from a table and a test is performed as to whether a certain bit is set.  Which bit is tested for is determined by the class.

For the wide-character classification functions this is made visible. There is a type classification type defined, a function to retrieve this value for a given class, and a function to test whether a given character is in this class, using the classification value.  On top of this the normal character classification functions as used for char objects can be defined.

 -- Data type: wctype_t
     The wctype_t can hold a value which represents a character class. The only defined way to generate such a value is by using the wctype function.

     This type is defined in wctype.h.

 -- Function: wctype_t wctype (const char *PROPERTY)
     The wctype returns a value representing a class of wide characters that is identified by the string PROPERTY.  Besides some standard properties, each locale can define its own ones.  In case no property with the given name is known for the current locale selected for the LC_CTYPE category, the function returns zero.

     The properties known in every locale are

     "alnum"   "alpha"   "cntrl"   "digit"

     "graph"   "lower"   "print"   "punct"

     "space"   "upper"   "xdigit"

     This function is declared in wctype.h.

To test the membership of a character to one of the nonstandard classes, the ISO C standard defines a completely new function.

 -- Function: int iswctype (wint_t WC, wctype_t DESC)
     This function returns a nonzero value if WC is in the character class specified by DESC.  DESC must previously be returned by a successful call to wctype.

     This function is declared in wctype.h.

To make it easier to use the commonly used classification functions, they are defined in the C library.  There is no need to use wctype if the property string is one of the known character classes.  In some situations it is desirable to construct the property strings, and then it is important that wctype can also handle the standard classes.

 -- Function: int iswalnum (wint_t WC)
     This function returns a nonzero value if WC is an alphanumeric character (a letter or number). In other words, if either iswalpha or iswdigit is true of a character, then iswalnum is also true.

     This function can be implemented using:

          iswctype (wc, wctype ("alnum"))

     It is declared in wctype.h.

 -- Function: int iswalpha (wint_t WC)
     Returns true if WC is an alphabetic character (a letter).  If iswlower or iswupper is true of a character, then iswalpha is also true.

     In some locales, there may be additional characters for which iswalpha is true---letters that are neither uppercase nor lowercase.  But in the standard "C" locale, there are no such additional characters.

     This function can be implemented using:

          iswctype (wc, wctype ("alpha"))

     It is declared in wctype.h.

 -- Function: int iswcntrl (wint_t WC)
     Returns true if WC is a control character; that is, a character that is not a printing character.

     This function can be implemented using:

          iswctype (wc, wctype ("cntrl"))

     It is declared in wctype.h.

 -- Function: int iswdigit (wint_t WC)
     Returns true if WC is a digit (e.g., 0 through 9). Please note that this function does not only return a nonzero value for decimal digits, but for all kinds of digits.  A consequence is that code like the following will not work unconditionally for wide characters:

          n = 0;
          while (iswdigit (*wc))
            {
              n *= 10;
              n += *wc++ - L'0';
            }

     This function can be implemented using:

          iswctype (wc, wctype ("digit"))

     It is declared in wctype.h.

 -- Function: int iswgraph (wint_t WC)
     Returns true if WC is a graphic character; that is, a character that has a glyph associated with it.  The white-space characters are not considered graphic.

     This function can be implemented using:

          iswctype (wc, wctype ("graph"))

     It is declared in wctype.h.

 -- Function: int iswlower (wint_t WC)
     Returns true if WC is a lowercase letter.  The letter need not be from the Latin alphabet---any representable alphabet is valid.

     This function can be implemented using:

          iswctype (wc, wctype ("lower"))

     It is declared in wctype.h.

 -- Function: int iswprint (wint_t WC)
     Returns true if WC is a printing character.  Printing characters include all the graphic characters, plus the space ( ) character.

     This function can be implemented using:

          iswctype (wc, wctype ("print"))

     It is declared in wctype.h.

 -- Function: int iswpunct (wint_t WC)
     Returns true if WC is a punctuation character. This means any printing character that is not alphanumeric or a space character.

     This function can be implemented using:

          iswctype (wc, wctype ("punct"))

     It is declared in wctype.h.

 -- Function: int iswspace (wint_t WC)
     Returns true if WC is a white-space character.  In the standard "C" locale, iswspace returns true for only the standard white-space characters:

     L' '
          space

     L'\f'
          formfeed

     L'\n'
          newline

     L'\r'
          carriage return

     L'\t'
          horizontal tab

     L'\v'
          vertical tab

     This function can be implemented using:

          iswctype (wc, wctype ("space"))

     It is declared in wctype.h.

 -- Function: int iswupper (wint_t WC)
     Returns true if WC is an uppercase letter.  The letter need not be from the Latin alphabet---any representable alphabet is valid.

     This function can be implemented using:

          iswctype (wc, wctype ("upper"))

     It is declared in wctype.h.

 -- Function: int iswxdigit (wint_t WC)
     Returns true if WC is a hexadecimal digit. Hexadecimal digits include the normal decimal digits 0 through 9 and the letters A through F and a through f.

     This function can be implemented using:

          iswctype (wc, wctype ("xdigit"))

     It is declared in wctype.h.

The GNU C Library also provides a function that is not defined in the ISO C standard but that is available as a version for single-byte characters as well.

 -- Function: int iswblank (wint_t WC)
     Returns true if WC is a blank character; that is, a space or a tab. This function is a GNU extension.  It is declared in wchar.h.

Notes on Using the Wide-Character Classes
=========================================

The first note is probably not astonishing but still occasionally a cause of problems.  The iswXXX functions can be implemented using macros and in fact, the GNU C Library does this.  They are still available as real functions, but when the wctype.h header is included the macros will be used.  This is the same as the char type versions of these functions.

The second note covers something new.  It can be best illustrated by a (real-world) example.  The first piece of code is an excerpt from the original code.  It is truncated a bit but the intention should be clear:

     int
     is_in_class (int c, const char *class)
     {
       if (strcmp (class, "alnum") == 0)
         return isalnum (c);
       if (strcmp (class, "alpha") == 0)
         return isalpha (c);
       if (strcmp (class, "cntrl") == 0)
         return iscntrl (c);
       ...
       return 0;
     }

Now, with the wctype and iswctype you can avoid the if cascades, but rewriting the code as follows is wrong:

     int
     is_in_class (int c, const char *class)
     {
       wctype_t desc = wctype (class);
       return desc ? iswctype ((wint_t) c, desc) : 0;
     }

The problem is that there is no guarantee that the wide-character representation of a single-byte character can be found using casting. In fact, usually this fails miserably.  The correct solution to this problem is to write the code as follows:

     int
     is_in_class (int c, const char *class)
     {
       wctype_t desc = wctype (class);
       return desc ? iswctype (btowc (c), desc) : 0;
     }

See Converting a Character, for more information on btowc. Note that this change probably does not improve the performance of the program a lot since the wctype function still has to make the string comparisons.  It gets really interesting if the is_in_class function is called more than once for the same class name.  In this case, the variable DESC could be computed once and reused for all the calls.  Therefore, the above form of the function is probably not the final one.

Mapping of Wide Characters
==========================

The classification functions are also generalized by the ISO C standard.  Instead of just allowing the two standard mappings, a locale can contain others.  Again, the localedef program already supports generating such locale data files.

 -- Data Type: wctrans_t
     This data type is defined as a scalar type that can hold a value representing the locale-dependent character mapping.  There is no way to construct such a value apart from using the return value of the wctrans function.

     This type is defined in wctype.h.

 -- Function: wctrans_t wctrans (const char *PROPERTY)
     The wctrans function has to be used to find out whether a named mapping is defined in the current locale selected for the LC_CTYPE category.  If the return value is nonzero, you can use it afterwards in calls to towctrans.  If the return value is zero, no such mapping is known in the current locale.

     Besides locale-specific mappings, there are two mappings that are guaranteed to be available in every locale: "tolower" and "toupper".

     These functions are declared in wctype.h.

 -- Function: wint_t towctrans (wint_t WC, wctrans_t DESC)
     towctrans maps the input character WC according to the rules of the mapping for which DESC is a descriptor, and returns the value it finds.  DESC must be obtained by a successful call to wctrans.

     This function is declared in wctype.h.

For the generally available mappings, the ISO C standard defines convenient shortcuts so that it is not necessary to call wctrans for them.

 -- Function: wint_t towlower (wint_t WC)
     If WC is an uppercase letter, towlower returns the corresponding lowercase letter.  If WC is not an uppercase letter, WC is returned unchanged.

     towlower can be implemented using:

          towctrans (wc, wctrans ("tolower"))

     This function is declared in wctype.h.

 -- Function: wint_t towupper (wint_t WC)
     If WC is a lowercase letter, towupper returns the corresponding uppercase letter.  Otherwise WC is returned unchanged.

     towupper can be implemented using:

          towctrans (wc, wctrans ("toupper"))

     This function is declared in wctype.h.

The same warnings given in the last section for the use of the wide-character classification functions apply here.  It is not possible to simply cast a char type value to a wint_t and use it as an argument to towctrans calls.

String and Array Utilities
**************************

Operations on strings (or arrays of characters) are an important part of many programs.  The GNU C Library provides an extensive set of string utility functions, including functions for copying, concatenating, comparing and searching strings.  Many of these functions can also operate on arbitrary regions of storage; for example, the memcpy function can be used to copy the contents of any kind of array.

It's fairly common for beginning C programmers to ``reinvent the wheel'' by duplicating this functionality in their own code, but it pays to become familiar with the library functions and to make use of them, since this offers benefits in maintenance, efficiency, and portability.

For instance, you could easily compare one string to another in two lines of C code, but if you use the built-in strcmp function, you're less likely to make a mistake.  And, since these library functions are typically highly optimized, your program may run faster too.

Representation of Strings
=========================

This section is a quick summary of string concepts for beginning C programmers.  It describes how character strings are represented in C and some common pitfalls.  If you are already familiar with this material, you can skip this section.

A string is an array of char objects.  But string-valued variables are usually declared to be pointers of type char *. Such variables do not include space for the text of a string; that has to be stored somewhere else---in an array variable, a string constant, or dynamically allocated memory (see Memory Allocation).  It's up to you to store the address of the chosen memory space into the pointer variable.  Alternatively, you can store a null pointer in the pointer variable.  The null pointer does not point anywhere, so attempting to reference the string it points to gets an error.

string normally refers to multibyte-character strings as opposed to wide-character strings.  Wide-character strings are arrays of type wchar_t and, as for multibyte-character strings, usually pointers of type wchar_t * are used.

By convention, a null character, '\0', marks the end of a multibyte-character string and the null wide character, L'\0', marks the end of a wide-character string.  For example, in testing to see whether the char * variable P points to a null character marking the end of a string, you can write !*P or *P == '\0'.

A null character is quite different conceptually from a null pointer, although both are represented by the integer 0.

String literals appear in C program source as strings of characters between double-quote characters (") where the initial double-quote character is immediately preceded by a capital L (ell) character (as in L"foo").  In ISO C, string literals can also be formed by string concatenation: "a" "b" is the same as "ab".  For wide-character strings one can either use L"a" L"b" or L"a" "b".  Modification of string literals is not allowed by the GNU C Compiler, because literals are placed in read-only storage.

Character arrays that are declared const cannot be modified either.  It's generally good style to declare nonmodifiable string pointers to be of type const char *, since this often allows the C compiler to detect accidental modifications as well as providing some amount of documentation about what your program intends to do with the string.

The amount of memory allocated for the character array may extend past the null character that normally marks the end of the string.  In this document, the term allocated size is always used to refer to the total amount of memory allocated for the string, while the term length refers to the number of characters up to (but not including) the terminating null character.

A notorious source of program bugs is trying to put more characters in a string than fit in its allocated size.  When writing code that extends strings or moves characters into a preallocated array, you should be very careful to keep track of the length of the text and make explicit checks for overflowing the array.  Many of the library functions do not do this for you!  Remember also that you need to allocate an extra byte to hold the null character that marks the end of the string.

Originally strings were sequences of bytes where each byte represents a single character.  This is still true today if the strings are encoded using a single-byte character encoding.  Things are different if the strings are encoded using a multibyte encoding (for more information on encodings see Extended Char Intro).  There is no difference in the programming interface for these two kind of strings; the programmer has to be aware of this and interpret the byte sequences accordingly.

But since there is no separate interface taking care of these differences the byte-based string functions are sometimes hard to use. Since the count parameters of these functions specify bytes, a call to strncpy could cut a multibyte character in the middle and put an incomplete (and therefore unusable) byte-sequence in the target buffer.

To avoid these problems, later versions of the ISO C standard introduce a second set of functions that operate on wide characters (see Extended Char Intro).  These functions don't have the problems the single-byte versions have, since every wide character is a legal, interpretable value.  This does not mean that cutting wide-character strings at arbitrary points is without problems.  It normally is for alphabet-based languages (except for nonnormalized text) but languages based on syllables still have the problem that more than one wide character is necessary to complete a logical unit.  This is a higher-level problem that the C library functions are not designed to solve.  But it is at least good that no invalid byte-sequences can be created.  Also, the higher-level functions can also operate more easily on wide characters than on multibyte characters, so that in general, it is advisable to use wide characters internally whenever text is more than simply copied.

The remainder of this chapter will discuss the functions for handling wide-character strings in parallel with the discussion of the multibyte-character strings, since there is almost always an exact equivalent available.

String and Array Conventions
============================

This chapter describes both functions that work on arbitrary arrays or blocks of memory, and functions that are specific to null-terminated arrays of characters and wide characters.

Functions that operate on arbitrary blocks of memory have names beginning with mem and wmem (such as memcpy and wmemcpy) and invariably take an argument that specifies the size (in bytes and wide characters respectively) of the block of memory to operate on.  The array arguments and return values for these functions have type void * or wchar_t.  As a matter of style, the elements of the arrays used with the mem functions are referred to as bytes.  You can pass any kind of pointer to these functions, and the sizeof operator is useful in computing the value for the size argument.  Parameters to the wmem functions must be of type wchar_t *.  These functions are not really usable with anything but arrays of this type.

In contrast, functions that operate specifically on strings and wide character strings have names beginning with str and wcs respectively (such as strcpy and wcscpy) and look for a null character to terminate the string instead of requiring an explicit size argument to be passed.  (Some of these functions accept a specified maximum length, but they also check for premature termination with a null character.)  The array arguments and return values for these functions have type char * and wchar_t * respectively, and the array elements are referred to as characters and wide characters.

In many cases, there are both mem and str/wcs versions of a function.  The one that is more appropriate to use depends on the exact situation.  When your program is manipulating arbitrary arrays or blocks of storage, then you should always use the mem functions.  On the other hand, when you are manipulating null-terminated strings it is usually more convenient to use the str/wcs functions, unless you already know the length of the string in advance. The wmem functions should be used for wide-character arrays with known size.

Some of the memory and string functions take single characters as arguments.  Since a value of type char is automatically promoted into a value of type int when used as a parameter, the functions are declared with int as the type of the parameter in question. In case of the wide-character function, the situation is similar---the parameter type for a single wide character is wint_t and not wchar_t.  This would, for many implementations, not be necessary since the wchar_t is large enough to not be automatically promoted, but since the ISO C standard does not require such a choice of types, the wint_t type is used.

String Length
=============

You can get the length of a string using the strlen function. This function is declared in the header file string.h.

 -- Function: size_t strlen (const char *S)
     The strlen function returns the length of the null-terminated string S in bytes.  In other words, it returns the offset of the terminating null character within the array.

     For example:

          strlen ("hello, world")
              => 12

     When applied to a character array, the strlen function returns the length of the string stored there, not its allocated size.  You can get the allocated size of the character array that holds a string using the sizeof operator:

          char string[32] = "hello, world";
          sizeof (string)
              => 32
          strlen (string)
              => 12

     But beware, this will not work unless STRING is the character array itself, not a pointer to it.  For example:

          char string[32] = "hello, world";
          char *ptr = string;
          sizeof (string)
              => 32
          sizeof (ptr)
              => 4  /* (on a machine with 4 byte pointers) */

     This is an easy mistake to make when you are working with functions that take string arguments; those arguments are always pointers, not arrays.

     It must also be noted that for multibyte-encoded strings the return value does not have to correspond to the number of characters in the string.  To get this value, the string can be converted to wide characters and wcslen can be used, or something like the following code can be used:

          /* The input is in string.
             The length is expected in n.  */
          {
            mbstate_t t;
            char *scopy = string;
            /* In initial state.  */
            memset (&t, '\0', sizeof (t));
            /* Determine number of characters.  */
            n = mbsrtowcs (NULL, &scopy, strlen (scopy), &t);
          }

     This is cumbersome, so if the number of characters (as opposed to bytes) is needed, often it is better to work with wide characters.

The wide character equivalent is declared in wchar.h.

 -- Function: size_t wcslen (const wchar_t *WS)
     The wcslen function is the wide character equivalent to strlen.  The return value is the number of wide characters in the wide-character string pointed to by WS (this is also the offset of the terminating null wide character of WS).

     Since there are no multi-wide-character sequences making up one character, the return value is not only the offset in the array, but also the number of wide characters.

     This function was introduced in Amendment 1 to ISO C90.

 -- Function: size_t strnlen (const char *S, size_t MAXLEN)
     The strnlen function returns the length of the string S in bytes if this length is smaller than MAXLEN bytes.  Otherwise it returns MAXLEN.  Therefore, this function is equivalent to (strlen (S) < n ? strlen (S) : MAXLEN), but it is more efficient and works even if the string S is not null-terminated.

          char string[32] = "hello, world";
          strnlen (string, 32)
              => 12
          strnlen (string, 5)
              => 5

     This function is a GNU extension and is declared in string.h.

 -- Function: size_t wcsnlen (const wchar_t *WS, size_t MAXLEN)
     wcsnlen is the wide-character equivalent to strnlen.  The MAXLEN parameter specifies the maximum number of wide characters.

     This function is a GNU extension and is declared in wchar.h.

Copying and Concatenation
=========================

You can use the functions described in this section to copy the contents of strings and arrays, or to append the contents of one string to another.  The str and mem functions are declared in the header file string.h while the wstr and wmem functions are declared in the file wchar.h.

A helpful way to remember the ordering of the arguments to the functions in this section is that it corresponds to an assignment expression, with the destination array specified to the left of the source array.  All of these functions return the address of the destination array.

Most of these functions do not work properly if the source and destination arrays overlap.  For example, if the beginning of the destination array overlaps the end of the source array, the original contents of that part of the source array may get overwritten before it is copied.  Even worse, in the case of the string functions, the null character marking the end of the string may be lost, and the copy function might get stuck in a loop, trashing all the memory allocated to your program.

All functions that have problems copying between overlapping arrays are explicitly identified in this manual.  In addition to functions in this section, there are a few others like sprintf (see Formatted Output Functions) and scanf (see Formatted Input Functions).

 -- Function: void * memcpy (void *restrict TO, const void *restrict FROM, size_t SIZE)
     The memcpy function copies SIZE bytes from the object beginning at FROM into the object beginning at TO.  The behavior of this function is undefined if the two arrays TO and FROM overlap; use memmove instead if overlapping is possible.

     The value returned by memcpy is the value of TO.

     Here is an example of how you might use memcpy to copy the contents of an array:

          struct foo *oldarray, *newarray;
          int arraysize;
          ...
          memcpy (new, old, arraysize * sizeof (struct foo));

 -- Function: wchar_t * wmemcpy (wchar_t *restrict WTO, const wchar_t *restruct WFROM, size_t SIZE)
     The wmemcpy function copies SIZE wide characters from the object beginning at WFROM into the object beginning at WTO.  The behavior of this function is undefined if the two arrays WTO and WFROM overlap; use wmemmove instead if overlapping is possible.

     The following is one possible implementation of wmemcpy, but there are also other optimizations possible:

          wchar_t *
          wmemcpy (wchar_t *restrict wto, const wchar_t *restrict wfrom,
                   size_t size)
          {
            return (wchar_t *) memcpy (wto, wfrom, size * sizeof (wchar_t));
          }

     The value returned by wmemcpy is the value of WTO.

     This function was introduced in Amendment 1 to ISO C90.

 -- Function: void * mempcpy (void *restrict TO, const void *restrict FROM, size_t SIZE)
     The mempcpy function is nearly identical to the memcpy function.  It copies SIZE bytes from the object beginning at from into the object pointed to by TO.  But instead of returning the value of TO, it returns a pointer to the byte following the last written byte in the object beginning at TO---the value is ((void *) ((char *) TO + SIZE)).

     This function is useful in situations where a number of objects will be copied to consecutive memory positions:

          void *
          combine (void *o1, size_t s1, void *o2, size_t s2)
          {
            void *result = malloc (s1 + s2);
            if (result != NULL)
              mempcpy (mempcpy (result, o1, s1), o2, s2);
            return result;
          }

     This function is a GNU extension.

 -- Function: wchar_t * wmempcpy (wchar_t *restrict WTO, const wchar_t *restrict WFROM, size_t SIZE)
     The wmempcpy function is nearly identical to the wmemcpy function.  It copies SIZE wide characters from the object beginning at wfrom into the object pointed to by WTO.  But instead of returning the value of WTO, it returns a pointer to the wide character following the last written wide character in the object beginning at WTO---the value is WTO + SIZE.

     This function is useful in situations where a number of objects will be copied to consecutive memory positions.

     The following is one possible implementation of wmemcpy, but there are also other optimizations possible:

          wchar_t *
          wmempcpy (wchar_t *restrict wto, const wchar_t *restrict wfrom,
                    size_t size)
          {
            return (wchar_t *) mempcpy (wto, wfrom, size * sizeof (wchar_t));
          }

     This function is a GNU extension.

 -- Function: void * memmove (void *TO, const void *FROM, size_t SIZE)
     memmove copies the SIZE bytes at FROM into the SIZE bytes at TO, even if those two blocks of space overlap.  In the case of overlap, memmove is careful to copy the original values of the bytes in the block at FROM, including those bytes that also belong to the block at TO.

     The value returned by memmove is the value of TO.

 -- Function: wchar_t * wmemmove (wchar *WTO, const wchar_t *WFROM, size_t SIZE)
     wmemmove copies the SIZE wide characters at WFROM into the SIZE wide characters at WTO, even if those two blocks of space overlap.  In the case of overlap, memmove is careful to copy the original values of the wide characters in the block at WFROM, including those wide characters that also belong to the block at WTO.

     The following is one possible implementation of wmemcpy, but there are also other optimizations possible:

          wchar_t *
          wmempcpy (wchar_t *restrict wto, const wchar_t *restrict wfrom,
                    size_t size)
          {
            return (wchar_t *) mempcpy (wto, wfrom, size * sizeof (wchar_t));
          }

     The value returned by wmemmove is the value of WTO.

     This function is a GNU extension.

 -- Function: void * memccpy (void *restrict TO, const void *restrict FROM, int C, size_t SIZE)
     This function copies no more than SIZE bytes from FROM to TO, stopping if a byte matching C is found.  The return value is a pointer into TO one byte past where C was copied, or a null pointer if no byte matching C appeared in the first SIZE bytes of FROM.

 -- Function: void * memset (void *BLOCK, int C, size_t SIZE)
     This function copies the value of C (converted to an unsigned char) into each of the first SIZE bytes of the object beginning at BLOCK.  It returns the value of BLOCK.

 -- Function: wchar_t * wmemset (wchar_t *BLOCK, wchar_t WC, size_t SIZE)
     This function copies the value of WC into each of the first SIZE wide characters of the object beginning at BLOCK.  It returns the value of BLOCK.

 -- Function: char * strcpy (char *restrict TO, const char *restrict FROM)
     This copies characters from the string FROM (up to and including the terminating null character) into the string TO.  Like memcpy, this function has undefined results if the strings overlap.  The return value is the value of TO.

 -- Function: wchar_t * wcscpy (wchar_t *restrict WTO, const wchar_t *restrict WFROM)
     This copies wide characters from the string WFROM (up to and including the terminating null wide character) into the string WTO.  Like wmemcpy, this function has undefined results if the strings overlap.  The return value is the value of WTO.

 -- Function: char * strncpy (char *restrict TO, const char *restrict FROM, size_t SIZE)
     This function is similar to strcpy but always copies exactly SIZE characters into TO.

     If the length of FROM is more than SIZE, then strncpy copies just the first SIZE characters.  Note that in this case there is no null terminator written into TO.

     If the length of FROM is less than SIZE, then strncpy copies all of FROM, followed by enough null characters to add up to SIZE characters in all.  This behavior is rarely useful, but it is specified by the ISO C standard.

     The behavior of strncpy is undefined if the strings overlap.

     Using strncpy as opposed to strcpy is a way to avoid bugs relating to writing past the end of the allocated space for TO. However, it can also make your program much slower in one common case: copying a string that is probably small into a potentially large buffer. In this case, SIZE may be large, and when it is, strncpy will waste a considerable amount of time copying null characters.

 -- Function: wchar_t * wcsncpy (wchar_t *restrict WTO, const wchar_t *restrict WFROM, size_t SIZE)
     This function is similar to wcscpy but always copies exactly SIZE wide characters into WTO.

     If the length of WFROM is more than SIZE, then wcsncpy copies just the first SIZE wide characters.  Note that in this case there is no null terminator written into WTO.

     If the length of WFROM is less than SIZE, then wcsncpy copies all of WFROM, followed by enough null wide characters to add up to SIZE wide characters in all.  This behavior is rarely useful, but it is specified by the ISO C standard.

     The behavior of wcsncpy is undefined if the strings overlap.

     Using wcsncpy as opposed to wcscpy is a way to avoid bugs relating to writing past the end of the allocated space for WTO. However, it can also make your program much slower in one common case: copying a string that is probably small into a potentially large buffer. In this case, SIZE may be large, and when it is, wcsncpy will waste a considerable amount of time copying null wide characters.

 -- Function: char * strdup (const char *S)
     This function copies the null-terminated string S into a newly allocated string.  The string is allocated using malloc (see Unconstrained Allocation).  If malloc cannot allocate space for the new string, strdup returns a null pointer. Otherwise, it returns a pointer to the new string.

 -- Function: wchar_t * wcsdup (const wchar_t *WS)
     This function copies the null-terminated wide-character string WS into a newly allocated string.  The string is allocated using malloc (see Unconstrained Allocation).  If malloc cannot allocate space for the new string, wcsdup returns a null pointer.  Otherwise, it returns a pointer to the new wide-character string.

     This function is a GNU extension.

 -- Function: char * strndup (const char *S, size_t SIZE)
     This function is similar to strdup but always copies at most SIZE characters into the newly allocated string.

     If the length of S is more than SIZE, then strndup copies just the first SIZE characters and adds a closing null terminator.  Otherwise, all characters are copied and the string is terminated.

     This function is different to strncpy in that it always terminates the destination string.

     strndup is a GNU extension.

 -- Function: char * stpcpy (char *restrict TO, const char *restrict FROM)
     This function is like strcpy, except that it returns a pointer to the end of the string TO (that is, the address of the terminating null character to + strlen (from)) rather than the beginning.

     For example, this program uses stpcpy to concatenate foo and bar to produce foobar, which it then prints:

          #include <string.h>
          #include <stdio.h>

          int
          main (void)
          {
            char buffer[10];
            char *to = buffer;
            to = stpcpy (to, "foo");
            to = stpcpy (to, "bar");
            puts (buffer);
            return 0;
          }

     This function is not part of the ISO or POSIX standards, and is not customary on Unix systems, but we did not invent it either. Perhaps it comes from MS-DOS.

     Its behavior is undefined if the strings overlap.  The function is declared in string.h.

 -- Function: wchar_t * wcpcpy (wchar_t *restrict WTO, const wchar_t *restrict WFROM)
     This function is like wcscpy, except that it returns a pointer to the end of the string WTO (that is, the address of the terminating null character wto + strlen (wfrom)) rather than the beginning.

     This function is not part of ISO or POSIX but was found useful while developing the GNU C Library itself.

     The behavior of wcpcpy is undefined if the strings overlap.

     wcpcpy is a GNU extension and is declared in wchar.h.

 -- Function: char * stpncpy (char *restrict TO, const char *restrict FROM, size_t SIZE)
     This function is similar to stpcpy but copies always exactly SIZE characters into TO.

     If the length of FROM is more then SIZE, then stpncpy copies just the first SIZE characters and returns a pointer to the character directly following the one that was copied last.  In this case, there is no null terminator written into TO.

     If the length of FROM is less than SIZE, then stpncpy copies all of FROM, followed by enough null characters to add up to SIZE characters in all.  This behavior is rarely useful, but it is implemented to be useful in contexts where this behavior of the strncpy is used.  stpncpy returns a pointer to the first written null character.

     This function is not part of ISO or POSIX but was found useful while developing the GNU C Library itself.

     Its behavior is undefined if the strings overlap.  The function is declared in string.h.

 -- Function: wchar_t * wcpncpy (wchar_t *restrict WTO, const wchar_t *restrict WFROM, size_t SIZE)
     This function is similar to wcpcpy but copies always exactly WSIZE characters into WTO.

     If the length of WFROM is more then SIZE, then wcpncpy copies just the first SIZE wide characters and returns a pointer to the wide character directly following the last nonnull wide character which was copied last.  Note that in this case there is no null terminator written into WTO.

     If the length of WFROM is less than SIZE, then wcpncpy copies all of WFROM, followed by enough null characters to add up to SIZE characters in all.  This behavior is rarely useful, but it is implemented to be useful in contexts where this behavior of the wcsncpy is used.  wcpncpy returns a pointer to the first written null character.

     This function is not part of ISO or POSIX but was found useful while developing the GNU C Library itself.

     Its behavior is undefined if the strings overlap.

     wcpncpy is a GNU extension and is declared in wchar.h.

 -- Macro: char * strdupa (const char *S)
     This macro is similar to strdup but allocates the new string using alloca instead of malloc (see Variable Size Automatic).  This means that the returned string has the same limitations as any block of memory allocated using alloca.

     For obvious reasons, strdupa is implemented only as a macro; you cannot get the address of this function.  Despite this limitation, it is a useful function.  The following code shows a situation where using malloc would be much more expensive:

          #include <paths.h>
          #include <string.h>
          #include <stdio.h>

          const char path[] = _PATH_STDPATH;

          int
          main (void)
          {
            char *wr_path = strdupa (path);
            char *cp = strtok (wr_path, ":");

            while (cp != NULL)
              {
                puts (cp);
                cp = strtok (NULL, ":");
              }
            return 0;
          }

     strtok must be passed a writeable string.  Passing a const string, such as PATH in the above example, would be invalid. Also, using strdupa directly would also cause conflict due to the use of alloca in strdupa (see Variable Size Automatic).

     This function is only available if GNU CC is used.

 -- Macro: char * strndupa (const char *S, size_t SIZE)
     This function is similar to strndup, but like strdupa it allocates the new string using alloca (see Variable Size Automatic).  The same advantages and limitations of strdupa are valid for strndupa too.

     This function is implemented only as a macro, just like strdupa. Just as strdupa, this macro also must not be used inside the parameter list in a function call.

     strndupa is only available if GNU CC is used.

 -- Function: char * strcat (char *restrict TO, const char *restrict FROM)
     The strcat function is similar to strcpy, except that the characters from FROM are concatenated or appended to the end of TO, instead of overwriting it.  The first character from FROM overwrites the null character marking the end of TO.

     An equivalent definition for strcat would be

          char *
          strcat (char *restrict to, const char *restrict from)
          {
            strcpy (to + strlen (to), from);
            return to;
          }

     This function has undefined results if the strings overlap.

 -- Function: wchar_t * wcscat (wchar_t *restrict WTO, const wchar_t *restrict WFROM)
     The wcscat function is similar to wcscpy, except that the characters from WFROM are concatenated or appended to the end of WTO, instead of overwriting it.  The first character from WFROM overwrites the null character marking the end of WTO.

     An equivalent definition for wcscat would be

          wchar_t *
          wcscat (wchar_t *wto, const wchar_t *wfrom)
          {
            wcscpy (wto + wcslen (wto), wfrom);
            return wto;
          }

     This function has undefined results if the strings overlap.

Programmers using the strcat or wcscat function (or the following strncat or wcsncar functions for that matter) can easily be recognized as lazy and reckless.  In almost all situations, the lengths of the participating strings are known (since otherwise you cannot ensure that the allocated size of the buffer is sufficient).  Or at least you could know them if you keep track of the results of the various function calls.  But then it is very inefficient to use strcat/wcscat.  Much time is wasted finding the end of the destination string so that the actual copying can start. This is a common example:

     /* This function concatenates arbitrarily many strings.  The last
        parameter must be NULL.  */
     char *
     concat (const char *str, ...)
     {
       va_list ap, ap2;
       size_t total = 1;
       const char *s;
       char *result;

       va_start (ap, str);
       /* Actually va_copy, but this is the name more gcc versions
          understand.  */
       __va_copy (ap2, ap);

       /* Determine how much space we need.  */
       for (s = str; s != NULL; s = va_arg (ap, const char *))
         total += strlen (s);

       va_end (ap);

       result = (char *) malloc (total);
       if (result != NULL)
         {
           result[0] = '\0';

           /* Copy the strings.  */
           for (s = str; s != NULL; s = va_arg (ap2, const char *))
             strcat (result, s);
         }

       va_end (ap2);

       return result;
     }

This looks quite simple, especially the second loop, where the strings are actually copied.  But these innocent lines hide a major performance penalty.  Just imagine that ten strings of 100 bytes each have to be concatenated.  For the second string, we search the already stored 100 bytes for the end of the string so that we can append the next string. For all strings in total, the comparisons necessary to find the end of the intermediate results add up to 5500!  If we combine the copying with the search for the allocation, we can write this function more efficiently:

     char *
     concat (const char *str, ...)
     {
       va_list ap;
       size_t allocated = 100;
       char *result = (char *) malloc (allocated);
       char *wp;

       if (allocated != NULL)
         {
           char *newp;

           va_start (ap, atr);

           wp = result;
           for (s = str; s != NULL; s = va_arg (ap, const char *))
             {
               size_t len = strlen (s);

               /* Resize the allocated memory if necessary.  */
               if (wp + len + 1 > result + allocated)
                 {
                   allocated = (allocated + len) * 2;
                   newp = (char *) realloc (result, allocated);
                   if (newp == NULL)
                     {
                       free (result);
                       return NULL;
                     }
                   wp = newp + (wp - result);
                   result = newp;
                 }

               wp = mempcpy (wp, s, len);
             }

           /* Terminate the result string.  */
           *wp++ = '\0';

           /* Resize memory to the optimal size.  */
           newp = realloc (result, wp - result);
           if (newp != NULL)
             result = newp;

           va_end (ap);
         }

       return result;
     }

With a bit more knowledge about the input strings, one could fine-tune the memory allocation.  The difference we are pointing to here is that we don't use strcat anymore.  We always keep track of the length of the current intermediate result so we can save the search for the end of the string and use mempcpy.  We also don't use stpcpy, which might seem more natural since we handle strings.  But this is not necessary, since we already know the length of the string and therefore can use the faster memory copying function.  The example would work for wide characters the same way.

Whenever a programmer feels the need to use strcat, she should think twice and look through the program to see whether the code cannot be rewritten to take advantage of already calculated results.  It is almost always unnecessary to use strcat.

 -- Function: char * strncat (char *restrict TO, const char *restrict FROM, size_t SIZE)
     This function is like strcat except that not more than SIZE characters from FROM are appended to the end of TO.  A single null character is also always appended to TO, so the total allocated size of TO must be at least SIZE + 1 bytes longer than its initial length.

     The strncat function could be implemented like this:

          char *
          strncat (char *to, const char *from, size_t size)
          {
            to[strlen (to) + size] = '\0';
            strncpy (to + strlen (to), from, size);
            return to;
          }

     The behavior of strncat is undefined if the strings overlap.

 -- Function: wchar_t * wcsncat (wchar_t *restrict WTO, const wchar_t *restrict WFROM, size_t SIZE)
     This function is like wcscat except that not more than SIZE characters from FROM are appended to the end of TO.  A single null character is also always appended to TO, so the total allocated size of TO must be at least SIZE + 1 bytes longer than its initial length.

     The wcsncat function could be implemented like this:

          wchar_t *
          wcsncat (wchar_t *restrict wto, const wchar_t *restrict wfrom,
                   size_t size)
          {
            wto[wcslen (to) + size] = L'\0';
            wcsncpy (wto + wcslen (wto), wfrom, size);
            return wto;
          }

     The behavior of wcsncat is undefined if the strings overlap.

Here is an example showing the use of strncpy and strncat (the wide-character version is equivalent).  Notice how, in the call to strncat, the SIZE parameter is computed to avoid overflowing the character array buffer.

     #include <string.h>
     #include <stdio.h>

     #define SIZE 10

     static char buffer[SIZE];

     main ()
     {
       strncpy (buffer, "hello", SIZE);
       puts (buffer);
       strncat (buffer, ", world", SIZE - strlen (buffer) - 1);
       puts (buffer);
     }

The output produced by this program looks like:

     hello
     hello, wo

 -- Function: void bcopy (const void *FROM, void *TO, size_t SIZE)
     This is a partially obsolete alternative for memmove, derived from BSD.  It is not quite equivalent to memmove, because the arguments are not in the same order and there is no return value.

 -- Function: void bzero (void *BLOCK, size_t SIZE)
     This is a partially obsolete alternative for memset, derived from BSD.  It is not as general as memset, because the only value it can store is zero.

String/Array Comparison
=======================

You can use the functions in this section to perform comparisons on the contents of strings and arrays.  As well as checking for equality, these functions can also be used as the ordering functions for sorting operations (see Searching and Sorting for an example of this).

Unlike most comparison operations in C, the string comparison functions return a nonzero value if the strings are not equivalent rather than if they are.  The sign of the value indicates the relative ordering of the first characters in the strings that are not equivalent:  a negative value indicates that the first string is ``less'' than the second, while a positive value indicates that the first string is ``greater''.

The most common use of these functions is to check only for equality. This is canonically done with an expression like ! strcmp (s1, s2).

All of these functions are declared in the header file string.h.

 -- Function: int memcmp (const void *A1, const void *A2, size_t SIZE)
     The function memcmp compares the SIZE bytes of memory beginning at A1 against the SIZE bytes of memory beginning at A2.  The value returned has the same sign as the difference between the first differing pair of bytes (interpreted as unsigned char objects, then promoted to int).

     If the contents of the two blocks are equal, memcmp returns 0.

 -- Function: int wmemcmp (const wchar_t *A1, const wchar_t *A2, size_t SIZE)
     The function wmemcmp compares the SIZE wide characters beginning at A1 against the SIZE wide characters beginning at A2.  The value returned is smaller than or larger than zero depending on whether the first differing wide character is A1 is smaller or larger than the corresponding character in A2.

     If the contents of the two blocks are equal, wmemcmp returns 0.

On arbitrary arrays, the memcmp function is mostly useful for testing equality.  It usually isn't meaningful to do byte-wise ordering comparisons on arrays of things other than bytes.  For example, a byte-wise comparison on the bytes that make up floating-point numbers isn't likely to tell you anything about the relationship between the values of the floating-point numbers.

wmemcmp is really only useful to compare arrays of type wchar_t, since the function looks at sizeof (wchar_t) bytes at a time, and this number of bytes is system dependent.

You should also be careful about using memcmp to compare objects that can contain ``holes'', such as the padding inserted into structure objects to enforce alignment requirements, extra space at the end of unions and extra characters at the ends of strings whose length is less than their allocated size.  The contents of these ``holes'' are indeterminate and may cause strange behavior when performing byte-wise comparisons.  For more predictable results, perform an explicit component-wise comparison.

For example, given a structure-type definition like:

     struct foo
       {
         unsigned char tag;
         union
           {
             double f;
             long i;
             char *p;
           } value;
       };

you are better off writing a specialized comparison function to compare struct foo objects instead of comparing them with memcmp.

 -- Function: int strcmp (const char *S1, const char *S2)
     The strcmp function compares the string S1 against S2, returning a value that has the same sign as the difference between the first differing pair of characters (interpreted as unsigned char objects, then promoted to int).

     If the two strings are equal, strcmp returns 0.

     A consequence of the ordering used by strcmp is that if S1 is an initial substring of S2, then S1 is considered to be ``less than'' S2.

     strcmp does not take sorting conventions of the language the strings are written in into account.  To get that one has to use strcoll.

 -- Function: int wcscmp (const wchar_t *WS1, const wchar_t *WS2)

     The wcscmp function compares the wide-character string WS1 against WS2.  The value returned is smaller than or larger than zero, depending on whether the first differing wide character WS1 is smaller or larger than the corresponding character in WS2.

     If the two strings are equal, wcscmp returns 0.

     A consequence of the ordering used by wcscmp is that if WS1 is an initial substring of WS2, then WS1 is considered to be ``less than'' WS2.

     wcscmp does not take sorting conventions of the language the strings are written in into account.  To get that one has to use wcscoll.

 -- Function: int strcasecmp (const char *S1, const char *S2)
     This function is like strcmp, except that differences in case are ignored.  How uppercase and lowercase characters are related is determined by the currently selected locale.  In the standard "C" locale, the characters Ä and ä do not match, but in a locale that regards these characters as parts of the alphabet, they do match.

     strcasecmp is derived from BSD.

 -- Function: int wcscasecmp (const wchar_t *WS1, const wchar_T *WS2)
     This function is like wcscmp, except that differences in case are ignored.  How uppercase and lowercase characters are related is determined by the currently selected locale.  In the standard "C" locale, the characters Ä and ä do not match, but in a locale that regards these characters as parts of the alphabet, they do match.

     wcscasecmp is a GNU extension.

 -- Function: int strncmp (const char *S1, const char *S2, size_t SIZE)
     This function is the similar to strcmp, except that no more than SIZE wide characters are compared.  In other words, if the two strings are the same in their first SIZE wide characters, the return value is zero.

 -- Function: int wcsncmp (const wchar_t *WS1, const wchar_t *WS2, size_t SIZE)
     This function is the similar to wcscmp, except that no more than SIZE wide characters are compared.  In other words, if the two strings are the same in their first SIZE wide characters, the return value is zero.

 -- Function: int strncasecmp (const char *S1, const char *S2, size_t N)
     This function is like strncmp, except that differences in case are ignored.  Like strcasecmp, how uppercase and lowercase characters are related is locale dependent.

     strncasecmp is a GNU extension.

 -- Function: int wcsncasecmp (const wchar_t *WS1, const wchar_t *S2, size_t N)
     This function is like wcsncmp, except that differences in case are ignored.  Like wcscasecmp, how uppercase and lowercase characters are related is locale dependent.

     wcsncasecmp is a GNU extension.

Here are some examples showing the use of strcmp and strncmp (equivalent examples can be constructed for the wide character functions).  These examples assume the use of the ASCII character set.  If some other character set---say, EBCDIC---is used instead, then the glyphs are associated with different numeric codes, and the return values and ordering may differ.

     strcmp ("hello", "hello")
         => 0    /* These two strings are the same. */
     strcmp ("hello", "Hello")
         => 32   /* Comparisons are case-sensitive. */
     strcmp ("hello", "world")
         => -15  /* The character 'h' comes before 'w'. */
     strcmp ("hello", "hello, world")
         => -44  /* Comparing a null character against a comma. */
     strncmp ("hello", "hello, world", 5)
         => 0    /* The initial 5 characters are the same. */
     strncmp ("hello, world", "hello, stupid world!!!", 5)
         => 0    /* The initial 5 characters are the same. */

 -- Function: int strverscmp (const char *S1, const char *S2)
     The strverscmp function compares the string S1 against S2, considering them as holding indices/version numbers.  Return value follows the same conventions as found in the strverscmp function.  In fact, if S1 and S2 contain no digits, strverscmp behaves like strcmp.

     Basically, we compare strings normally (character-by-character) until we find a digit in each string.  Then we enter a special comparison mode, where each sequence of digits is taken as a whole.  If we reach the end of these two parts without noticing a difference, we return to the standard comparison mode.  There are two types of numeric parts: integral and fractional (those begin with a '0'). The types of the numeric parts affect the way we sort them:

       * integral/integral: We compare values as you would expect.

       * fractional/integral: The fractional part is less than the integral one.

       * fractional/fractional: Things become a bit more complex. If the common prefix contains only leading zeros, the longest part is less than the other one; else the comparison behaves normally.

          strverscmp ("no digit", "no digit")
              => 0    /* same behavior as strcmp. */
          strverscmp ("item#99", "item#100")
              => <0   /* same prefix, but 99 < 100. */
          strverscmp ("alpha1", "alpha001")
              => >0   /* fractional part inferior to integral one. */
          strverscmp ("part1_f012", "part1_f01")
              => >0   /* two fractional parts. */
          strverscmp ("foo.009", "foo.0")
              => <0   /* idem, but with leading zeros only. */

     This function is especially useful when dealing with file-name sorting, because file names frequently hold indices and version numbers.

     strverscmp is a GNU extension.

 -- Function: int bcmp (const void *A1, const void *A2, size_t SIZE)
     This is an obsolete alias for memcmp, derived from BSD.

Collation Functions
===================

In some locales, the conventions for lexicographic ordering differ from the strict numeric ordering of character codes.  For example, in Spanish most glyphs with diacritical marks such as accents are not considered distinct letters for the purposes of collation.  On the other hand, the two-character sequence ll is treated as a single letter that is collated immediately after l.

You can use the functions strcoll and strxfrm (declared in the headers file string.h) and wcscoll and wcsxfrm (declared in the headers file wchar) to compare strings using a collation ordering appropriate for the current locale.  The locale used by these functions in particular can be specified by setting the locale for the LC_COLLATE category (see Locales).

In the standard C locale, the collation sequence for strcoll is the same as that for strcmp.  Similarly, wcscoll and wcscmp are the same in this situation.

Effectively, the way these functions work is by applying a mapping to transform the characters in a string to a byte sequence that represents the string's position in the collating sequence of the current locale. Comparing two such byte-sequences in a simple fashion is equivalent to comparing the strings with the locale's collating sequence.

The functions strcoll and wcscoll perform this translation implicitly, in order to do one comparison.  By contrast, strxfrm and wcsxfrm perform the mapping explicitly.  If you are making multiple comparisons using the same string or set of strings, it is likely to be more efficient to use strxfrm or wcsxfrm to transform all the strings just once, and subsequently compare the transformed strings with strcmp or wcscmp.

 -- Function: int strcoll (const char *S1, const char *S2)
     The strcoll function is similar to strcmp but uses the collating sequence of the current locale for collation (the LC_COLLATE locale).

 -- Function: int wcscoll (const wchar_t *WS1, const wchar_t *WS2)
     The wcscoll function is similar to wcscmp but uses the collating sequence of the current locale for collation (the LC_COLLATE locale).

Here is an example of sorting an array of strings, using strcoll to compare them.  The actual sort algorithm is not written here; it comes from qsort (see Array Sort Function).  The job of the code shown here is to say how to compare the strings while sorting them (later on in this section, we will show a way to do this more efficiently using strxfrm).

     /* This is the comparison function used with qsort. */

     int
     compare_elements (char **p1, char **p2)
     {
       return strcoll (*p1, *p2);
     }

     /* This is the entry point---the function to sort
        strings using the locale's collating sequence. */

     void
     sort_strings (char **array, int nstrings)
     {
       /* Sort temp_array by comparing the strings. */
       qsort (array, nstrings,
              sizeof (char *), compare_elements);
     }

 -- Function: size_t strxfrm (char *restrict TO, const char *restrict FROM, size_t SIZE)
     The function strxfrm transforms the string FROM using the collation transformation determined by the locale currently selected for collation, and stores the transformed string in the array TO.  Up to SIZE characters (including a terminating null character) are stored.

     The behavior is undefined if the strings TO and FROM overlap (see Copying and Concatenation).

     The return value is the length of the entire transformed string.  This value is not affected by the value of SIZE, but if it is greater than or equal to SIZE, it means that the transformed string did not entirely fit in the array TO.  In this case, only as much of the string as actually fits was stored.  To get the whole transformed string, call strxfrm again with a bigger output array.

     The transformed string may be longer than the original string, and it may also be shorter.

     If SIZE is zero, no characters are stored in TO.  In this case, strxfrm simply returns the number of characters that would be the length of the transformed string.  This is useful for determining what size the allocated array should be.  It does not matter what TO is if SIZE is zero; TO may even be a null pointer.

 -- Function: size_t wcsxfrm (wchar_t *restrict WTO, const wchar_t *WFROM, size_t SIZE)
     The function wcsxfrm transforms wide-character string WFROM using the collation transformation determined by the locale currently selected for collation, and stores the transformed string in the array WTO.  Up to SIZE wide characters (including a terminating null character) are stored.

     The behavior is undefined if the strings WTO and WFROM overlap (see Copying and Concatenation).

     The return value is the length of the entire transformed wide-character string.  This value is not affected by the value of SIZE, but if it is greater than or equal to SIZE, it means that the transformed wide-character string did not entirely fit in the array WTO.  In this case, only as much of the wide-character string as actually fits was stored.  To get the whole transformed wide-character string, call wcsxfrm again with a bigger output array.

     The transformed wide-character string may be longer than the original wide-character string, and it may also be shorter.

     If SIZE is zero, no characters are stored in TO.  In this case, wcsxfrm simply returns the number of wide characters that would be the length of the transformed wide-character string.  This is useful for determining what size the allocated array should be (remember to multiply with sizeof (wchar_t)).  It does not matter what WTO is if SIZE is zero; WTO may even be a null pointer.

Here is an example of how you can use strxfrm when you plan to do many comparisons.  It does the same thing as the previous example, but much faster, because it has to transform each string only once, no matter how many times it is compared with other strings.  Even the time needed to allocate and free storage is much less than the time we save, when there are many strings.

     struct sorter { char *input; char *transformed; };

     /* This is the comparison function used with qsort
        to sort an array of struct sorter. */

     int
     compare_elements (struct sorter *p1, struct sorter *p2)
     {
       return strcmp (p1->transformed, p2->transformed);
     }

     /* This is the entry point---the function to sort
        strings using the locale's collating sequence. */

     void
     sort_strings_fast (char **array, int nstrings)
     {
       struct sorter temp_array[nstrings];
       int i;

       /* Set up temp_array.  Each element contains
          one input string and its transformed string. */
       for (i = 0; i < nstrings; i++)
         {
           size_t length = strlen (array[i]) * 2;
           char *transformed;
           size_t transformed_length;

           temp_array[i].input = array[i];

           /* First make a buffer that you guess is big enough.  */
           transformed = (char *) xmalloc (length);

           /* Transform array[i].  */
           transformed_length = strxfrm (transformed, array[i], length);

           /* If the buffer was not large enough, resize it
              and try again.  */
           if (transformed_length >= length)
             {
               /* Allocate the needed space. +1 for terminating
                  NUL character.  */
               transformed = (char *) xrealloc (transformed,
                                                transformed_length + 1);

               /* The return value is not interesting because we know
                  how long the transformed string is.  */
               (void) strxfrm (transformed, array[i],
                               transformed_length + 1);
             }

           temp_array[i].transformed = transformed;
         }

       /* Sort temp_array by comparing transformed strings. */
       qsort (temp_array, sizeof (struct sorter),
              nstrings, compare_elements);

       /* Put the elements back in the permanent array
          in their sorted order. */
       for (i = 0; i < nstrings; i++)
         array[i] = temp_array[i].input;

       /* Free the strings we allocated. */
       for (i = 0; i < nstrings; i++)
         free (temp_array[i].transformed);
     }

The interesting part of this code for the wide-character version would look like this:

     void
     sort_strings_fast (wchar_t **array, int nstrings)
     {
       ...
           /* Transform array[i].  */
           transformed_length = wcsxfrm (transformed, array[i], length);

           /* If the buffer was not large enough, resize it
              and try again.  */
           if (transformed_length >= length)
             {
               /* Allocate the needed space. +1 for terminating
                  NUL character.  */
               transformed = (wchar_t *) xrealloc (transformed,
                                                   (transformed_length + 1)
                                                   * sizeof (wchar_t));

               /* The return value is not interesting because we know
                  how long the transformed string is.  */
               (void) wcsxfrm (transformed, array[i],
                               transformed_length + 1);
             }
       ...

Note the additional multiplication with sizeof (wchar_t) in the realloc call.

Compatibility Note: The string collation functions are a new feature of ISO C90.  Older C dialects have no equivalent feature. The wide-character versions were introduced in Amendment 1 to ISO C90.

Search Functions
================

This section describes library functions that perform various kinds of search operations on strings and arrays.  These functions are declared in the header file string.h.

 -- Function: void * memchr (const void *BLOCK, int C, size_t SIZE)
     This function finds the first occurrence of the byte C (converted to an unsigned char) in the initial SIZE bytes of the object beginning at BLOCK.  The return value is a pointer to the located byte, or a null pointer if no match was found.

 -- Function: wchar_t * wmemchr (const wchar_t *BLOCK, wchar_t WC, size_t SIZE)
     This function finds the first occurrence of the wide character WC in the initial SIZE wide characters of the object beginning at BLOCK.  The return value is a pointer to the located wide character, or a null pointer if no match was found.

 -- Function: void * rawmemchr (const void *BLOCK, int C)
     Often the memchr function is used with the knowledge that the byte C is available in the memory block specified by the parameters.  But this means that the SIZE parameter is not really needed and that the tests performed with it at run time (to check whether the end of the block is reached) are not needed.

     The rawmemchr function exists for just this situation, which is surprisingly frequent.  The interface is similar to memchr except that the SIZE parameter is missing.  The function will look beyond the end of the block pointed to by BLOCK in case the programmer made an error in assuming that the byte C is present in the block. In this case, the result is unspecified.  Otherwise, the return value is a pointer to the located byte.

     This function is of special interest when looking for the end of a string.  Since all strings are terminated by a null byte a call like:

             rawmemchr (str, '\0')

     will never go beyond the end of the string.

     This function is a GNU extension.

 -- Function: void * memrchr (const void *BLOCK, int C, size_t SIZE)
     The function memrchr is like memchr, except that it searches backward from the end of the block defined by BLOCK and SIZE (instead of forward from the front).

 -- Function: char * strchr (const char *STRING, int C)
     The strchr function finds the first occurrence of the character C (converted to a char) in the null-terminated string beginning at STRING.  The return value is a pointer to the located character, or a null pointer if no match was found.

     For example:

          strchr ("hello, world", 'l')
              => "llo, world"
          strchr ("hello, world", '?')
              => NULL

     The terminating null character is considered to be part of the string, so you can use this function get a pointer to the end of a string by specifying a null character as the value of the C argument.  It would be better (but less portable) to use strchrnul in this case, though.

 -- Function: wchar_t * wcschr (const wchar_t *WSTRING, int WC)
     The wcschr function finds the first occurrence of the wide character WC in the null-terminated wide-character string beginning at WSTRING.  The return value is a pointer to the located wide character, or a null pointer if no match was found.

     The terminating null character is considered to be part of the wide character string, so you can use this function get a pointer to the end of a wide-character string by specifying a null wide character as the value of the WC argument.  It would be better (but less portable) to use wcschrnul in this case, though.

 -- Function: char * strchrnul (const char *STRING, int C)
     strchrnul is the same as strchr except that if it does not find the character, it returns a pointer to STRING's terminating null character rather than a null pointer.

     This function is a GNU extension.

 -- Function: wchar_t * wcschrnul (const wchar_t *WSTRING, wchar_t WC)
     wcschrnul is the same as wcschr except that if it does not find the wide character, it returns a pointer to the wide-character string's terminating null wide character rather than a null pointer.

     This function is a GNU extension.

One useful but unusual use of the strchr function is when you want to have a pointer pointing to the NUL byte terminating a string. This is often written in this way:

       s += strlen (s);

This is almost optimal, but the addition operation duplicated a bit of the work already done in the strlen function.  A better solution is this:

       s = strchr (s, '\0');

There is no restriction on the second parameter of strchr, so it could very well also be the NUL character.  The strchr function is more expensive than the strlen function, since we have two abort criteria.  But in the GNU C Library, the implementation of strchr is optimized in a special way, so that strchr actually is faster.

 -- Function: char * strrchr (const char *STRING, int C)
     The function strrchr is like strchr, except that it searches backward from the end of the string STRING (instead of forward from the front).

     For example:

          strrchr ("hello, world", 'l')
              => "ld"

 -- Function: wchar_t * wcsrchr (const wchar_t *WSTRING, wchar_t C)
     The function wcsrchr is like wcschr, except that it searches backwards from the end of the string WSTRING (instead of forward from the front).

 -- Function: char * strstr (const char *HAYSTACK, const char *NEEDLE)
     This is like strchr, except that it searches HAYSTACK for a substring NEEDLE rather than just a single character.  It returns a pointer into the string HAYSTACK that is the first character of the substring, or a null pointer if no match was found.  If NEEDLE is an empty string, the function returns HAYSTACK.

     For example:

          strstr ("hello, world", "l")
              => "llo, world"
          strstr ("hello, world", "wo")
              => "world"

 -- Function: wchar_t * wcsstr (const wchar_t *HAYSTACK, const wchar_t *NEEDLE)
     This is like wcschr, except that it searches HAYSTACK for a substring NEEDLE rather than just a single wide character.  It returns a pointer into the string HAYSTACK that is the first wide character of the substring, or a null pointer if no match was found.  If NEEDLE is an empty string, the function returns HAYSTACK.

 -- Function: wchar_t * wcswcs (const wchar_t *HAYSTACK, const wchar_t *NEEDLE)
     wcsstr is a deprecated alias for wcsstr.  This is the name originally used in the X/Open Portability Guide (X/Open Company, X/Open Portability Guide, Issue 4, Version 2 (Reading, UK: X/Open Company, Ltd., 1994).) before the Amendment 1 to ISO C90 was published.

 -- Function: char * strcasestr (const char *HAYSTACK, const char *NEEDLE)
     This is like strstr, except that it ignores case in searching for the substring.  Like strcasecmp, how uppercase and lowercase characters are related is locale dependent.

     For example:

          strstr ("hello, world", "L")
              => "llo, world"
          strstr ("hello, World", "wo")
              => "World"

 -- Function: void * memmem (const void *HAYSTACK, size_t HAYSTACK-LEN,
const void *NEEDLE, size_t NEEDLE-LEN)
     This is like strstr, but NEEDLE and HAYSTACK are byte arrays rather than null-terminated strings.  NEEDLE-LEN is the length of NEEDLE and HAYSTACK-LEN is the length of HAYSTACK.

     This function is a GNU extension.

 -- Function: size_t strspn (const char *STRING, const char *SKIPSET)
     The strspn (``string span'') function returns the length of the initial substring of STRING that consists entirely of characters that are members of the set specified by the string SKIPSET.  The order of the characters in SKIPSET is not important.

     For example:

          strspn ("hello, world", "abcdefghijklmnopqrstuvwxyz")
              => 5

     Character used here in the sense of byte.  In a string using a multibyte-character encoding (abstract), characters consisting of more than one byte are not treated as an entity.  Each byte is treated separately.  The function is not locale dependent.

 -- Function: size_t wcsspn (const wchar_t *WSTRING, const wchar_t *SKIPSET)
     The wcsspn (``wide-character string span'') function returns the length of the initial substring of WSTRING that consists entirely of wide characters that are members of the set specified by the string SKIPSET.  The order of the wide characters in SKIPSET is not important.

 -- Function: size_t strcspn (const char *STRING, const char *STOPSET)
     The strcspn (``string complement span'') function returns the length of the initial substring of STRING that consists entirely of characters that are not members of the set specified by the string STOPSET.  (In other words, it returns the offset of the first character in STRING that is a member of the set STOPSET.)

     For example:

          strcspn ("hello, world", " \t\n,.;!?")
              => 5

     Character is used here in the sense of byte.  In a string using a multibyte-character encoding (abstract), characters consisting of more than one byte are not treated as an entity.  Each byte is treated separately.  The function is not locale dependent.

 -- Function: size_t wcscspn (const wchar_t *WSTRING, const wchar_t *STOPSET)
     The wcscspn (``wide-character string complement span'') function returns the length of the initial substring of WSTRING that consists entirely of wide characters that are not members of the set specified by the string STOPSET.  (In other words, it returns the offset of the first character in STRING that is a member of the set STOPSET.)

 -- Function: char * strpbrk (const char *STRING, const char *STOPSET)
     The strpbrk (``string pointer break'') function is related to strcspn, except that it returns a pointer to the first character in STRING that is a member of the set STOPSET instead of the length of the initial substring.  It returns a null pointer if no such character from STOPSET is found.

     For example:

          strpbrk ("hello, world", " \t\n,.;!?")
              => ", world"

     Character is used here in the sense of byte.  In a string using a multibyte-character encoding (abstract), characters consisting of more than one byte are not treated as an entity.  Each byte is treated separately.  The function is not locale dependent.

 -- Function: wchar_t * wcspbrk (const wchar_t *WSTRING, const wchar_t *STOPSET)
     The wcspbrk (``wide-character string pointer break'') function is related to wcscspn, except that it returns a pointer to the first wide character in WSTRING that is a member of the set STOPSET instead of the length of the initial substring.  It returns a null pointer if no such character from STOPSET is found.

Compatibility String Search Functions
-------------------------------------

 -- Function: char * index (const char *STRING, int C)
     index is another name for strchr; they are exactly the same.  New code should always use strchr, since this name is defined in ISO C while index is a BSD invention that was never available on System V derived systems.

 -- Function: char * rindex (const char *STRING, int C)
     rindex is another name for strrchr; they are exactly the same.  New code should always use strrchr, since this name is defined in ISO C while rindex is a BSD invention that was never available on System V derived systems.

Finding Tokens in a String
==========================

It's fairly common for programs to have a need to do some simple kinds of lexical analysis and parsing, such as splitting a command string up into tokens.  You can do this with the strtok function, declared in the header file string.h.

 -- Function: char * strtok (char *restrict NEWSTRING, const char *restrict DELIMITERS)
     A string can be split into tokens by making a series of calls to the function strtok.

     The string to be split up is passed as the NEWSTRING argument on the first call only.  The strtok function uses this to set up some internal state information.  Subsequent calls to get additional tokens from the same string are indicated by passing a null pointer as the NEWSTRING argument.  Calling strtok with another nonnull NEWSTRING argument reinitializes the state information. It is guaranteed that no other library function ever calls strtok behind your back (which would mess up this internal state information).

     The DELIMITERS argument is a string that specifies a set of delimiters that may surround the token being extracted.  All the initial characters that are members of this set are discarded.  The first character that is not a member of this set of delimiters marks the beginning of the next token.  The end of the token is found by looking for the next character that is a member of the delimiter set.  This character in the original string NEWSTRING is overwritten by a null character, and the pointer to the beginning of the token in NEWSTRING is returned.

     On the next call to strtok, the searching begins at the next character beyond the one that marked the end of the previous token. Note that the set of delimiters DELIMITERS do not have to be the same on every call in a series of calls to strtok.

     If the end of the string NEWSTRING is reached, or if the remainder of the string consists only of delimiter characters, strtok returns a null pointer.

     Character is used here in the sense of byte.  In a string using a multibyte-character encoding (abstract), characters consisting of more than 1 byte are not treated as an entity.  Each byte is treated separately.  The function is not locale dependent.

 -- Function: wchar_t * wcstok (wchar_t *NEWSTRING, const char *DELIMITERS)
     A string can be split into tokens by making a series of calls to the function wcstok.

     The string to be split up is passed as the NEWSTRING argument on the first call only.  The wcstok function uses this to set up some internal state information.  Subsequent calls to get additional tokens from the same wide-character string are indicated by passing a null pointer as the NEWSTRING argument.  Calling wcstok with another nonnull NEWSTRING argument reinitializes the state information.  It is guaranteed that no other library function ever calls wcstok behind your back (which would mess up this internal state information).

     The DELIMITERS argument is a wide-character string that specifies a set of delimiters that may surround the token being extracted.  All the initial wide characters that are members of this set are discarded. The first wide character that is not a member of this set of delimiters marks the beginning of the next token.  The end of the token is found by looking for the next wide character that is a member of the delimiter set.  This wide character in the original wide-character string NEWSTRING is overwritten by a null wide character, and the pointer to the beginning of the token in NEWSTRING is returned.

     On the next call to wcstok, the searching begins at the next wide character beyond the one that marked the end of the previous token.  The set of delimiters DELIMITERS do not have to be the same on every call in a series of calls to wcstok.

     If the end of the wide-character string NEWSTRING is reached, or if the remainder of the string consists only of delimiter wide characters, wcstok returns a null pointer.

     Character is used here in the sense of byte.  In a string using a multibyte-character encoding (abstract), characters consisting of more than one byte are not treated as an entity.  Each byte is treated separately.  The function is not locale dependent.

Warning: Since strtok and wcstok alter the string they are parsing, you should always copy the string to a temporary buffer before parsing it with strtok/wcstok (see Copying and Concatenation).  If you allow strtok or wcstok to modify a string that came from another part of your program, you are asking for trouble; that string might be used for other purposes after strtok or wcstok has modified it, and it would not have the expected value.

The string that you are operating on might even be a constant.  Then when strtok or wcstok tries to modify it, your program will get a fatal signal for writing in read-only memory. (See Loosemore et al., ``Program-Error Signals'' (see chap. 1, n. 1).)  Even if the operation of strtok or wcstok would not require a modification of the string (e.g., if there is exactly one token), the string can (and in the GNU libc case will) be modified.

This is a special case of a general principle: if a part of a program does not have as its purpose the modification of a certain data structure, then to modify the data structure temporarily is to risk errors.

The functions strtok and wcstok are not reentrant. (See Loosemore et al., ``Signal Handling and Nonreentrant Functions'', for a discussion of where and why reentrancy is important.)

Here is a simple example showing the use of strtok:

     #include <string.h>
     #include <stddef.h>

     ...

     const char string[] = "words separated by spaces -- and, punctuation!";
     const char delimiters[] = " .,;:!-";
     char *token, *cp;

     ...

     cp = strdupa (string);                /* Make writable copy.  */
     token = strtok (cp, delimiters);      /* token => "words" */
     token = strtok (NULL, delimiters);    /* token => "separated" */
     token = strtok (NULL, delimiters);    /* token => "by" */
     token = strtok (NULL, delimiters);    /* token => "spaces" */
     token = strtok (NULL, delimiters);    /* token => "and" */
     token = strtok (NULL, delimiters);    /* token => "punctuation" */
     token = strtok (NULL, delimiters);    /* token => NULL */

The GNU C Library contains two more functions for tokenizing a string which overcome the limitation of nonreentrancy.  They are only available for multibyte-character strings.

 -- Function: char * strtok_r (char *NEWSTRING, const char *DELIMITERS, char **SAVE_PTR)
     Just like strtok, this function splits the string into several tokens that can be accessed by successive calls to strtok_r.  The difference is that the information about the next token is stored in the space pointed to by the third argument, SAVE_PTR, which is a pointer to a string pointer.  Calling strtok_r with a null pointer for NEWSTRING and leaving SAVE_PTR between the calls unchanged does the job without hindering reentrancy.

     This function is defined in POSIX.1 and can be found on many systems which support multithreading.

 -- Function: char * strsep (char **STRING_PTR, const char *DELIMITER)
     This function has a similar functionality to strtok_r, with the NEWSTRING argument replaced by the SAVE_PTR argument.  The initialization of the moving pointer has to be done by the user. Successive calls to strsep move the pointer along the tokens separated by DELIMITER, returning the address of the next token and updating STRING_PTR to point to the beginning of the next token.

     One difference between strsep and strtok_r is that if the input string contains more than one character from DELIMITER in a row, strsep returns an empty string for each pair of characters from DELIMITER.  This means that a program normally should test for strsep returning an empty string before processing it.

     This function was introduced in 4.3BSD and therefore is widely available.

Here is how the above example looks like when strsep is used:

     #include <string.h>
     #include <stddef.h>

     ...

     const char string[] = "words separated by spaces -- and, punctuation!";
     const char delimiters[] = " .,;:!-";
     char *running;
     char *token;

     ...

     running = strdupa (string);
     token = strsep (&running, delimiters);    /* token => "words" */
     token = strsep (&running, delimiters);    /* token => "separated" */
     token = strsep (&running, delimiters);    /* token => "by" */
     token = strsep (&running, delimiters);    /* token => "spaces" */
     token = strsep (&running, delimiters);    /* token => "" */
     token = strsep (&running, delimiters);    /* token => "" */
     token = strsep (&running, delimiters);    /* token => "" */
     token = strsep (&running, delimiters);    /* token => "and" */
     token = strsep (&running, delimiters);    /* token => "" */
     token = strsep (&running, delimiters);    /* token => "punctuation" */
     token = strsep (&running, delimiters);    /* token => "" */
     token = strsep (&running, delimiters);    /* token => NULL */

 -- Function: char * basename (const char *FILENAME)
     The GNU version of the basename function returns the last component of the path in FILENAME.  This function is the preferred usage, since it does not modify the argument, FILENAME, and respects trailing slashes.  The prototype for basename can be found in string.h.  This function is overriden by the XPG version, if libgen.h is included.

     Here is an example using GNU basename:

          #include <string.h>

          int
          main (int argc, char *argv[])
          {
            char *prog = basename (argv[0]);

            if (argc < 2)
              {
                fprintf (stderr, "Usage %s <arg>\n", prog);
                exit (1);
              }

            ...
          }

     Portability Note: This function may produce different results on different systems.

 -- Function: char * basename (char *PATH)
     This is the standard XPG-defined basename. It is similar in spirit to the GNU version, but may modify the PATH by removing trailing / characters.  If the PATH is made up entirely of / characters, then ``/'' will be returned.  Also, if PATH is NULL or an empty string, then ``.'' is returned.  The prototype for the XPG version can be found in libgen.h.

     Here is an example using XPG basename:

          #include <libgen.h>

          int
          main (int argc, char *argv[])
          {
            char *prog;
            char *path = strdupa (argv[0]);

            prog = basename (path);

            if (argc < 2)
              {
                fprintf (stderr, "Usage %s <arg>\n", prog);
                exit (1);
              }

            ...

          }

 -- Function: char * dirname (char *PATH)
     The dirname function is the compliment to the XPG version of basename.  It returns the parent directory of the file specified by PATH.  If PATH is NULL, an empty string, or contains no / characters, then ``.'' is returned. The prototype for this function can be found in libgen.h.

strfry
======

The function below addresses the perennial programming quandary, ``How do I take good data in string form and painlessly turn it into garbage?''  This is actually a fairly simple task for C programmers who do not use the GNU C Library string functions, but for programs based on the GNU C Library, the strfry function is the preferred method for destroying string data.

The prototype for this function is in string.h.

 -- Function: char * strfry (char *STRING)

     strfry creates a pseudorandom anagram of a string, replacing the input with the anagram.  For each position in the string, strfry swaps it with a position in the string selected at random (from a uniform distribution).  The two positions may be the same.

     The return value of strfry is always STRING.

     Portability Note:  This function is unique to the GNU C Library.

Trivial Encryption
==================

The memfrob function converts an array of data to something unrecognizable and back again.  It is not encryption in its usual sense since it is easy for someone to convert the encrypted data back to clear text.  The transformation is analogous to Usenet's ``Rot13'' encryption method for obscuring offensive jokes from sensitive eyes and such. Unlike Rot13, memfrob works on arbitrary binary data, not just text. (See Loosemore et al., ``DES Encryption and Password Handling'', for a discussion of true encryption.)

This function is declared in string.h.

 -- Function: void * memfrob (void *MEM, size_t LENGTH)

     memfrob transforms (frobnicates) each byte of the data structure at MEM, which is LENGTH bytes long, by bit-wise exclusive-oring it with binary 00101010.  It does the transformation in place and its return value is always MEM.

     memfrob a second time on the same data structure returns it to its original state.

     This is a good function for hiding information from someone who doesn't want to see it.  To really prevent people from retrieving the information, use stronger encryption. (Ibid., ``DESEncryption and Password Handling''.)

     Portability Note: This function is unique to the GNU C Library.

Encode Binary Data
==================

To store or transfer binary data in environments that only support text, you have to encode the binary data by mapping the input bytes to characters in the range allowed for storing or transferring.  SVID systems (and nowadays XPG-compliant systems) provide minimal support for this task.

 -- Function: char * l64a (long int N)
     This function encodes a 32-bit input value using characters from the basic character set.  It returns a pointer to a 6-character buffer that contains an encoded version of N.  To encode a series of bytes, the user must copy the returned string to a destination buffer.  It returns the empty string if N is zero, which is somewhat bizarre but mandated by the standard.
      Warning: Since a static buffer is used, this function should not be used in multithreaded programs. There is no thread-safe alternative to this function in the C library.

     Compatibility Note: The XPG standard states that the return value of l64a is undefined if N is negative.  In the GNU implementation, l64a treats its argument as unsigned, so it will return a sensible encoding for any nonzero N; however, portable programs should not rely on this.

     To encode a large buffer l64a must be called in a loop, once for each 32-bit word of the buffer.  For example, one could do something like this:

          char *
          encode (const void *buf, size_t len)
          {
            /* We know in advance how long the buffer has to be. */
            unsigned char *in = (unsigned char *) buf;
            char *out = malloc (6 + ((len + 3) / 4) * 6 + 1);
            char *cp = out;

            /* Encode the length. */
            /* Using `htonl' is necessary so that the data can be
               decoded even on machines with different byte order. */

            cp = mempcpy (cp, l64a (htonl (len)), 6);

            while (len > 3)
              {
                unsigned long int n = *in++;
                n = (n << 8) | *in++;
                n = (n << 8) | *in++;
                n = (n << 8) | *in++;
                len -= 4;
                if (n)
                  cp = mempcpy (cp, l64a (htonl (n)), 6);
                else
                      /* `l64a' returns the empty string for n==0, so we
                         must generate its encoding ("......") by hand. */
                  cp = stpcpy (cp, "......");
              }
            if (len > 0)
              {
                unsigned long int n = *in++;
                if (--len > 0)
                  {
                    n = (n << 8) | *in++;
                    if (--len > 0)
                      n = (n << 8) | *in;
                  }
                memcpy (cp, l64a (htonl (n)), 6);
                cp += 6;
              }
            *cp = '\0';
            return out;
          }

To decode data produced with l64a, the following function should be used.

 -- Function: long int a64l (const char *STRING)
     The parameter STRING should contain a string that was produced by a call to l64a.  The function processes at least 6 characters of this string, and decodes the characters it finds according to the table below.  It stops decoding when it finds a character not in the table, rather like atoi; if you have a buffer that has been broken into lines, you must be careful to skip over the end-of-line characters.

     The decoded number is returned as a long int value.

The l64a and a64l functions use a base-64 encoding, in which each character of an encoded string represents 6 bits of an input word.  These symbols are used for the base-64 digits:

  0   1   2   3   4   5   6   7
0        .   /   0   1
2   3   4   5
8        6   7   8   9
A   B   C   D
16       E   F   G   H
I   J   K   L
24       M   N   O   P
Q   R   S   T
32       U   V   W   X
Y   Z   a   b
40       c   d   e   f
g   h   i   j
48       k   l   m   n
o   p   q   r
56       s   t   u   v
w   x   y   z

This encoding scheme is not standard.  There are some other encoding methods that are much more widely used (UU encoding, MIME encoding). Generally, it is better to use one of these encodings.

Argz and Envz Vectors
=====================

argz vectors are vectors of strings in a contiguous block of memory, each element separated from its neighbors by null-characters ('\0').

envz vectors are an extension of argz vectors where each element is a name-value pair, separated by a '=' character (as in a Unix environment).

Argz Functions
--------------

Each argz vector is represented by a pointer to the first element, of type char *, and a size, of type size_t, both of which can be initialized to 0 to represent an empty argz vector.  All argz functions accept either a pointer and a size argument, or pointers to them, if they will be modified.

The argz functions use malloc/realloc to allocate and grow argz vectors, so any argz vector created using these functions may be freed by using free; conversely, any argz function that may grow a string expects that string to have been allocated using malloc (those argz functions that only examine their arguments or modify them in place will work on any sort of memory).  See Unconstrained Allocation, for more information.

All argz functions that do memory allocation have a return type of error_t, and return 0 for success or ENOMEM if an allocation error occurs.

These functions are declared in the standard include file argz.h.

 -- Function: error_t argz_create (char *const ARGV[], char **ARGZ, size_t *ARGZ_LEN)
     The argz_create function converts the Unix-style argument vector ARGV (a vector of pointers to normal C strings, terminated by (char *)0; see Program Arguments) into an argz vector with the same elements, which is returned in ARGZ and ARGZ_LEN.

 -- Function: error_t argz_create_sep (const char *STRING, int SEP, char **ARGZ, size_t *ARGZ_LEN)
     The argz_create_sep function converts the null-terminated string STRING into an argz vector (returned in ARGZ and ARGZ_LEN) by splitting it into elements at every occurrence of the character SEP.

 -- Function: size_t argz_count (const char *ARGZ, size_t ARG_LEN)
     Returns the number of elements in the argz vector ARGZ and ARGZ_LEN.

 -- Function: void argz_extract (char *ARGZ, size_t ARGZ_LEN, char **ARGV)
     The argz_extract function converts the argz vector ARGZ and ARGZ_LEN into a Unix-style argument vector stored in ARGV, by putting pointers to every element in ARGZ into successive positions in ARGV, followed by a terminator of 0. ARGV must be preallocated with enough space to hold all the elements in ARGZ plus the terminating (char *)0 ((argz_count (ARGZ, ARGZ_LEN) + 1) * sizeof (char *) bytes should be enough).  The string pointers stored into ARGV point into ARGZ---they are not copies---and so ARGZ must be copied if it will be changed while ARGV is still active.  This function is useful for passing the elements in ARGZ to an exec function. (Ibid., ``Executing a File''.)

 -- Function: void argz_stringify (char *ARGZ, size_t LEN, int SEP)
     The argz_stringify converts ARGZ into a normal string with the elements separated by the character SEP, by replacing each '\0' inside ARGZ (except the last one, which terminates the string) with SEP.  This is handy for printing ARGZ in a readable manner.

 -- Function: error_t argz_add (char **ARGZ, size_t *ARGZ_LEN, const char *STR)
     The argz_add function adds the string STR to the end of the argz vector *ARGZ, and updates *ARGZ and *ARGZ_LEN accordingly.

 -- Function: error_t argz_add_sep (char **ARGZ, size_t *ARGZ_LEN, const char *STR, int DELIM)
     The argz_add_sep function is similar to argz_add, but STR is split into separate elements in the result at occurrences of the character DELIM.  This is useful, for instance, for adding the components of a Unix search path to an argz vector, by using a value of ':' for DELIM.

 -- Function: error_t argz_append (char **ARGZ, size_t *ARGZ_LEN, const char *BUF, size_t BUF_LEN)
     The argz_append function appends BUF_LEN bytes starting at BUF to the argz vector *ARGZ, reallocating *ARGZ to accommodate it, and adding BUF_LEN to *ARGZ_LEN.

 -- Function: error_t argz_delete (char **ARGZ, size_t *ARGZ_LEN, char *ENTRY)
     If ENTRY points to the beginning of one of the elements in the argz vector *ARGZ, the argz_delete function will remove this entry and reallocate *ARGZ, modifying *ARGZ and *ARGZ_LEN accordingly.  As destructive argz functions usually reallocate their argz argument, pointers into argz vectors such as ENTRY will then become invalid.

 -- Function: error_t argz_insert (char **ARGZ, size_t *ARGZ_LEN, char *BEFORE, const char *ENTRY)
     The argz_insert function inserts the string ENTRY into the argz vector *ARGZ at a point just before the existing element pointed to by BEFORE, reallocating *ARGZ and updating *ARGZ and *ARGZ_LEN.  If BEFORE is 0, ENTRY is added to the end instead (as if by argz_add).  Since the first element is in fact the same as *ARGZ, passing in *ARGZ as the value of BEFORE will result in ENTRY being inserted at the beginning.

 -- Function: char * argz_next (char *ARGZ, size_t ARGZ_LEN, const char *ENTRY)
     The argz_next function provides a convenient way of iterating over the elements in the argz vector ARGZ.  It returns a pointer to the next element in ARGZ after the element ENTRY, or 0 if there are no elements following ENTRY.  If ENTRY is 0, the first element of ARGZ is returned.

     This behavior suggests two styles of iteration:

              char *entry = 0;
              while ((entry = argz_next (ARGZ, ARGZ_LEN, entry)))
                ACTION;

     (the double parentheses are necessary to make some C compilers shut up about what they consider a questionable while-test) and:

              char *entry;
              for (entry = ARGZ;
                   entry;
                   entry = argz_next (ARGZ, ARGZ_LEN, entry))
                ACTION;

     The latter depends on ARGZ having a value of 0 if it is empty (rather than a pointer to an empty block of memory); this invariant is maintained for argz vectors created by the functions here.

 -- Function: error_t argz_replace (char **ARGZ, size_t *ARGZ_LEN, const char *STR, const char *WITH, unsigned *REPLACE_COUNT)
     Replace any occurrences of the string STR in ARGZ with WITH, reallocating ARGZ as necessary.  If REPLACE_COUNT is nonzero, *REPLACE_COUNT will be incremented by the number of replacements performed.

Envz Functions
--------------

Envz vectors are just argz vectors with additional constraints on the form of each element; as such, argz functions can also be used on them, where it makes sense.

Each element in an envz vector is a name-value pair, separated by a '=' character; if multiple '=' characters are present in an element, those after the first are considered part of the value, and treated like all other non-'\0' characters.

If no '=' characters are present in an element, that element is considered the name of a ``null'' entry, as distinct from an entry with an empty value: envz_get will return 0 if given the name of null entry, whereas an entry with an empty value would result in a value of ""; envz_entry will still find such entries, however.  Null entries can be removed with envz_strip function.

As with argz functions, envz functions that may allocate memory (and thus fail) have a return type of error_t, and return either 0 or ENOMEM.

These functions are declared in the standard include file envz.h.

 -- Function: char * envz_entry (const char *ENVZ, size_t ENVZ_LEN, const char *NAME)
     The envz_entry function finds the entry in ENVZ with the name NAME, and returns a pointer to the whole entry---the argz element that begins with NAME followed by an '=' character. If there is no entry with that name, 0 is returned.

 -- Function: char * envz_get (const char *ENVZ, size_t ENVZ_LEN, const char *NAME)
     The envz_get function finds the entry in ENVZ with the name NAME (like envz_entry) and returns a pointer to the value portion of that entry (following the '=').  If there is no entry with that name (or only a null entry), 0 is returned.

 -- Function: error_t envz_add (char **ENVZ, size_t *ENVZ_LEN, const char *NAME, const char *VALUE)
     The envz_add function adds an entry to *ENVZ (updating *ENVZ and *ENVZ_LEN) with the name NAME, and value VALUE.  If an entry with the same name already exists in ENVZ, it is removed first.  If VALUE is 0, then the new entry will be the special null type of entry mentioned above.

 -- Function: error_t envz_merge (char **ENVZ, size_t *ENVZ_LEN, const char *ENVZ2, size_t ENVZ2_LEN, int OVERRIDE)
     The envz_merge function adds each entry in ENVZ2 to ENVZ, as if with envz_add, updating *ENVZ and *ENVZ_LEN.  If OVERRIDE is true, then values in ENVZ2 will supersede those with the same name in ENVZ, otherwise they will not.

     Null entries are treated just like other entries in this respect, so a null entry in ENVZ can prevent an entry of the same name in ENVZ2 from being added to ENVZ, if OVERRIDE is false.

 -- Function: void envz_strip (char **ENVZ, size_t *ENVZ_LEN)
     The envz_strip function removes any null entries from ENVZ, updating *ENVZ and *ENVZ_LEN.

Character-Set Handling
**********************

Character sets used in the early days of computing had only 6, 7, or 8 bits for each character---there was never a case where more than 8 bits (1 byte) were used to represent a single character.  The limitations of this approach became more apparent as more people grappled with non-Roman character sets, where all the characters that make up a language's character set cannot be represented by 2^8 choices.  This chapter shows the functionality that was added to the C library to support multiple character sets.

Introduction to Extended Characters
===================================

A variety of solutions is available to overcome the differences between character sets with a 1:1 relation between bytes and characters and character sets with ratios of 2:1 or 4:1.  The remainder of this section gives a few examples to help you understand the design decisions made while developing the functionality of the C library.

A distinction we have to make right away is between internal and external representation.  Internal representation means the representation used by a program while keeping the text in memory. External representations are used when text is stored or transmitted through some communication channel.  Examples of external representations include files waiting in a directory to be read and parsed.

Traditionally, there has been no difference between the two representations. It was equally comfortable and useful to use the same single-byte representation internally and externally.  This comfort level decreases with more and larger character sets.

One of the problems to overcome with the internal representation is handling text that is externally encoded using different character sets.  Assume a program that reads two texts and compares them using some metric.  The comparison can be usefully done only if the texts are internally kept in a common format.

For such a common format (character set), 8 bits are certainly no longer enough.  So the smallest entity will have to grow---wide characters will now be used.  Instead of 1 byte per character, 2 or 4 will be used instead (three are not good to address in memory and using more than 4 bytes seems to be unnecessary).

As shown in String and Array Utilities, a completely new family has been created of functions that can handle wide-character texts in memory.  The most commonly used character sets for such internal wide-character representations are Unicode and ISO 10646 (also known as UCS for Universal Character Set). Unicode was originally planned as a 16-bit character set, whereas ISO 10646 was designed to be a 31-bit large code space.  The two standards are practically identical.  They have the same character repertoire and code table, but Unicode specifies added semantics.  At the moment, only characters in the first 0x10000 code positions (the so-called Basic Multilingual Plane or BMP) have been assigned, but the assignment of more specialized characters outside this 16-bit space is already in progress.  A number of encodings have been defined for Unicode and ISO 10646 characters: UCS-2 is a 16-bit word that can only represent characters from the BMP, UCS-4 is a 32-bit word than can represent any Unicode and ISO 10646 character, UTF-8 is an ASCII-compatible encoding where ASCII characters are represented by ASCII bytes and non-ASCII characters by sequences of 2-6 non-ASCII bytes, and UTF-16 is an extension of UCS-2 in which pairs of certain UCS-2 words can be used to encode non-BMP characters up to 0x10ffff.

To represent wide characters, the char type is not suitable.  For this reason, the ISO C standard introduces a new type that is designed to keep one character of a wide-character string.  To maintain the similarity, there is also a type corresponding to int for those functions that take a single wide character.

 -- Data type: wchar_t
     This data type is used as the base type for wide-character strings. In other words, arrays of objects of this type are the equivalent of char[] for multibyte-character strings.  The type is defined in stddef.h.

     The ISO C90 standard, where wchar_t was introduced, does not say anything specific about the representation.  It only requires that this type be capable of storing all elements of the basic character set.  Therefore, it would be legitimate to define wchar_t as char, which might make sense for embedded systems.

     But for GNU systems, wchar_t is always 32 bits wide and therefore capable of representing all UCS-4 values, which means it is capable of covering all of ISO 10646.  Some Unix systems define wchar_t as a 16-bit type and thereby follow Unicode very strictly.  This definition is perfectly fine with the standard, but it also means that to represent all characters from Unicode and ISO 10646, you have to use UTF-16 surrogate characters, which is in fact a multi-wide-character encoding.  But resorting to multi-wide-character encoding contradicts the purpose of the wchar_t type.

 -- Data type: wint_t
     wint_t is a data type used for parameters and variables that contain a single wide character.  As the name suggests, this type is the equivalent of int when using the normal char strings.  The types wchar_t and wint_t often have the same representation if their size is 32 bits wide, but if wchar_t is defined as char, the type wint_t must be defined as int due to the parameter promotion.

     This type is defined in wchar.h and was introduced in Amendment 1 to ISO C90.

As there are for the char data type, macros are available for specifying the minimum and maximum value representable in an object of type wchar_t.

 -- Macro: wint_t WCHAR_MIN
     The macro WCHAR_MIN evaluates to the minimum value representable by an object of type wint_t.

     This macro was introduced in Amendment 1 to ISO C90.

 -- Macro: wint_t WCHAR_MAX
     The macro WCHAR_MAX evaluates to the maximum value representable by an object of type wint_t.

     This macro was introduced in Amendment 1 to ISO C90.

Another special wide-character value is the equivalent to EOF.

 -- Macro: wint_t WEOF
     The macro WEOF evaluates to a constant expression of type wint_t, whose value is different from any member of the extended character set.

     WEOF need not be the same value as EOF, and unlike EOF, it also need not be negative.  In other words, sloppy code like:

          {
            int c;
            ...
            while ((c = getc (fp)) < 0)
              ...
          }

     has to be rewritten to use WEOF explicitly when wide characters are used:

          {
            wint_t c;
            ...
            while ((c = wgetc (fp)) != WEOF)
              ...
          }

     This macro was introduced in Amendment 1 to ISO C90 and is defined in wchar.h.

These internal representations present problems when it comes to storing and transmittal.  Because each single wide character consists of more than 1 byte, they are effected by byte-ordering.  Thus, machines with different endianesses would see different values when accessing the same data.  This byte-ordering concern also applies for communication protocols that are all byte-based and therefore require the sender to decide about splitting the wide character in bytes.  A last point is that wide characters often require more storage space than a customized byte-oriented character set.

For all the above reasons, an external encoding that is different from the internal encoding is often used if the latter is UCS-2 or UCS-4. The external encoding is byte-based and can be chosen appropriately for the environment and for the texts to be handled.  A variety of different character sets can be used for this external encoding (information that will not be exhaustively presented here---instead, a description of the major groups will suffice).  All of the ASCII-based character sets fulfill one requirement: they are file-system safe.  This means that the character '/' is used in the encoding only to represent itself.  Things are a bit different for character sets like EBCDIC (Extended Binary Coded Decimal Interchange Code, a character-set family used by IBM), but if the operation system does not understand EBCDIC directly, the parameters-to-system calls have to be converted first anyhow.

  * The simplest character sets are single-byte character sets.  There can only be up to 256 characters (for 8-bit character sets), which is not sufficient to cover all languages but might be sufficient to handle a specific text.  Handling of an 8-bit character set is simple. This is not true for other kinds presented later, and therefore, the application you use might require 8-bit character sets.

  * The ISO 2022 standard defines a mechanism for extended character sets where one character can be represented by more than one byte.  This is achieved by associating a state with the text. Characters that can be used to change the state can be embedded in the text.  Each byte in the text might have a different interpretation in each state.  The state might even influence whether a given byte stands for a character on its own or whether it has to be combined with some more bytes.

     In most uses of ISO 2022, the defined character sets do not allow state changes that cover more than the next character.  This has the big advantage that whenever you can identify the beginning of the byte sequence of a character, you can interpret a text correctly. Examples of character sets using this policy are the various EUC character sets (used by Sun's operations systems, EUC-JP, EUC-KR, EUC-TW, and EUC-CN) or Shift_JIS (SJIS, a Japanese encoding).

     But there are also character sets using a state that is valid for more than one character and has to be changed by another byte sequence. Examples for this are ISO-2022-JP, ISO-2022-KR and ISO-2022-CN.

  * Early attempts to fix 8-bit character sets for other languages using the Roman alphabet lead to character sets like ISO 6937.  Here, bytes representing characters like the acute accent do not produce output themselves---you have to combine them with other characters to get the desired result.  For example, the byte sequence 0xc2 0x61 (nonspacing acute accent, followed by lowercase `a') to get the ``small a with acute'' character.  To get the acute accent character on its own, you have to write 0xc2 0x20 (the nonspacing acute followed by a space).

     Character sets like ISO 6937 are used in some embedded systems, such as teletex.

  * Instead of converting the Unicode or ISO 10646 text used internally, it is often sufficient to simply use an encoding different than UCS-2/UCS-4.  The Unicode and ISO 10646 standards even specify such an encoding: UTF-8.  This encoding is able to represent all of ISO 10646 31 bits in a byte string of length 1 to 6.

     There were a few other attempts to encode ISO 10646, such as UTF-7, but UTF-8 is today the only encoding that should be used.  In fact, with any luck, UTF-8 will soon be the only external encoding that has to be supported.  It proves to be universally usable and its only disadvantage is that it favors Roman languages by making the byte string representation of other scripts (Cyrillic, Greek, Asian scripts) longer than necessary if using a specific character set for these scripts.  Methods like the Unicode compression scheme can alleviate these problems.

The question remaining is how to select the character set or encoding to use.  You cannot decide yourself---it is decided by the developers of the system or the majority of the users.  Since the goal is interoperability, you have to use whatever the other people you work with use.  If there are no constraints, the selection is based on the requirements the expected circle of users will have.  In other words, if a project is expected to be used in only Russia, it is fine to use KOI8-R or a similar character set.  But if at the same time people from Greece are participating, you should use a character set that allows all people to collaborate.

The most widely useful solution seems to be to go with the most general character set, ISO 10646.  Use UTF-8 as the external encoding and problems with users not being able to use their own language adequately are a thing of the past.

One final comment about the choice of the wide-character representation is necessary at this point.  We have said above that the natural choice is using Unicode or ISO 10646.  This is not required but is encouraged by the ISO C standard.  The standard defines at least a macro __STDC_ISO_10646__ that is only defined on systems where the wchar_t type encodes ISO 10646 characters.  If this symbol is not defined, you should avoid making assumptions about the wide-character representation.  If the programmer uses only the functions provided by the C library to handle wide-character strings, there should be no compatibility problems with other systems.

Overview About Character-Handling Functions
===========================================

A Unix C library contains three different sets of functions in two families to handle character-set conversion.  One of the function families (the most commonly used) is specified in the ISO C90 standard and, therefore, is portable even beyond the Unix world. Unfortunately, this family is the least useful one.  These functions should be avoided whenever possible, especially when developing libraries (as opposed to applications).

The second family of functions was introduced in the early Unix standards (XPG2) and is still part of the latest and greatest Unix standard, Unix 98.  It is also the most powerful and useful set of functions.  But we will start with the functions defined in Amendment 1 to ISO C90.

Restartable Multibyte Conversion Functions
==========================================

The ISO C standard defines functions to convert strings from a multibyte representation to wide-character strings.  There are a number of peculiarities:

  * The character set assumed for the multibyte encoding is not specified as an argument to the functions.  Instead, the character set specified by the LC_CTYPE category of the current locale is used (see Locale Categories).

  * The functions handling more than one character at a time require NUL terminated strings as the argument (i.e., converting blocks of text does not work unless one can add a NUL byte at an appropriate place). The GNU C Library contains some extensions to the standard that allow specifying a size, but basically they also expect terminated strings.

Despite these limitations, the ISO C functions can be used in many contexts.  In graphical user interfaces, for instance, it is not uncommon to have functions that require text to be displayed in a wide-character string if the text is not simple ASCII.  The text itself might come from a file with translations, and the user should decide about the current locale, which determines the translation and therefore also the external encoding used.  In such a situation (and many others), the functions described here are perfect.  If more freedom while performing the conversion is necessary, take a look at the iconv functions (see Generic Charset Conversion).

Selecting the Conversion and Its Properties
-------------------------------------------

We already said above that the currently selected locale for the LC_CTYPE category decides about the conversion that is performed by the functions we are about to describe.  Each locale uses its own character set (given as an argument to localedef) and this is the one assumed as the external multibyte encoding.  The wide-character character set always is UCS-4, at least on GNU systems.

A characteristic of each multibyte-character set is the maximum number of bytes that can be necessary to represent one character.  This information is quite important when writing code that uses the conversion functions (as shown in the examples below).  The ISO C standard defines two macros that provide this information.

 -- Macro: int MB_LEN_MAX
     MB_LEN_MAX specifies the maximum number of bytes in the multibyte sequence for a single character in any of the supported locales.  It is a compile-time constant and is defined in limits.h.

 -- Macro: int MB_CUR_MAX
     MB_CUR_MAX expands into a positive integer expression that is the maximum number of bytes in a multibyte character in the current locale. The value is never greater than MB_LEN_MAX.  Unlike MB_LEN_MAX, this macro need not be a compile-time constant, and in the GNU C Library it is not.

     MB_CUR_MAX is defined in stdlib.h.

Two different macros are necessary, since strictly ISO C90 compilers do not allow variable-length array definitions, but still it is desirable to avoid dynamic allocation.  This incomplete piece of code shows the problem:

     {
       char buf[MB_LEN_MAX];
       ssize_t len = 0;

       while (! feof (fp))
         {
           fread (&buf[len], 1, MB_CUR_MAX - len, fp);
           /* ... process buf */
           len -= used;
         }
     }

The code in the inner loop is expected to always have enough bytes in the array BUF to convert one multibyte character.  The array BUF has to be sized statically since many compilers do not allow a variable size.  The fread call makes sure that MB_CUR_MAX bytes are always available in BUF.  It isn't a problem if MB_CUR_MAX is not a compile-time constant.

Representing the State of the Conversion
----------------------------------------

In the introduction of this chapter, it was said that certain character sets use a stateful encoding.  That is, the encoded values depend in some way on the previous bytes in the text.

Since the conversion functions allow converting a text in more than one step, we must have a way to pass this information from one call of the functions to another.

 -- Data type: mbstate_t
     A variable of type mbstate_t can contain all the information about the shift state needed from one call to a conversion function to another.

     mbstate_t is defined in wchar.h.  It was introduced in Amendment 1 to ISO C90.

To use objects of type mbstate_t, the programmer has to define such objects (normally as local variables on the stack) and pass a pointer to the object to the conversion functions.  This way, the conversion function can update the object if the current multibyte-character set is stateful.

There is no specific function or initializer to put the state object in any specific state.  The rules are that the object should always represent the initial state before the first use, and this is achieved by clearing the whole variable with code such as:

     {
       mbstate_t state;
       memset (&state, '\0', sizeof (state));
       /* from now on STATE can be used.  */
       ...
     }

When using the conversion functions to generate output, it is often necessary to test whether the current state corresponds to the initial state.  This is necessary, for example, to decide whether to emit escape sequences to set the state to the initial state at certain sequence points.  Communication protocols often require this.

 -- Function: int mbsinit (const mbstate_t *PS)
     The mbsinit function determines whether the state object pointed to by PS is in the initial state.  If PS is a null pointer or the object is in the initial state, the return value is nonzero.  Otherwise it is zero.

     mbsinit was introduced in Amendment 1 to ISO C90 and is declared in wchar.h.

Code using mbsinit often looks similar to this:

     {
       mbstate_t state;
       memset (&state, '\0', sizeof (state));
       /* Use STATE.  */
       ...
       if (! mbsinit (&state))
         {
           /* Emit code to return to initial state.  */
           const wchar_t empty[] = L"";
           const wchar_t *srcp = empty;
           wcsrtombs (outbuf, &srcp, outbuflen, &state);
         }
       ...
     }

The code to emit the escape sequence to get back to the initial state is interesting.  The wcsrtombs function can be used to determine the necessary output code (see Converting Strings).  On GNU systems it is not necessary to perform this extra action for the conversion from multibyte text to wide-character text since the wide-character encoding is not stateful.  But there is nothing mentioned in any standard that prohibits making wchar_t using a stateful encoding.

Converting Single Characters
----------------------------

The most fundamental of the conversion functions are those dealing with single characters.  This does not always mean single bytes.  But since there is very often a subset of the multibyte-character set that consists of single-byte sequences, there are functions to help with converting bytes.  Frequently, ASCII is a subpart of the multibyte-character set.  In such a scenario, each ASCII character stands for itself, and all other characters have at least a first byte that is beyond the range 0 to 127.

 -- Function: wint_t btowc (int C)
     The btowc function (``byte to wide character'') converts a valid single-byte character C in the initial shift state into the wide-character equivalent using the conversion rules from the currently selected locale of the LC_CTYPE category.

     If (unsigned char) C is not a valid single-byte multibyte character or if C is EOF, the function returns WEOF.

     Please note the restriction of C being tested for validity only in the initial shift state.  No mbstate_t object is used from which the state information is taken, and the function also does not use any static state.

     The btowc function was introduced in Amendment 1 to ISO C90 and is declared in wchar.h.

Despite the limitation that the single-byte value is always interpreted in the initial state, this function is actually useful most of the time. Most characters are either entirely single-byte character sets or extensions to ASCII.  But then, it is possible to write code like this (not that this specific example is very useful):

     wchar_t *
     itow (unsigned long int val)
     {
       static wchar_t buf[30];
       wchar_t *wcp = &buf[29];
       *wcp = L'\0';
       while (val != 0)
         {
           *--wcp = btowc ('0' + val % 10);
           val /= 10;
         }
       if (wcp == &buf[29])
         *--wcp = L'0';
       return wcp;
     }

Why is it necessary to use such a complicated implementation and not simply cast '0' + val % 10 to a wide character?  The answer is that there is no guarantee that one can perform this kind of arithmetic on the character of the character set used for wchar_t representation.  In other situations, the bytes are not constant at compile time and so the compiler cannot do the work.  In situations like this, it is necessary to use btowc.

There also is a function for the conversion in the other direction.

 -- Function: int wctob (wint_t C)
     The wctob function (``wide character to byte'') takes as the parameter a valid wide character.  If the multibyte representation for this character in the initial state is exactly 1 byte long, the return value of this function is this character.  Otherwise, the return value is EOF.

     wctob was introduced in Amendment 1 to ISO C90 and is declared in wchar.h.

There are more general functions to convert single characters from multibyte representations to wide characters and vice versa.  These functions pose no limit on the length of the multibyte representation, and they also do not require it to be in the initial state.

 -- Function: size_t mbrtowc (wchar_t *restrict PWC, const char *restrict S, size_t N, mbstate_t *restrict PS)
     The mbrtowc function (``multibyte restartable to wide character'') converts the next multibyte character in the string pointed to by S into a wide character and stores it in the wide-character string pointed to by PWC.  The conversion is performed according to the locale currently selected for the LC_CTYPE category.  If the conversion for the character set used in the locale requires a state, the multibyte string is interpreted in the state represented by the object pointed to by PS.  If PS is a null pointer, a static, internal state variable used only by the mbrtowc function is used.

     If the next multibyte character corresponds to the NUL wide character, the return value of the function is 0 and the state object is afterwards in the initial state.  If the next N or fewer bytes form a correct multibyte character, the return value is the number of bytes starting from S that form the multibyte character.  The conversion state is updated according to the bytes consumed in the conversion.  In both cases the wide character (either the L'\0' or the one found in the conversion) is stored in the string pointed to by PWC if PWC is not null.

     If the first N bytes of the multibyte string possibly form a valid multibyte character but there are more than N bytes needed to complete it, the return value of the function is (size_t) -2 and no value is stored.  This can happen even if N has a value greater than or equal to MB_CUR_MAX since the input might contain redundant shift sequences.

     If the first n bytes of the multibyte string cannot possibly form a valid multibyte character, no value is stored, the global variable errno is set to the value EILSEQ, and the function returns (size_t) -1.  The conversion state is afterwards undefined.

     mbrtowc was introduced in Amendment 1 to ISO C90 and is declared in wchar.h.

Use of mbrtowc is straightforward.  A function that copies a multibyte string into a wide-character string while at the same time converting all lowercase characters into uppercase could look like this (this is not the final version, just an example---it has no error checking, and sometimes leaks memory):

     wchar_t *
     mbstouwcs (const char *s)
     {
       size_t len = strlen (s);
       wchar_t *result = malloc ((len + 1) * sizeof (wchar_t));
       wchar_t *wcp = result;
       wchar_t tmp[1];
       mbstate_t state;
       size_t nbytes;

       memset (&state, '\0', sizeof (state));
       while ((nbytes = mbrtowc (tmp, s, len, &state)) > 0)
         {
           if (nbytes >= (size_t) -2)
             /* Invalid input string.  */
             return NULL;
           *wcp++ = towupper (tmp[0]);
           len -= nbytes;
           s += nbytes;
         }
       return result;
     }

The use of mbrtowc should be clear.  A single wide character is stored in TMP[0], and the number of consumed bytes is stored in the variable NBYTES.  If the conversion is successful, the uppercase variant of the wide character is stored in the RESULT array and the pointer to the input string and the number of available bytes is adjusted.

The only non-obvious thing about mbrtowc might be the way memory is allocated for the result.  The above code uses the fact that there can never be more wide characters in the converted results than there are bytes in the multibyte input string.  This method yields a pessimistic guess about the size of the result, and if many wide-character strings have to be constructed this way or if the strings are long, the extra memory allocation required because the input string contains multibyte characters might be significant.  The allocated memory block can be resized to the correct size before returning it, but a better solution might be to allocate just the right amount of space for the result right away.  Unfortunately, there is no function to compute the length of the wide-character string directly from the multibyte string.  There is, however, a function that does part of the work.

 -- Function: size_t mbrlen (const char *restrict S, size_t N, mbstate_t *PS)
     The mbrlen function (``multibyte restartable length'') computes the number of at most N bytes starting at S, which form the next valid and complete multibyte character.

     If the next multibyte character corresponds to the NUL wide character, the return value is 0.  If the next N bytes form a valid multibyte character, the number of bytes belonging to this multibyte-character byte sequence is returned.

     If the first N bytes possibly form a valid multibyte character but the character is incomplete, the return value is (size_t) -2. Otherwise, the multibyte-character sequence is invalid and the return value is (size_t) -1.

     The multibyte sequence is interpreted in the state represented by the object pointed to by PS.  If PS is a null pointer, a state object local to mbrlen is used.

     mbrlen was introduced in Amendment 1 to ISO C90 and is declared in wchar.h.

The attentive reader now will note that mbrlen can be implemented as:

     mbrtowc (NULL, s, n, ps != NULL ? ps : &internal)

This is true and in fact is mentioned in the official specification. How can this function be used to determine the length of the wide-character string created from a multibyte-character string?  It is not directly usable, but we can define a function mbslen using it:

     size_t
     mbslen (const char *s)
     {
       mbstate_t state;
       size_t result = 0;
       size_t nbytes;
       memset (&state, '\0', sizeof (state));
       while ((nbytes = mbrlen (s, MB_LEN_MAX, &state)) > 0)
         {
           if (nbytes >= (size_t) -2)
             /* Something is wrong.  */
             return (size_t) -1;
           s += nbytes;
           ++result;
         }
       return result;
     }

This function simply calls mbrlen for each multibyte character in the string and counts the number of function calls.  We here use MB_LEN_MAX as the size argument in the mbrlen call.  This is acceptable since a) this value is larger than the length of the longest multibyte-character sequence and b) we know that the string S ends with a NUL byte, which cannot be part of any other multibyte-character sequence but the one representing the NUL wide character.  Therefore, the mbrlen function will never read invalid memory.

Now that this function is available (just to make this clear, this function is not part of the GNU C Library), we can compute the number of wide characters required to store the converted multibyte-character string S using:

     wcs_bytes = (mbslen (s) + 1) * sizeof (wchar_t);

Please note that the mbslen function is quite inefficient.  The implementation of mbstouwcs with mbslen would have to perform the conversion of the multibyte-character input string twice, and this conversion might be quite expensive.  So it is necessary to think about the consequences of using the easier but imprecise method before doing the work twice.

 -- Function: size_t wcrtomb (char *restrict S, wchar_t WC, mbstate_t *restrict PS)
     The wcrtomb function (``wide character restartable to multibyte'') converts a single wide character into a multibyte string corresponding to that wide character.

     If S is a null pointer, the function resets the state stored in the objects pointed to by PS (or the internal mbstate_t object) to the initial state.  This can also be achieved by a call like this:

          wcrtombs (temp_buf, L'\0', ps)

     since, if S is a null pointer, wcrtomb performs as if it writes into an internal buffer, which is guaranteed to be large enough.

     If WC is the NUL wide character, wcrtomb emits, if necessary, a shift sequence to get the state PS into the initial state followed by a single NUL byte, which is stored in the string S.

     Otherwise, a byte sequence (possibly including shift sequences) is written into the string S.  This only happens if WC is a valid wide character (i.e., it has a multibyte representation in the character set selected by locale of the LC_CTYPE category).  If WC is not a valid wide character, nothing is stored in the string S, errno is set to EILSEQ, the conversion state in PS is undefined, and the return value is (size_t) -1.

     If no error occurred, the function returns the number of bytes stored in the string S.  This includes all bytes representing shift sequences.

     One word about the interface of the function: there is no parameter specifying the length of the array S.  Instead, the function assumes that there are at least MB_CUR_MAX bytes available since this is the maximum length of any byte sequence representing a single character.  So the caller has to make sure that there is enough space available; otherwise, buffer overruns can occur.

     wcrtomb was introduced in Amendment 1 to ISO C90 and is declared in wchar.h.

Using wcrtomb is as easy as using mbrtowc.  The following example appends a wide-character string to a multibyte-character string. Again, the code is not really useful (or correct) but is simply here to demonstrate use and some problems.

     char *
     mbscatwcs (char *s, size_t len, const wchar_t *ws)
     {
       mbstate_t state;
       /* Find the end of the existing string.  */
       char *wp = strchr (s, '\0');
       len -= wp - s;
       memset (&state, '\0', sizeof (state));
       do
         {
           size_t nbytes;
           if (len < MB_CUR_LEN)
             {
               /* We cannot guarantee that the next
                  character fits into the buffer, so
                  return an error.  */
               errno = E2BIG;
               return NULL;
             }
           nbytes = wcrtomb (wp, *ws, &state);
           if (nbytes == (size_t) -1)
             /* Error in the conversion.  */
             return NULL;
           len -= nbytes;
           wp += nbytes;
         }
       while (*ws++ != L'\0');
       return s;
     }

First the function has to find the end of the string currently in the array S.  The strchr call does this very efficiently, since a requirement for multibyte-character representations is that the NUL byte never be used except to represent itself (and in this context, the end of the string).

After initializing the state object, the loop is entered where the first task is to make sure there is enough room in the array S.  We abort if there are not at least MB_CUR_LEN bytes available.  This is not always optimal, but we have no other choice.  We might have less than MB_CUR_LEN bytes available, but the next multibyte character might also be only 1 byte long.  At the time the wcrtomb call returns, it is too late to decide whether the buffer was large enough. If this solution is unsuitable, there is a very slow but more accurate solution:

       ...
       if (len < MB_CUR_LEN)
         {
           mbstate_t temp_state;
           memcpy (&temp_state, &state, sizeof (state));
           if (wcrtomb (NULL, *ws, &temp_state) > len)
             {
               /* We cannot guarantee that the next
                  character fits into the buffer, so
                  return an error.  */
               errno = E2BIG;
               return NULL;
             }
         }
       ...

Here we perform the conversion that might overflow the buffer so that we are afterwards in the position to make an exact decision about the buffer size.  Please note the NULL argument for the destination buffer in the new wcrtomb call; a writeable string would usually be used, but we've used NULL since we're not interested in the converted text at this point.  The most unusual thing about this piece of code certainly is the duplication of the conversion-state object, but if a change of the state is necessary to emit the next multibyte character, we want to have the same shift-state change performed in the real conversion.  Therefore, we have to preserve the initial shift-state information.

There are certainly many more and better solutions to this problem. This example is only provided for educational purposes.

Converting Multibyte- and Wide-Character Strings
------------------------------------------------

The functions described in the previous section only convert a single character at a time.  Most operations to be performed in real-world programs include strings and therefore the ISO C standard also defines conversions on entire strings.  However, the defined set of functions is quite limited; therefore, the GNU C Library contains a few extensions that can help in some important situations.

 -- Function: size_t mbsrtowcs (wchar_t *restrict DST, const char **restrict SRC, size_t LEN, mbstate_t *restrict PS)
     The mbsrtowcs function (``multibyte string restartable to wide-character string'') converts a NUL-terminated multibyte-character string at *SRC into an equivalent wide-character string, including the NUL wide character at the end.  The conversion is started using the state information from the object pointed to by PS or from an internal object of mbsrtowcs if PS is a null pointer.  Before returning, the state object is updated to match the state after the last converted character.  The state is the initial state if the terminating NUL byte is reached and converted.

     If DST is not a null pointer, the result is stored in the array pointed to by DST; otherwise, the conversion result is not available since it is stored in an internal buffer.

     If LEN wide characters are stored in the array DST before reaching the end of the input string, the conversion stops and LEN is returned.  If DST is a null pointer, LEN is never checked.

     Another reason for a premature return from the function call is if the input string contains an invalid multibyte-sequence.  In this case the global variable errno is set to EILSEQ and the function returns (size_t) -1.

     In all other cases, the function returns the number of wide characters converted during this call.  If DST is not null, mbsrtowcs stores in the pointer pointed to by SRC either a null pointer (if the NUL byte in the input string was reached) or the address of the byte following the last converted multibyte character.

     mbsrtowcs was introduced in Amendment 1 to ISO C90 and is declared in wchar.h.

The definition of the mbsrtowcs function has one important limitation.  The requirement that DST has to be a NUL-terminated string causes problems if you want to convert buffers with text.  A buffer is normally not a collection of NUL-terminated strings but instead a continuous collection of lines, separated by newline characters.  Now assume that a function to convert one line from a buffer is needed.  Since the line is not NUL-terminated, the source pointer cannot directly point into the unmodified text buffer.  This means that either you insert the NUL byte at the appropriate place for the time of the mbsrtowcs function call (which is not doable for a read-only buffer or in a multithreaded application) or you copy the line in an extra buffer where it can be terminated by a NUL byte.  It is not in general possible to limit the number of characters to convert by setting the parameter LEN to any specific value.  Since it is not known how many bytes each multibyte-character sequence is in length, you can only guess.

There is still a problem with the method of NUL-terminating a line right after the newline character, which could lead to very strange results. As said in the description of the mbsrtowcs function above, the conversion state is guaranteed to be in the initial shift state after processing the NUL byte at the end of the input string.  But this NUL byte is not really part of the text (i.e., the conversion state after the newline in the original text could be something different than the initial shift state and therefore the first character of the next line is encoded using this state).  But the state in question is never accessible to the user since the conversion stops after the NUL byte (which resets the state).  Most stateful character sets in use today require that the shift state after a newline be the initial state---but this is not a strict guarantee.  Therefore, simply NUL-terminating a piece of a running text is not always an adequate solution and, therefore, should never be done in generally used code.

The generic conversion interface (see Generic Charset Conversion) does not have this limitation (it simply works on buffers, not strings), and the GNU C Library contains a set of functions that take additional parameters specifying the maximum number of bytes that are consumed from the input string.  This way the problem of mbsrtowcs's example above could be solved by determining the line length and passing this length to the function.

 -- Function: size_t wcsrtombs (char *restrict DST, const wchar_t **restrict SRC, size_t LEN, mbstate_t *restrict PS)
     The wcsrtombs function (``wide-character string restartable to multibyte string'') converts the NUL-terminated wide-character string at *SRC into an equivalent multibyte-character string and stores the result in the array pointed to by DST.  The NUL wide character is also converted.  The conversion starts in the state described in the object pointed to by PS or by a state object locally to wcsrtombs in case PS is a null pointer.  If DST is a null pointer, the conversion is performed as usual, but the result is not available.  If all characters of the input string were successfully converted and if DST is not a null pointer, the pointer pointed to by SRC gets assigned a null pointer.

     If one of the wide characters in the input string has no valid multibyte-character equivalent, the conversion stops early, sets the global variable errno to EILSEQ, and returns (size_t) -1.

     Another reason for a premature stop is if DST is not a null pointer and the next converted character would require more than LEN bytes in total to the array DST.  In this case (and if DEST is not a null pointer), the pointer pointed to by SRC is assigned a value pointing to the wide character right after the last successfully converted one.

     Except in the case of an encoding error, the return value of the wcsrtombs function is the number of bytes in all the multibyte-character sequences stored in DST.  Before returning, the state in the object pointed to by PS (or the internal object in case PS is a null pointer) is updated to reflect the state after the last conversion.  The state is the initial shift state in case the terminating NUL wide character was converted.

     The wcsrtombs function was introduced in Amendment 1 to ISO C90 and is declared in wchar.h.

The restriction mentioned above for the mbsrtowcs function applies here also.  There is no possibility of directly controlling the number of input characters.  You have to place the NUL wide character at the correct place, or control the consumed input indirectly via the available output array size (the LEN parameter).

 -- Function: size_t mbsnrtowcs (wchar_t *restrict DST, const char **restrict SRC, size_t NMC, size_t LEN, mbstate_t *restrict PS)
     The mbsnrtowcs function is very similar to the mbsrtowcs function.  All the parameters are the same except for NMC, which is new.  The return value is the same as for mbsrtowcs.

     This new parameter specifies how many bytes at most can be used from the multibyte-character string.  In other words, the multibyte-character string *SRC need not be NUL-terminated.  But if a NUL byte is found within the NMC first bytes of the string, the conversion stops there.

     This function is a GNU extension.  It is meant to work around the problems mentioned above.  Now it is possible to convert a buffer with multibyte-character text piece-for-piece without having to care about inserting NUL bytes and the effect of NUL bytes on the conversion state.

A function to convert a multibyte string into a wide-character string and display it could be written like this (this example is useful only to show the syntax):

     void
     showmbs (const char *src, FILE *fp)
     {
       mbstate_t state;
       int cnt = 0;
       memset (&state, '\0', sizeof (state));
       while (1)
         {
           wchar_t linebuf[100];
           const char *endp = strchr (src, '\n');
           size_t n;

           /* Exit if there is no more line.  */
           if (endp == NULL)
             break;

           n = mbsnrtowcs (linebuf, &src, endp - src, 99, &state);
           linebuf[n] = L'\0';
           fprintf (fp, "line %d: \"%S\"\n", linebuf);
         }
     }

There is no problem with the state after a call to mbsnrtowcs. Since we don't insert characters in the strings that were not there from the beginning, and we use STATE only for the conversion of the given buffer, there is no problem with altering the state.

 -- Function: size_t wcsnrtombs (char *restrict DST, const wchar_t **restrict SRC, size_t NWC, size_t LEN, mbstate_t *restrict PS)
     The wcsnrtombs function implements the conversion from wide-character strings to multibyte-character strings.  It is similar to wcsrtombs, but like mbsnrtowcs, it takes an extra parameter, which specifies the length of the input string.

     No more than NWC wide characters from the input string *SRC are converted.  If the input string contains a NUL wide character in the first NWC characters, the conversion stops at this place.

     The wcsnrtombs function is a GNU extension and like mbsnrtowcs helps in situations where no NUL-terminated input strings are available.

A Complete Multibyte Conversion Example
---------------------------------------

The example programs given in the last sections are only brief and do not contain things like error checking.  Presented here is a complete and documented example.  It features the mbrtowc function, but it should be easy to derive versions using the other functions:

     int
     file_mbsrtowcs (int input, int output)
     {
       /* Note the use of MB_LEN_MAX.
          MB_CUR_MAX cannot portably be used here.  */
       char buffer[BUFSIZ + MB_LEN_MAX];
       mbstate_t state;
       int filled = 0;
       int eof = 0;

       /* Initialize the state.  */
       memset (&state, '\0', sizeof (state));

       while (!eof)
         {
           ssize_t nread;
           ssize_t nwrite;
           char *inp = buffer;
           wchar_t outbuf[BUFSIZ];
           wchar_t *outp = outbuf;

           /* Fill up the buffer from the input file.  */
           nread = read (input, buffer + filled, BUFSIZ);
           if (nread < 0)
             {
               perror ("read");
               return 0;
             }
           /* If we reach end of file, make a note to read no more. */
           if (nread == 0)
             eof = 1;

           /* filled is now the number of bytes in buffer. */
           filled += nread;

           /* Convert those bytes to wide characters---as many as we can. */
           while (1)
             {
               size_t thislen = mbrtowc (outp, inp, filled, &state);
               /* Stop converting at invalid character;
                  this can mean we have read just the first part
                  of a valid character.  */
               if (thislen == (size_t) -1)
                 break;
               /* We want to handle embedded NUL bytes
                  but the return value is 0.  Correct this.  */
               if (thislen == 0)
                 thislen = 1;
               /* Advance past this character. */
               inp += thislen;
               filled -= thislen;
               ++outp;
             }

           /* Write the wide characters we just made.  */
           nwrite = write (output, outbuf,
                           (outp - outbuf) * sizeof (wchar_t));
           if (nwrite < 0)
             {
               perror ("write");
               return 0;
             }

           /* See if we have a real invalid character. */
           if ((eof && filled > 0) || filled >= MB_CUR_MAX)
             {
               error (0, 0, "invalid multibyte character");
               return 0;
             }

           /* If any characters must be carried forward,
              put them at the beginning of buffer. */
           if (filled > 0)
             memmove (inp, buffer, filled);
         }

       return 1;
     }

Nonreentrant Conversion Function
================================

The functions described in the previous chapter are defined in Amendment 1 to ISO C90, but the original ISO C90 standard also contained functions for character-set conversion.  The reason that these original functions are not described first is that they are almost entirely useless.

The problem is that all the conversion functions described in the original ISO C90 use a local state.  Using a local state implies that multiple conversions at the same time (not only when using threads) cannot be done, and that you cannot first convert single characters and then strings since you cannot tell the conversion functions which state to use.

These original functions are therefore usable only in a very limited set of situations.  You have to finish converting the entire string before starting a new one, and each string or text must be converted with the same function (there is no problem with the library itself; it is guaranteed that no library function changes the state of any of these functions).  For the above reasons, it is strongly recommended that the functions described in the previous section be used in place of nonreentrant conversion functions.

Nonreentrant Conversion of Single Characters
--------------------------------------------

 -- Function: int mbtowc (wchar_t *restrict RESULT, const char *restrict STRING, size_t SIZE)
     The mbtowc (``multibyte to wide character'') function, when called with nonnull STRING, converts the first multibyte character beginning at STRING to its corresponding wide-character code.  It stores the result in *RESULT.

     mbtowc never examines more than SIZE bytes.  (The idea is to supply for SIZE the number of bytes of data you have in hand.)

     mbtowc with nonnull STRING distinguishes three possibilities: either the first SIZE bytes at STRING start with valid multibyte characters, they start with an invalid byte-sequence or just part of a character, or STRING points to an empty string (a null character).

     For a valid multibyte character, mbtowc converts it to a wide character and stores that in *RESULT, and returns the number of bytes in that character (always at least 1 and never more than SIZE).

     For an invalid byte-sequence, mbtowc returns -1.  For an empty string, it returns 0, also storing '\0' in *RESULT.

     If the multibyte-character code uses shift characters, then mbtowc maintains and updates a shift state as it scans.  If you call mbtowc with a null pointer for STRING, that initializes the shift state to its standard initial value.  It also returns nonzero if the multibyte-character code in use actually has a shift state (see Shift State).

 -- Function: int wctomb (char *STRING, wchar_t WCHAR)
     The wctomb (``wide character to multibyte'') function converts the wide-character code WCHAR to its corresponding multibyte-character sequence, and stores the result in bytes starting at STRING.  At most MB_CUR_MAX characters are stored.

     wctomb with nonnull STRING distinguishes three possibilities for WCHAR: a valid wide-character code (one that can be translated to a multibyte character), an invalid code, and L'\0'.

     Given a valid code, wctomb converts it to a multibyte character, storing the bytes starting at STRING.  Then it returns the number of bytes in that character (always at least 1 and never more than MB_CUR_MAX).

     If WCHAR is an invalid wide-character code, wctomb returns -1.  If WCHAR is L'\0', it returns 0, also storing '\0' in *STRING.

     If the multibyte-character code uses shift characters, then wctomb maintains and updates a shift state as it scans.  If you call wctomb with a null pointer for STRING, that initializes the shift state to its standard initial value.  It also returns nonzero if the multibyte-character code in use actually has a shift state (see Shift State).

     Calling this function with a WCHAR argument of zero when STRING is not null has the side effect of reinitializing the stored shift state as well as storing the multibyte character '\0' and returning 0.

Similar to mbrlen, there is also a nonreentrant function that computes the length of a multibyte character.  It can be defined in terms of mbtowc.

 -- Function: int mblen (const char *STRING, size_t SIZE)
     The mblen function with a nonnull STRING argument returns the number of bytes that make up the multibyte character beginning at STRING, never examining more than SIZE bytes.  (The idea is to supply for SIZE the number of bytes of data you have in hand.)

     The return value of mblen distinguishes three possibilities: either the first SIZE bytes at STRING start with valid multibyte characters, they start with an invalid byte-sequence or just part of a character, or STRING points to an empty string (a null character).

     For a valid multibyte character, mblen returns the number of bytes in that character (always at least 1 and never more than SIZE).  For an invalid byte-sequence, mblen returns -1.  For an empty string, it returns 0.

     If the multibyte-character code uses shift characters, then mblen maintains and updates a shift state as it scans.  If you call mblen with a null pointer for STRING, that initializes the shift state to its standard initial value.  It also returns a nonzero value if the multibyte-character code in use actually has a shift state (see Shift State).

     The function mblen is declared in stdlib.h.

Nonreentrant Conversion of Strings
----------------------------------

For convenience, the ISO C90 standard also defines functions to convert entire strings instead of single characters.  These functions suffer from the same problems as their reentrant counterparts from Amendment 1 to ISO C90 (see Converting Strings).

 -- Function: size_t mbstowcs (wchar_t *WSTRING, const char *STRING, size_t SIZE)
     The mbstowcs (``multibyte string to wide-character string'') function converts the null-terminated string of multibyte characters STRING to an array of wide-character codes, storing not more than SIZE wide characters into the array beginning at WSTRING. The terminating null character counts toward the size, so if SIZE is less than the actual number of wide characters resulting from STRING, no terminating null character is stored.

     The conversion of characters from STRING begins in the initial shift state.

     If an invalid multibyte-character sequence is found, the mbstowcs function returns a value of -1.  Otherwise, it returns the number of wide characters stored in the array WSTRING.  This number does not include the terminating null character, which is present if the number is less than SIZE.

     Here is an example showing how to convert a string of multibyte characters, allocating enough space for the result:

          wchar_t *
          mbstowcs_alloc (const char *string)
          {
            size_t size = strlen (string) + 1;
            wchar_t *buf = xmalloc (size * sizeof (wchar_t));

            size = mbstowcs (buf, string, size);
            if (size == (size_t) -1)
              return NULL;
            buf = xrealloc (buf, (size + 1) * sizeof (wchar_t));
            return buf;
          }

 -- Function: size_t wcstombs (char *STRING, const wchar_t *WSTRING, size_t SIZE)
     The wcstombs (``wide-character string to multibyte string'') function converts the null-terminated wide-character array WSTRING into a string containing multibyte characters, storing not more than SIZE bytes starting at STRING, followed by a terminating null character if there is room.  The conversion of characters begins in the initial shift state.

     The terminating null character counts toward the size, so if SIZE is less than or equal to the number of bytes needed in WSTRING, no terminating null character is stored.

     If a code that does not correspond to a valid multibyte character is found, the wcstombs function returns a value of -1. Otherwise, the return value is the number of bytes stored in the array STRING.  This number does not include the terminating null character, which is present if the number is less than SIZE.

States in Nonreentrant Functions
--------------------------------

In some multibyte-character codes, the meaning of any particular byte sequence is not fixed; it depends on what other sequences have come earlier in the same string.  Typically there are just a few sequences that can change the meaning of other sequences; these few are called shift sequences and we say that they set the shift state for other sequences that follow.

To illustrate shift state and shift sequences, suppose we decide that the sequence 0200 (just one byte) enters Japanese mode, in which pairs of bytes in the range 0240 to 0377 are single characters, while 0201 enters Latin-1 mode, in which single bytes in the range 0240 to 0377 are characters, and interpreted according to the ISO Latin-1 character set.  This is a multibyte code that has two alternative shift states (``Japanese mode'' and ``Latin-1 mode''), and two shift sequences that specify particular shift states.

When the multibyte-character code in use has shift states, then mblen, mbtowc, and wctomb must maintain and update the current shift state as they scan the string.  To make this work properly, you must follow these rules:

  * Before starting to scan a string, call the function with a null pointer for the multibyte-character address---for example, mblen (NULL, 0).  This initializes the shift state to its standard initial value.

  * Scan the string one character at a time, in order.  Do not ``back up'' and rescan characters already scanned, and do not intersperse the processing of different strings.

Here is an example of using mblen following these rules:

     void
     scan_string (char *s)
     {
       int length = strlen (s);

       /* Initialize shift state.  */
       mblen (NULL, 0);

       while (1)
         {
           int thischar = mblen (s, length);
           /* Deal with end of string and invalid characters.  */
           if (thischar == 0)
             break;
           if (thischar == -1)
             {
               error ("invalid multibyte character");
               break;
             }
           /* Advance past this character.  */
           s += thischar;
           length -= thischar;
         }
     }

The functions mblen, mbtowc and wctomb are not reentrant when using a multibyte code that uses a shift state.  However, no other library functions call these functions, so you don't have to worry that the shift state might be mysteriously changed.

Generic Charset Conversion
==========================

The conversion functions mentioned so far in this chapter all have in common that they operate on character sets that are not directly specified by the functions.  The multibyte encoding used is specified by the currently selected locale for the LC_CTYPE category.  The wide-character set is fixed by the implementation (in the case of GNU C library it is always UCS-4 encoded ISO 10646.

This has of course several problems when it comes to general character conversion:

  * For every conversion where neither the source nor the destination character set is the character set of the locale for the LC_CTYPE category, one has to change the LC_CTYPE locale using setlocale.

     Changing the LC_TYPE locale introduces major problems for the rest of the programs since several more functions, like the character-classification functions (see Classification of Characters), use the LC_CTYPE category.

  * Parallel conversions to and from different character sets are not possible since the LC_CTYPE selection is global and shared by all threads.

  * If neither the source nor the destination character set is the character set used for wchar_t representation, there is at least a two-step process necessary to convert a text using the functions above.  One would have to select the source character set as the multibyte encoding, convert the text into a wchar_t text, select the destination character set as the multibyte encoding, and convert the wide-character text to the multibyte (= destination) character set.

     Even if this is possible (which is not guaranteed), it is a very tiring work.  Plus it suffers from the other two aforementioned problems even more, due to the steady changing of the locale.

The XPG2 standard defines a completely new set of functions, which has none of these limitations.  They are not at all coupled to the selected locales, and they have no constraints on the character sets selected for source and destination.  Only the set of available conversions limits them.  The standard does not specify that any conversion at all must be available.  Such availability is a measure of the quality of the implementation.

In the following text, first the interface to iconv and then the conversion function will be described.  Comparisons with other implementations will show what obstacles stand in the way of portable applications.  Finally, the implementation is described in so far as might interest the advanced user who wants to extend conversion capabilities.

Generic Character-Set Conversion Interface
------------------------------------------

This set of functions follows the traditional cycle of using a resource---open, use, close.  The interface consists of three functions, each of which implements one step.

Before the interfaces are described, it is necessary to introduce a data type.  Just like other open-use-close interfaces, the functions introduced here work using handles, and the iconv.h header defines a special type for the handles used.

 -- Data Type: iconv_t
     This data type is an abstract type defined in iconv.h.  The user must not assume anything about the definition of this type; it must be completely opaque.

     Objects of this type can get assigned handles for the conversions using the iconv functions.  The objects themselves need not be freed, but the conversions that the handles stand for do.

The first step is the function to create a handle.

 -- Function: iconv_t iconv_open (const char *TOCODE, const char *FROMCODE)
     The iconv_open function has to be used before starting a conversion.  The two parameters this function takes determine the source and destination character set for the conversion, and if the implementation is able to perform such a conversion, the function returns a handle.

     If the wanted conversion is not available, the iconv_open function returns (iconv_t) -1. In this case, the global variable errno can have the following values:

     EMFILE
          The process already has OPEN_MAX file descriptors open.

     ENFILE
          The system limit of open files is reached.

     ENOMEM
          There is not enough memory to carry out the operation.

     EINVAL
          The conversion from FROMCODE to TOCODE is not supported.

     It is not possible to use the same descriptor in different threads to perform independent conversions.  The data structures associated with the descriptor include information about the conversion state. This must not be messed up by using it in different conversions.

     An iconv descriptor is like a file descriptor, as for every use a new descriptor must be created.  The descriptor does not stand for all of the conversions from FROMSET to TOSET.

     The GNU C Library implementation of iconv_open has one significant extension to other implementations.  To ease the extension of the set of available conversions, the implementation allows storing the necessary files with data and code in an arbitrary number of directories.  How this extension must be written will be explained below (see glibc iconv Implementation).  Here it is only important to say that all directories mentioned in the GCONV_PATH environment variable are considered only if they contain a file gconv-modules.  These directories need not necessarily be created by the system administrator.  In fact, this extension is introduced to help users writing and using their own, new conversions.  Of course, this does not work for security reasons in SUID binaries; in this case only the system directory is considered---this is normally PREFIX/lib/gconv.  The GCONV_PATH environment variable is examined exactly once at the first call of the iconv_open function.  Later modifications of the variable have no effect.

     The iconv_open function was introduced early in the X/Open Portability Guide, Issue 2. (X/Open Company, X/Open Portability Guide, Issue 2 (Reading, UK: X/Open Company, Ltd., 1987).) It is supported by all commercial Unices, as it is required for the Unix branding.  However, the quality and completeness of the implementation varies widely.  The iconv_open function is declared in iconv.h.

The iconv implementation can associate large data structures with the handle returned by iconv_open.  Therefore, it is crucial to free all the resources once all conversions are carried out and the conversion is not needed anymore.

 -- Function: int iconv_close (iconv_t CD)
     The iconv_close function frees all resources associated with the handle CD, which must have been returned by a successful call to the iconv_open function.

     If the function call was successful, the return value is 0. Otherwise, it is -1 and errno is set appropriately. Defined errors are

     EBADF
          The conversion descriptor is invalid.

     The iconv_close function was introduced together with the rest of the iconv functions in XPG2 and is declared in iconv.h.

The standard defines only one actual conversion function.  This has, therefore, the most general interface: it allows conversion from one buffer to another.  Conversion from a file to a buffer, from a buffer to a file, or even from a file to file can be implemented on top of it.

 -- Function: size_t iconv (iconv_t CD, char **INBUF, size_t *INBYTESLEFT, char **OUTBUF, size_t *OUTBYTESLEFT)
     The iconv function converts the text in the input buffer according to the rules associated with the descriptor CD and stores the result in the output buffer.  It is possible to call the function for the same text several times in a row, since for stateful character sets the necessary state information is kept in the data structures associated with the descriptor.

     The input buffer is specified by *INBUF and it contains *INBYTESLEFT bytes.  The extra indirection is necessary for communicating the used input back to the caller (see below).  It is important to note that the buffer pointer is of type char and the length is measured in bytes even if the input text is encoded in wide characters.

     The output buffer is specified in a similar way.  *OUTBUF points to the beginning of the buffer with at least *OUTBYTESLEFT bytes of room for the result.  The buffer pointer again is of type char and the length is measured in bytes.  If OUTBUF or *OUTBUF is a null pointer, the conversion is performed but no output is available.

     If INBUF is a null pointer, the iconv function performs the necessary action to put the state of the conversion into the initial state.  This is obviously a no-op for nonstateful encodings, but if the encoding has a state, such a function call might put some byte sequences in the output buffer, which perform the necessary state changes.  The next call with INBUF not being a null pointer then goes on from the initial state.  It is important that the programmer never make any assumption as to whether the conversion has to deal with states.  Even if the input and output character sets are not stateful, the implementation might still have to keep states.  This is due to the implementation chosen for the GNU C Library as it is described below.  Therefore an iconv call to reset the state should always be performed if some protocol requires this for the output text.

     The conversion stops for one of three reasons. The first is that all characters from the input buffer are converted.  This actually can mean two things: either all bytes from the input buffer are consumed or there are some bytes at the end of the buffer that can possibly form a complete character, but the input is incomplete.  The second reason for a stop is that the output buffer is full.  The third reason is that the input contains invalid characters.

     In all of these cases, the buffer pointers after the last successful conversion, for input and output buffer, are stored in INBUF and OUTBUF, and the available room in each buffer is stored in INBYTESLEFT and OUTBYTESLEFT.

     Since the character sets selected in the iconv_open call can be almost arbitrary, there can be situations where the input buffer contains valid characters, which have no identical representation in the output character set.  The behavior in this situation is undefined.  The current behavior of the GNU C Library in this situation is to return with an error immediately.  This certainly is not the most desirable solution.  Future versions will provide better solutions, but they are not yet finished.

     If all input from the input buffer is successfully converted and stored in the output buffer, the function returns the number of nonreversible conversions performed.  In all other cases, the return value is (size_t) -1 and errno is set appropriately.  In such cases, the value pointed to by INBYTESLEFT is nonzero.

     EILSEQ
          The conversion stopped because of an invalid byte-sequence in the input. After the call, *INBUF points at the first byte of the invalid byte-sequence.

     E2BIG
          The conversion stopped because it ran out of space in the output buffer.

     EINVAL
          The conversion stopped because of an incomplete byte-sequence at the end of the input buffer.

     EBADF
          The CD argument is invalid.

     The iconv function was introduced in the XPG2 standard and is declared in the iconv.h header.

The definition of the iconv function is quite good overall.  It provides flexible functionality.  The only problems lie in the boundary cases, which are incomplete byte-sequences at the end of the input buffer and invalid input.  A third problem, which is not really a design problem, is the way conversions are selected.  The standard does not say anything about the legitimate names---a minimal set of available conversions.  We will see how this negatively impacts other implementations, as demonstrated below.

A Complete iconv Example
------------------------

The example below features a solution for a common problem.  Given that we know the internal encoding used by the system for wchar_t strings, we often are in the position to read text from a file and store it in wide-character buffers.  We can do this using mbsrtowcs, but then we run into the problems discussed above.

     int
     file2wcs (int fd, const char *charset, wchar_t *outbuf, size_t avail)
     {
       char inbuf[BUFSIZ];
       size_t insize = 0;
       char *wrptr = (char *) outbuf;
       int result = 0;
       iconv_t cd;

       cd = iconv_open ("WCHAR_T", charset);
       if (cd == (iconv_t) -1)
         {
           /* Something went wrong.  */
           if (errno == EINVAL)
             error (0, 0, "conversion from '%s' to wchar_t not available",
                    charset);
           else
             perror ("iconv_open");

           /* Terminate the output string.  */
           *outbuf = L'\0';

           return -1;
         }

       while (avail > 0)
         {
           size_t nread;
           size_t nconv;
           char *inptr = inbuf;

           /* Read more input.  */
           nread = read (fd, inbuf + insize, sizeof (inbuf) - insize);
           if (nread == 0)
             {
               /* When we come here the file is completely read.
                  This still could mean there are some unused
                  characters in the inbuf.  Put them back.  */
               if (lseek (fd, -insize, SEEK_CUR) == -1)
                 result = -1;

               /* Now write out the byte sequence to get into the
                  initial state if this is necessary.  */
               iconv (cd, NULL, NULL, &wrptr, &avail);

               break;
             }
           insize += nread;

           /* Do the conversion.  */
           nconv = iconv (cd, &inptr, &insize, &wrptr, &avail);
           if (nconv == (size_t) -1)
             {
               /* Not everything went right.  It might only be
                  an unfinished byte-sequence at the end of the
                  buffer, or it could be a real problem.  */
               if (errno == EINVAL)
                 /* This is harmless.  Simply move the unused
                    bytes to the beginning of the buffer so that
                    they can be used in the next round.  */
                 memmove (inbuf, inptr, insize);
               else
                 {
                   /* It is a real problem.  Maybe we ran out of
                      space in the output buffer or we have invalid
                      input.  In any case back the file pointer to
                      the position of the last processed byte.  */
                   lseek (fd, -insize, SEEK_CUR);
                   result = -1;
                   break;
                 }
             }
         }

       /* Terminate the output string.  */
       if (avail >= sizeof (wchar_t))
         *((wchar_t *) wrptr) = L'\0';

       if (iconv_close (cd) != 0)
         perror ("iconv_close");

       return (wchar_t *) wrptr - outbuf;
     }

This example shows the most important aspects of using the iconv functions.  It shows how successive calls to iconv can be used to convert large amounts of text.  The user does not have to care about stateful encodings, since the functions take care of everything.

An interesting point is the case where iconv returns an error and errno is set to EINVAL.  This is not really an error in the transformation.  It can happen whenever the input character set contains byte sequences of more than 1 byte for some character, and texts are not processed in one piece.  In this case, there is a chance that a multibyte sequence is cut.  The caller can then simply read the remainder of the takes and feed the offending bytes together with new characters from the input to iconv and continue the work.  The internal state kept in the descriptor is not unspecified after such an event, as is the case with the conversion functions from the ISO C standard.

The example also shows the problem of using wide-character strings with iconv.  As explained in the description of the iconv function above, the function always takes a pointer to a char array, and the available space is measured in bytes.  In the example, the output buffer is a wide-character buffer; therefore, we use a local variable WRPTR of type char *, which is used in the iconv calls.

This looks rather innocent but can lead to problems on platforms that have tight restrictions on alignment.  Therefore, the caller of iconv has to make sure that the pointers passed are suitable for accessing characters from the appropriate character set.  Since, in the above case, the input parameter to the function is a wchar_t pointer, this is the case (unless the user violates alignment when computing the parameter).  In other situations, especially when writing generic functions where you do not know what type of character set you are using, iconv treats text as a sequence of bytes, which might become tricky.

Some Details About Other iconv Implementations
----------------------------------------------

This is not really the place to discuss the iconv implementation of other systems, but it is necessary to know a bit about them to write portable programs.  The aforementioned problems with the specification of the iconv functions can lead to portability issues.

The first thing to notice is that, due to the large number of character sets in use, it is certainly not practical to encode the conversions directly in the C library.  Therefore, the conversion information must come from files outside the C library.  This is usually done in one or both of the following ways:

  * The C library contains a set of generic conversion functions that can read the needed conversion tables and other information from data files. These files get loaded when necessary.

     This solution is problematic, since it requires a great deal of effort to apply to all character sets (potentially an infinite set).  The differences in the structures of the different character sets are so large that many different variants of the table-processing functions must be developed.  In addition, the generic nature of these functions makes them slower than specifically implemented functions.

  * The C library only contains a framework that can dynamically load object files and execute the conversion functions contained therein.

     This solution provides much more flexibility.  The C library itself contains only very little code and therefore reduces the general memory footprint.  Also, with a documented interface between the C library and the loadable modules, it is possible for third parties to extend the set of available conversion modules.  A drawback of this solution is that dynamic loading must be available.

Some implementations in commercial Unices implement a mixture of these possibilities; the majority implement only the second solution.  Using loadable modules moves the code out of the library itself and keeps the door open for extensions and improvements, but this design is also limiting on some platforms, since not many platforms support dynamic loading in statically linked programs.  On platforms without this capability, it is therefore not possible to use this interface in statically linked programs.  The GNU C Library has, on ELF platforms, no problems with dynamic loading in these situations; therefore, this point is moot.  The danger is that you get acquainted with this situation and forget about the restrictions on other systems.

A second thing to know about other iconv implementations is that the number of available conversions is often very limited.  Some implementations provide, in the standard release (not special international or developer releases), at most 100 to 200 conversion possibilities.  This does not mean 200 different character sets are supported; for example, conversions from one character set to a set of ten others might count as ten conversions.  Together with the other direction, this makes twenty conversion possibilities used up by one character set.  You can imagine the thin coverage these platform provide.  Some Unix vendors even provide only a handful of conversions, which renders them useless for almost all uses.

This leads directly to the third and probably most problematic point. The way the iconv conversion functions are implemented on all known Unix systems and the availability of the conversion functions from character set A to B and the conversion from B to C does not imply that the conversion from A to C is available.

This might not seem unreasonable or problematic at first, but it is a big problem.  To show the problem, we assume a program that has to convert from A to C.  A call like:

     cd = iconv_open ("C", "A");

fails according to the assumption above.  But what does the program do now?  The conversion is necessary; therefore, simply giving up is not an option.

This is a nuisance.  The iconv function should take care of this. But how should the program proceed from here on?  If it tries to convert to character set B, first the two iconv_open calls:

     cd1 = iconv_open ("B", "A");

and

     cd2 = iconv_open ("C", "B");

will succeed, but how to find B?

Unfortunately, the answer is that there is no general solution.  On some systems, guessing might help.  On those systems, most character sets can convert to and from UTF-8 encoded ISO 10646 or Unicode text. Besides this, only some very system-specific methods can help. Since the conversion functions come from loadable modules and these modules must be stored somewhere in the file system, one could try to find them and determine from the available file which conversions are available and whether there is an indirect route from A to C.

This example shows one of the design errors of iconv mentioned above.  It should at least be possible to determine the list of available conversions programmatically so that if iconv_open says there is no such conversion, one could make sure this also is true for indirect routes.

The iconv Implementation in the GNU C Library
---------------------------------------------

After reading about the problems of iconv implementations in the last section, it is certainly good to note that the implementation in the GNU C Library has none of the problems mentioned above.  What follows is a step-by-step analysis of the points raised above.  The evaluation is based on the current state of the development (as of January 1999).  The development of the iconv functions is not complete, but basic functionality has solidified.

The GNU C Library's iconv implementation uses shared loadable modules to implement the conversions.  A very small number of conversions are built into the library itself, but these are only rather trivial conversions.

All the benefits of loadable modules are available in the GNU C Library implementation.  This is especially appealing since the interface is well documented (see below); therefore, it is easy to write new conversion modules.  The drawback of using loadable objects is not a problem in the GNU C Library, at least on ELF systems.  Since the library is able to load shared objects even in statically linked binaries, static linking need not be forbidden in case one wants to use iconv.

The second mentioned problem is the number of supported conversions. Currently, the GNU C Library supports more than 150 character sets.  The way the implementation is designed the number of supported conversions is greater than 22350 (150 times 149).  If any conversion from or to a character set is missing, it can be added easily.

Particularly impressive as it may be, this high number is due to the fact that the GNU C Library implementation of iconv does not have the third problem mentioned above (i.e., whenever there is a conversion from a character set A to B and from B to C it is always possible to convert from A to C directly).  If the iconv_open returns an error and sets errno to EINVAL, there is no known way, directly or indirectly, to perform the wanted conversion.

Triangulation is achieved by providing for each character set a conversion from and to UCS-4 encoded ISO 10646.  Using ISO 10646 as an intermediate representation, it is possible to triangulate (i.e., convert with an intermediate representation).

There is no inherent requirement to provide a conversion to ISO 10646 for a new character set, and it is also possible to provide other conversions where neither source nor destination character set is ISO 10646.  The existing set of conversions is simply meant to cover all conversions that might be of interest.

All currently available conversions use the triangulation method above, making conversion run unnecessarily slow.  If, for example, somebody often needs the conversion from ISO-2022-JP to EUC-JP, a quicker solution would involve direct conversion between the two character sets, skipping the input to ISO 10646 first.  The two character sets of interest are much more similar to each other than to ISO 10646.

In such a situation, one easily can write a new conversion and provide it as a better alternative.  The GNU C Library iconv implementation would automatically use the module implementing the conversion if it is specified to be more efficient.

Format of gconv-modules Files
.............................

All information about the available conversions comes from a file named gconv-modules, which can be found in any of the directories along the GCONV_PATH.  The gconv-modules files are line-oriented text files, where each of the lines has one of the following formats:

  * If the first non-white-space character is a #, the line contains only comments and is ignored.

  * Lines starting with alias define an alias name for a character set.  Two more words are expected on the line.  The first word defines the alias name, and the second defines the original name of the character set.  The effect is that it is possible to use the alias name in the FROMSET or TOSET parameters of iconv_open and achieve the same result as when using the real character-set name.

     This is quite important, since a character set often has many different names.  There is normally an official name, but this need not correspond to the most popular name.  Besides this, many character sets have special names that are somehow constructed.  For example, all character sets specified by the ISO have an alias of the form ISO-IR-NNN, where NNN is the registration number. This allows programs that know about the registration number to construct character-set names and use them in iconv_open calls. More on the available names and aliases follows below.

  * Lines starting with module introduce an available conversion module.  These lines must contain three or four more words.

     The first word specifies the source character set, the second word the destination character set of conversion implemented in this module, and the third word is the name of the loadable module.  The file name is constructed by appending the usual shared-object suffix (normally .so) and this file is then supposed to be found in the same directory the gconv-modules file is in.  The last word on the line, which is optional, is a numeric value representing the cost of the conversion.  If this word is missing, a cost of 1 is assumed. The numeric value itself does not matter that much; the relative values of the sums of costs for all possible conversion paths are what counts. Below is a more precise description of the use of the cost value.

Returning to the example above of a module written to directly convert from ISO-2022-JP to EUC-JP and back, all that has to be done is to put the new module (let its name be ISO2022JP-EUCJP.so) in a directory and add a file gconv-modules with the following content in the same directory:

     module  ISO-2022-JP//   EUC-JP//        ISO2022JP-EUCJP    1
     module  EUC-JP//        ISO-2022-JP//   ISO2022JP-EUCJP    1

To see why this is sufficient, it is necessary to understand how the conversion used by iconv (and described in the descriptor) is selected.  The approach to this problem is quite simple.

At the first call of the iconv_open function, the program reads all available gconv-modules files and builds up two tables---one containing all the known aliases and another that contains the information about the conversions and which shared object implements them.

Finding the Conversion Path in iconv
....................................

The set of available conversions form a directed graph with weighted edges.  The weights on the edges are the costs specified in the gconv-modules files.  The iconv_open function uses an algorithm suitable for searching for the best path in such a graph and so constructs a list of conversions that must be performed in succession to get the transformation from the source to the destination character set.

Explaining why the above gconv-modules file allows the iconv implementation to resolve the specific ISO-2022-JP to EUC-JP conversion module instead of the conversion coming with the library itself is straightforward.  Since the latter conversion takes two steps (from ISO-2022-JP to ISO 10646 and then from ISO 10646 to EUC-JP), the cost is 1+1 = 2.  The above gconv-modules file, however, specifies that the new conversion modules can perform this conversion with only the cost of 1.

A mysterious item about the gconv-modules file above (and also the file that comes with the GNU C Library) is the names of the character sets specified in the module lines.  Almost all of the names end in //.  Some names are actually regular expressions.  The part of the implementation where this is used is not yet finished.  For now please simply follow the existing examples.  It should become clearer as you read on.

A last remark about the gconv-modules is about the names not ending with //.  A character set named INTERNAL is often mentioned.  From the discussion above and the chosen name, it should have become clear that this is the name for the representation used in the intermediate step of the triangulation.  We have said that this is UCS-4, but actually that is not quite right.  The UCS-4 specification also includes the specification of the byte ordering used.  Since a UCS-4 value consists of 4 bytes, a stored value is effected by byte ordering.  The internal representation is not the same as UCS-4 in case the byte ordering of the processor (or at least the running process) is not the same as the one required for UCS-4.  This is done for performance reasons as one does not want to perform unnecessary byte-swapping operations if one is not interested in actually seeing the result in UCS-4.  To avoid trouble with endianess, the internal representation consistently is named INTERNAL even on big-endian systems where the representations are identical.

iconv Module Data Structures
............................

So far, this section has described how modules are located and considered to be used.  What remains to be described is the interface of the modules so that you can write new ones. This section describes the interface as it is in use in January 1999.  The interface will change a bit in the future but, with luck, only in an upwardly compatible way.

The definitions necessary to write new modules are publicly available in the nonstandard header gconv.h.  The following text, therefore, describes the definitions from this header file.  First, however, it is necessary to get an overview.

From the perspective of the user of iconv, the interface is quite simple; the iconv_open function returns a handle that can be used in calls to iconv, and then the handle is freed with a call to iconv_close.  The problem is that the handle has to be able to represent the possibly long sequences of conversion steps and also the state of each conversion, since the handle is all that is passed to the iconv function.  Therefore, the data structures are really the elements necessary to understanding the implementation.

We need two different kinds of data structures.  The first describes the conversion and the second describes the state, etc.  There are really two type definitions like this in gconv.h.

 -- Data type: struct __gconv_step
     This data structure describes one conversion a module can perform.  For each function in a loaded module with conversion functions, there is exactly one object of this type.  This object is shared by all users of the conversion (i.e., this object does not contain any information corresponding to an actual conversion; it only describes the conversion itself).

     struct __gconv_loaded_object *__shlib_handle
     const char *__modname
     int __counter
          All these elements of the structure are used internally in the C library to coordinate loading and unloading the shared object.  You should not expect any of the other elements to be available or initialized.

     const char *__from_name
     const char *__to_name
          __from_name and __to_name contain the names of the source and destination character sets.  They can be used to identify the actual conversion to be carried out, since one module might implement conversions for more than one character set and/or direction.

     gconv_fct __fct
     gconv_init_fct __init_fct
     gconv_end_fct __end_fct
          These elements contain pointers to the functions in the loadable module. The interface will be explained below.

     int __min_needed_from
     int __max_needed_from
     int __min_needed_to
     int __max_needed_to;
          These values have to be supplied in the init function of the module. The __min_needed_from value specifies the minimum number of bytes a character of the source character set needs.  The __max_needed_from specifies the maximum value that also includes possible shift sequences.

          The __min_needed_to and __max_needed_to values serve the same purpose as __min_needed_from and __max_needed_from but this time for the destination character set.

          It is crucial that these values be accurate, since otherwise the conversion functions will have problems or not work at all.

     int __stateful
          This element must also be initialized by the init function.  int __stateful is nonzero if the source character set is stateful. Otherwise, it is zero.

     void *__data
          This element can be used freely by the conversion functions in the module.  void *__data can be used to communicate extra information from one call to another.  void *__data need not be initialized if it is not needed at all.  If void *__data element is assigned a pointer to dynamically allocated memory (presumably in the init function), it has to be ensured that the end function de-allocates the memory.  Otherwise, the application will leak memory.

          It is important to be aware that this data structure is shared by all users of this specification conversion and therefore, the __data element must not contain data specific to any particular use of the conversion function.

 -- Data type: struct __gconv_step_data
     This is the data structure that contains the information specific to each use of the conversion functions.

     char *__outbuf
     char *__outbufend
          These elements specify the output buffer for the conversion step.  The __outbuf element points to the beginning of the buffer, and __outbufend points to the byte following the last byte in the buffer.  The conversion function must not assume anything about the size of the buffer, but it can be safely assumed that there is room for at least one complete character in the output buffer.

          Once the conversion is finished, if the conversion is the last step, the __outbuf element must be modified to point after the last byte written into the buffer to signal how much output is available.  If this conversion step is not the last one, the element must not be modified. The __outbufend element must not be modified.

     int __is_last
          This element is nonzero if this conversion step is the last one.  This information is necessary for the recursion.  See the description of the conversion function internals below.  This element must never be modified.

     int __invocation_counter
          The conversion function can use this element to see how many calls of the conversion function already happened.  Some character sets require a certain prolog when generating output, and by comparing this value with zero, one can find out whether it is the first call and whether, therefore, the prolog should be emitted.  This element must never be modified.

     int __internal_use
          This element is another one rarely used but needed in certain situations.  It is assigned a nonzero value in case the conversion functions are used to implement mbsrtowcs or others like it (i.e., the function is not used directly through the iconv interface).

          This sometimes makes a difference, since it is expected that the iconv functions are used to translate entire texts, while the mbsrtowcs functions are normally used only to convert single strings and might be used multiple times to convert entire texts.

          But in this situation we would have problems complying with some rules of the character-set specification.  Some character sets require a prolog, which must appear exactly once for an entire text.  If a number of mbsrtowcs calls are used to convert the text, only the first call must add the prolog.  However, because there is no communication between the different calls of mbsrtowcs, the conversion functions have no chance to find this out.  The situation is different for sequences of iconv calls, since the handle allows access to the needed information.

          The int __internal_use element is mostly used together with __invocation_counter as follows:

               if (!data->__internal_use
                    && data->__invocation_counter == 0)
                 /* Emit prolog.  */
                 ...

          This element must never be modified.

     mbstate_t *__statep
          The __statep element points to an object of type mbstate_t (see Keeping the state).  The conversion of a stateful character set must use the object pointed to by __statep to store information about the conversion state.  The __statep element itself must never be modified.

     mbstate_t __state
          This element must never be used directly.  It is only part of this structure to have the needed space allocated.

iconv Module Interfaces
.......................

With the knowledge about the data structures, we can now describe the conversion function itself.  To understand the interface, a bit of knowledge about the functionality in the C library that loads the objects with the conversions is necessary.

It is often the case that one conversion is used more than once (i.e., there are several iconv_open calls for the same set of character sets during one program run).  The mbsrtowcs functions in the GNU C Library also use the iconv functionality, which increases the number of uses of the same functions even more.

Because of this multiple use of conversions, the modules do not get loaded exclusively for one conversion.  Instead, a module once loaded can be used by an arbitrary number of iconv or mbsrtowcs calls at the same time.  The splitting of the information between conversion-function-specific information and conversion data makes this possible.  The last section showed the two data structures used to do this.

This is also reflected in the interface and semantics of the functions that the modules must provide.  There are three functions, which must have the following names:

gconv_init
     The gconv_init function initializes the conversion-function-specific data structure.  This very same object is shared by all conversions that use this conversion and, therefore, no state information about the conversion itself must be stored in here. If a module implements more than one conversion, the gconv_init function will be called multiple times.

gconv_end
     The gconv_end function is responsible for freeing all resources allocated by the gconv_init function.  If there is nothing to do, this function can be missing.  Special care must be taken if the module implements more than one conversion and the gconv_init function does not allocate the same resources for all conversions.

gconv
     This is the actual conversion function.  It is called to convert one block of text.  It gets passed the conversion-step information initialized by gconv_init and the conversion data, specific to this use of the conversion functions.

There are three data types defined for the three module interface functions, and these define the interface.

 -- Data type: int (*__gconv_init_fct) (struct __gconv_step *)
     This specifies the interface of the initialization function of the module.  It is called exactly once for each conversion the module implements.

     As explained in the description of the struct __gconv_step data structure above, the initialization function has to initialize parts of it.

     __min_needed_from
     __max_needed_from
     __min_needed_to
     __max_needed_to
          These elements must be initialized to the exact numbers of the minimum and maximum number of bytes used by one character in the source and destination character sets, respectively.  If the characters all have the same size, the minimum and maximum values are the same.

     __stateful
          This element must be initialized to a nonzero value if the source character set is stateful.  Otherwise, it must be zero.

     If the initialization function needs to communicate some information to the conversion function, this communication can happen using the __data element of the __gconv_step structure.  But since this data is shared by all the conversions, it must not be modified by the conversion function.  The example below shows how this can be used.

          #define MIN_NEEDED_FROM         1
          #define MAX_NEEDED_FROM         4
          #define MIN_NEEDED_TO           4
          #define MAX_NEEDED_TO           4

          int
          gconv_init (struct __gconv_step *step)
          {
            /* Determine which direction.  */
            struct iso2022jp_data *new_data;
            enum direction dir = illegal_dir;
            enum variant var = illegal_var;
            int result;

            if (__strcasecmp (step->__from_name, "ISO-2022-JP//") == 0)
              {
                dir = from_iso2022jp;
                var = iso2022jp;
              }
            else if (__strcasecmp (step->__to_name, "ISO-2022-JP//") == 0)
              {
                dir = to_iso2022jp;
                var = iso2022jp;
              }
            else if (__strcasecmp (step->__from_name, "ISO-2022-JP-2//") == 0)
              {
                dir = from_iso2022jp;
                var = iso2022jp2;
              }
            else if (__strcasecmp (step->__to_name, "ISO-2022-JP-2//") == 0)
              {
                dir = to_iso2022jp;
                var = iso2022jp2;
              }

            result = __GCONV_NOCONV;
            if (dir != illegal_dir)
              {
                new_data = (struct iso2022jp_data *)
                  malloc (sizeof (struct iso2022jp_data));

                result = __GCONV_NOMEM;
                if (new_data != NULL)
                  {
                    new_data->dir = dir;
                    new_data->var = var;
                    step->__data = new_data;

                    if (dir == from_iso2022jp)
                      {
                        step->__min_needed_from = MIN_NEEDED_FROM;
                        step->__max_needed_from = MAX_NEEDED_FROM;
                        step->__min_needed_to = MIN_NEEDED_TO;
                        step->__max_needed_to = MAX_NEEDED_TO;
                      }
                    else
                      {
                        step->__min_needed_from = MIN_NEEDED_TO;
                        step->__max_needed_from = MAX_NEEDED_TO;
                        step->__min_needed_to = MIN_NEEDED_FROM;
                        step->__max_needed_to = MAX_NEEDED_FROM + 2;
                      }

                    /* Yes, this is a stateful encoding.  */
                    step->__stateful = 1;

                    result = __GCONV_OK;
                  }
              }

            return result;
          }

     The function first checks which conversion is wanted.  The module from which this function is taken implements four different conversions; the one that is selected can be determined by comparing the names.  The comparison should always be done without paying attention to the case.

     Next, a data structure, which contains the necessary information about which conversion is selected, is allocated.  The data structure struct iso2022jp_data is locally defined since, outside the module, this data is not used at all.  If all four conversions this module supports are requested, there are four data blocks.

     The initialization of the __min_ and __max_ elements of the step-data object is interesting.  A single ISO-2022-JP character can consist of 1 to 4 bytes.  Therefore, the MIN_NEEDED_FROM and MAX_NEEDED_FROM macros are defined this way.  The output is always the INTERNAL character set (aka UCS-4), and therefore each character consists of exactly 4 bytes. For the conversion from INTERNAL to ISO-2022-JP, we have to take into account that escape sequences might be necessary to switch the character sets.  Therefore, the __max_needed_to element for this direction gets assigned MAX_NEEDED_FROM + 2.  This takes into account the 2 bytes needed for the escape sequences to signal the switching.  The asymmetry in the maximum values for the two directions can be explained easily, when reading ISO-2022-JP text, escape sequences can be handled alone (i.e., it is not necessary to process a real character, since the effect of the escape sequence can be recorded in the state information).  The situation is different for the other direction.  Since it is in general not known which character comes next, you cannot emit escape sequences to change the state in advance.  This means the escape sequences have to be emitted together with the next character.  Therefore, you need room for more than only the character itself.

     The possible return values of the initialization function are

     __GCONV_OK
          The initialization succeeded.

     __GCONV_NOCONV
          The requested conversion is not supported in the module.  This can happen if the gconv-modules file has errors.

     __GCONV_NOMEM
          Memory required to store additional information could not be allocated.

The function called before the module is unloaded is significantly easier.  It often has nothing at all to do, in which case it can be left out completely.

 -- Data type: void (*__gconv_end_fct) (struct gconv_step *)
     The task of this function is to free all resources allocated in the initialization function.  Therefore, only the __data element of the object pointed to by the argument is of interest.  Continuing the example from the initialization function, the finalization function looks like this:

          void
          gconv_end (struct __gconv_step *data)
          {
            free (data->__data);
          }

The most important function is the conversion function itself, which can get quite complicated for complex character sets.  But since this is not of interest here, we will only describe a possible skeleton for the conversion function.

 -- Data type: int (*__gconv_fct) (struct __gconv_step *, struct __gconv_step_data *, const char **, const char *, size_t *, int)
     The conversion function can be called for two basic reasons: to convert text or to reset the state.  From the description of the iconv function, it can be seen why the flushing mode is necessary.  The mode selected is determined by the sixth argument, an integer.  This argument being nonzero means that flushing is selected.

     Common to both modes is the location of the output buffer.  The information about this buffer is stored in the conversion-step data.  A pointer to this information is passed as the second argument to this function.  The description of the struct __gconv_step_data structure has more information on the conversion-step data.

     What has to be done for flushing depends on the source character set. If the source character set is not stateful, nothing has to be done. Otherwise, the function has to emit a byte sequence to bring the state object into the initial state.  Once this has all happened, the other conversion modules in the chain of conversions have to get the same chance.  Whether another step follows can be determined from the __is_last element of the step data structure to which the first parameter points.

     The more interesting mode is when actual text has to be converted.  The first step in this case is to convert as much text as possible from the input buffer and store the result in the output buffer.  The start of the input buffer is determined by the third argument, which is a pointer to a pointer variable referencing the beginning of the buffer.  The fourth argument is a pointer to the byte right after the last byte in the buffer.

     The conversion has to be performed according to the current state if the character set is stateful.  The state is stored in an object pointed to by the __statep element of the step data (second argument).  Once either the input buffer is empty or the output buffer is full, the conversion stops.  At this point, the pointer variable referenced by the third parameter must point to the byte following the last processed byte (i.e., if all of the input is consumed, this pointer and the fourth parameter have the same value).

     What happens now depends on whether this step is the last one.  If it is the last step, the only thing to be done is to update the __outbuf element of the step data structure to point after the last written byte.  This update gives the caller the information on how much text is available in the output buffer.  In addition, the variable pointed to by the fifth parameter, which is of type size_t, must be incremented by the number of characters (not bytes) that were converted in a nonreversible way.  Then, the function can return.

     In case the step is not the last one, the later conversion functions have to get a chance to do their work.  Therefore, the appropriate conversion function has to be called.  The information about the functions is stored in the conversion data structures, passed as the first parameter. This information and the step data are stored in arrays, so the next element in both cases can be found by simple pointer arithmetic:

          int
          gconv (struct __gconv_step *step, struct __gconv_step_data *data,
                 const char **inbuf, const char *inbufend, size_t *written,
                 int do_flush)
          {
            struct __gconv_step *next_step = step + 1;
            struct __gconv_step_data *next_data = data + 1;
            ...

     The next_step pointer references the next step information, and next_data references the next data record.  The call of the next function therefore will look similar to this:

            next_step->__fct (next_step, next_data, &outerr, outbuf,
                              written, 0)

     But this is not yet all.  Once the function call returns, the conversion function might have some more to do.  If the return value of the function is __GCONV_EMPTY_INPUT, more room is available in the output buffer.  Unless the input buffer is empty, the conversion functions start all over again and process the rest of the input buffer. If the return value is not __GCONV_EMPTY_INPUT, something went wrong and we will have to recover from this.

     A requirement for the conversion function is that the input buffer pointer (the third argument) always point to the last character that was put in converted form into the output buffer.  This is true after the conversion performed in the current step, but if the conversion functions deeper downstream stop prematurely, not all characters from the output buffer are consumed and, therefore, the input buffer pointers must be backed off to the right position.

     Correcting the input buffers is easy to do if the input and output character sets have a fixed width for all characters.  In this situation, we can compute how many characters are left in the output buffer and, therefore, can correct the input buffer pointer appropriately with a similar computation.  Things are getting tricky if either character set has characters represented with variable-length byte sequences, and it gets even more complicated if the conversion has to take care of the state.  In these cases, the conversion has to be performed once again, from the known state before the initial conversion (i.e., if necessary the state of the conversion has to be reset and the conversion loop has to be executed again).  The difference now is that it is known how much input must be created, and the conversion can stop before converting the first unused character.  Once this is done, the input buffer pointers must be updated again and the function can return.

     One final thing should be mentioned.  If it is necessary for the conversion to know whether it is the first invocation (in case a prolog has to be emitted), the conversion function should increment the __invocation_counter element of the step data structure just before returning to the caller.  See the description of the struct __gconv_step_data structure in iconv Module Data Structures for more information on how this can be used.

     The return value must be one of the following values:

     __GCONV_EMPTY_INPUT
          All input was consumed and there is room left in the output buffer.

     __GCONV_FULL_OUTPUT
          No more room in the output buffer.  In case this is not the last step, this value is propagated down from the call of the next conversion function in the chain.

     __GCONV_INCOMPLETE_INPUT
          The input buffer is not entirely empty, since it contains an incomplete character sequence.

     The following example provides a framework for a conversion function. In case a new conversion has to be written, the following is a template that can be filled in to do the job:

          int
          gconv (struct __gconv_step *step, struct __gconv_step_data *data,
                 const char **inbuf, const char *inbufend, size_t *written,
                 int do_flush)
          {
            struct __gconv_step *next_step = step + 1;
            struct __gconv_step_data *next_data = data + 1;
            gconv_fct fct = next_step->__fct;
            int status;

            /* If the function is called with no input, we have
               to reset to the initial state.  The potentially partly
               converted input is dropped.  */
            if (do_flush)
              {
                status = __GCONV_OK;

                /* Possibly emit a byte sequence that puts the state object
                   into the initial state.  */

                /* Call the steps down the chain if there are any but only
                   if we successfully emitted the escape sequence.  */
                if (status == __GCONV_OK && ! data->__is_last)
                  status = fct (next_step, next_data, NULL, NULL,
                                written, 1);
              }
            else
              {
                /* We preserve the initial values of the pointer variables.  */
                const char *inptr = *inbuf;
                char *outbuf = data->__outbuf;
                char *outend = data->__outbufend;
                char *outptr;

                do
                  {
                    /* Remember the start value for this round.  */
                    inptr = *inbuf;
                    /* The outbuf buffer is empty.  */
                    outptr = outbuf;

                    /* For stateful encodings the state must be safe here.  */

                    /* Run the conversion loop.  status is set
                       appropriately afterwards.  */

                    /* If this is the last step, leave the loop. There is
                       nothing we can do.  */
                    if (data->__is_last)
                      {
                        /* Store information about how many bytes are
                           available.  */
                        data->__outbuf = outbuf;

                       /* If any nonreversible conversions were performed,
                          add the number to *written.  */

                       break;
                     }

                    /* Write out all output that was produced.  */
                    if (outbuf > outptr)
                      {
                        const char *outerr = data->__outbuf;
                        int result;

                        result = fct (next_step, next_data, &outerr,
                                      outbuf, written, 0);

                        if (result != __GCONV_EMPTY_INPUT)
                          {
                            if (outerr != outbuf)
                              {
                                /* Reset the input buffer pointer. Here we
                                   document the complex case.  */
                                size_t nstatus;

                                /* Reload the pointers.  */
                                *inbuf = inptr;
                                outbuf = outptr;

                                /* Possibly reset the state.  */

                                /* Redo the conversion, but this time
                                   the end of the output buffer is at
                                   outerr.  */
                              }

                            /* Change the status.  */
                            status = result;
                          }
                        else
                          /* All the output is consumed, we can make
                              another run if everything was ok.  */
                          if (status == __GCONV_FULL_OUTPUT)
                            status = __GCONV_OK;
                     }
                  }
                while (status == __GCONV_OK);

                /* We finished one use of this step.  */
                ++data->__invocation_counter;
              }

            return status;
          }

This information should be sufficient to write new modules.  Anybody doing so should also take a look at the available source code in the GNU C Library sources.  It contains many examples of working and optimized modules.

Locales and Internationalization
********************************

Different countries and cultures have varying conventions for how to communicate.  These conventions range from very simple ones, such as the format for representing dates and times, to very complex ones, such as the language spoken.

Internationalization of software means programming it to be able to adapt to the user's favorite conventions.  In ISO C, internationalization works by means of locales.  Each locale specifies a collection of conventions, one convention for each purpose. The user chooses a set of conventions by specifying a locale (via environment variables).

All programs inherit the chosen locale as part of their environment. Provided the programs are written to obey the choice of locale, they will follow the conventions preferred by the user.

What Effects a Locale Has
=========================

Each locale specifies conventions for several purposes, including the following:

  * What multibyte-character sequences are valid, and how they are interpreted (see Character-Set Handling)

  * Classification of which characters in the local character set are considered alphabetic, and uppercase and lowercase conversion conventions (see Character Handling)

  * The collating sequence for the local language and character set (see Collation Functions)

  * Formatting of numbers and currency amounts (see General Numeric)

  * Formatting of dates and times (see Formatting Calendar Time)

  * What language to use for output, including error messages (see Message Translation)

  * What language to use for user answers to yes-or-no questions (see Yes-or-No Questions)

  * What language to use for more complex user input (the C library doesn't yet help you implement this)

Some aspects of adapting to the specified locale are handled automatically by the library subroutines.  For example, all your program needs to do in order to use the collating sequence of the chosen locale is to use strcoll or strxfrm to compare strings.

Other aspects of locales are beyond the comprehension of the library. For example, the library can't automatically translate your program's output messages into other languages.  The only way you can support output in the user's favorite language is to program this more or less by hand.  The C library provides functions to handle translations for multiple languages easily.

This chapter discusses the mechanism by which you can modify the current locale.  The effects of the current locale on specific library functions are discussed in more detail in the descriptions of those functions.

Choosing a Locale
=================

The simplest way for the user to choose a locale is to set the environment variable LANG.  This specifies a single locale to use for all purposes.  For example, a user could specify a hypothetical locale named espana-castellano to use the standard conventions of most of Spain.

The set of locales supported depends on the operating system you are using, and so do their names.  We can't make any promises about what locales will exist, except for one standard locale called C or POSIX.  Later we will describe how to construct locales.

A user also has the option of specifying different locales for different purposes---in effect, choosing a mixture of multiple locales.

For example, the user might specify the locale espana-castellano for most purposes, but specify the locale usa-english for currency formatting.  This might make sense if the user is a Spanish-speaking American, working in Spanish, but representing monetary amounts in US dollars.

Both locales espana-castellano and usa-english, like all locales, would include conventions for all of the purposes to which locales apply.  However, the user can choose to use each locale for a particular subset of those purposes.

Categories of Activities That Locales Affect
============================================

The purposes that locales serve are grouped into categories, so that a user or a program can choose the locale for each category independently.  Here is a table of categories; each name is both an environment variable that a user can set, and a macro name that you can use as an argument to setlocale.

LC_COLLATE
     This category applies to collation of strings (functions strcoll and strxfrm) (see Collation Functions).

LC_CTYPE
     This category applies to classification and conversion of characters, and to multibyte and wide characters (see Character Handling, and Character-Set Handling).

LC_MONETARY
     This category applies to formatting monetary values (see General Numeric).

LC_NUMERIC
     This category applies to formatting numeric values that are not monetary (see General Numeric).

LC_TIME
     This category applies to formatting date and time values (see Formatting Calendar Time).

LC_MESSAGES
     This category applies to selecting the language used in the user interface for message translation (see The Uniforum approach, and Message catalogs a la X/Open) and contains regular expressions for affirmative and negative responses.

LC_ALL
     This is not an environment variable; it is only a macro that you can use with setlocale to set a single locale for all purposes.  Setting this environment variable overwrites all selections by the other LC_* variables or LANG.

LANG
     If this environment variable is defined, its value specifies the locale to use for all purposes except as overridden by the variables above.

When developing the message translation functions it was felt that the functionality provided by the variables above is not sufficient.  For example, it should be possible to specify more than one locale name. Take a Swedish user who speaks German better than English, and a program whose messages are output in English by default.  It should be possible to specify that the first choice of language is Swedish, the second German, and if this also fails, to use English.  This is possible with the variable LANGUAGE.  For further description of this GNU extension, see Using gettextized software.

How Programs Set the Locale
===========================

A C program inherits its locale environment variables when it starts up. This happens automatically.  However, these variables do not automatically control the locale used by the library functions, because ISO C says that all programs start by default in the standard C locale.  To use the locales specified by the environment, you must call setlocale.  Call it as follows:

     setlocale (LC_ALL, "");

to select a locale based on the user choice of the appropriate environment variables.

You can also use setlocale to specify a particular locale, for general use or for a specific category.

The symbols in this section are defined in the header file locale.h.

 -- Function: char * setlocale (int CATEGORY, const char *LOCALE)
     The function setlocale sets the current locale for category CATEGORY to LOCALE.  A list of all the locales the system provides can be created by running:

            locale -a

     If CATEGORY is LC_ALL, this specifies the locale for all purposes.  The other possible values of CATEGORY specify a single purpose (see Locale Categories).

     You can also use this function to find out the current locale by passing a null pointer as the LOCALE argument.  In this case, setlocale returns a string that is the name of the locale currently selected for category CATEGORY.

     The string returned by setlocale can be overwritten by subsequent calls, so you should make a copy of the string (see Copying and Concatenation) if you want to save it past any further calls to setlocale.  (The standard library is guaranteed never to call setlocale itself.)

     You should not modify the string returned by setlocale.  It might be the same string that was passed as an argument in a previous call to setlocale.  One requirement is that the CATEGORY must be the same in the call the string was returned from and the call where the string is passed in as LOCALE parameter.

     When you read the current locale for category LC_ALL, the value encodes the entire combination of selected locales for all categories. In this case, the value is not just a single locale name.  In fact, we don't make any promises about what it looks like.  But if you specify the same locale name with LC_ALL in a subsequent call to setlocale, it restores the same combination of locale selections.

     To be sure you can use the returned string encoding the currently selected locale at a later time, you must make a copy of the string.  It is not guaranteed that the returned pointer remains valid over time.

     When the LOCALE argument is not a null pointer, the string returned by setlocale reflects the newly-modified locale.

     If you specify an empty string for LOCALE, this means to read the appropriate environment variable and use its value to select the locale for CATEGORY.

     If a nonempty string is given for LOCALE, then the locale of that name is used if possible.

     If you specify an invalid locale name, setlocale returns a null pointer and leaves the current locale unchanged.

Here is an example showing how you might use setlocale to temporarily switch to a new locale:

     #include <stddef.h>
     #include <locale.h>
     #include <stdlib.h>
     #include <string.h>

     void
     with_other_locale (char *new_locale,
                        void (*subroutine) (int),
                        int argument)
     {
       char *old_locale, *saved_locale;

       /* Get the name of the current locale.  */
       old_locale = setlocale (LC_ALL, NULL);

       /* Copy the name so it won't be clobbered by setlocale. */
       saved_locale = strdup (old_locale);
       if (saved_locale == NULL)
         fatal ("Out of memory");

       /* Now change the locale and do some stuff with it. */
       setlocale (LC_ALL, new_locale);
       (*subroutine) (argument);

       /* Restore the original locale. */
       setlocale (LC_ALL, saved_locale);
       free (saved_locale);
     }

Portability Note: Some ISO C systems may define additional locale categories, and future versions of the library will do so.  For portability, assume that any symbol beginning with LC_ might be defined in locale.h.

Standard Locales
================

The only locale names you can count on finding on all operating systems are these three standard ones:

``C''
     This is the standard C locale.  The attributes and behavior it provides are specified in the ISO C standard.  When your program starts up, it initially uses this locale by default.

``POSIX''
     This is the standard POSIX locale.  Currently, it is an alias for the standard C locale.

``''
     The empty name says to select a locale based on environment variables (see Locale Categories).

Defining and installing named locales is normally a responsibility of the system administrator at your site (or the person who installed the GNU C Library).  It is also possible for the user to create private locales.  All this will be discussed later when describing the tool for such creation.

If your program needs to use something other than the C locale, it will be more portable if you use whatever locale the user specifies with the environment, rather than trying to specify some nonstandard locale explicitly by name.  Remember, different machines might have different sets of locales installed.

Accessing Locale Information
============================

There are several ways to access locale information.  The simplest way is to let the C library itself do the work.  Several of the functions in this library implicitly access the locale data, and use what information is provided by the currently selected locale.  This is how the locale model is meant to work normally.

As an example, take the strftime function, which is meant to nicely format date and time information (see Formatting Calendar Time).  Part of the standard information contained in the LC_TIME category is the names of the months.  Instead of requiring the programmer to take care of providing the translations, the strftime function does this all by itself.  %A in the format string is replaced by the appropriate weekday name of the locale currently selected by LC_TIME.  This is an easy example, and wherever possible, functions do things automatically in this way.

But there are quite often situations when there is simply no function to perform the task, or it is simply not possible to do the work automatically.  For these cases it is necessary to access the information in the locale directly.  To do this, the C library provides two functions: localeconv and nl_langinfo.  The former is part of ISO C and is therefore portable, but has a brain-damaged interface.  The second is part of the Unix interface and is portable in as far as the system follows the Unix standards.

localeconv: ``It is portable, but ...''
---------------------------------------

Together with the setlocale function, the ISO C people invented the localeconv function.  It is a masterpiece of poor design.  It is expensive to use, not extendable, and not generally usable, since it provides access to only LC_MONETARY and LC_NUMERIC related information.  Nevertheless, if it is applicable to a given situation, it should be used since it is very portable.  The function strfmon formats monetary amounts according to the selected locale using this information.

 -- Function: struct lconv * localeconv (void)
     The localeconv function returns a pointer to a structure whose components contain information about how numeric and monetary values should be formatted in the current locale.

     You should not modify the structure or its contents.  The structure might be overwritten by subsequent calls to localeconv, or by calls to setlocale, but no other function in the library overwrites this value.

 -- Data Type: struct lconv
     localeconv's return value is of this data type.  Its elements are described in the following subsections.

If a member of the structure struct lconv has type char, and the value is CHAR_MAX, it means that the current locale has no value for that parameter.

Generic Numeric Formatting Parameters
.....................................

These are the standard members of struct lconv; there may be others.

char *decimal_point
char *mon_decimal_point
     These are the decimal-point separators used in formatting nonmonetary and monetary quantities, respectively.  In the C locale, the value of decimal_point is ".", and the value of mon_decimal_point is "".

char *thousands_sep
char *mon_thousands_sep
     These are the separators used to delimit groups of digits to the left of the decimal point in formatting nonmonetary and monetary quantities, respectively.  In the C locale, both members have a value of "" (the empty string).

char *grouping
char *mon_grouping
     These are strings that specify how to group the digits to the left of the decimal point.  grouping applies to nonmonetary quantities, and mon_grouping applies to monetary quantities.  Use either thousands_sep or mon_thousands_sep to separate the digit groups.

     Each member of these strings is to be interpreted as an integer value of type char.  Successive numbers (from left to right) give the sizes of successive groups (from right to left, starting at the decimal point.)  The last member is either 0, in which case the previous member is used over and over again for all the remaining groups, or CHAR_MAX, in which case there is no more grouping---or, put another way, any remaining digits form one large group without separators.

     For example, if grouping is "\04\03\02", the correct grouping for the number 123456787654321 is 12, 34, 56, 78, 765, 4321.  This uses a group of four digits at the end, preceded by a group of three digits, preceded by groups of two digits (as many as needed).  With a separator of ,, the number would be printed as 12,34,56,78,765,4321.

     A value of "\03" indicates repeated groups of three digits, as normally used in the United States.

     In the standard C locale, both grouping and mon_grouping have a value of "".  This value specifies no grouping at all.

char int_frac_digits
char frac_digits
     These are small integers indicating how many fractional digits (to the right of the decimal point) should be displayed in a monetary value in international and local formats, respectively.  (Most often, both members have the same value.)

     In the standard C locale, both of these members have the value CHAR_MAX, meaning ``unspecified''.  The ISO standard doesn't say what to do when you find this value; we recommend printing no fractional digits.  (This locale also specifies the empty string for mon_decimal_point, so printing any fractional digits would be confusing!)

Printing the Currency Symbol
............................

These members of the struct lconv structure specify how to print the symbol to identify a monetary value---the international analog of $ for US dollars.

Each country has two standard currency symbols.  The local currency symbol is used commonly within the country, while the international currency symbol is used internationally to refer to that country's currency when it is necessary to indicate the country unambiguously.

For example, many countries use the dollar as their monetary unit, and when dealing with international currencies it's important to specify that one is dealing with (say) Canadian dollars instead of US dollars or Australian dollars.  But when the context is known to be Canada, there is no need to make this explicit---dollar amounts are implicitly assumed to be in Canadian dollars.

char *currency_symbol
     The local currency symbol for the selected locale.

     In the standard C locale, this member has a value of "" (the empty string), meaning ``unspecified''.  The ISO standard doesn't say what to do when you find this value; we recommend you simply print the empty string as you would print any other string pointed to by this variable.

char *int_curr_symbol
     The international currency symbol for the selected locale.

     The value of int_curr_symbol should normally consist of a three-letter abbreviation determined by the international standard ISO 4217 Codes for the Representation of Currency and Funds, followed by a one-character separator (often a space).

     In the standard C locale, this member has a value of "" (the empty string), meaning ``unspecified''.  We recommend you simply print the empty string as you would print any other string pointed to by this variable.

char p_cs_precedes
char n_cs_precedes
char int_p_cs_precedes
char int_n_cs_precedes
     These members are 1 if the currency_symbol or int_curr_symbol strings should precede the value of a monetary amount, or 0 if the strings should follow the value.  The p_cs_precedes and int_p_cs_precedes members apply to positive amounts (or zero), and the n_cs_precedes and int_n_cs_precedes members apply to negative amounts.

     In the standard C locale, all of these members have a value of CHAR_MAX, meaning ``unspecified''.  The ISO standard doesn't say what to do when you find this value.  We recommend printing the currency symbol before the amount, which is right for most countries. In other words, treat all nonzero values alike in these members.

     The members with the int_ prefix apply to the int_curr_symbol while the other two apply to currency_symbol.

char p_sep_by_space
char n_sep_by_space
char int_p_sep_by_space
char int_n_sep_by_space
     These members are 1 if a space should appear between the currency_symbol or int_curr_symbol strings and the amount, or 0 if no space should appear.  The p_sep_by_space and int_p_sep_by_space members apply to positive amounts (or zero), and the n_sep_by_space and int_n_sep_by_space members apply to negative amounts.

     In the standard C locale, all of these members have a value of CHAR_MAX, meaning ``unspecified''.  The ISO standard doesn't say what you should do when you find this value; we suggest you treat it as 1 (print a space).  In other words, treat all nonzero values alike in these members.

     The members with the int_ prefix apply to the int_curr_symbol while the other two apply to currency_symbol.  There is one special case with the int_curr_symbol, though.  Since all legal values contain a space at the end the string one either adds this space with printf (if the currency symbol must appear in front and must be separated) or one has to avoid printing this character at all (especially when at the end of the string).

Printing the Sign of a Monetary Amount
......................................

These members of the struct lconv structure specify how to print the sign (if any) of a monetary value.

char *positive_sign
char *negative_sign
     These are strings used to indicate positive (or zero) and negative monetary quantities, respectively.

     In the standard C locale, both of these members have a value of "" (the empty string), meaning ``unspecified''.

     The ISO standard doesn't say what to do when you find this value; we recommend printing positive_sign as you find it, even if it is empty.  For a negative value, print negative_sign as you find it unless both it and positive_sign are empty, in which case print - instead.  (Failing to indicate the sign at all seems rather unreasonable.)

char p_sign_posn
char n_sign_posn
char int_p_sign_posn
char int_n_sign_posn
     These members are small integers that indicate how to position the sign for nonnegative and negative monetary quantities, respectively.  (The string used by the sign is what was specified with positive_sign or negative_sign.)  The possible values are as follows:

     0
          The currency symbol and quantity should be surrounded by parentheses.

     1
          Print the sign string before the quantity and currency symbol.

     2
          Print the sign string after the quantity and currency symbol.

     3
          Print the sign string right before the currency symbol.

     4
          Print the sign string right after the currency symbol.

     CHAR_MAX
          ``Unspecified''.  Both members have this value in the standard C locale.

     The ISO standard doesn't say what you should do when the value is CHAR_MAX.  We recommend you print the sign after the currency symbol.

     The members with the int_ prefix apply to the int_curr_symbol while the other two apply to currency_symbol.

Pinpoint Access to Locale Data
------------------------------

When writing the X/Open Portability Guide, (X/Open Company, X/Open Portability Guide, Issue 4, Version 2 (Reading, UK: X/Open Company, Ltd., 1994).) the authors realized that the localeconv function is not enough to provide reasonable access to locale information.  The information that was meant to be available in the locale (as later specified in the POSIX.1 standard) requires more ways to access it.  Therefore, the nl_langinfo function was introduced.

 -- Function: char * nl_langinfo (nl_item ITEM)
     The nl_langinfo function can be used to access individual elements of the locale categories.  Unlike the localeconv function, which returns all the information, nl_langinfo lets the caller select what information it requires.  This is very fast and it is not a problem to call this function multiple times.

     A second advantage is that in addition to the numeric and monetary formatting information, information from the LC_TIME and LC_MESSAGES categories is available.

     The type nl_type is defined in nl_types.h.  The argument ITEM is a numeric value defined in the header langinfo.h. The X/Open standard defines the following values:

     CODESET
          nl_langinfo returns a string with the name of the coded character set used in the selected locale.

     ABDAY_1
     ABDAY_2
     ABDAY_3
     ABDAY_4
     ABDAY_5
     ABDAY_6
     ABDAY_7
          nl_langinfo returns the abbreviated weekday name.  ABDAY_1 corresponds to Sunday.

     DAY_1
     DAY_2
     DAY_3
     DAY_4
     DAY_5
     DAY_6
     DAY_7
          Similar to ABDAY_1, etc., but here the return value is the unabbreviated weekday name.

     ABMON_1
     ABMON_2
     ABMON_3
     ABMON_4
     ABMON_5
     ABMON_6
     ABMON_7
     ABMON_8
     ABMON_9
     ABMON_10
     ABMON_11
     ABMON_12
          The return value is the abbreviated name of the month.  ABMON_1 corresponds to January.

     MON_1
     MON_2
     MON_3
     MON_4
     MON_5
     MON_6
     MON_7
     MON_8
     MON_9
     MON_10
     MON_11
     MON_12
          Similar to ABMON_1, etc., but here the month names are not abbreviated.  Here the first value MON_1 also corresponds to January.

     AM_STR
     PM_STR
          The return values are strings that can be used in the representation of time as an hour from 1 to 12 plus an a.m. or p.m. specifier.

          In locales that do not use this time representation, these strings might be empty, in which case the a.m./p.m. format cannot be used at all.

     D_T_FMT
          The return value can be used as a format string for strftime to represent time and date in a locale-specific way.

     D_FMT
          The return value can be used as a format string for strftime to represent a date in a locale-specific way.

     T_FMT
          The return value can be used as a format string for strftime to represent time in a locale-specific way.

     T_FMT_AMPM
          The return value can be used as a format string for strftime to represent time in the a.m./p.m. format.

          If the a.m./p.m. format does not make any sense for the selected locale, the return value might be the same as the one for T_FMT.

     ERA
          The return value represents the era used in the current locale.

          Most locales do not define this value.  An example of a locale that does define this value is the Japanese locale.  In Japan, the traditional representation of dates includes the name of the era corresponding to the then-emperor's reign.

          Normally it should not be necessary to use this value directly. Specifying the E modifier in their format strings causes the strftime functions to use this information.  The format of the returned string is not specified, and therefore you should not assume knowledge of it on different systems.

     ERA_YEAR
          The return value gives the year in the relevant era of the locale.  As for ERA, it should not be necessary to use this value directly.

     ERA_D_T_FMT
          This return value can be used as a format string for strftime to represent dates and times in a locale-specific era-based way.

     ERA_D_FMT
          This return value can be used as a format string for strftime to represent a date in a locale-specific era-based way.

     ERA_T_FMT
          This return value can be used as a format string for strftime to represent time in a locale-specific era-based way.

     ALT_DIGITS
          The return value is a representation of up to 100 values used to represent the values 0 to 99.  As for ERA, this value is not intended to be used directly, but instead indirectly through the strftime function.  When the modifier O is used in a format that would otherwise use numerals to represent hours, minutes, seconds, weekdays, months, or weeks, the appropriate value for the locale is used instead.

     INT_CURR_SYMBOL
          This is the same as the value returned by localeconv in the int_curr_symbol element of the struct lconv.

     CURRENCY_SYMBOL
     CRNCYSTR
          This is the same as the value returned by localeconv in the currency_symbol element of the struct lconv.

          CRNCYSTR is a deprecated alias still required by Unix98.

     MON_DECIMAL_POINT
          This is the same as the value returned by localeconv in the mon_decimal_point element of the struct lconv.

     MON_THOUSANDS_SEP
          This is the same as the value returned by localeconv in the mon_thousands_sep element of the struct lconv.

     MON_GROUPING
          This is the same as the value returned by localeconv in the mon_grouping element of the struct lconv.

     POSITIVE_SIGN
          This is the same as the value returned by localeconv in the positive_sign element of the struct lconv.

     NEGATIVE_SIGN
          This is the same as the value returned by localeconv in the negative_sign element of the struct lconv.

     INT_FRAC_DIGITS
          This is the same as the value returned by localeconv in the int_frac_digits element of the struct lconv.

     FRAC_DIGITS
          This is the same as the value returned by localeconv in the frac_digits element of the struct lconv.

     P_CS_PRECEDES
          This is the same as the value returned by localeconv in the p_cs_precedes element of the struct lconv.

     P_SEP_BY_SPACE
          This is the same as the value returned by localeconv in the p_sep_by_space element of the struct lconv.

     N_CS_PRECEDES
          This is the same as the value returned by localeconv in the n_cs_precedes element of the struct lconv.

     N_SEP_BY_SPACE
          This is the same as the value returned by localeconv in the n_sep_by_space element of the struct lconv.

     P_SIGN_POSN
          This is the same as the value returned by localeconv in the p_sign_posn element of the struct lconv.

     N_SIGN_POSN
          This is the same as the value returned by localeconv in the n_sign_posn element of the struct lconv.

     INT_P_CS_PRECEDES
          This is the same as the value returned by localeconv in the int_p_cs_precedes element of the struct lconv.

     INT_P_SEP_BY_SPACE
          This is the same as the value returned by localeconv in the int_p_sep_by_space element of the struct lconv.

     INT_N_CS_PRECEDES
          This is the same as the value returned by localeconv in the int_n_cs_precedes element of the struct lconv.

     INT_N_SEP_BY_SPACE
          This is the same as the value returned by localeconv in the int_n_sep_by_space element of the struct lconv.

     INT_P_SIGN_POSN
          This is the same as the value returned by localeconv in the int_p_sign_posn element of the struct lconv.

     INT_N_SIGN_POSN
          This is the same as the value returned by localeconv in the int_n_sign_posn element of the struct lconv.

     DECIMAL_POINT
     RADIXCHAR
          This is the same as the value returned by localeconv in the decimal_point element of the struct lconv.

          The name RADIXCHAR is a deprecated alias still used in Unix98.

     THOUSANDS_SEP
     THOUSEP
          This is the same as the value returned by localeconv in the thousands_sep element of the struct lconv.

          The name THOUSEP is a deprecated alias still used in Unix98.

     GROUPING
          This is the same as the value returned by localeconv in the grouping element of the struct lconv.

     YESEXPR
          The return value is a regular expression that can be used with the regex function to recognize a positive response to a yes-or-no question.  The GNU C Library provides the rpmatch function for easier handling in applications.

     NOEXPR
          The return value is a regular expression that can be used with the regex function to recognize a negative response to a yes-or-no question.

     YESSTR
          The return value is a locale-specific translation of the positive response to a yes-or-no question.

          Using this symbol or its value is deprecated, since it is a very special case of message translation and is better handled by the message translation functions (see Message Translation).

     NOSTR
          The return value is a locale-specific translation of the negative response to a yes-or-no question.  Use of this symbol or its value is deprecated in the same fashion as YESSTR. Instead, message translation should be used (see Message Translation).

     The file langinfo.h defines a many more symbols, but none of them are official.  Using them is not portable, and the format of the return values might change.  Therefore, we recommend you do not use them.

     The return value for any valid argument can be used for strftime in all situations (with the possible exception of the a.m. and p.m. time-formatting codes).  If the user has not selected any locale for the appropriate category, nl_langinfo returns the information from the "C" locale.  It is therefore possible to use this function as shown in the example below.

     If the argument ITEM is not valid, a pointer to an empty string is returned.

An example of nl_langinfo usage is a function that has to print a given date and time in a locale-specific way.  At first you might think that, since strftime internally uses the locale information, writing something like the following is enough:

     size_t
     i18n_time_n_data (char *s, size_t len, const struct tm *tp)
     {
       return strftime (s, len, "%X %D", tp);
     }

The format contains no weekday or month names and therefore is internationally usable.  This is incorrect.  The output produced is something like "hh:mm:ss MM/DD/YY".  This format is only recognizable in the United States.  Other countries use different formats.  Therefore, the function should be rewritten like this:

     size_t
     i18n_time_n_data (char *s, size_t len, const struct tm *tp)
     {
       return strftime (s, len, nl_langinfo (D_T_FMT), tp);
     }

Now it uses the date and time format of the locale selected when the program runs.  If the user selects the locale correctly, there should never be a misunderstanding over the time and date format.

A Dedicated Function to Format Numbers
======================================

We have seen that the structure returned by localeconv as well as the values given to nl_langinfo allow you to retrieve the various pieces of locale-specific information to format numbers and monetary amounts.  We have also seen that the underlying rules are quite complex.

Therefore, the X/Open standards introduce a function that uses such locale information, making it easier for the user to format numbers according to these rules.

 -- Function: ssize_t strfmon (char *S, size_t MAXSIZE, const char *FORMAT, ...)
     The strfmon function is similar to the strftime function in that it takes a buffer, its size, a format string and values to write into the buffer as text in a form specified by the format string.  Like strftime, the function also returns the number of bytes written into the buffer.

     There are two differences: strfmon can take more than one argument, and the format specification is different.  Like strftime, the format string consists of normal text, which is output as is, and format specifiers, which are indicated by a %. Immediately after the %, you can optionally specify various flags and formatting information before the main formatting character, in a similar way to printf:

       * Immediately following the % there can be one or more of the following flags:

          =F
               The single-byte character F is used for this field as the numeric fill character.  By default, this character is a space character. Filling with this character is only performed if a left precision is specified.  It is not just to fill to the given field width.

          ^
               The number is printed without grouping the digits according to the rules of the current locale.  By default, grouping is enabled.

          +, (
               At most one of these flags can be used.  They select which format to represent the sign of a currency amount.  By default, and if + is given, the locale equivalent of +/- is used.  If ( is given, negative amounts are enclosed in parentheses.  The exact format is determined by the values of the LC_MONETARY category of the locale selected at program run time.

          !
               The output will not contain the currency symbol.

          -
               The output will be formatted left-justified instead of right-justified if it does not fill the entire field width.

     The next part of a specification is an optional field width.  If no width is specified, 0 is taken.  During output, the function first determines how much space is required.  If it requires at least as many characters as given by the field width, it is output using as much space as necessary.  Otherwise, it is extended to use the full width by filling with the space character.  The presence or absence of the - flag determines the side at which such padding occurs.  If present, the spaces are added at the right, making the output left-justified, and vice versa.

     So far the format looks familiar, being similar to the printf and strftime formats.  However, the next two optional fields introduce something new.  The first one is a # character followed by a decimal digit string.  The value of the digit string specifies the number of digit positions to the left of the decimal point (or equivalent).  This does not include the grouping character when the ^ flag is not given.  If the space needed to print the number does not fill the whole width, the field is padded at the left side with the fill character, which can be selected using the = flag and by default is a space.  For example, if the field width is selected as 6 and the number is 123, the fill character is *, and the result will be ***123.

     The second optional field starts with a . (period) and consists of another decimal digit string.  Its value describes the number of characters printed after the decimal point.  The default is selected from the current locale (frac_digits, int_frac_digits; see General Numeric).  If the exact representation needs more digits than given by the field width, the displayed value is rounded.  If the number of fractional digits is selected to be zero, no decimal point is printed.

     As a GNU extension, the strfmon implementation in the GNU libc allows an optional L next as a format modifier.  If this modifier is given, the argument is expected to be a long double instead of a double value.

     Finally, the last component is a format specifier.  There are three specifiers defined:

     i
          Use the locale's rules for formatting an international currency value.

     n
          Use the locale's rules for formatting a national currency value.

     %
          Place a % in the output.  There must be no flag, width specifier or modifier given; only %% is allowed.

     As for printf, the function reads the format string from left to right and uses the values passed to the function following the format string.  The values are expected to be either of type double or long double, depending on the presence of the modifier L.  The result is stored in the buffer pointed to by S.  At most MAXSIZE characters are stored.

     The return value of the function is the number of characters stored in S, including the terminating NULL byte.  If the number of characters stored would exceed MAXSIZE, the function returns -1 and the content of the buffer S is unspecified.  In this case errno is set to E2BIG.

A few examples should make clear how the function works.  It is assumed that all the following pieces of code are executed in a program that uses the US locale (en_US).  The simplest form of the format is this:

     strfmon (buf, 100, "@%n@%n@%n@", 123.45, -567.89, 12345.678);

The output produced is

     "@$123.45@-$567.89@$12,345.68@"

We can notice several things here.  First, the widths of the output numbers are different, since we have not specified a width in the format string.  Second, the third number is printed using thousands separators. The thousands separator for the en_US locale is a comma.  The number is also rounded.  The number .678 is rounded to .68 since the format does not specify a precision and the default value in the locale is 2.  Finally, note that the national currency symbol is printed since %n was used, not i.  The next example shows how we can align the output:

     strfmon (buf, 100, "@%=*11n@%=*11n@%=*11n@", 123.45, -567.89, 12345.678);

The output this time is

     "@    $123.45@   -$567.89@ $12,345.68@"

Two things stand out.  First, all fields have the same width (eleven characters) since this is the width given in the format, and since no number required more characters to be printed.  The second important point is that the fill character is not used.  This is correct since the white space was not used to achieve a precision given by a # modifier, but instead to fill to the given width.  The difference becomes obvious if we now add a width specification:

     strfmon (buf, 100, "@%=*11#5n@%=*11#5n@%=*11#5n@",
              123.45, -567.89, 12345.678);

The output is

     "@ $***123.45@-$***567.89@ $12,456.68@"

Here we can see that all the currency symbols are now aligned, and that the space between the currency sign and the number is filled with the selected fill character.  Although the width is selected to be 5 and 123.45 has 3 digits left of the decimal point, the space is filled with 3 asterisks.  This is correct since, as explained above, the width does not include the positions used to store thousands separators. One last example should explain the remaining functionality:

     strfmon (buf, 100, "@%=0(16#5.3i@%=0(16#5.3i@%=0(16#5.3i@",
              123.45, -567.89, 12345.678);

This rather complex format string produces the following output:

     "@ USD 000123,450 @(USD 000567.890)@ USD 12,345.678 @"

The most noticeable change is the alternative way of representing negative numbers.  In financial circles this is often done using parentheses, and this is what the ( flag selected.  The fill character is now 0.  This 0 character is not regarded as a numeric zero, and therefore the first and second numbers are not printed using a thousands separator.  Since we used the format specifier i instead of n, the international form of the currency symbol is used.  This is a four-letter string, in this case "USD ".  The last point is that since the precision right of the decimal point is selected to be three, the first and second numbers are printed with an extra zero at the end and the third number is printed without rounding.

Yes-or-No Questions
===================

Some non-GUI programs ask a yes-or-no question.  If the messages (especially the questions) are translated into foreign languages, be sure that you localize the answers too.  It would be very bad habit to ask a question in one language and request the answer in another, often English.

The GNU C Library contains rpmatch to give applications easy access to the corresponding locale definitions.

 -- Function: int rpmatch (const char *RESPONSE)
     The function rpmatch checks the string in RESPONSE to see whether it is a correct yes-or-no answer and if yes, which one.  The check uses the YESEXPR and NOEXPR data in the LC_MESSAGES category of the currently selected locale.  The return value is as follows:

     1
          The user entered an affirmative answer.

     0
          The user entered a negative answer.

     -1
          The answer matched neither the YESEXPR nor the NOEXPR regular expression.

     This function is not standardized, but is available in GNU libc and at least also in the IBM AIX library.

This function would normally be used like this:

       ...
       /* Use a safe default.  */
       _Bool doit = false;

       fputs (gettext ("Do you really want to do this? "), stdout);
       fflush (stdout);
       /* Prepare the getline call.  */
       line = NULL;
       len = 0;
       while (getline (&line, &len, stdout) >= 0)
         {
           /* Check the response.  */
           int res = rpmatch (line);
           if (res >= 0)
             {
               /* We got a definitive answer.  */
               if (res > 0)
                 doit = true;
               break;
             }
         }
       /* Free what getline allocated.  */
       free (line);

Note that the loop continues until a read error is detected or until a definitive (positive or negative) answer is read.

Mathematics
***********

This chapter contains information about functions for performing mathematical computations, such as trigonometric functions.  Most of these functions have prototypes declared in the header file math.h.  The complex-valued functions are defined in complex.h.

All mathematical functions that take a floating-point argument have three variants, one each for double, float and long double arguments.  The double versions are mostly defined in ISO C89.  The float and long double versions are from the numeric extensions to C included in ISO C99.

Which of the three versions of a function should be used depends on the situation.  For most calculations, the float functions are the fastest.  On the other hand, the long double functions have the highest precision.  double is somewhere in between.  It is usually wise to pick the narrowest type that can accommodate your data. Not all machines have a distinct long double type; it may be the same as double.

Predefined Mathematical Constants
=================================

The header math.h defines several useful mathematical constants. All values are defined as preprocessor macros starting with M_. The values provided are

M_E
     The base of natural logarithms

M_LOG2E
     The logarithm to base 2 of M_E

M_LOG10E
     The logarithm to base 10 of M_E

M_LN2
     The natural logarithm of 2

M_LN10
     The natural logarithm of 10

M_PI
     Pi, the ratio of a circle's circumference to its diameter

M_PI_2
     Pi divided by two

M_PI_4
     Pi divided by four

M_1_PI
     The reciprocal of pi (1/pi)

M_2_PI
     Two times the reciprocal of pi

M_2_SQRTPI
     Two times the reciprocal of the square root of pi

M_SQRT2
     The square root of two

M_SQRT1_2
     The reciprocal of the square root of two (also the square root of 1/2)

These constants come from the Unix98 standard and were also available in 4.4BSD; therefore they are only defined if _BSD_SOURCE or _XOPEN_SOURCE=500 or a more general feature-select macro is defined.  The default set of features includes these constants (see Feature-Test Macros).

All values are of type double.  As an extension, the GNU C library also defines these constants with type long double.  The long double macros have a lowercase l appended to their names: M_El, M_PIl, and so forth.  These are only available if _GNU_SOURCE is defined.

Some programs use a constant named PI that has the same value as M_PI.  This constant is not standard; it may have appeared in some old AT&T headers, and is mentioned in Stroustrup's book on C++.  It infringes on the user's name space, so the GNU C Library does not define it.  Fixing programs written to expect it is simple; replace PI with M_PI throughout, or put -DPI=M_PI on the compiler command line.

Trigonometric Functions
=======================

These are the familiar sin, cos and tan functions. The arguments to all of these functions are in units of radians; recall that pi radians equals 180 degrees.

The math library normally defines M_PI to a double approximation of pi.  If strict ISO and/or POSIX compliance are requested, this constant is not defined, but you can easily define it yourself:

     #define M_PI 3.14159265358979323846264338327

You can also compute the value of pi with the expression acos (-1.0).

 -- Function: double sin (double X)
 -- Function: float sinf (float X)
 -- Function: long double sinl (long double X)
     These functions return the sine of X, where X is given in radians.  The return value is in the range -1 to 1.

 -- Function: double cos (double X)
 -- Function: float cosf (float X)
 -- Function: long double cosl (long double X)
     These functions return the cosine of X, where X is given in radians.  The return value is in the range -1 to 1.

 -- Function: double tan (double X)
 -- Function: float tanf (float X)
 -- Function: long double tanl (long double X)
     These functions return the tangent of X, where X is given in radians.

     Mathematically, the tangent function has singularities at odd multiples of pi/2.  If the argument X is too close to one of these singularities, tan will signal overflow.

In many applications where sin and cos are used, the sine and cosine of the same angle are needed at the same time.  It is more efficient to compute them simultaneously, so the library provides a function to do that.

 -- Function: void sincos (double X, double *SINX, double *COSX)
 -- Function: void sincosf (float X, float *SINX, float *COSX)
 -- Function: void sincosl (long double X, long double *SINX, long double *COSX)
     These functions return the sine of X in *SINX and the cosine of X in *COS, where X is given in radians.  Both values, *SINX and *COSX, are in the range of -1 to 1.

     This function is a GNU extension.  Portable programs should be prepared to cope with its absence.

ISO C99 defines variants of the trig functions that work on complex numbers.  The GNU C Library provides these functions, but they are only useful if your compiler supports the new complex types defined by the standard. (As of this writing, GCC supports complex numbers, but there are bugs in the implementation.)

 -- Function: complex double csin (complex double Z)
 -- Function: complex float csinf (complex float Z)
 -- Function: complex long double csinl (complex long double Z)
     These functions return the complex sine of Z. The mathematical definition of the complex sine is

     sin (z) = 1/(2*i) * (exp (z*i) - exp (-z*i)).

 -- Function: complex double ccos (complex double Z)
 -- Function: complex float ccosf (complex float Z)
 -- Function: complex long double ccosl (complex long double Z)
     These functions return the complex cosine of Z. The mathematical definition of the complex cosine is

     cos (z) = 1/2 * (exp (z*i) + exp (-z*i))

 -- Function: complex double ctan (complex double Z)
 -- Function: complex float ctanf (complex float Z)
 -- Function: complex long double ctanl (complex long double Z)
     These functions return the complex tangent of Z. The mathematical definition of the complex tangent is

     tan (z) = -i * (exp (z*i) - exp (-z*i)) / (exp (z*i) + exp (-z*i))

     The complex tangent has poles at pi/2 + 2n, where n is an integer.  ctan may signal overflow if Z is too close to a pole.

Inverse Trigonometric Functions
===============================

These are the usual arc sine, arc cosine and arc tangent functions, which are the inverses of the sine, cosine and tangent functions respectively.

 -- Function: double asin (double X)
 -- Function: float asinf (float X)
 -- Function: long double asinl (long double X)
     These functions compute the arc sine of X---that is, the value whose sine is X.  The value is in units of radians.  Mathematically, there are infinitely many such values; the one actually returned is the one between -pi/2 and pi/2 (inclusive).

     The arc sine function is defined mathematically only over the domain -1 to 1.  If X is outside the domain, asin signals a domain error.

 -- Function: double acos (double X)
 -- Function: float acosf (float X)
 -- Function: long double acosl (long double X)
     These functions compute the arc cosine of X---the value whose cosine is X.  The value is in units of radians.  Mathematically, there are infinitely many such values; the one actually returned is the one between 0 and pi (inclusive).

     The arc cosine function is defined mathematically only over the domain -1 to 1.  If X is outside the domain, acos signals a domain error.

 -- Function: double atan (double X)
 -- Function: float atanf (float X)
 -- Function: long double atanl (long double X)
     These functions compute the arc tangent of X---the value whose tangent is X.  The value is in units of radians.  Mathematically, there are infinitely many such values; the one actually returned is the one between -pi/2 and pi/2 (inclusive).

 -- Function: double atan2 (double Y, double X)
 -- Function: float atan2f (float Y, float X)
 -- Function: long double atan2l (long double Y, long double X)
     This function computes the arc tangent of Y/X, but the signs of both arguments are used to determine the quadrant of the result, and X is permitted to be zero.  The return value is given in radians and is in the range -pi to pi, inclusive.

     If X and Y are coordinates of a point in the plane, atan2 returns the signed angle between the line from the origin to that point and the x-axis.  Thus, atan2 is useful for converting Cartesian coordinates to polar coordinates.  To compute the radial coordinate, use hypot (see Exponents and Logarithms).

     If both X and Y are zero, atan2 returns zero.

ISO C99 defines complex versions of the inverse trig functions.

 -- Function: complex double casin (complex double Z)
 -- Function: complex float casinf (complex float Z)
 -- Function: complex long double casinl (complex long double Z)
     These functions compute the complex arc sine of Z---the value whose sine is Z.  The value returned is in radians.

     Unlike the real-valued functions, casin is defined for all values of Z.

 -- Function: complex double cacos (complex double Z)
 -- Function: complex float cacosf (complex float Z)
 -- Function: complex long double cacosl (complex long double Z)
     These functions compute the complex arc cosine of Z---the value whose cosine is Z.  The value returned is in radians.

     Unlike the real-valued functions, cacos is defined for all values of Z.

 -- Function: complex double catan (complex double Z)
 -- Function: complex float catanf (complex float Z)
 -- Function: complex long double catanl (complex long double Z)
     These functions compute the complex arc tangent of Z---the value whose tangent is Z.  The value is in units of radians.

Exponentiation and Logarithms
=============================

 -- Function: double exp (double X)
 -- Function: float expf (float X)
 -- Function: long double expl (long double X)
     These functions compute e (the base of natural logarithms) raised to the power X.

     If the magnitude of the result is too large to be representable, exp signals overflow.

 -- Function: double exp2 (double X)
 -- Function: float exp2f (float X)
 -- Function: long double exp2l (long double X)
     These functions compute 2 raised to the power X. Mathematically, exp2 (x) is the same as exp (x * log (2)).

 -- Function: double exp10 (double X)
 -- Function: float exp10f (float X)
 -- Function: long double exp10l (long double X)
 -- Function: double pow10 (double X)
 -- Function: float pow10f (float X)
 -- Function: long double pow10l (long double X)
     These functions compute 10 raised to the power X. Mathematically, exp10 (x) is the same as exp (x * log (10)).

     These functions are GNU extensions.  The name exp10 is preferred, since it is analogous to exp and exp2.

 -- Function: double log (double X)
 -- Function: float logf (float X)
 -- Function: long double logl (long double X)
     These functions compute the natural logarithm of X.  exp (log (X)) equals X, exactly in mathematics and approximately in C.

     If X is negative, log signals a domain error.  If X is zero, it returns negative infinity; if X is too close to zero, it may signal overflow.

 -- Function: double log10 (double X)
 -- Function: float log10f (float X)
 -- Function: long double log10l (long double X)
     These functions return the base-10 logarithm of X. log10 (X) equals log (X) / log (10).

 -- Function: double log2 (double X)
 -- Function: float log2f (float X)
 -- Function: long double log2l (long double X)
     These functions return the base-2 logarithm of X. log2 (X) equals log (X) / log (2).

 -- Function: double logb (double X)
 -- Function: float logbf (float X)
 -- Function: long double logbl (long double X)
     These functions extract the exponent of X and return it as a floating-point value.  If FLT_RADIX is two, logb is equal to floor (log2 (x)), except it's probably faster.

     If X is de-normalized, logb returns the exponent X would have if it were normalized.  If X is infinity (positive or negative), logb returns oo.  If X is zero, logb returns oo.  It does not signal.

 -- Function: int ilogb (double X)
 -- Function: int ilogbf (float X)
 -- Function: int ilogbl (long double X)
     These functions are equivalent to the corresponding logb functions, except that they return signed integer values.

Since integers cannot represent infinity and NaN, ilogb instead returns an integer that can't be the exponent of a normal floating-point number.  math.h defines constants so you can check for this.

 -- Macro: int FP_ILOGB0
     ilogb returns this value if its argument is 0.  The numeric value is either INT_MIN or -INT_MAX.

     This macro is defined in ISO C99.

 -- Macro: int FP_ILOGBNAN
     ilogb returns this value if its argument is NaN.  The numeric value is either INT_MIN or INT_MAX.

     This macro is defined in ISO C99.

These values are system specific.  They might even be the same.  The proper way to test the result of ilogb is as follows:

     i = ilogb (f);
     if (i == FP_ILOGB0 || i == FP_ILOGBNAN)
       {
         if (isnan (f))
           {
             /* Handle NaN.  */
           }
         else if (f  == 0.0)
           {
             /* Handle 0.0.  */
           }
         else
           {
             /* Some other value with large exponent,
                perhaps +Inf.  */
           }
       }

 -- Function: double pow (double BASE, double POWER)
 -- Function: float powf (float BASE, float POWER)
 -- Function: long double powl (long double BASE, long double POWER)
     These are general exponentiation functions, returning BASE raised to POWER.

     Mathematically, pow would return a complex number when BASE is negative and POWER is not an integral value.  pow can't do that, so instead it signals a domain error. pow may also underflow or overflow the destination type.

 -- Function: double sqrt (double X)
 -- Function: float sqrtf (float X)
 -- Function: long double sqrtl (long double X)
     These functions return the nonnegative square root of X.

     If X is negative, sqrt signals a domain error. Mathematically, it should return a complex number.

 -- Function: double cbrt (double X)
 -- Function: float cbrtf (float X)
 -- Function: long double cbrtl (long double X)
     These functions return the cube root of X.  They cannot fail; every representable real value has a representable real cube root.

 -- Function: double hypot (double X, double Y)
 -- Function: float hypotf (float X, float Y)
 -- Function: long double hypotl (long double X, long double Y)
     These functions return sqrt (X*X + Y*Y). This is the length of the hypotenuse of a right triangle with sides of length X and Y, or the distance of the point (X, Y) from the origin.  Using this function instead of the direct formula is wise, since the error is much smaller (see also the function cabs in Absolute Value).

 -- Function: double expm1 (double X)
 -- Function: float expm1f (float X)
 -- Function: long double expm1l (long double X)
     These functions return a value equivalent to exp (X) - 1. They are computed in a way that is accurate even if X is near zero---a case where exp (X) - 1 would be inaccurate owing to subtraction of two numbers that are nearly equal.

 -- Function: double log1p (double X)
 -- Function: float log1pf (float X)
 -- Function: long double log1pl (long double X)
     These functions returns a value equivalent to log (1 + X). They are computed in a way that is accurate even if X is near zero.

ISO C99 defines complex variants of some of the exponentiation and logarithm functions.

 -- Function: complex double cexp (complex double Z)
 -- Function: complex float cexpf (complex float Z)
 -- Function: complex long double cexpl (complex long double Z)
     These functions return e (the base of natural logarithms) raised to the power of Z. Mathematically, this corresponds to the value:

     exp (z) = exp (creal (z)) * (cos (cimag (z)) + I * sin (cimag (z)))

 -- Function: complex double clog (complex double Z)
 -- Function: complex float clogf (complex float Z)
 -- Function: complex long double clogl (complex long double Z)
     These functions return the natural logarithm of Z. Mathematically, this corresponds to the value:

     log (z) = log (cabs (z)) + I * carg (z)

     clog has a pole at 0, and will signal overflow if Z equals or is very close to 0.  It is well-defined for all other values of Z.

 -- Function: complex double clog10 (complex double Z)
 -- Function: complex float clog10f (complex float Z)
 -- Function: complex long double clog10l (complex long double Z)
     These functions return the base-10 logarithm of the complex value Z. Mathematically, this corresponds to the value:

     log (z) = log10 (cabs (z)) + I * carg (z)

     These functions are GNU extensions.

 -- Function: complex double csqrt (complex double Z)
 -- Function: complex float csqrtf (complex float Z)
 -- Function: complex long double csqrtl (complex long double Z)
     These functions return the complex square root of the argument Z.  Unlike the real-valued functions, they are defined for all values of Z.

 -- Function: complex double cpow (complex double BASE, complex double POWER)
 -- Function: complex float cpowf (complex float BASE, complex float POWER)
 -- Function: complex long double cpowl (complex long double BASE, complex long double POWER)
     These functions return BASE raised to the power of POWER.  This is equivalent to cexp (y * clog (x))

Hyperbolic Functions
====================

The functions in this section are related to the exponential functions (see Exponents and Logarithms).

 -- Function: double sinh (double X)
 -- Function: float sinhf (float X)
 -- Function: long double sinhl (long double X)
     These functions return the hyperbolic sine of X, defined mathematically as (exp (X) - exp (-X)) / 2.  They may signal overflow if X is too large.

 -- Function: double cosh (double X)
 -- Function: float coshf (float X)
 -- Function: long double coshl (long double X)
     These functions return the hyperbolic cosine of X, defined mathematically as (exp (X) + exp (-X)) / 2.  They may signal overflow if X is too large.

 -- Function: double tanh (double X)
 -- Function: float tanhf (float X)
 -- Function: long double tanhl (long double X)
     These functions return the hyperbolic tangent of X, defined mathematically as sinh (X) / cosh (X). They may signal overflow if X is too large.

There are counterparts for the hyperbolic functions that take complex arguments.

 -- Function: complex double csinh (complex double Z)
 -- Function: complex float csinhf (complex float Z)
 -- Function: complex long double csinhl (complex long double Z)
     These functions return the complex hyperbolic sine of Z, defined mathematically as (exp (Z) - exp (-Z)) / 2.

 -- Function: complex double ccosh (complex double Z)
 -- Function: complex float ccoshf (complex float Z)
 -- Function: complex long double ccoshl (complex long double Z)
     These functions return the complex hyperbolic cosine of Z, defined mathematically as (exp (Z) + exp (-Z)) / 2.

 -- Function: complex double ctanh (complex double Z)
 -- Function: complex float ctanhf (complex float Z)
 -- Function: complex long double ctanhl (complex long double Z)
     These functions return the complex hyperbolic tangent of Z, defined mathematically as csinh (Z) / ccosh (Z).

 -- Function: double asinh (double X)
 -- Function: float asinhf (float X)
 -- Function: long double asinhl (long double X)
     These functions return the inverse hyperbolic sine of X---the value whose hyperbolic sine is X.

 -- Function: double acosh (double X)
 -- Function: float acoshf (float X)
 -- Function: long double acoshl (long double X)
     These functions return the inverse hyperbolic cosine of X---the value whose hyperbolic cosine is X.  If X is less than 1, acosh signals a domain error.

 -- Function: double atanh (double X)
 -- Function: float atanhf (float X)
 -- Function: long double atanhl (long double X)
     These functions return the inverse hyperbolic tangent of X---the value whose hyperbolic tangent is X.  If the absolute value of X is greater than 1, atanh signals a domain error; if it is equal to 1, atanh returns infinity.

 -- Function: complex double casinh (complex double Z)
 -- Function: complex float casinhf (complex float Z)
 -- Function: complex long double casinhl (complex long double Z)
     These functions return the inverse complex hyperbolic sine of Z---the value whose complex hyperbolic sine is Z.

 -- Function: complex double cacosh (complex double Z)
 -- Function: complex float cacoshf (complex float Z)
 -- Function: complex long double cacoshl (complex long double Z)
     These functions return the inverse complex hyperbolic cosine of Z---the value whose complex hyperbolic cosine is Z.  Unlike the real-valued functions, there are no restrictions on the value of Z.

 -- Function: complex double catanh (complex double Z)
 -- Function: complex float catanhf (complex float Z)
 -- Function: complex long double catanhl (complex long double Z)
     These functions return the inverse complex hyperbolic tangent of Z---the value whose complex hyperbolic tangent is Z.  Unlike the real-valued functions, there are no restrictions on the value of Z.

Special Functions
=================

These are some more exotic mathematical functions that are sometimes useful.  Currently they only have real-valued versions.

 -- Function: double erf (double X)
 -- Function: float erff (float X)
 -- Function: long double erfl (long double X)
     erf returns the error function of X.  The error function is defined as:

          erf (x) = 2/sqrt(pi) * integral from 0 to x of exp(-t^2) dt

 -- Function: double erfc (double X)
 -- Function: float erfcf (float X)
 -- Function: long double erfcl (long double X)
     erfc returns 1.0 - erf(X), but computed in a fashion that avoids round-off error when X is large.

 -- Function: double lgamma (double X)
 -- Function: float lgammaf (float X)
 -- Function: long double lgammal (long double X)
     lgamma returns the natural logarithm of the absolute value of the gamma function of X.  The gamma function is defined as

          gamma (x) = integral from 0 to oo of t^(x-1) e^-t dt

     The sign of the gamma function is stored in the global variable SIGNGAM, which is declared in math.h.  It is 1 if the intermediate result was positive or zero, or -1 if it was negative.

     To compute the real gamma function, you can use the tgamma function or you can compute the values as follows:

          lgam = lgamma(x);
          gam  = signgam*exp(lgam);

     The gamma function has singularities at the nonpositive integers. lgamma will raise the zero divide exception if evaluated at a singularity.

 -- Function: double lgamma_r (double X, int *SIGNP)
 -- Function: float lgammaf_r (float X, int *SIGNP)
 -- Function: long double lgammal_r (long double X, int *SIGNP)
     lgamma_r is just like lgamma, but it stores the sign of the intermediate result in the variable pointed to by SIGNP instead of in the SIGNGAM global.  This means it is reentrant.

 -- Function: double gamma (double X)
 -- Function: float gammaf (float X)
 -- Function: long double gammal (long double X)
     These functions exist for compatibility reasons.  They are equivalent to lgamma, lgammaf and lgammal.  It is better to use lgamma for two reasons.  The name better reflects the actual computation, and lgamma is standardized in ISO C99 while gamma is not.

 -- Function: double tgamma (double X)
 -- Function: float tgammaf (float X)
 -- Function: long double tgammal (long double X)
     tgamma applies the gamma function to X.  The gamma function is defined as:

          gamma (x) = integral from 0 to oo of t^(x-1) e^-t dt

     This function was introduced in ISO C99.

 -- Function: double j0 (double X)
 -- Function: float j0f (float X)
 -- Function: long double j0l (long double X)
     j0 returns the Bessel function of the first kind of order 0 of X.  It may signal underflow if X is too large.

 -- Function: double j1 (double X)
 -- Function: float j1f (float X)
 -- Function: long double j1l (long double X)
     j1 returns the Bessel function of the first kind of order 1 of X.  It may signal underflow if X is too large.

 -- Function: double jn (int n, double X)
 -- Function: float jnf (int n, float X)
 -- Function: long double jnl (int n, long double X)
     jn returns the Bessel function of the first kind of order N of X.  It may signal underflow if X is too large.

 -- Function: double y0 (double X)
 -- Function: float y0f (float X)
 -- Function: long double y0l (long double X)
     y0 returns the Bessel function of the second kind of order 0 of X.  It may signal underflow if X is too large.  If X is negative, y0 signals a domain error; if it is zero, y0 signals overflow and returns -oo.

 -- Function: double y1 (double X)
 -- Function: float y1f (float X)
 -- Function: long double y1l (long double X)
     y1 returns the Bessel function of the second kind of order 1 of X.  It may signal underflow if X is too large.  If X is negative, y1 signals a domain error; if it is zero, y1 signals overflow and returns -oo.

 -- Function: double yn (int n, double X)
 -- Function: float ynf (int n, float X)
 -- Function: long double ynl (int n, long double X)
     yn returns the Bessel function of the second kind of order N of X.  It may signal underflow if X is too large.  If X is negative, yn signals a domain error; if it is zero, yn signals overflow and returns -oo.

Known Maximum Errors in Math Functions
======================================

This section lists the known errors of the functions in the math library.  Errors are measured in ``units of the last place''.  This is a measure for the relative error.  For a number z with the representation d.d...d*2^e (we assume IEEE floating-point numbers with base 2), the ULP is represented by:

     |d.d...d - (z / 2^e)| / 2^(p - 1)

where p is the number of bits in the mantissa of the floating-point number representation.  Ideally the error for all functions is always less than 0.5ulps.  Using rounding bits, this is also possible and normally implemented for the basic operations.  To achieve the same for the complex math functions requires a lot more work and this has not yet been done.

Therefore many of the functions in the math library have errors.  The table lists the maximum error for each function that is exposed by one of the existing tests in the test suite.  The table tries to cover as much as possible and list the actual maximum error (or at least a ballpark figure) but this is often not achieved due to the large search space.

The table lists the ULP values for different architectures.  Different architectures have different results since their hardware support for floating-point operations varies and also the existing hardware support is different.

Function    Alpha   ARM   Generic   hppa/fpu   ix86
acosf    -   -   -   -   -
acos    -   -   -   -   -
acosl    -   -   -   -   622
acoshf    -   -   -   -   -
acosh    -   -   -   -   -
acoshl    -   -   -   -   -
asinf    -   2   -   -   -
asin    -   1   -   -   -
asinl    -   -   -   -   1
asinhf    -   -   -   -   -
asinh    -   -   -   -   -
asinhl    -   -   -   -   -
atanf    -   -   -   -   -
atan    -   -   -   -   -
atanl    -   -   -   -   -
atanhf    1   -   -   1   -
atanh    -   1   -   -   -
atanhl    -   -   -   -   1
atan2f    3   -   -   3   -
atan2    -   -   -   -   -
atan2l    -   -   -   -   -
cabsf    -   1   -   -   -
cabs    -   1   -   -   -
cabsl    -   -   -   -   -
cacosf    -   1 + i 1   -   -   0 + i 1
cacos    -   1 + i 0   -   -   -
cacosl    -   -   -   -   0 + i 2
cacoshf    7 + i 3   7 + i 3   -   7 + i 3   9 + i 4
cacosh    1 + i 1   1 + i 1   -   1 + i 1   1 + i 1
cacoshl    -   -   -   -   6 + i 1
cargf    -   -   -   -   -
carg    -   -   -   -   -
cargl    -   -   -   -   -
casinf    1 + i 0   2 + i 1   -   1 + i 0   1 + i 1
casin    1 + i 0   3 + i 0   -   1 + i 0   1 + i 0
casinl    -   -   -   -   2 + i 2
casinhf    1 + i 6   1 + i 6   -   1 + i 6   1 + i 6
casinh    5 + i 3   5 + i 3   -   5 + i 3   5 + i 3
casinhl    -   -   -   -   5 + i 5
catanf    4 + i 1   4 + i 1   -   4 + i 1   0 + i 1
catan    0 + i 1   0 + i 1   -   0 + i 1   0 + i 1
catanl    -   -   -   -   -
catanhf    0 + i 6   1 + i 6   -   0 + i 6   1 + i 0
catanh    4 + i 0   4 + i 1   -   4 + i 0   2 + i 0
catanhl    -   -   -   -   1 + i 0
cbrtf    -   -   -   -   -
cbrt    1   1   -   1   -
cbrtl    -   -   -   -   1
ccosf    1 + i 1   0 + i 1   -   1 + i 1   0 + i 1
ccos    1 + i 0   1 + i 1   -   1 + i 0   1 + i 0
ccosl    -   -   -   -   1 + i 1
ccoshf    1 + i 1   1 + i 1   -   1 + i 1   1 + i 1
ccosh    1 + i 0   1 + i 1   -   1 + i 0   1 + i 1
ccoshl    -   -   -   -   0 + i 1
ceilf    -   -   -   -   -
ceil    -   -   -   -   -
ceill    -   -   -   -   -
cexpf    1 + i 1   1 + i 1   -   1 + i 1   -
cexp    -   1 + i 0   -   -   -
cexpl    -   -   -   -   1 + i 1
cimagf    -   -   -   -   -
cimag    -   -   -   -   -
cimagl    -   -   -   -   -
clogf    1 + i 3   0 + i 3   -   1 + i 3   1 + i 0
clog    -   0 + i 1   -   -   -
clogl    -   -   -   -   1 + i 0
clog10f    1 + i 5   1 + i 5   -   1 + i 5   1 + i 1
clog10    0 + i 1   1 + i 1   -   0 + i 1   1 + i 1
clog10l    -   -   -   -   1 + i 1
conjf    -   -   -   -   -
conj    -   -   -   -   -
conjl    -   -   -   -   -
copysignf    -   -   -   -   -
copysign    -   -   -   -   -
copysignl    -   -   -   -   -
cosf    1   1   -   1   1
cos    2   2   -   2   2
cosl    -   -   -   -   1
coshf    -   -   -   -   -
cosh    -   -   -   -   -
coshl    -   -   -   -   -
cpowf    4 + i 2   4 + i 2   -   4 + i 2   4 + i 3
cpow    2 + i 2   1 + i 1.1031   -   2 + i 2   1 + i 2
cpowl    -   -   -   -   763 + i 2
cprojf    -   -   -   -   -
cproj    -   -   -   -   -
cprojl    -   -   -   -   -
crealf    -   -   -   -   -
creal    -   -   -   -   -
creall    -   -   -   -   -
csinf    -   0 + i 1   -   -   1 + i 1
csin    -   -   -   -   -
csinl    -   -   -   -   1 + i 0
csinhf    1 + i 1   1 + i 1   -   1 + i 1   1 + i 1
csinh    0 + i 1   0 + i 1   -   0 + i 1   1 + i 1
csinhl    -   -   -   -   1 + i 2
csqrtf    1 + i 0   1 + i 1   -   1 + i 0   -
csqrt    -   1 + i 0   -   -   -
csqrtl    -   -   -   -   -
ctanf    -   1 + i 1   -   -   0 + i 1
ctan    1 + i 1   1 + i 1   -   1 + i 1   1 + i 1
ctanl    -   -   -   -   439 + i 3
ctanhf    2 + i 1   2 + i 1   -   2 + i 1   1 + i 1
ctanh    1 + i 0   2 + i 2   -   1 + i 0   0 + i 1
ctanhl    -   -   -   -   5 + i 25
erff    -   -   -   -   -
erf    1   -   -   1   1
erfl    -   -   -   -   -
erfcf    -   12   -   -   1
erfc    1   24   -   1   1
erfcl    -   -   -   -   1
expf    -   -   -   -   -
exp    -   -   -   -   -
expl    -   -   -   -   -
exp10f    2   2   -   2   -
exp10    6   6   -   6   -
exp10l    -   -   -   -   8
exp2f    -   -   -   -   -
exp2    -   -   -   -   -
exp2l    -   -   -   -   -
expm1f    1   1   -   1   -
expm1    1   -   -   1   -
expm1l    -   -   -   -   -
fabsf    -   -   -   -   -
fabs    -   -   -   -   -
fabsl    -   -   -   -   -
fdimf    -   -   -   -   -
fdim    -   -   -   -   -
fdiml    -   -   -   -   -
floorf    -   -   -   -   -
floor    -   -   -   -   -
floorl    -   -   -   -   -
fmaf    -   -   -   -   -
fma    -   -   -   -   -
fmal    -   -   -   -   -
fmaxf    -   -   -   -   -
fmax    -   -   -   -   -
fmaxl    -   -   -   -   -
fminf    -   -   -   -   -
fmin    -   -   -   -   -
fminl    -   -   -   -   -
fmodf    -   1   -   -   -
fmod    -   2   -   -   -
fmodl    -   -   -   -   -
frexpf    -   -   -   -   -
frexp    -   -   -   -   -
frexpl    -   -   -   -   -
gammaf    -   -   -   -   -
gamma    -   -   -   -   1
gammal    -   -   -   -   1
hypotf    1   1   -   1   1
hypot    -   1   -   -   -
hypotl    -   -   -   -   -
ilogbf    -   -   -   -   -
ilogb    -   -   -   -   -
ilogbl    -   -   -   -   -
j0f    2   2   -   2   1
j0    2   2   -   2   1
j0l    -   -   -   -   1
j1f    2   2   -   2   1
j1    1   1   -   1   1
j1l    -   -   -   -   1
jnf    4   4   -   4   2
jn    4   6   -   4   2
jnl    -   -   -   -   2
lgammaf    2   2   -   2   2
lgamma    1   1   -   1   1
lgammal    -   -   -   -   1
lrintf    -   -   -   -   -
lrint    -   -   -   -   -
lrintl    -   -   -   -   -
llrintf    -   -   -   -   -
llrint    -   -   -   -   -
llrintl    -   -   -   -   -
logf    -   1   -   -   1
log    -   1   -   -   -
logl    -   -   -   -   -
log10f    2   1   -   2   1
log10    1   1   -   1   -
log10l    -   -   -   -   1
log1pf    1   1   -   1   -
log1p    -   1   -   -   -
log1pl    -   -   -   -   -
log2f    -   1   -   -   -
log2    -   1   -   -   -
log2l    -   -   -   -   -
logbf    -   -   -   -   -
logb    -   -   -   -   -
logbl    -   -   -   -   -
lroundf    -   -   -   -   -
lround    -   -   -   -   -
lroundl    -   -   -   -   -
llroundf    -   -   -   -   -
llround    -   -   -   -   -
llroundl    -   -   -   -   -
modff    -   -   -   -   -
modf    -   -   -   -   -
modfl    -   -   -   -   -
nearbyintf    -   -   -   -   -
nearbyint    -   -   -   -   -
nearbyintl    -   -   -   -   -
nextafterf    -   -   -   -   -
nextafter    -   -   -   -   -
nextafterl    -   -   -   -   -
nexttowardf    -   -   -   -   -
nexttoward    -   -   -   -   -
nexttowardl    -   -   -   -   -
powf    -   -   -   -   -
pow    -   -   -   -   -
powl    -   -   -   -   -
remainderf    -   -   -   -   -
remainder    -   -   -   -   -
remainderl    -   -   -   -   -
remquof    -   -   -   -   -
remquo    -   -   -   -   -
remquol    -   -   -   -   -
rintf    -   -   -   -   -
rint    -   -   -   -   -
rintl    -   -   -   -   -
roundf    -   -   -   -   -
round    -   -   -   -   -
roundl    -   -   -   -   -
scalbf    -   -   -   -   -
scalb    -   -   -   -   -
scalbl    -   -   -   -   -
scalbnf    -   -   -   -   -
scalbn    -   -   -   -   -
scalbnl    -   -   -   -   -
scalblnf    -   -   -   -   -
scalbln    -   -   -   -   -
scalblnl    -   -   -   -   -
sinf    -   -   -   -   -
sin    -   -   -   -   -
sinl    -   -   -   -   -
sincosf    1   1   -   1   1
sincos    1   1   -   1   1
sincosl    -   -   -   -   1
sinhf    -   1   -   -   -
sinh    -   1   -   -   -
sinhl    -   -   -   -   -
sqrtf    -   -   -   -   -
sqrt    -   -   -   -   -
sqrtl    -   -   -   -   -
tanf    -   -   -   -   -
tan    1   0.5   -   1   1
tanl    -   -   -   -   -
tanhf    -   1   -   -   -
tanh    -   1   -   -   -
tanhl    -   -   -   -   -
tgammaf    1   1   -   1   1
tgamma    1   1   -   1   2
tgammal    -   -   -   -   1
truncf    -   -   -   -   -
trunc    -   -   -   -   -
truncl    -   -   -   -   -
y0f    1   1   -   1   1
y0    2   2   -   2   2
y0l    -   -   -   -   1
y1f    2   2   -   2   2
y1    3   3   -   3   2
y1l    -   -   -   -   1
ynf    2   2   -   2   3
yn    3   3   -   3   2
ynl    -   -   -   -   4

Function    IA64   M68k   MIPS   PowerPC   powerpc/nofpu
acosf    -   -   -   -   -
acos    -   -   -   -   -
acosl    -   -   -   -   -
acoshf    -   -   -   -   -
acosh    -   -   -   -   -
acoshl    -   1   -   -   -
asinf    -   -   -   -   -
asin    -   -   -   -   -
asinl    -   -   -   -   -
asinhf    -   -   -   -   -
asinh    -   -   -   -   -
asinhl    -   1   -   -   -
atanf    -   -   -   -   -
atan    -   -   -   -   -
atanl    -   -   -   -   -
atanhf    -   -   1   1   1
atanh    -   -   -   -   -
atanhl    -   1   -   -   -
atan2f    -   -   3   3   3
atan2    -   -   -   -   -
atan2l    -   1   -   -   -
cabsf    -   -   -   -   -
cabs    -   -   -   -   -
cabsl    -   -   -   -   -
cacosf    0 + i 1   2 + i 1   -   -   -
cacos    -   -   -   -   -
cacosl    0 + i 2   1 + i 2   -   -   -
cacoshf    7 + i 0   7 + i 1   7 + i 3   7 + i 3   7 + i 3
cacosh    1 + i 1   1 + i 1   1 + i 1   1 + i 1   1 + i 1
cacoshl    7 + i 1   6 + i 2   -   -   -
cargf    -   -   -   -   -
carg    -   -   -   -   -
cargl    -   -   -   -   -
casinf    1 + i 1   5 + i 1   1 + i 0   1 + i 0   1 + i 0
casin    1 + i 0   1 + i 0   1 + i 0   1 + i 0   1 + i 0
casinl    2 + i 2   3 + i 2   -   -   -
casinhf    1 + i 6   19 + i 1   1 + i 6   1 + i 6   1 + i 6
casinh    5 + i 3   6 + i 13   5 + i 3   5 + i 3   5 + i 3
casinhl    5 + i 5   5 + i 6   -   -   -
catanf    0 + i 1   0 + i 1   4 + i 1   4 + i 1   4 + i 1
catan    0 + i 1   0 + i 1   0 + i 1   0 + i 1   0 + i 1
catanl    -   1 + i 0   -   -   -
catanhf    -   -   0 + i 6   0 + i 6   0 + i 6
catanh    4 + i 0   -   4 + i 0   4 + i 0   4 + i 0
catanhl    1 + i 0   1 + i 0   -   -   -
cbrtf    -   -   -   -   -
cbrt    -   -   1   1   1
cbrtl    -   1   -   -   -
ccosf    0 + i 1   1 + i 1   1 + i 1   1 + i 1   1 + i 1
ccos    1 + i 0   -   1 + i 0   1 + i 0   1 + i 0
ccosl    1 + i 1   1 + i 1   -   -   -
ccoshf    1 + i 1   1 + i 1   1 + i 1   1 + i 1   1 + i 1
ccosh    1 + i 1   -   1 + i 0   1 + i 0   1 + i 0
ccoshl    0 + i 1   0 + i 1   -   -   -
ceilf    -   -   -   -   -
ceil    -   -   -   -   -
ceill    -   -   -   -   -
cexpf    1 + i 1   2 + i 1   1 + i 1   1 + i 1   1 + i 1
cexp    -   -   -   -   -
cexpl    0 + i 1   0 + i 1   -   -   -
cimagf    -   -   -   -   -
cimag    -   -   -   -   -
cimagl    -   -   -   -   -
clogf    1 + i 0   1 + i 0   1 + i 3   1 + i 3   1 + i 3
clog    -   -   -   -   -
clogl    1 + i 0   1 + i 1   -   -   -
clog10f    1 + i 1   1 + i 1   1 + i 5   1 + i 5   1 + i 5
clog10    1 + i 1   1 + i 1   0 + i 1   0 + i 1   0 + i 1
clog10l    1 + i 1   1 + i 2   -   -   -
conjf    -   -   -   -   -
conj    -   -   -   -   -
conjl    -   -   -   -   -
copysignf    -   -   -   -   -
copysign    -   -   -   -   -
copysignl    -   -   -   -   -
cosf    1   1   1   1   1
cos    2   2   2   2   2
cosl    1   1   -   -   -
coshf    -   -   -   -   -
cosh    -   -   -   -   -
coshl    -   -   -   -   -
cpowf    5 + i 3   2 + i 6   4 + i 2   4 + i 2   4 + i 2
cpow    2 + i 2   1 + i 2   2 + i 2   2 + i 2   2 + i 2
cpowl    3 + i 4   15 + i 2   -   -   -
cprojf    -   -   -   -   -
cproj    -   -   -   -   -
cprojl    -   -   -   -   -
crealf    -   -   -   -   -
creal    -   -   -   -   -
creall    -   -   -   -   -
csinf    1 + i 1   1 + i 1   -   -   -
csin    -   -   -   -   -
csinl    1 + i 0   1 + i 0   -   -   -
csinhf    1 + i 1   1 + i 1   1 + i 1   1 + i 1   1 + i 1
csinh    1 + i 1   -   0 + i 1   0 + i 1   0 + i 1
csinhl    1 + i 2   1 + i 0   -   -   -
csqrtf    1 + i 0   -   1 + i 0   1 + i 0   1 + i 0
csqrt    -   -   -   -   -
csqrtl    -   -   -   -   -
ctanf    0 + i 1   -   -   -   -
ctan    1 + i 1   1 + i 0   1 + i 1   1 + i 1   1 + i 1
ctanl    2 + i 1   1 + i 2   -   -   -
ctanhf    0 + i 1   0 + i 1   2 + i 1   2 + i 1   2 + i 1
ctanh    1 + i 1   0 + i 1   1 + i 0   1 + i 0   1 + i 0
ctanhl    1 + i 24   0 + i 1   -   -   -
erff    -   -   -   -   -
erf    1   -   1   1   1
erfl    -   -   -   -   -
erfcf    1   1   -   1   -
erfc    1   -   1   1   1
erfcl    1   1   -   -   -
expf    -   -   -   -   -
exp    -   -   -   -   -
expl    -   -   -   -   -
exp10f    2   -   2   2   2
exp10    6   -   6   6   6
exp10l    3   -   -   -   -
exp2f    -   -   -   -   -
exp2    -   -   -   -   -
exp2l    -   -   -   -   -
expm1f    -   -   1   1   1
expm1    -   -   1   1   1
expm1l    1   1   -   -   -
fabsf    -   -   -   -   -
fabs    -   -   -   -   -
fabsl    -   -   -   -   -
fdimf    -   -   -   -   -
fdim    -   -   -   -   -
fdiml    -   -   -   -   -
floorf    -   -   -   -   -
floor    -   -   -   -   -
floorl    -   -   -   -   -
fmaf    -   -   -   -   -
fma    -   -   -   -   -
fmal    -   -   -   -   -
fmaxf    -   -   -   -   -
fmax    -   -   -   -   -
fmaxl    -   -   -   -   -
fminf    -   -   -   -   -
fmin    -   -   -   -   -
fminl    -   -   -   -   -
fmodf    -   -   -   -   -
fmod    -   -   -   -   -
fmodl    -   -   -   -   -
frexpf    -   -   -   -   -
frexp    -   -   -   -   -
frexpl    -   -   -   -   -
gammaf    -   -   -   -   -
gamma    -   -   -   -   -
gammal    1   1   -   -   -
hypotf    1   1   1   1   1
hypot    -   -   -   -   -
hypotl    -   -   -   -   -
ilogbf    -   -   -   -   -
ilogb    -   -   -   -   -
ilogbl    -   -   -   -   -
j0f    1   1   2   1   2
j0    2   1   2   2   2
j0l    2   1   -   -   -
j1f    2   2   2   2   2
j1    1   -   1   1   1
j1l    1   1   -   -   -
jnf    3   5   4   3   4
jn    3   1   4   3   4
jnl    2   2   -   -   -
lgammaf    2   2   2   2   2
lgamma    1   1   1   1   1
lgammal    1   1   -   -   -
lrintf    -   -   -   -   -
lrint    -   -   -   -   -
lrintl    -   -   -   -   -
llrintf    -   -   -   -   -
llrint    -   -   -   -   -
llrintl    -   -   -   -   -
logf    1   1   -   -   -
log    -   -   -   -   -
logl    -   1   -   -   -
log10f    1   1   2   2   2
log10    -   -   1   1   1
log10l    1   2   -   -   -
log1pf    -   -   1   1   1
log1p    -   -   -   -   -
log1pl    -   1   -   -   -
log2f    -   -   -   -   -
log2    -   -   -   -   -
log2l    -   1   -   -   -
logbf    -   -   -   -   -
logb    -   -   -   -   -
logbl    -   -   -   -   -
lroundf    -   -   -   -   -
lround    -   -   -   -   -
lroundl    -   -   -   -   -
llroundf    -   -   -   -   -
llround    -   -   -   -   -
llroundl    -   -   -   -   -
modff    -   -   -   -   -
modf    -   -   -   -   -
modfl    -   -   -   -   -
nearbyintf    -   -   -   -   -
nearbyint    -   -   -   -   -
nearbyintl    -   -   -   -   -
nextafterf    -   -   -   -   -
nextafter    -   -   -   -   -
nextafterl    -   -   -   -   -
nexttowardf    -   -   -   -   -
nexttoward    -   -   -   -   -
nexttowardl    -   -   -   -   -
powf    -   -   -   -   -
pow    -   -   -   -   -
powl    -   1   -   -   -
remainderf    -   -   -   -   -
remainder    -   -   -   -   -
remainderl    -   -   -   -   -
remquof    -   -   -   -   -
remquo    -   -   -   -   -
remquol    -   -   -   -   -
rintf    -   -   -   -   -
rint    -   -   -   -   -
rintl    -   -   -   -   -
roundf    -   -   -   -   -
round    -   -   -   -   -
roundl    -   -   -   -   -
scalbf    -   -   -   -   -
scalb    -   -   -   -   -
scalbl    -   -   -   -   -
scalbnf    -   -   -   -   -
scalbn    -   -   -   -   -
scalbnl    -   -   -   -   -
scalblnf    -   -   -   -   -
scalbln    -   -   -   -   -
scalblnl    -   -   -   -   -
sinf    -   -   -   -   -
sin    -   -   -   -   -
sinl    -   -   -   -   -
sincosf    1   1   1   1   1
sincos    1   1   1   1   1
sincosl    1   1   -   -   -
sinhf    -   -   -   -   -
sinh    -   -   -   -   -
sinhl    -   1   -   -   -
sqrtf    -   -   -   -   -
sqrt    -   -   -   -   -
sqrtl    -   -   -   -   -
tanf    -   -   -   -   -
tan    1   1   1   1   1
tanl    -   1   -   -   -
tanhf    -   -   -   -   -
tanh    -   -   -   -   -
tanhl    -   -   -   -   -
tgammaf    1   1   1   1   1
tgamma    1   1   1   1   1
tgammal    1   1   -   -   -
truncf    -   -   -   -   -
trunc    -   -   -   -   -
truncl    -   -   -   -   -
y0f    1   1   1   1   1
y0    2   1   2   2   2
y0l    1   2   -   -   -
y1f    2   2   2   2   2
y1    3   1   3   3   3
y1l    1   1   -   -   -
ynf    2   2   2   2   2
yn    3   1   3   3   3
ynl    2   4   -   -   -

Function    S/390   SH4   Sparc 32-bit   Sparc 64-bit   x86_64/fpu
acosf    -   -   -   -   -
acos    -   -   -   -   -
acosl    -   -   -   -   -
acoshf    -   -   -   -   -
acosh    -   -   -   -   -
acoshl    -   -   -   -   -
asinf    -   2   -   -   -
asin    -   1   -   -   -
asinl    -   -   -   -   1
asinhf    -   -   -   -   -
asinh    -   -   -   -   -
asinhl    -   -   -   -   -
atanf    -   -   -   -   -
atan    -   -   -   -   -
atanl    -   -   -   -   -
atanhf    1   -   1   1   1
atanh    -   1   -   -   -
atanhl    -   -   -   -   1
atan2f    3   4   3   3   3
atan2    -   -   -   -   -
atan2l    -   -   -   1   -
cabsf    -   1   -   -   -
cabs    -   1   -   -   -
cabsl    -   -   -   -   -
cacosf    -   1 + i 1   -   -   0 + i 1
cacos    -   1 + i 0   -   -   -
cacosl    -   -   -   0 + i 1   0 + i 2
cacoshf    7 + i 3   7 + i 3   7 + i 3   7 + i 3   7 + i 3
cacosh    1 + i 1   1 + i 1   1 + i 1   1 + i 1   1 + i 1
cacoshl    -   -   -   5 + i 1   6 + i 1
cargf    -   -   -   -   -
carg    -   -   -   -   -
cargl    -   -   -   -   -
casinf    1 + i 0   2 + i 1   1 + i 0   1 + i 0   1 + i 1
casin    1 + i 0   3 + i 0   1 + i 0   1 + i 0   1 + i 0
casinl    -   -   -   0 + i 1   2 + i 2
casinhf    1 + i 6   1 + i 6   1 + i 6   1 + i 6   1 + i 6
casinh    5 + i 3   5 + i 3   5 + i 3   5 + i 3   5 + i 3
casinhl    -   -   -   4 + i 2   5 + i 5
catanf    4 + i 1   4 + i 1   4 + i 1   4 + i 1   4 + i 1
catan    0 + i 1   0 + i 1   0 + i 1   0 + i 1   0 + i 1
catanl    -   -   -   0 + i 1   -
catanhf    0 + i 6   1 + i 6   0 + i 6   0 + i 6   0 + i 6
catanh    4 + i 0   4 + i 1   4 + i 0   4 + i 0   4 + i 0
catanhl    -   -   -   1 + i 1   1 + i 0
cbrtf    -   -   -   -   -
cbrt    1   1   1   1   1
cbrtl    -   -   -   1   1
ccosf    1 + i 1   0 + i 1   1 + i 1   1 + i 1   1 + i 1
ccos    1 + i 0   1 + i 1   1 + i 0   1 + i 0   1 + i 0
ccosl    -   -   -   -   1 + i 1
ccoshf    1 + i 1   1 + i 1   1 + i 1   1 + i 1   1 + i 1
ccosh    1 + i 0   1 + i 1   1 + i 0   1 + i 0   1 + i 1
ccoshl    -   -   -   -   0 + i 1
ceilf    -   -   -   -   -
ceil    -   -   -   -   -
ceill    -   -   -   -   -
cexpf    1 + i 1   1 + i 1   1 + i 1   1 + i 1   1 + i 1
cexp    -   1 + i 0   -   -   -
cexpl    -   -   -   1 + i 1   0 + i 1
cimagf    -   -   -   -   -
cimag    -   -   -   -   -
cimagl    -   -   -   -   -
clogf    1 + i 3   0 + i 3   1 + i 3   1 + i 3   1 + i 3
clog    -   0 + i 1   -   -   -
clogl    -   -   -   1 + i 0   1 + i 0
clog10f    1 + i 5   1 + i 5   1 + i 5   1 + i 5   1 + i 5
clog10    0 + i 1   1 + i 1   0 + i 1   0 + i 1   1 + i 1
clog10l    -   -   -   0 + i 1   1 + i 1
conjf    -   -   -   -   -
conj    -   -   -   -   -
conjl    -   -   -   -   -
copysignf    -   -   -   -   -
copysign    -   -   -   -   -
copysignl    -   -   -   -   -
cosf    1   1   1   1   1
cos    2   2   2   2   2
cosl    -   -   -   1   1
coshf    -   -   -   -   -
cosh    -   -   -   -   -
coshl    -   -   -   -   -
cpowf    4 + i 2   4 + i 2   4 + i 2   4 + i 2   5 + i 2
cpow    2 + i 2   1 + i 1.1031   2 + i 2   2 + i 2   2 + i 2
cpowl    -   -   -   1 + i 1   5 + i 2
cprojf    -   -   -   -   -
cproj    -   -   -   -   -
cprojl    -   -   -   -   -
crealf    -   -   -   -   -
creal    -   -   -   -   -
creall    -   -   -   -   -
csinf    -   0 + i 1   -   -   0 + i 1
csin    -   -   -   -   -
csinl    -   -   -   1 + i 0   1 + i 0
csinhf    1 + i 1   1 + i 1   1 + i 1   1 + i 1   1 + i 1
csinh    0 + i 1   0 + i 1   0 + i 1   0 + i 1   1 + i 1
csinhl    -   -   -   -   1 + i 2
csqrtf    1 + i 0   1 + i 1   1 + i 0   1 + i 0   1 + i 0
csqrt    -   1 + i 0   -   -   -
csqrtl    -   -   -   1 + i 1   -
ctanf    -   1 + i 1   -   -   0 + i 1
ctan    1 + i 1   1 + i 1   1 + i 1   1 + i 1   1 + i 1
ctanl    -   -   -   0 + i 2   439 + i 3
ctanhf    2 + i 1   2 + i 1   2 + i 1   2 + i 1   2 + i 1
ctanh    1 + i 0   2 + i 2   1 + i 0   1 + i 0   1 + i 1
ctanhl    -   -   -   -   5 + i 25
erff    -   -   -   -   -
erf    1   -   1   1   1
erfl    -   -   -   -   -
erfcf    -   12   -   -   -
erfc    1   24   1   1   1
erfcl    -   -   -   1   1
expf    -   -   -   -   -
exp    -   -   -   -   -
expl    -   -   -   -   -
exp10f    2   2   2   2   2
exp10    6   6   6   6   6
exp10l    -   -   -   1   3
exp2f    -   -   -   -   -
exp2    -   -   -   -   -
exp2l    -   -   -   -   -
expm1f    1   1   1   1   1
expm1    1   -   1   1   1
expm1l    -   -   -   1   -
fabsf    -   -   -   -   -
fabs    -   -   -   -   -
fabsl    -   -   -   -   -
fdimf    -   -   -   -   -
fdim    -   -   -   -   -
fdiml    -   -   -   -   -
floorf    -   -   -   -   -
floor    -   -   -   -   -
floorl    -   -   -   -   -
fmaf    -   -   -   -   -
fma    -   -   -   -   -
fmal    -   -   -   -   -
fmaxf    -   -   -   -   -
fmax    -   -   -   -   -
fmaxl    -   -   -   -   -
fminf    -   -   -   -   -
fmin    -   -   -   -   -
fminl    -   -   -   -   -
fmodf    -   1   -   -   -
fmod    -   2   -   -   -
fmodl    -   -   -   -   -
frexpf    -   -   -   -   -
frexp    -   -   -   -   -
frexpl    -   -   -   -   -
gammaf    -   -   -   -   -
gamma    -   -   -   -   -
gammal    -   -   -   1   1
hypotf    1   1   1   1   1
hypot    -   1   -   -   -
hypotl    -   -   -   -   -
ilogbf    -   -   -   -   -
ilogb    -   -   -   -   -
ilogbl    -   -   -   -   -
j0f    2   2   2   2   2
j0    2   2   2   2   2
j0l    -   -   -   2   1
j1f    2   2   2   2   2
j1    1   1   1   1   1
j1l    -   -   -   4   1
jnf    4   4   4   4   4
jn    4   6   4   4   4
jnl    -   -   -   4   2
lgammaf    2   2   2   2   2
lgamma    1   1   1   1   1
lgammal    -   -   -   1   1
lrintf    -   -   -   -   -
lrint    -   -   -   -   -
lrintl    -   -   -   -   -
llrintf    -   -   -   -   -
llrint    -   -   -   -   -
llrintl    -   -   -   -   -
logf    -   1   -   -   -
log    -   1   -   -   -
logl    -   -   -   -   -
log10f    2   1   2   2   2
log10    1   1   1   1   1
log10l    -   -   -   1   1
log1pf    1   1   1   1   1
log1p    -   1   -   -   -
log1pl    -   -   -   1   -
log2f    -   1   -   -   -
log2    -   1   -   -   -
log2l    -   -   -   1   -
logbf    -   -   -   -   -
logb    -   -   -   -   -
logbl    -   -   -   -   -
lroundf    -   -   -   -   -
lround    -   -   -   -   -
lroundl    -   -   -   -   -
llroundf    -   -   -   -   -
llround    -   -   -   -   -
llroundl    -   -   -   -   -
modff    -   -   -   -   -
modf    -   -   -   -   -
modfl    -   -   -   -   -
nearbyintf    -   -   -   -   -
nearbyint    -   -   -   -   -
nearbyintl    -   -   -   -   -
nextafterf    -   -   -   -   -
nextafter    -   -   -   -   -
nextafterl    -   -   -   -   -
nexttowardf    -   -   -   -   -
nexttoward    -   -   -   -   -
nexttowardl    -   -   -   -   -
powf    -   -   -   -   -
pow    -   -   -   -   -
powl    -   -   -   -   -
remainderf    -   -   -   -   -
remainder    -   -   -   -   -
remainderl    -   -   -   -   -
remquof    -   -   -   -   -
remquo    -   -   -   -   -
remquol    -   -   -   -   -
rintf    -   -   -   -   -
rint    -   -   -   -   -
rintl    -   -   -   -   -
roundf    -   -   -   -   -
round    -   -   -   -   -
roundl    -   -   -   -   -
scalbf    -   -   -   -   -
scalb    -   -   -   -   -
scalbl    -   -   -   -   -
scalbnf    -   -   -   -   -
scalbn    -   -   -   -   -
scalbnl    -   -   -   -   -
scalblnf    -   -   -   -   -
scalbln    -   -   -   -   -
scalblnl    -   -   -   -   -
sinf    -   -   -   -   -
sin    -   -   -   -   -
sinl    -   -   -   -   -
sincosf    1   1   1   1   1
sincos    1   1   1   1   1
sincosl    -   -   -   1   1
sinhf    -   1   -   -   -
sinh    -   1   -   -   -
sinhl    -   -   -   -   -
sqrtf    -   -   -   -   -
sqrt    -   -   -   -   -
sqrtl    -   -   -   1   -
tanf    -   -   -   -   -
tan    1   0.5   1   1   1
tanl    -   -   -   -   -
tanhf    -   1   -   -   -
tanh    -   1   -   -   -
tanhl    -   -   -   1   -
tgammaf    1   1   1   1   1
tgamma    1   1   1   1   1
tgammal    -   -   -   1   1
truncf    -   -   -   -   -
trunc    -   -   -   -   -
truncl    -   -   -   -   -
y0f    1   1   1   1   1
y0    2   2   2   2   2
y0l    -   -   -   3   1
y1f    2   2   2   2   2
y1    3   3   3   3   3
y1l    -   -   -   1   1
ynf    2   2   2   2   2
yn    3   3   3   3   3
ynl    -   -   -   5   4

Pseudorandom Numbers
====================

This section describes the GNU facilities for generating a series of pseudorandom numbers.  The numbers generated are not truly random; typically, they form a sequence that repeats periodically, with a period so large that you can ignore it for ordinary purposes.  The random-number generator works by remembering a seed value, which it uses to compute the next random number and also to compute a new seed.

Although the generated numbers look unpredictable within one run of a program, the sequence of numbers is exactly the same from one run to the next.  This is because the initial seed is always the same.  This is convenient when you are debugging a program, but it is unhelpful if you want the program to behave unpredictably.  If you want a different pseudorandom series each time your program runs, you must specify a different seed each time.  For ordinary purposes, basing the seed on the current time works well.

You can obtain repeatable sequences of numbers on a particular machine type by specifying the same initial seed value for the random-number generator.  There is no standard meaning for a particular seed value; the same seed, used in different C libraries or on different CPU types, will give you different random numbers.

The GNU library supports the standard ISO C random-number functions plus two other sets derived from BSD and SVID.  The BSD and ISO C functions provide identical, somewhat limited functionality.  If only a small number of random bits are required, we recommend you use the ISO C interface, rand and srand.  The SVID functions provide a more flexible interface, which allows better random-number generator algorithms, provides more random bits (up to 48) per call, and can provide random floating-point numbers.  These functions are required by the XPG standard and therefore will be present in all modern Unix systems.

ISO C Random-Number Functions
-----------------------------

This section describes the random-number functions that are part of the ISO C standard.

To use these facilities, you should include the header file stdlib.h in your program.

 -- Macro: int RAND_MAX
     The value of this macro is an integer constant representing the largest value the rand function can return.  In the GNU library, it is 2147483647, which is the largest signed integer representable in 32 bits.  In other libraries, it may be as low as 32767.

 -- Function: int rand (void)
     The rand function returns the next pseudorandom number in the series.  The value ranges from 0 to RAND_MAX.

 -- Function: void srand (unsigned int SEED)
     This function establishes SEED as the seed for a new series of pseudorandom numbers.  If you call rand before a seed has been established with srand, it uses the value 1 as a default seed.

     To produce a different pseudorandom series each time your program is run, do srand (time (0)).

POSIX.1 extended the C standard functions to support reproducible random numbers in multithreaded programs.  However, the extension is badly designed and unsuitable for serious work.

 -- Function: int rand_r (unsigned int *SEED)
     This function returns a random number in the range 0 to RAND_MAX just as rand does.  However, all its state is stored in the SEED argument.  This means the RNG's state can only have as many bits as the type unsigned int has.  This is far too few to provide a good RNG.

     If your program requires a reentrant RNG, we recommend you use the reentrant GNU extensions to the SVID random-number generator.  The POSIX.1 interface should only be used when the GNU extensions are not available.

BSD Random-Number Functions
---------------------------

This section describes a set of random-number generation functions that are derived from BSD.  There is no advantage to using these functions with the GNU C Library; we support them for BSD compatibility only.

The prototypes for these functions are in stdlib.h.

 -- Function: long int random (void)
     This function returns the next pseudorandom number in the sequence. The value returned ranges from 0 to RAND_MAX.

     Temporarily, this function was defined to return a int32_t value to indicate that the return value always contains 32 bits even if long int is wider.  The standard demands it differently. Users must always be aware of the 32-bit limitation, though.

 -- Function: void srandom (unsigned int SEED)
     The srandom function sets the state of the random-number generator based on the integer SEED.  If you supply a SEED value of 1, this will cause random to reproduce the default set of random numbers.

     To produce a different set of pseudorandom numbers each time your program runs, do srandom (time (0)).

 -- Function: void * initstate (unsigned int SEED, void *STATE, size_t SIZE)
     The initstate function is used to initialize the random-number generator state.  The argument STATE is an array of SIZE bytes, used to hold the state information.  It is initialized based on SEED.  The size must be between 8 and 256 bytes, and should be a power of two.  The bigger the STATE array, the better.

     The return value is the previous value of the state information array. You can use this value later as an argument to setstate to restore that state.

 -- Function: void * setstate (void *STATE)
     The setstate function restores the random-number state information STATE.  The argument must have been the result of a previous call to INITSTATE or SETSTATE.

     The return value is the previous value of the state information array. You can use this value later as an argument to setstate to restore that state.

     If the function fails, the return value is NULL.

The four functions described so far in this section all work on a state that is shared by all threads.  The state is not directly accessible to the user and can only be modified by these functions. This makes it hard to deal with situations where each thread should have its own pseudorandom-number generator.

The GNU C Library contains four additional functions that contain the state as an explicit parameter and therefore make it possible to handle thread-local PRNGs.  Besides this, there are no differences.  In fact, the four functions already discussed are implemented internally using the following interfaces.

The stdlib.h header contains a definition of the following type:

 -- Data Type: struct random_data

     Objects of type struct random_data contain the information necessary to represent the state of the PRNG.  Although a complete definition of the type is present, the type should be treated as opaque.

The functions modifying the state follow exactly the already described functions.

 -- Function: int random_r (struct random_data *restrict BUF, int32_t *restrict RESULT)
     The random_r function behaves exactly like the random function, except that it uses and modifies the state in the object pointed to by the first parameter instead of the global state.

 -- Function: int srandom_r (unsigned int SEED, struct random_data *BUF)
     The srandom_r function behaves exactly like the srandom function, except that it uses and modifies the state in the object pointed to by the second parameter instead of the global state.

 -- Function: int initstate_r (unsigned int SEED, char *restrict STATEBUF, size_t STATELEN, struct random_data *restrict BUF)
     The initstate_r function behaves exactly like the initstate function, except that it uses and modifies the state in the object pointed to by the fourth parameter instead of the global state.

 -- Function: int setstate_r (char *restrict STATEBUF, struct random_data *restrict BUF)
     The setstate_r function behaves exactly like the setstate function, except that it uses and modifies the state in the object pointed to by the first parameter instead of the global state.

SVID Random-Number Functions
----------------------------

The C library on SVID systems contains yet another kind of random-number generator functions.  They use a state of 48 bits of data.  The user can choose among a collection of functions that return the random bits in different forms.

Generally, there are two kinds of function.  The first uses a state of the random-number generator that is shared among several functions and by all threads of the process.  The second requires the user to handle the state.

All functions have in common that they use the same congruential formula with the same constants.  The formula is

     Y = (a * X + c) mod m

where X is the state of the generator at the beginning and Y the state at the end.  a and c are constants determining the way the generator works.  By default they are

     a = 0x5DEECE66D = 25214903917
     c = 0xb = 11

but they can also be changed by the user.  m is of course 2^48 since the state consists of a 48-bit array.

The prototypes for these functions are in stdlib.h.

 -- Function: double drand48 (void)
     This function returns a double value in the range of 0.0 to 1.0 (exclusive).  The random bits are determined by the global state of the random-number generator in the C library.

     Since the double type according to IEEE 754 has a 52-bit mantissa, 4 bits are not initialized by the random-number generator. These are chosen to be the least significant bits and are initialized to 0.

 -- Function: double erand48 (unsigned short int XSUBI[3])
     This function returns a double value in the range of 0.0 to 1.0 (exclusive), similarly to drand48.  The argument is an array describing the state of the random-number generator.

     This function can be called subsequently since it updates the array to guarantee random numbers.  The array should have been initialized before initial use to obtain reproducible results.

 -- Function: long int lrand48 (void)
     The lrand48 function returns an integer value in the range of 0 to 2^31 (exclusive).  Even if the size of the long int type can take more than 32 bits, no higher numbers are returned. The random bits are determined by the global state of the random-number generator in the C library.

 -- Function: long int nrand48 (unsigned short int XSUBI[3])
     This function is similar to the lrand48 function in that it returns a number in the range of 0 to 2^31 (exclusive), but the state of the random-number generator used to produce the random bits is determined by the array provided as the parameter to the function.

     The numbers in the array are updated afterwards so that subsequent calls to this function yield different results (as is expected of a random-number generator).  The array should have been initialized before the first call to obtain reproducible results.

 -- Function: long int mrand48 (void)
     The mrand48 function is similar to lrand48.  The only difference is that the numbers returned are in the range -2^31 to 2^31 (exclusive).

 -- Function: long int jrand48 (unsigned short int XSUBI[3])
     The jrand48 function is similar to nrand48.  The only difference is that the numbers returned are in the range -2^31 to 2^31 (exclusive).  For the xsubi parameter the same requirements are necessary.

The internal state of the random-number generator can be initialized in several ways.  The methods differ in the completeness of the information provided.

 -- Function: void srand48 (long int SEEDVAL)
     The srand48 function sets the most significant 32 bits of the internal state of the random-number generator to the least significant 32 bits of the SEEDVAL parameter.  The lower 16 bits are initialized to the value 0x330E.  Even if the long int type contains more than 32 bits, only the lower 32 bits are used.

     Owing to this limitation, initialization of the state of this function is not very useful.  But it makes it easy to use a construct like srand48 (time (0)).

     A side effect of this function is that the values a and c from the internal state, which are used in the congruential formula, are reset to the default values given above.  This is of importance once the user has called the lcong48 function (see below).

 -- Function: unsigned short int * seed48 (unsigned short int SEED16V[3])
     The seed48 function initializes all 48 bits of the state of the internal random-number generator from the contents of the parameter SEED16V.  Here the lower 16 bits of the first element of SEE16V initialize the least significant 16 bits of the internal state, the lower 16 bits of SEED16V[1] initialize the mid-order 16 bits of the state and the 16 lower bits of SEED16V[2] initialize the most significant 16 bits of the state.

     Unlike srand48 this function lets the user initialize all 48 bits of the state.

     The value returned by seed48 is a pointer to an array containing the values of the internal state before the change.  This might be useful to restart the random-number generator at a certain state. Otherwise the value can simply be ignored.

     As for srand48, the values a and c from the congruential formula are reset to the default values.

There is one more function to initialize the random-number generator that enables you to specify even more information by allowing you to change the parameters in the congruential formula.

 -- Function: void lcong48 (unsigned short int PARAM[7])
     The lcong48 function allows the user to change the complete state of the random-number generator.  Unlike srand48 and seed48, this function also changes the constants in the congruential formula.

     From the seven elements in the array PARAM the least significant 16 bits of the entries PARAM[0] to PARAM[2] determine the initial state, the least significant 16 bits of PARAM[3] to PARAM[5] determine the 48-bit constant a and PARAM[6] determines the 16-bit value c.

All the above functions have in common that they use the global parameters for the congruential formula.  In multithreaded programs, it might sometimes be useful to have different parameters in different threads.  For this reason, all the above functions have a counterpart that works on a description of the random-number generator in the user-supplied buffer instead of the global state.

It is no problem if several threads use the global state if all threads use the functions that take a pointer to an array containing the state.  The random numbers are computed following the same loop, but if the state in the array is different, all threads will obtain an individual random-number generator.

The user-supplied buffer must be of type struct drand48_data. This type should be regarded as opaque and not manipulated directly.

 -- Function: int drand48_r (struct drand48_data *BUFFER, double *RESULT)
     This function is equivalent to the drand48 function with the difference that it does not modify the global random-number generator parameters but instead the parameters in the buffer supplied through the pointer BUFFER.  The random number is returned in the variable pointed to by RESULT.

     The return value of the function indicates whether the call succeeded. If the value is less than 0 an error occurred and ERRNO is set to indicate the problem.

     This function is a GNU extension and should not be used in portable programs.

 -- Function: int erand48_r (unsigned short int XSUBI[3], struct drand48_data *BUFFER, double *RESULT)
     The erand48_r function works like erand48, but in addition it takes an argument BUFFER that describes the random-number generator.  The state of the random-number generator is taken from the xsubi array, the parameters for the congruential formula from the global random-number generator data.  The random number is returned in the variable pointed to by RESULT.

     The return value is nonnegative if the call succeeded.

     This function is a GNU extension and should not be used in portable programs.

 -- Function: int lrand48_r (struct drand48_data *BUFFER, double *RESULT)
     This function is similar to lrand48, but in addition it takes a pointer to a buffer describing the state of the random-number generator just like drand48.

     If the return value of the function is nonnegative, the variable pointed to by RESULT contains the result.  Otherwise an error occurred.

     This function is a GNU extension and should not be used in portable programs.

 -- Function: int nrand48_r (unsigned short int XSUBI[3], struct drand48_data *BUFFER, long int *RESULT)
     The nrand48_r function works like nrand48 in that it produces a random number in the range 0 to 2^31.  But instead of using the global parameters for the congruential formula, it uses the information from the buffer pointed to by BUFFER. The state is described by the values in XSUBI.

     If the return value is nonnegative, the variable pointed to by RESULT contains the result.

     This function is a GNU extension and should not be used in portable programs.

 -- Function: int mrand48_r (struct drand48_data *BUFFER, double *RESULT)
     This function is similar to mrand48, but like the other reentrant functions it uses the random-number generator described by the value in the buffer pointed to by BUFFER.

     If the return value is nonnegative, the variable pointed to by RESULT contains the result.

     This function is a GNU extension and should not be used in portable programs.

 -- Function: int jrand48_r (unsigned short int XSUBI[3], struct drand48_data *BUFFER, long int *RESULT)
     The jrand48_r function is similar to jrand48.  Like the other reentrant functions of this function family, it uses the congruential formula parameters from the buffer pointed to by BUFFER.

     If the return value is nonnegative, the variable pointed to by RESULT contains the result.

     This function is a GNU extension and should not be used in portable programs.

Before any of the above functions are used, the buffer of type struct drand48_data should be initialized.  The easiest way to do this is to fill the whole buffer with null bytes, e.g.:

     memset (buffer, '\0', sizeof (struct drand48_data));

Using any of the reentrant functions of this family now will automatically initialize the random-number generator to the default values for the state and the parameters of the congruential formula.

The other possibility is to use any of the functions that explicitly initialize the buffer.  Though it might be obvious how to initialize the buffer from looking at the parameter to the function, it is highly recommended to use these functions since the result might not always be what you expect.

 -- Function: int srand48_r (long int SEEDVAL, struct drand48_data *BUFFER)
     The description of the random-number generator represented by the information in BUFFER is initialized in a similar way to that of srand48. The state is initialized from the parameter SEEDVAL, and the parameters for the congruential formula are initialized to their default values.

     If the return value is nonnegative, the function call succeeded.

     This function is a GNU extension and should not be used in portable programs.

 -- Function: int seed48_r (unsigned short int SEED16V[3], struct drand48_data *BUFFER)
     This function is similar to srand48_r, but like seed48 it initializes all 48 bits of the state from the parameter SEED16V.

     If the return value is nonnegative, the function call succeeded.  It does not return a pointer to the previous state of the random-number generator like the seed48 function does.  If the user wants to preserve the state for a later rerun, she can copy the whole buffer pointed to by BUFFER.

     This function is a GNU extension and should not be used in portable programs.

 -- Function: int lcong48_r (unsigned short int PARAM[7], struct drand48_data *BUFFER)
     This function initializes all aspects of the random-number generator described in BUFFER with the data in PARAM.  Here it is especially true that the function does more than just copying the contents of PARAM and BUFFER.  More work is required and therefore it is important to use this function rather than initializing the random-number generator directly.

     If the return value is nonnegative, the function call succeeded.

     This function is a GNU extension and should not be used in portable programs.

Is Fast Code or Small Code Preferred?
=====================================

If an application uses many floating-point functions, it is often the case that the cost of the function calls themselves is not negligible. Modern processors can often execute the operations themselves very fast, but the function call disrupts the instruction pipeline.

For this reason, the GNU C Library provides optimizations for many of the frequently used math functions.  When GNU CC is used and the user activates the optimizer, several new in-line functions and macros are defined.  These new functions and macros have the same names as the library functions and so are used instead of the latter. In the case of in-line functions, the compiler will decide whether it is reasonable to use them, and this decision is usually correct.

This means that calls to the library functions may be unnecessary, and can increase the speed of generated code significantly.  The drawback is that code size will increase, and the increase is not always negligible.

There are two kind of in-line functions: those that give the same result as the library functions, and others that might not set errno and might have a reduced precision or argument range in comparison with the library functions.  The latter in-line functions are only available if the flag -ffast-math is given to GNU CC.

In cases where the in-line functions and macros are not wanted, the symbol __NO_MATH_INLINES should be defined before any system header is included.  This will ensure that only library functions are used.  Of course, it can be determined for each file in the project whether giving this option is preferable or not.

Not all hardware implements the entire IEEE 754 standard, and even if it does there may be a substantial performance penalty for using some of its features.  For example, enabling traps on some processors forces the FPU to run unpipelined, which can more than double calculation time.

Arithmetic Functions
********************

This chapter contains information about functions for doing basic arithmetic operations, such as splitting a float into its integer and fractional parts or retrieving the imaginary part of a complex value. These functions are declared in the header files math.h and complex.h.

Integers
========

The C language defines several integer data types: integer, short integer, long integer, and character; all in both signed and unsigned varieties.  The GNU C Compiler extends the language to contain long long integers as well.

The C integer types were intended to allow code to be portable among machines with different inherent data sizes (word sizes), so each type may have different ranges on different machines.  The problem with this is that a program often needs to be written for a particular range of integers, and sometimes must be written for a particular size of storage, regardless of what machine the program runs on.

To address this problem, the GNU C Library contains C type definitions you can use to declare integers that meet your exact needs.  Because the GNU C Library header files are customized to a specific machine, your program source code doesn't have to be.

These typedefs are in stdint.h.

If you require that an integer be represented in exactly N bits, use one of the following types, with the obvious mapping to bit size and signedness:

  * int8_t

  * int16_t

  * int32_t

  * int64_t

  * uint8_t

  * uint16_t

  * uint32_t

  * uint64_t

If your C compiler and target machine do not allow integers of a certain size, the corresponding above type does not exist.

If you don't need a specific storage size, but want the smallest data structure with at least N bits, use one of these:

  * int_least8_t

  * int_least16_t

  * int_least32_t

  * int_least64_t

  * uint_least8_t

  * uint_least16_t

  * uint_least32_t

  * uint_least64_t

If you don't need a specific storage size, but want the data structure that allows the fastest access while having at least N bits (and among data structures with the same access speed, the smallest one), use one of these:

  * int_fast8_t

  * int_fast16_t

  * int_fast32_t

  * int_fast64_t

  * uint_fast8_t

  * uint_fast16_t

  * uint_fast32_t

  * uint_fast64_t

If you want an integer with the widest range possible on the platform on which it is being used, use one of the following.  If you use these, you should write code that takes into account the variable size and range of the integer.

  * intmax_t

  * uintmax_t

The GNU C Library also provides macros that tell you the maximum and minimum possible values for each integer data type.  The macro names follow these examples: INT32_MAX, UINT8_MAX, INT_FAST32_MIN, INT_LEAST64_MIN, UINTMAX_MAX, INTMAX_MAX, INTMAX_MIN.  There are no macros for unsigned integer minima.  These are always zero.

There are similar macros for use with C's built in integer types that should come with your C compiler. (See Loosemore, et al., ``Data Type Measurements'' (see chap. 1, n. 1).)

Don't forget that you can use the C sizeof function with any of these data types to get the number of bytes of storage each uses.

Integer Division
================

This section describes functions for performing integer division.  These functions are redundant when GNU CC is used, because in GNU C the / operator always rounds toward zero.  But in other C implementations, / may round differently with negative arguments. div and ldiv are useful because they specify how to round the quotient---toward zero.  The remainder has the same sign as the numerator.

These functions are specified to return a result R such that the value R.quot*DENOMINATOR + R.rem equals NUMERATOR.

To use these facilities, you should include the header file stdlib.h in your program.

 -- Data Type: div_t
     This is a structure type used to hold the result returned by the div function.  It has the following members:

     int quot
          The quotient from the division

     int rem
          The remainder from the division

 -- Function: div_t div (int NUMERATOR, int DENOMINATOR)
     This function div computes the quotient and remainder from the division of NUMERATOR by DENOMINATOR, returning the result in a structure of type div_t.

     If the result cannot be represented (as in a division by zero), the behavior is undefined.

     Here is an example, albeit not a very useful one:

          div_t result;
          result = div (20, -6);

     Now result.quot is -3 and result.rem is 2.

 -- Data Type: ldiv_t
     This is a structure type used to hold the result returned by the ldiv function.  It has the following members:

     long int quot
          The quotient from the division

     long int rem
          The remainder from the division

     This is identical to div_t, except that the components are of type long int rather than int.

 -- Function: ldiv_t ldiv (long int NUMERATOR, long int DENOMINATOR)
     The ldiv function is similar to div, except that the arguments are of type long int and the result is returned as a structure of type ldiv_t.

 -- Data Type: lldiv_t
     This is a structure type used to hold the result returned by the lldiv function.  It has the following members:

     long long int quot
          The quotient from the division

     long long int rem
          The remainder from the division

     This is identical to div_t, except that the components are of type long long int rather than int.

 -- Function: lldiv_t lldiv (long long int NUMERATOR, long long int DENOMINATOR)
     The lldiv function is like the div function, but the arguments are of type long long int and the result is returned as a structure of type lldiv_t.

     The lldiv function was added in ISO C99.

 -- Data Type: imaxdiv_t
     This is a structure type used to hold the result returned by the imaxdiv function.  It has the following members:

     intmax_t quot
          The quotient from the division

     intmax_t rem
          The remainder from the division

     This is identical to div_t, except that the components are of type intmax_t rather than int (see Integers for a description of the intmax_t type).

 -- Function: imaxdiv_t imaxdiv (intmax_t NUMERATOR, intmax_t DENOMINATOR)
     The imaxdiv function is like the div function, but the arguments are of type intmax_t and the result is returned as a structure of type imaxdiv_t (see Integers for a description of the intmax_t type).

     The imaxdiv function was added in ISO C99.

Floating-Point Numbers
======================

Most computer hardware has support for two different kinds of numbers: integers (...-3, -2, -1, 0, 1, 2, 3...) and floating-point numbers.  Floating-point numbers have three parts: the mantissa, the exponent and the sign bit.  The real number represented by a floating-point value is given by (s ? -1 : 1) * 2^e * M where s is the sign bit, e the exponent, and M the mantissa. (Ibid., ``Floating-Point Representation Concepts''.) It is possible to have a different base for the exponent, but all modern hardware uses 2.

Floating-point numbers can represent a finite subset of the real numbers.  While this subset is large enough for most purposes, it is important to remember that the only reals that can be represented exactly are rational numbers that have a terminating binary expansion shorter than the width of the mantissa.  Even simple fractions such as 1/5 can only be approximated by floating point.

Mathematical operations and functions frequently need to produce values that are not representable.  Often these values can be approximated closely enough for practical purposes, but sometimes they can't. Historically there was no way to tell when the results of a calculation were inaccurate.  Modern computers implement the IEEE 754 standard for numerical computations, which defines a framework for indicating to the program when the results of calculation are not trustworthy.  This framework consists of a set of exceptions that indicate why a result could not be represented, and the special values infinity and not a number (NaN).

Floating-Point Number Classification Functions
==============================================

ISO C99 defines macros that let you determine what sort of floating-point number a variable holds.

 -- Macro: int fpclassify (float-type X)
     This is a generic macro that works on all floating-point types and that returns a value of type int.  The possible values are

     FP_NAN
          The floating-point number X is Not a Number (see Infinity and NaN).

     FP_INFINITE
          The value of X is either plus or minus infinity (see Infinity and NaN).

     FP_ZERO
          The value of X is zero.  In floating-point formats like IEEE 754, where zero can be signed, this value is also returned if X is negative zero.

     FP_SUBNORMAL
          Numbers whose absolute value is too small to be represented in the normal format are represented in an alternate, de-normalized format. (Ibid., ``Floating-Point Representation Concepts''.)  This format is less precise but can represent values closer to zero.  fpclassify returns this value for values of X in this alternate format.

     FP_NORMAL
          This value is returned for all other values of X.  It indicates that there is nothing special about the number.

fpclassify is most useful if more than one property of a number must be tested.  There are more specific macros, which only test one property at a time.  Generally these macros execute faster than fpclassify, since there is special hardware support for them. You should therefore use the specific macros whenever possible.

 -- Macro: int isfinite (float-type X)
     This macro returns a nonzero value if X is finite---not plus or minus infinity, and not NaN.  It is equivalent to:

          (fpclassify (x) != FP_NAN && fpclassify (x) != FP_INFINITE)

     isfinite is implemented as a macro that accepts any floating-point type.

 -- Macro: int isnormal (float-type X)
     This macro returns a nonzero value if X is finite and normalized. It is equivalent to:

          (fpclassify (x) == FP_NORMAL)

 -- Macro: int isnan (float-type X)
     This macro returns a nonzero value if X is NaN.  It is equivalent to:

          (fpclassify (x) == FP_NAN)

Another set of floating-point classification functions was provided by BSD.  The GNU C Library also supports these functions; however, we recommend that you use the ISO C99 macros in new code.  Those are standard and will be available more widely.  Also, since they are macros, you do not have to worry about the type of their argument.

 -- Function: int isinf (double X)
 -- Function: int isinff (float X)
 -- Function: int isinfl (long double X)
     This function returns -1 if X represents negative infinity, 1 if X represents positive infinity, and 0 otherwise.

 -- Function: int isnan (double X)
 -- Function: int isnanf (float X)
 -- Function: int isnanl (long double X)
     This function returns a nonzero value if X is a Not a Number value, and zero otherwise.

     The isnan macro defined by ISO C99 overrides the BSD function.  This is normally not a problem, because the two routines behave identically.  However, if you really need to get the BSD function for some reason, you can write:

          (isnan) (x)

 -- Function: int finite (double X)
 -- Function: int finitef (float X)
 -- Function: int finitel (long double X)
     This function returns a nonzero value if X is finite or a Not a Number value, and zero otherwise.

Portability Note: The functions listed in this section are BSD extensions.

Errors in Floating-Point Calculations
=====================================

FP Exceptions
-------------

The IEEE 754 standard defines five exceptions that can occur during a calculation.  Each corresponds to a particular sort of error, such as overflow.

When exceptions occur (when exceptions are raised, in the language of the standard), one of two things can happen.  By default, the exception is simply noted in the floating-point status word, and the program continues as if nothing had happened.  The operation produces a default value, which depends on the exception (see the table below).  Your program can check the status word to find out which exceptions happened.

Alternatively, you can enable traps for exceptions.  In that case, when an exception is raised, your program will receive the SIGFPE signal.  The default action for this signal is to terminate the program. (For how you can change the effect of the signal, see Loosemore et al., ``Signal Handling''.)

In the System V math library, the user-defined function matherr is called when certain exceptions occur inside math library functions. However, the Unix98 standard deprecates this interface.  We support it for historical compatibility, but recommend that you do not use it in new programs.

The exceptions defined in IEEE 754 are

Invalid Operation
     This exception is raised if the given operands are invalid for the operation to be performed.  Examples are (see IEEE 754, section 7):

      1. Addition or subtraction: oo - oo (but oo + oo = oo)

      2. Multiplication: 0 * oo

      3. Division: 0/0 or oo/oo

      4. Remainder: x REM y, where y is zero or x is infinite

      5. Square root if the operand is less then zero; more generally, any mathematical function evaluated outside its domain produces this exception

      6. Conversion of a floating-point number to an integer or decimal string, when the number cannot be represented in the target format (due to overflow, infinity, or NaN)

      7. Conversion of an unrecognizable input string

      8. Comparison via predicates involving < or >, when one or the other of the operands is NaN; you can prevent this exception by using the unordered comparison functions instead (see FP Comparison Functions)

     If the exception does not trap, the result of the operation is NaN.

Division by Zero
     This exception is raised when a finite nonzero number is divided by zero.  If no trap occurs, the result is either +oo or -oo, depending on the signs of the operands.

Overflow
     This exception is raised whenever the result cannot be represented as a finite value in the precision format of the destination.  If no trap occurs, the result depends on the sign of the intermediate result and the current rounding mode (IEEE 754, section 7.3):

      1. Round to nearest carries all overflows to oo with the sign of the intermediate result.

      2. Round toward 0 carries all overflows to the largest representable finite number with the sign of the intermediate result.

      3. Round toward -oo carries positive overflows to the largest representable finite number and negative overflows to -oo.

      4. Round toward oo carries negative overflows to the most negative representable finite number and positive overflows to oo.

     Whenever the overflow exception is raised, the inexact exception is also raised.

Underflow
     The underflow exception is raised when an intermediate result is too small to be calculated accurately, or if the operation's result rounded to the destination precision is too small to be normalized.

     When no trap is installed for the underflow exception, underflow is signaled (via the underflow flag) only when both tininess and loss of accuracy have been detected.  If no trap handler is installed, the operation continues with an imprecise small value, or zero if the destination precision cannot hold the small exact result.

Inexact
     This exception is signalled if a rounded result is not exact (such as when calculating the square root of two) or a result overflows without an overflow trap.

Infinity and NaN
----------------

IEEE 754 floating-point numbers can represent positive or negative infinity, and NaN (not a number).  These three values arise from calculations whose result is undefined or cannot be represented accurately.  You can also deliberately set a floating-point variable to any of them, which is sometimes useful.  Some examples of calculations that produce infinity or NaN:

     1/0 = oo
     log (0) = -oo
     sqrt (-1) = NaN

When a calculation produces any of these values, an exception also occurs (see FP Exceptions).

The basic operations and math functions all accept infinity and NaN and produce sensible output.  Infinities propagate through calculations as one would expect: for example, 2 + oo = oo, 4/oo = 0, atan (oo) = pi/2.  NaN, on the other hand, infects any calculation that involves it.  Unless the calculation would produce the same result no matter what real value replaced NaN, the result is NaN.

In comparison operations, positive infinity is larger than all values except itself and NaN, and negative infinity is smaller than all values except itself and NaN.  NaN is unordered: it is not equal to, greater than, or less than anything, including itself. x == x is false if the value of x is NaN.  You can use this to test whether a value is NaN or not, but the recommended way to test for NaN is with the isnan function (see Floating-Point Classes).  In addition, <, >, <= and >= will raise an exception when applied to NaNs.

math.h defines macros that allow you to explicitly set a variable to infinity or NaN.

 -- Macro: float INFINITY
     An expression representing positive infinity.  It is equal to the value produced  by mathematical operations like 1.0 / 0.0. -INFINITY represents negative infinity.

     You can test whether a floating-point value is infinite by comparing it to this macro.  However, this is not recommended; you should use the isfinite macro instead (see Floating-Point Classes).

     This macro was introduced in the ISO C99 standard.

 -- Macro: float NAN
     An expression representing a value which is not a number.  This macro is a GNU extension, available only on machines that support the not a number value---that is to say, on all machines that support IEEE floating point.

     You can use #ifdef NAN to test whether the machine supports NaN.  (Of course, you must arrange for GNU extensions to be visible, such as by defining _GNU_SOURCE, and then you must include math.h.)

IEEE 754 also allows for another unusual value: negative zero.  This value is produced when you divide a positive number by negative infinity, or when a negative result is smaller than the limits of representation.  Negative zero behaves identically to zero in all calculations, unless you explicitly test the sign bit with signbit or copysign.

Examining the FPU Status Word
-----------------------------

ISO C99 defines functions to query and manipulate the floating-point status word.  You can use these functions to check for untrapped exceptions when it's convenient, rather than worrying about them in the middle of a calculation.

These constants represent the various IEEE 754 exceptions.  Not all FPUs report all the different exceptions.  Each constant is defined if and only if the FPU you are compiling for supports that exception, so you can test for FPU support with #ifdef.  They are defined in fenv.h.

FE_INEXACT
     The inexact exception

FE_DIVBYZERO
     The divide by zero exception

FE_UNDERFLOW
     The underflow exception

FE_OVERFLOW
     The overflow exception

FE_INVALID
     The invalid exception

The macro FE_ALL_EXCEPT is the bit-wise OR of all exception macros that are supported by the FP implementation.

These functions allow you to clear exception flags, test for exceptions, and save and restore the set of exceptions flagged.

 -- Function: int feclearexcept (int EXCEPTS)
     This function clears all of the supported exception flags indicated by EXCEPTS.

     The function returns zero when the operation was successful and a nonzero value otherwise.

 -- Function: int feraiseexcept (int EXCEPTS)
     This function raises the supported exceptions indicated by EXCEPTS.  If more than one exception bit in EXCEPTS is set, the order in which the exceptions are raised is undefined except that overflow (FE_OVERFLOW) or underflow (FE_UNDERFLOW) are raised before inexact (FE_INEXACT).  For overflow or underflow, whether the inexact exception is also raised is implementation dependent.

     The function returns zero when the operation was successful and a nonzero value otherwise.

 -- Function: int fetestexcept (int EXCEPTS)
     Test whether the exception flags indicated by the parameter EXCEPT are currently set.  If any of them are, a nonzero value is returned, which specifies which exceptions are set.  Otherwise, the result is zero.

To understand these functions, imagine that the status word is an integer variable named STATUS.  feclearexcept is then equivalent to status &= ~excepts and fetestexcept is equivalent to (status & excepts).  The actual implementation may be very different, of course.

Exception flags are only cleared when the program explicitly requests it, by calling feclearexcept.  If you want to check for exceptions from a set of calculations, you should clear all the flags first.  Here is a simple example of the way to use fetestexcept:

     {
       double f;
       int raised;
       feclearexcept (FE_ALL_EXCEPT);
       f = compute ();
       raised = fetestexcept (FE_OVERFLOW | FE_INVALID);
       if (raised & FE_OVERFLOW) { /* ... */ }
       if (raised & FE_INVALID) { /* ... */ }
       /* ... */
     }

You cannot explicitly set bits in the status word.  You can, however, save the entire status word and restore it later.  This is done with the following functions:

 -- Function: int fegetexceptflag (fexcept_t *FLAGP, int EXCEPTS)
     This function stores in the variable pointed to by FLAGP an implementation-defined value representing the current setting of the exception flags indicated by EXCEPTS.

     The function returns zero if the operation was successful and a nonzero value otherwise.

 -- Function: int fesetexceptflag (const fexcept_t *FLAGP, int EXCEPTS)
     This function restores the flags for the exceptions indicated by EXCEPTS to the values stored in the variable pointed to by FLAGP.

     The function returns zero if the operation was successful and a nonzero value otherwise.

The value stored in fexcept_t bears no resemblance to the bit mask returned by fetestexcept.  The type may not even be an integer.  Do not attempt to modify an fexcept_t variable.

Error Reporting by Mathematical Functions
-----------------------------------------

Many of the math functions are defined only over a subset of the real or complex numbers.  Even if they are mathematically defined, their result may be larger or smaller than the range representable by their return type.  These are known as domain errors, overflows, and underflows, respectively.  Math functions do several things when one of these errors occurs.  In this manual we will refer to the complete response as signalling a domain error, overflow, or underflow.

When a math function suffers a domain error, it raises the invalid exception and returns NaN.  It also sets ERRNO to EDOM; this is for compatibility with old systems that do not support IEEE 754 exception handling.  Likewise, when overflow occurs, math functions raise the overflow exception and return oo or -oo as appropriate.  They also set ERRNO to ERANGE.  When underflow occurs, the underflow exception is raised, and zero (appropriately signed) is returned.  ERRNO may be set to ERANGE, but this is not guaranteed.

Some of the math functions are defined mathematically to result in a complex value over parts of their domains.  The most familiar example of this is taking the square root of a negative number.  The complex math functions, such as csqrt, will return the appropriate complex value in this case.  The real-valued functions, such as sqrt, will signal a domain error.

Some older hardware does not support infinities.  On that hardware, overflows instead return a particular very large number (usually the largest representable number).  math.h defines macros you can use to test for overflow on both old and new hardware.

 -- Macro: double HUGE_VAL
 -- Macro: float HUGE_VALF
 -- Macro: long double HUGE_VALL
     An expression representing a particular very large number.  On machines that use IEEE 754 floating-point format, HUGE_VAL is infinity. On other machines, it's typically the largest positive number that can be represented.

     Mathematical functions return the appropriately typed version of HUGE_VAL or -HUGE_VAL when the result is too large to be represented.

Rounding Modes
==============

Floating-point calculations are carried out internally with extra precision, and then rounded to fit into the destination type.  This ensures that results are as precise as the input data.  IEEE 754 defines four possible rounding modes:

Round to nearest
     This is the default mode.  It should be used unless there is a specific need for one of the others.  In this mode, results are rounded to the nearest representable value.  If the result is midway between two representable values, the even representable is chosen. Even here means the lowest-order bit is zero.  This rounding mode prevents statistical bias and guarantees numeric stability---round-off errors in a lengthy calculation will remain smaller than half of FLT_EPSILON.

Round toward plus Infinity.
     All results are rounded to the smallest representable value that is greater than the result.

Round toward minus Infinity.
     All results are rounded to the largest representable value that is less than the result.

Round toward zero
     All results are rounded to the largest representable value whose magnitude is less than that of the result.  In other words, if the result is negative, it is rounded up; if it is positive, it is rounded down.

fenv.h defines constants which you can use to refer to the various rounding modes.  Each one will be defined if and only if the FPU supports the corresponding rounding mode.

FE_TONEAREST
     Round to nearest

FE_UPWARD
     Round toward +oo

FE_DOWNWARD
     Round toward -oo

FE_TOWARDZERO
     Round toward zero

Underflow is an unusual case.  Normally, IEEE 754 floating-point numbers are always normalized. (Ibid., ``Floating-Point Representation Concepts''.) Numbers smaller than 2^r (where r is the minimum exponent, FLT_MIN_RADIX-1 for FLOAT) cannot be represented as normalized numbers.  Rounding all such numbers to zero or 2^r would cause some algorithms to fail at 0.  Therefore, they are left in de-normalized form.  That produces loss of precision, since some bits of the mantissa are stolen to indicate the decimal point.

If a result is too small to be represented as a de-normalized number, it is rounded to zero.  However, the sign of the result is preserved; if the calculation was negative, the result is negative zero. Negative zero can also result from some operations on infinity, such as 4/-oo.  Negative zero behaves identically to zero except when the copysign or signbit functions are used to check the sign bit directly.

At any time one of the above four rounding modes is selected.  You can find out which one with this function:

 -- Function: int fegetround (void)
     Returns the currently selected rounding mode, represented by one of the values of the defined rounding mode macros.

To change the rounding mode, use this function:

 -- Function: int fesetround (int ROUND)
     Changes the currently selected rounding mode to ROUND.  If ROUND does not correspond to one of the supported rounding modes nothing is changed.  fesetround returns zero if it changed the rounding mode and a nonzero value if the mode is not supported.

You should avoid changing the rounding mode if possible.  It can be an expensive operation; also, some hardware requires you to compile your program differently for it to work.  The resulting code may run slower. See your compiler documentation for details.

Floating-Point Control Functions
================================

IEEE 754 floating-point implementations allow the programmer to decide whether traps will occur for each of the exceptions, by setting bits in the control word.  In C, traps result in the program receiving the SIGFPE signal. (Ibid., ``Signal Handling''.)

IEEE 754 says that trap handlers are given details of the exceptional situation and can set the result value.  C signals do not provide any mechanism to pass this information back and forth. Trapping exceptions in C is therefore not very useful.

It is sometimes necessary to save the state of the floating-point unit while you perform some calculation.  The library provides functions that save and restore the exception flags, the set of exceptions that generate traps, and the rounding mode.  This information is known as the floating-point environment.

The functions to save and restore the floating-point environment all use a variable of type fenv_t to store information.  This type is defined in fenv.h.  Its size and contents are implementation defined.  You should not attempt to manipulate a variable of this type directly.

To save the state of the FPU, use one of these functions:

 -- Function: int fegetenv (fenv_t *ENVP)
     Store the floating-point environment in the variable pointed to by ENVP.

     The function returns zero if the operation was successful and a nonzero value otherwise.

 -- Function: int feholdexcept (fenv_t *ENVP)
     Store the current floating-point environment in the object pointed to by ENVP.  Then clear all exception flags, and set the FPU to trap no exceptions.  Not all FPUs support trapping no exceptions; if feholdexcept cannot set this mode, it returns a nonzero value. If it succeeds, it returns zero.

The functions that restore the floating-point environment can take these kinds of arguments:

  * Pointers to fenv_t objects that were initialized previously by a call to fegetenv or feholdexcept.

  * The special macro FE_DFL_ENV, which represents the floating-point environment as it was available at program start.

  * Implementation-defined macros with names starting with FE_ and having type fenv_t *.

     If possible, the GNU C Library defines a macro FE_NOMASK_ENV, which represents an environment where every exception raised causes a trap to occur.  You can test for this macro using #ifdef.  It is only defined if _GNU_SOURCE is defined.

     Some platforms might define other predefined environments.

To set the floating-point environment, you can use either of these functions:

 -- Function: int fesetenv (const fenv_t *ENVP)
     Set the floating-point environment to that described by ENVP.

     The function returns zero if the operation was successful and a nonzero value otherwise.

 -- Function: int feupdateenv (const fenv_t *ENVP)
     Like fesetenv, this function sets the floating-point environment to that described by ENVP.  However, if any exceptions were flagged in the status word before feupdateenv was called, they remain flagged after the call.  In other words, after feupdateenv is called, the status word is the bit-wise OR of the previous status word and the one saved in ENVP.

     The function returns zero if the operation was successful and a nonzero value otherwise.

To control for individual exceptions if raising them causes a trap to occur, you can use the following two functions.

Portability Note: These functions are all GNU extensions.

 -- Function: int feenableexcept (int EXCEPTS)
     This function enables traps for each of the exceptions as indicated by the parameter EXCEPT.  The individual exceptions are described in Status bit operations.  Only the specified exceptions are enabled---the status of the other exceptions is not changed.

     The function returns the previously enabled exceptions if the operation was successful, -1 otherwise.

 -- Function: int fedisableexcept (int EXCEPTS)
     This function disables traps for each of the exceptions as indicated by the parameter EXCEPT.  The individual exceptions are described in Status bit operations.  Only the specified exceptions are disabled---the status of the other exceptions is not changed.

     The function returns the previously enabled exceptions if the operation was successful and -1 otherwise.

 -- Function: int fegetexcept (int EXCEPTS)
     The function returns a bitmask of all currently enabled exceptions.  It returns -1 in case of failure.

Arithmetic Functions
====================

The C library provides functions to do basic operations on floating-point numbers.  These include absolute value, maximum and minimum, normalization, bit twiddling, rounding and a few others.

Absolute Value
--------------

These functions are provided for obtaining the absolute value (or magnitude) of a number.  The absolute value of a real number X is X if X is positive, -X if X is negative.  For a complex number Z, whose real part is X and whose imaginary part is Y, the absolute value is sqrt (X*X + Y*Y).

Prototypes for abs, labs and llabs are in stdlib.h; imaxabs is declared in inttypes.h; fabs, fabsf and fabsl are declared in math.h. cabs, cabsf and cabsl are declared in complex.h.

 -- Function: int abs (int NUMBER)
 -- Function: long int labs (long int NUMBER)
 -- Function: long long int llabs (long long int NUMBER)
 -- Function: intmax_t imaxabs (intmax_t NUMBER)
     These functions return the absolute value of NUMBER.

     Most computers use a two's complement integer representation, in which the absolute value of INT_MIN (the smallest possible int) cannot be represented; thus, abs (INT_MIN) is not defined.

     llabs and imaxdiv are new to ISO C99.

     See Integers for a description of the intmax_t type.

 -- Function: double fabs (double NUMBER)
 -- Function: float fabsf (float NUMBER)
 -- Function: long double fabsl (long double NUMBER)
     This function returns the absolute value of the floating-point number NUMBER.

 -- Function: double cabs (complex double Z)
 -- Function: float cabsf (complex float Z)
 -- Function: long double cabsl (complex long double Z)
     These functions return the absolute value of the complex number Z (see Complex Numbers).  The absolute value of a complex number is

          sqrt (creal (Z) * creal (Z) + cimag (Z) * cimag (Z))

     This function should always be used instead of the direct formula because it takes special care to avoid losing precision.  It may also take advantage of hardware support for this operation (see hypot in Exponents and Logarithms).

Normalization Functions
-----------------------

The functions described in this section are primarily provided as a way to efficiently perform certain low-level manipulations on floating-point numbers that are represented internally using a binary radix. (Ibid., ``Floating-Point Representation Concepts''.)  These functions are required to have equivalent behavior even if the representation does not use a radix of 2, but of course they are unlikely to be particularly efficient in those cases.

All these functions are declared in math.h.

 -- Function: double frexp (double VALUE, int *EXPONENT)
 -- Function: float frexpf (float VALUE, int *EXPONENT)
 -- Function: long double frexpl (long double VALUE, int *EXPONENT)
     These functions are used to split the number VALUE into a normalized fraction and an exponent.

     If the argument VALUE is not zero, the return value is VALUE times a power of two, and is always in the range 1/2 (inclusive) to 1 (exclusive).  The corresponding exponent is stored in *EXPONENT; the return value multiplied by 2 raised to this exponent equals the original number VALUE.

     For example, frexp (12.8, &exponent) returns 0.8 and stores 4 in exponent.

     If VALUE is zero, then the return value is zero, and zero is stored in *EXPONENT.

 -- Function: double ldexp (double VALUE, int EXPONENT)
 -- Function: float ldexpf (float VALUE, int EXPONENT)
 -- Function: long double ldexpl (long double VALUE, int EXPONENT)
     These functions return the result of multiplying the floating-point number VALUE by 2 raised to the power EXPONENT.  (It can be used to reassemble floating-point numbers that were taken apart by frexp.)

     For example, ldexp (0.8, 4) returns 12.8.

The following functions, which come from BSD, provide facilities equivalent to those of ldexp and frexp.  See also the ISO C function logb, which originally also appeared in BSD.

 -- Function: double scalb (double VALUE, int EXPONENT)
 -- Function: float scalbf (float VALUE, int EXPONENT)
 -- Function: long double scalbl (long double VALUE, int EXPONENT)
     The scalb function is the BSD name for ldexp.

 -- Function: long long int scalbn (double X, int n)
 -- Function: long long int scalbnf (float X, int n)
 -- Function: long long int scalbnl (long double X, int n)
     scalbn is identical to scalb, except that the exponent N is an int instead of a floating-point number.

 -- Function: long long int scalbln (double X, long int n)
 -- Function: long long int scalblnf (float X, long int n)
 -- Function: long long int scalblnl (long double X, long int n)
     scalbln is identical to scalb, except that the exponent N is a long int instead of a floating-point number.

 -- Function: long long int significand (double X)
 -- Function: long long int significandf (float X)
 -- Function: long long int significandl (long double X)
     significand returns the mantissa of X scaled to the range [1, 2). It is equivalent to scalb (X, (double) -ilogb (X)).

     This function exists mainly for use in certain standardized tests of IEEE 754 conformance.

Rounding Functions
------------------

The functions listed here perform operations such as rounding and truncation of floating-point values. Some of these functions convert floating-point numbers to integer values.  They are all declared in math.h.

You can also convert floating-point numbers to integers simply by casting them to int.  This discards the fractional part, effectively rounding toward zero.  However, this only works if the result can actually be represented as an int---for very large numbers, this is impossible.  The functions listed here return the result as a double instead to get around this problem:

 -- Function: double ceil (double X)
 -- Function: float ceilf (float X)
 -- Function: long double ceill (long double X)
     These functions round X upward to the nearest integer, returning that value as a double.  Thus, ceil (1.5) is 2.0.

 -- Function: double floor (double X)
 -- Function: float floorf (float X)
 -- Function: long double floorl (long double X)
     These functions round X downward to the nearest integer, returning that value as a double.  Thus, floor (1.5) is 1.0 and floor (-1.5) is -2.0.

 -- Function: double trunc (double X)
 -- Function: float truncf (float X)
 -- Function: long double truncl (long double X)
     The trunc functions round X toward zero to the nearest integer (returned in floating-point format).  Thus, trunc (1.5) is 1.0 and trunc (-1.5) is -1.0.

 -- Function: double rint (double X)
 -- Function: float rintf (float X)
 -- Function: long double rintl (long double X)
     These functions round X to an integer value according to the current rounding mode. (See Loosemore et al., ``Floating-Point Parameters'', for information about the various rounding modes.)  The default rounding mode is to round to the nearest integer; some machines support other modes, but round-to-nearest is always used unless you explicitly select another.

     If X was not initially an integer, these functions raise the inexact exception.

 -- Function: double nearbyint (double X)
 -- Function: float nearbyintf (float X)
 -- Function: long double nearbyintl (long double X)
     These functions return the same value as the rint functions, but do not raise the inexact exception if X is not an integer.

 -- Function: double round (double X)
 -- Function: float roundf (float X)
 -- Function: long double roundl (long double X)
     These functions are similar to rint, but they round halfway cases away from zero instead of to the nearest even integer.

 -- Function: long int lrint (double X)
 -- Function: long int lrintf (float X)
 -- Function: long int lrintl (long double X)
     These functions are just like rint, but they return a long int instead of a floating-point number.

 -- Function: long long int llrint (double X)
 -- Function: long long int llrintf (float X)
 -- Function: long long int llrintl (long double X)
     These functions are just like rint, but they return a long long int instead of a floating-point number.

 -- Function: long int lround (double X)
 -- Function: long int lroundf (float X)
 -- Function: long int lroundl (long double X)
     These functions are just like round, but they return a long int instead of a floating-point number.

 -- Function: long long int llround (double X)
 -- Function: long long int llroundf (float X)
 -- Function: long long int llroundl (long double X)
     These functions are just like round, but they return a long long int instead of a floating-point number.

 -- Function: double modf (double VALUE, double *INTEGER-PART)
 -- Function: float modff (float VALUE, float *INTEGER-PART)
 -- Function: long double modfl (long double VALUE, long double *INTEGER-PART)
     These functions break the argument VALUE into an integer part and a fractional part (between -1 and 1, exclusive).  Their sum equals VALUE.  Each of the parts has the same sign as VALUE, and the integer part is always rounded toward zero.

     modf stores the integer part in *INTEGER-PART and returns the fractional part.  For example, modf (2.5, &intpart) returns 0.5 and stores 2.0 into intpart.

Remainder Functions
-------------------

The functions in this section compute the remainder on division of two floating-point numbers.  Each is a little different; pick the one that suits your problem.

 -- Function: double fmod (double NUMERATOR, double DENOMINATOR)
 -- Function: float fmodf (float NUMERATOR, float DENOMINATOR)
 -- Function: long double fmodl (long double NUMERATOR, long double DENOMINATOR)
     These functions compute the remainder from the division of NUMERATOR by DENOMINATOR.  Specifically, the return value is NUMERATOR - N * DENOMINATOR, where N is the quotient of NUMERATOR divided by DENOMINATOR, rounded toward zero to an integer.  Thus, fmod (6.5, 2.3) returns 1.9, which is 6.5 minus 4.6.

     The result has the same sign as the NUMERATOR and has magnitude less than the magnitude of the DENOMINATOR.

     If DENOMINATOR is zero, fmod signals a domain error.

 -- Function: double drem (double NUMERATOR, double DENOMINATOR)
 -- Function: float dremf (float NUMERATOR, float DENOMINATOR)
 -- Function: long double dreml (long double NUMERATOR, long double DENOMINATOR)
     These functions are like fmod except that they round the internal quotient N to the nearest integer instead of toward zero to an integer.  For example, drem (6.5, 2.3) returns -0.4, which is 6.5 minus 6.9.

     The absolute value of the result is less than or equal to half the absolute value of the DENOMINATOR.  The difference between fmod (NUMERATOR, DENOMINATOR) and drem (NUMERATOR, DENOMINATOR) is always either DENOMINATOR, minus DENOMINATOR, or zero.

     If DENOMINATOR is zero, drem signals a domain error.

 -- Function: double remainder (double NUMERATOR, double DENOMINATOR)
 -- Function: float remainderf (float NUMERATOR, float DENOMINATOR)
 -- Function: long double remainderl (long double NUMERATOR, long double DENOMINATOR)
     This function is another name for drem.

Setting and Modifying Single Bits of FP Values
----------------------------------------------

There are some operations that are too complicated or expensive to perform by hand on floating-point numbers.  ISO C99 defines functions to do these operations, which mostly involve changing single bits.

 -- Function: double copysign (double X, double Y)
 -- Function: float copysignf (float X, float Y)
 -- Function: long double copysignl (long double X, long double Y)
     These functions return X but with the sign of Y.  They work even if X or Y are NaN or zero.  Both of these can carry a sign (although not all implementations support it), and this is one of the few operations that can tell the difference.

     copysign never raises an exception.

     This function is defined in IEC 559 (and the appendix with recommended functions in IEEE 754/IEEE 854).

 -- Function: int signbit (float-type X)
     signbit is a generic macro that can work on all floating-point types.  It returns a nonzero value if the value of X has its sign bit set.

     This is not the same as x < 0.0, because IEEE 754 floating-point allows zero to be signed.  The comparison -0.0 < 0.0 is false, but signbit (-0.0) will return a nonzero value.

 -- Function: double nextafter (double X, double Y)
 -- Function: float nextafterf (float X, float Y)
 -- Function: long double nextafterl (long double X, long double Y)
     The nextafter function returns the next representable neighbor of X in the direction toward Y.  The size of the step between X and the result depends on the type of the result.  If X = Y, the function simply returns Y.  If either value is NaN, NaN is returned.  Otherwise, a value corresponding to the value of the least significant bit in the mantissa is added or subtracted, depending on the direction. nextafter will signal overflow or underflow if the result goes outside of the range of normalized numbers.

     This function is defined in IEC 559 (and the appendix with recommended functions in IEEE 754/IEEE 854).

 -- Function: double nexttoward (double X, long double Y)
 -- Function: float nexttowardf (float X, long double Y)
 -- Function: long double nexttowardl (long double X, long double Y)
     These functions are identical to the corresponding versions of nextafter except that their second argument is a long double.

 -- Function: double nan (const char *TAGP)
 -- Function: float nanf (const char *TAGP)
 -- Function: long double nanl (const char *TAGP)
     The nan function returns a representation of NaN, provided that NaN is supported by the target platform. nan ("N-CHAR-SEQUENCE") is equivalent to strtod ("NAN(N-CHAR-SEQUENCE)").

     The argument TAGP is used in an unspecified manner.  On IEEE 754 systems, there are many representations of NaN, and TAGP selects one.  On other systems it may do nothing.

Floating-Point Comparison Functions
-----------------------------------

The standard C comparison operators provoke exceptions when one or other of the operands is NaN.  For example:

     int v = a < 1.0;

will raise an exception if A is NaN (this does not happen with == and !=; those merely return false and true, respectively, when NaN is examined).  Frequently, this exception is undesirable.  ISO C99 therefore defines comparison functions that do not raise exceptions when NaN is examined.  All of the functions are implemented as macros that allow their arguments to be of any floating-point type.  The macros are guaranteed to evaluate their arguments only once.

 -- Macro: int isgreater (real-floating X, real-floating Y)
     This macro determines whether the argument X is greater than Y.  It is equivalent to (X) > (Y), but no exception is raised if X or Y are NaN.

 -- Macro: int isgreaterequal (real-floating X, real-floating Y)
     This macro determines whether the argument X is greater than or equal to Y.  It is equivalent to (X) >= (Y), but no exception is raised if X or Y are NaN.

 -- Macro: int isless (real-floating X, real-floating Y)
     This macro determines whether the argument X is less than Y. It is equivalent to (X) < (Y), but no exception is raised if X or Y are NaN.

 -- Macro: int islessequal (real-floating X, real-floating Y)
     This macro determines whether the argument X is less than or equal to Y.  It is equivalent to (X) <= (Y), but no exception is raised if X or Y are NaN.

 -- Macro: int islessgreater (real-floating X, real-floating Y)
     This macro determines whether the argument X is less than or greater than Y.  It is equivalent to (X) < (Y) || (X) > (Y) (although it only evaluates X and Y once), but no exception is raised if X or Y are NaN.

     This macro is not equivalent to X != Y, because that expression is true if X or Y are NaN.

 -- Macro: int isunordered (real-floating X, real-floating Y)
     This macro determines whether its arguments are unordered.  In other words, it is true if X or Y are NaN and false otherwise.

Not all machines provide hardware support for these operations.  On machines that do not, the macros can be very slow.  Therefore, you should not use these functions when NaN is not a concern.

There are no macros isequal or isunequal.  They are unnecessary, because the == and != operators do not throw an exception if one or both of the operands are NaN.

Miscellaneous FP Arithmetic Functions
-------------------------------------

The functions in this section perform miscellaneous but common operations that are awkward to express with C operators.  On some processors, these functions can use special machine instructions to perform these operations faster than the equivalent C code.

 -- Function: double fmin (double X, double Y)
 -- Function: float fminf (float X, float Y)
 -- Function: long double fminl (long double X, long double Y)
     The fmin function returns the lesser of the two values X and Y.  It is similar to the expression:

          ((x) < (y) ? (x) : (y))

     except that X and Y are only evaluated once.

     If an argument is NaN, the other argument is returned.  If both arguments are NaN, NaN is returned.

 -- Function: double fmax (double X, double Y)
 -- Function: float fmaxf (float X, float Y)
 -- Function: long double fmaxl (long double X, long double Y)
     The fmax function returns the greater of the two values X and Y.

     If an argument is NaN, the other argument is returned.  If both arguments are NaN, NaN is returned.

 -- Function: double fdim (double X, double Y)
 -- Function: float fdimf (float X, float Y)
 -- Function: long double fdiml (long double X, long double Y)
     The fdim function returns the positive difference between X and Y.  The positive difference is X - Y if X is greater than Y, and 0 otherwise.

     If X, Y, or both are NaN, NaN is returned.

 -- Function: double fma (double X, double Y, double Z)
 -- Function: float fmaf (float X, float Y, float Z)
 -- Function: long double fmal (long double X, long double Y, long double Z)
     The fma function performs floating-point multiply-add.  This is the operation (X * Y) + Z, but the intermediate result is not rounded to the destination type.  This can sometimes improve the precision of a calculation.

     This function was introduced because some processors have a special instruction to perform multiply-add.  The C compiler cannot use it directly, because the expression x*y + z is defined to round the intermediate result.  fma lets you choose when you want to round only once.

     On processors that do not implement multiply-add in hardware, fma can be very slow since it must avoid intermediate rounding. math.h defines the symbols FP_FAST_FMA, FP_FAST_FMAF, and FP_FAST_FMAL when the corresponding version of fma is no slower than the expression x*y + z. In the GNU C Library, this always means the operation is implemented in hardware.

Complex Numbers
===============

ISO C99 introduces support for complex numbers in C.  This is done with a new type qualifier, complex.  It is a keyword if and only if complex.h has been included.  There are three complex types, corresponding to the three real types: float complex, double complex and long double complex.

To construct complex numbers, you need a way to indicate the imaginary part of a number.  There is no standard notation for an imaginary floating-point constant.  Instead, complex.h defines two macros that can be used to create complex numbers.

 -- Macro: const float complex _Complex_I
     This macro is a representation of the complex number "0+1i". Multiplying a real floating-point value by _Complex_I gives a complex number whose value is purely imaginary.  You can use this to construct complex constants:

          3.0 + 4.0i = 3.0 + 4.0 * _Complex_I

     Note that _Complex_I * _Complex_I has the value -1, but the type of that value is complex.

_Complex_I is a bit of a mouthful.  complex.h also defines a shorter name for the same constant.

 -- Macro: const float complex I
     This macro has exactly the same value as _Complex_I.  Most of the time it is preferable.  However, it causes problems if you want to use the identifier I for something else.  You can safely write:

          #include <complex.h>
          #undef I

     if you need I for your own purposes.  In that case, we recommend you also define some other short name for _Complex_I, such as J.

Projections, Conjugates and Decomposing of Complex Numbers
==========================================================

ISO C99 also defines functions that perform basic operations on complex numbers, such as decomposition and conjugation.  The prototypes for all these functions are in complex.h.  All functions are available in three variants, one for each of the three complex types.

 -- Function: double creal (complex double Z)
 -- Function: float crealf (complex float Z)
 -- Function: long double creall (complex long double Z)
     These functions return the real part of the complex number Z.

 -- Function: double cimag (complex double Z)
 -- Function: float cimagf (complex float Z)
 -- Function: long double cimagl (complex long double Z)
     These functions return the imaginary part of the complex number Z.

 -- Function: complex double conj (complex double Z)
 -- Function: complex float conjf (complex float Z)
 -- Function: complex long double conjl (complex long double Z)
     These functions return the conjugate value of the complex number Z.  The conjugate of a complex number has the same real part and a negated imaginary part.  In other words, conj(a + bi) = a + -bi.

 -- Function: double carg (complex double Z)
 -- Function: float cargf (complex float Z)
 -- Function: long double cargl (complex long double Z)
     These functions return the argument of the complex number Z. The argument of a complex number is the angle in the complex plane between the positive real axis and a line passing through zero and the number.  This angle is measured in the usual fashion and ranges from 0 to 2pi.

     carg has a branch cut along the positive real axis.

 -- Function: complex double cproj (complex double Z)
 -- Function: complex float cprojf (complex float Z)
 -- Function: complex long double cprojl (complex long double Z)
     These functions return the projection of the complex value Z onto the Riemann sphere.  Values with an infinite imaginary part are projected to positive infinity on the real axis, even if the real part is NaN.  If the real part is infinite, the result is equivalent to:

          INFINITY + I * copysign (0.0, cimag (z))

Parsing of Numbers
==================

This section describes functions for ``reading'' integer and floating-point numbers from a string.  It may be more convenient in some cases to use sscanf or one of the related functions (see Formatted Input).  But often you can make a program more robust by finding the tokens in the string by hand, then converting the numbers one by one.

Parsing of Integers
-------------------

The str functions are declared in stdlib.h, and those beginning with wcs are declared in wchar.h.  You might wonder about the use of restrict in the prototypes of the functions in this section.  It is seemingly useless, but the ISO C standard uses it (for the functions defined there), so we have to use it as well.

 -- Function: long int strtol (const char *restrict STRING, char **restrict TAILPTR, int BASE)
     The strtol (``string-to-long'') function converts the initial part of STRING to a signed integer, which is returned as a value of type long int.

     This function attempts to decompose STRING as follows:

       * A (possibly empty) sequence of white-space characters---which characters are white space is determined by the isspace function (see Classification of Characters).  These are discarded.

       * An optional plus or minus sign (+ or -)

       * A nonempty sequence of digits in the radix specified by BASE---if BASE is zero, decimal radix is assumed unless the series of digits begins with 0 (specifying octal radix), or 0x or 0X (specifying hexadecimal radix); in other words, the same syntax used for integer constants in C.

          Otherwise BASE must have a value between 2 and 36. If BASE is 16, the digits may optionally be preceded by 0x or 0X.  If base has no legal value, the value returned is 0l, and the global variable errno is set to EINVAL.

       * Any remaining characters in the string---if TAILPTR is not a null pointer, strtol stores a pointer to this tail in *TAILPTR.

     If the string is empty, contains only white space, or does not contain an initial substring that has the expected syntax for an integer in the specified BASE, no conversion is performed.  In this case, strtol returns a value of zero and the value stored in *TAILPTR is the value of STRING.

     In a locale other than the standard "C" locale, this function may recognize additional implementation-dependent syntax.

     If the string has valid syntax for an integer but the value is not representable because of overflow, strtol returns either LONG_MAX or LONG_MIN, as appropriate for the sign of the value. (Ibid., ``Range of an Integer Type''.)  It also sets errno to ERANGE to indicate there was overflow.

     You should not check for errors by examining the return value of strtol, because the string might be a valid representation of 0l, LONG_MAX, or LONG_MIN.  Instead, check whether TAILPTR points to what you expect after the number (e.g., '\0' if the string should end after the number).  You also need to clear ERRNO before the call and check it afterward, in case there was overflow.

     There is an example at the end of this section.

 -- Function: long int wcstol (const wchar_t *restrict STRING, wchar_t **restrict TAILPTR, int BASE)
     The wcstol function is equivalent to the strtol function in nearly all aspects, but it handles wide-character strings.

     The wcstol function was introduced in Amendment 1 of ISO C90.

 -- Function: unsigned long int strtoul (const char *restrict STRING, char **restrict TAILPTR, int BASE)
     The strtoul (``string-to-unsigned-long'') function is like strtol except it converts to an unsigned long int value. The syntax is the same as described above for strtol.  The value returned on overflow is ULONG_MAX. (Ibid.)

     If STRING depicts a negative number, strtoul acts the same as STRTOL but casts the result to an unsigned integer. That means, for example, that strtoul on "-1" returns ULONG_MAX, and an input more negative than LONG_MIN returns (ULONG_MAX + 1) / 2.

     strtoul sets ERRNO to EINVAL if BASE is out of range, or to ERANGE on overflow.

 -- Function: unsigned long int wcstoul (const wchar_t *restrict STRING, wchar_t **restrict TAILPTR, int BASE)
     The wcstoul function is equivalent to the strtoul function in nearly all aspects, but it handles wide-character strings.

     The wcstoul function was introduced in Amendment 1 of ISO C90.

 -- Function: long long int strtoll (const char *restrict STRING, char **restrict TAILPTR, int BASE)
     The strtoll function is like strtol except that it returns a long long int value and accepts numbers with a correspondingly larger range.

     If the string has valid syntax for an integer but the value is not representable because of overflow, strtoll returns either LONG_LONG_MAX or LONG_LONG_MIN, as appropriate for the sign of the value. (Ibid.)  It also sets errno to ERANGE to indicate there was overflow.

     The strtoll function was introduced in ISO C99.

 -- Function: long long int wcstoll (const wchar_t *restrict STRING, wchar_t **restrict TAILPTR, int BASE)
     The wcstoll function is equivalent to the strtoll function in nearly all aspects, but it handles wide-character strings.

     The wcstoll function was introduced in Amendment 1 of ISO C90.

 -- Function: long long int strtoq (const char *restrict STRING, char **restrict TAILPTR, int BASE)
     strtoq (``string-to-quad-word'') is the BSD name for strtoll.

 -- Function: long long int wcstoq (const wchar_t *restrict STRING, wchar_t **restrict TAILPTR, int BASE)
     The wcstoq function is equivalent to the strtoq function in nearly all aspects, but it handles wide-character strings.

     The wcstoq function is a GNU extension.

 -- Function: unsigned long long int strtoull (const char *restrict STRING, char **restrict TAILPTR, int BASE)
     The strtoull function is related to strtoll the same way strtoul is related to strtol.

     The strtoull function was introduced in ISO C99.

 -- Function: unsigned long long int wcstoull (const wchar_t *restrict STRING, wchar_t **restrict TAILPTR, int BASE)
     The wcstoull function is equivalent to the strtoull function in nearly all aspects, but it handles wide-character strings.

     The wcstoull function was introduced in Amendment 1 of ISO C90.

 -- Function: unsigned long long int strtouq (const char *restrict STRING, char **restrict TAILPTR, int BASE)
     strtouq is the BSD name for strtoull.

 -- Function: unsigned long long int wcstouq (const wchar_t *restrict STRING, wchar_t **restrict TAILPTR, int BASE)
     The wcstouq function is equivalent to the strtouq function in nearly all aspects, but it handles wide-character strings.

     The wcstoq function is a GNU extension.

 -- Function: intmax_t strtoimax (const char *restrict STRING, char **restrict TAILPTR, int BASE)
     The strtoimax function is like strtol except that it returns an intmax_t value and accepts numbers of a corresponding range.

     If the string has valid syntax for an integer but the value is not representable because of overflow, strtoimax returns either INTMAX_MAX or INTMAX_MIN (see Integers), as appropriate for the sign of the value.  It also sets errno to ERANGE to indicate there was overflow.

     See Integers for a description of the intmax_t type.  The strtoimax function was introduced in ISO C99.

 -- Function: intmax_t wcstoimax (const wchar_t *restrict STRING, wchar_t **restrict TAILPTR, int BASE)
     The wcstoimax function is equivalent to the strtoimax function in nearly all aspects, but it handles wide-character strings.

     The wcstoimax function was introduced in ISO C99.

 -- Function: uintmax_t strtoumax (const char *restrict STRING, char **restrict TAILPTR, int BASE)
     The strtoumax function is related to strtoimax the same way that strtoul is related to strtol.

     See Integers for a description of the intmax_t type.  The strtoumax function was introduced in ISO C99.

 -- Function: uintmax_t wcstoumax (const wchar_t *restrict STRING, wchar_t **restrict TAILPTR, int BASE)
     The wcstoumax function is equivalent to the strtoumax function in nearly all aspects, but it handles wide-character strings.

     The wcstoumax function was introduced in ISO C99.

 -- Function: long int atol (const char *STRING)
     This function is similar to the strtol function with a BASE argument of 10, except that it need not detect overflow errors. The atol function is provided mostly for compatibility with existing code; using strtol is more robust.

 -- Function: int atoi (const char *STRING)
     This function is like atol except that it returns an int.  The atoi function is also considered obsolete; use strtol instead.

 -- Function: long long int atoll (const char *STRING)
     This function is similar to atol except that it returns a long long int.

     The atoll function was introduced in ISO C99.  It too is obsolete (despite having just been added); use strtoll instead.

All the functions mentioned in this section so far do not handle alternative representations of characters as described in the locale data.  Some locales specify the thousands-separator and the way they have to be used, which can help to make large numbers more readable. To read such numbers, one has to use the scanf functions with the ' flag.

Here is a function that parses a string as a sequence of integers and returns the sum of them:

     int
     sum_ints_from_string (char *string)
     {
       int sum = 0;

       while (1) {
         char *tail;
         int next;

         /* Skip white space by hand, to detect the end.  */
         while (isspace (*string)) string++;
         if (*string == 0)
           break;

         /* There is more non-white-space,  */
         /* so it ought to be another number.  */
         errno = 0;
         /* Parse it.  */
         next = strtol (string, &tail, 0);
         /* Add it in, if not overflow.  */
         if (errno)
           printf ("Overflow\n");
         else
           sum += next;
         /* Advance past it.  */
         string = tail;
       }

       return sum;
     }

Parsing of Floats
-----------------

The str functions are declared in stdlib.h, and those beginning with wcs are declared in wchar.h.  One might wonder about the use of restrict in the prototypes of the functions in this section.  It is seemingly useless, but the ISO C standard uses it (for the functions defined there), so we have to use it as well.

 -- Function: double strtod (const char *restrict STRING, char **restrict TAILPTR)
     The strtod (``string-to-double'') function converts the initial part of STRING to a floating-point number, which is returned as a value of type double.

     This function attempts to decompose STRING as follows:

       * A (possibly empty) sequence of white-space characters---which characters are white space is determined by the isspace function (see Classification of Characters).  These are discarded.

       * An optional plus or minus sign (+ or -)

       * A floating-point number in decimal or hexadecimal format; the decimal format is:

            - A nonempty sequence of digits optionally containing a decimal-point character---normally ., but it depends on the locale (see General Numeric).

            - An optional exponent part, consisting of a character e or E, an optional sign and a sequence of digits

          The hexadecimal format is as follows:

            - A 0x or 0X followed by a nonempty sequence of hexadecimal digits optionally containing a decimal-point character---normally ., but it depends on the locale (see General Numeric)

            - An optional binary-exponent part, consisting of a character p or P, an optional sign, and a sequence of digits

       * Any remaining characters in the string.  If TAILPTR is not a null pointer, a pointer to this tail of the string is stored in *TAILPTR.

     If the string is empty, contains only white space, or does not contain an initial substring that has the expected syntax for a floating-point number, no conversion is performed.  In this case, strtod returns a value of zero and the value returned in *TAILPTR is the value of STRING.

     In a locale other than the standard "C" or "POSIX" locales, this function may recognize additional locale-dependent syntax.

     If the string has valid syntax for a floating-point number but the value is outside the range of a double, strtod will signal overflow or underflow as described in Math Error Reporting.

     strtod recognizes four special input strings.  The strings "inf" and "infinity" are converted to oo, or to the largest representable value if the floating-point format doesn't support infinities.  You can prepend a "+" or "-" to specify the sign.  Case is ignored when scanning these strings.

     The strings "nan" and "nan(CHARS...)" are converted to NaN.  Again, case is ignored.  If CHARS... are provided, they are used in some unspecified fashion to select a particular representation of NaN (there can be several).

     Since zero is a valid result as well as the value returned on error, you should check for errors in the same way as for strtol, by examining ERRNO and TAILPTR.

 -- Function: float strtof (const char *STRING, char **TAILPTR)
 -- Function: long double strtold (const char *STRING, char **TAILPTR)
     These functions are analogous to strtod, but they return float and long double values respectively.  They report errors in the same way as strtod.  strtof can be substantially faster than strtod, but has less precision; conversely, strtold can be much slower, but has more precision (on systems where long double is a separate type).

     These functions have been GNU extensions and are new to ISO C99.

 -- Function: double wcstod (const wchar_t *restrict STRING, wchar_t **restrict TAILPTR)
 -- Function: float wcstof (const wchar_t *STRING, wchar_t **TAILPTR)
 -- Function: long double wcstold (const wchar_t *STRING, wchar_t **TAILPTR)
     The wcstod, wcstof and wcstol functions are equivalent in nearly all aspect to the strtod, strtof and strtold functions, but they handle wide-character strings.

     The wcstod function was introduced in Amendment 1 of ISO C90.  The wcstof and wcstold functions were introduced in ISO C99.

 -- Function: double atof (const char *STRING)
     This function is similar to the strtod function, except that it need not detect overflow and underflow errors.  The atof function is provided mostly for compatibility with existing code; using strtod is more robust.

The GNU C Library also provides _l versions of these functions, which take an additional argument, the locale to use in conversion (see Parsing of Integers).

Old-fashioned System V Number-to-String Functions
=================================================

The old System V C library provided three functions to convert numbers to strings, with unusual and hard-to-use semantics.  The GNU C library also provides these functions and some natural extensions.

These functions are only available in glibc and on systems descended from AT&T Unix.  Therefore, unless these functions do precisely what you need, it is better to use sprintf, which is standard.

All these functions are defined in stdlib.h.

 -- Function: char * ecvt (double VALUE, int NDIGIT, int *DECPT, int *NEG)
     The function ecvt converts the floating-point number VALUE to a string with at most NDIGIT decimal digits.  The returned string contains no decimal point or sign. The first digit of the string is nonzero (unless VALUE is actually zero) and the last digit is rounded to nearest.  *DECPT is set to the index in the string of the first digit after the decimal point. *NEG is set to a nonzero value if VALUE is negative and zero otherwise.

     If NDIGIT decimal digits would exceed the precision of a double, it is reduced to a system-specific value.

     The returned string is statically allocated and overwritten by each call to ecvt.

     If VALUE is zero, whether *DECPT is 0 or 1 is implementation defined.

     For example: ecvt (12.3, 5, &d, &n) returns "12300" and sets D to 2 and N to 0.

 -- Function: char * fcvt (double VALUE, int NDIGIT, int *DECPT, int *NEG)
     The function fcvt is like ecvt, but NDIGIT specifies the number of digits after the decimal point.  If NDIGIT is less than zero, VALUE is rounded to the NDIGIT+1'th place to the left of the decimal point.  For example, if NDIGIT is -1, VALUE will be rounded to the nearest 10.  If NDIGIT is negative and larger than the number of digits to the left of the decimal point in VALUE, VALUE will be rounded to one significant digit.

     If NDIGIT decimal digits would exceed the precision of a double, it is reduced to a system-specific value.

     The returned string is statically allocated and overwritten by each call to fcvt.

 -- Function: char * gcvt (double VALUE, int NDIGIT, char *BUF)
     gcvt is functionally equivalent to sprintf(buf, "%*g", ndigit, value).  It is provided only for compatibility's sake.  It returns BUF.

     If NDIGIT decimal digits would exceed the precision of a double, it is reduced to a system-specific value.

As extensions, the GNU C Library provides versions of these three functions that take long double arguments.

 -- Function: char * qecvt (long double VALUE, int NDIGIT, int *DECPT, int *NEG)
     This function is equivalent to ecvt, except that it takes a long double for the first parameter and that NDIGIT is restricted by the precision of a long double.

 -- Function: char * qfcvt (long double VALUE, int NDIGIT, int *DECPT, int *NEG)
     This function is equivalent to fcvt, except that it takes a long double for the first parameter, and that NDIGIT is restricted by the precision of a long double.

 -- Function: char * qgcvt (long double VALUE, int NDIGIT, char *BUF)
     This function is equivalent to gcvt, except that it takes a long double for the first parameter, and that NDIGIT is restricted by the precision of a long double.

The ecvt and fcvt functions, and their long double equivalents, all return a string located in a static buffer, which is overwritten by the next call to the function.  The GNU C Library provides another set of extended functions that write the converted string into a user-supplied buffer.  These have the conventional _r suffix.

gcvt_r is not necessary, because gcvt already uses a user-supplied buffer.

 -- Function: char * ecvt_r (double VALUE, int NDIGIT, int *DECPT, int *NEG, char *BUF, size_t LEN)
     The ecvt_r function is the same as ecvt, except that it places its result into the user-specified buffer pointed to by BUF, with length LEN.

     This function is a GNU extension.

 -- Function: char * fcvt_r (double VALUE, int NDIGIT, int *DECPT, int *NEG, char *BUF, size_t LEN)
     The fcvt_r function is the same as fcvt, except that it places its result into the user-specified buffer pointed to by BUF, with length LEN.

     This function is a GNU extension.

 -- Function: char * qecvt_r (long double VALUE, int NDIGIT, int *DECPT, int *NEG, char *BUF, size_t LEN)
     The qecvt_r function is the same as qecvt, except that it places its result into the user-specified buffer pointed to by BUF, with length LEN.

     This function is a GNU extension.

 -- Function: char * qfcvt_r (long double VALUE, int NDIGIT, int *DECPT, int *NEG, char *BUF, size_t LEN)
     The qfcvt_r function is the same as qfcvt, except that it places its result into the user-specified buffer pointed to by BUF, with length LEN.

     This function is a GNU extension.

Date and Time
*************

This chapter describes functions for manipulating dates and times, including functions for determining what time it is and conversion between different time representations.

Time Basics
===========

Discussing time in a technical manual can be difficult because the word time in English refers to lots of different things.  In this manual, we use a rigorous terminology to avoid confusion, and the only thing we use the simple word time for is to talk about the abstract concept.

A calendar time is a point in the time continuum, for example November 4, 1990 at 18:02.5 UTC.  Sometimes this is called absolute time.

We don't speak of a date, because that is inherent in a calendar time.

An interval is a contiguous part of the time continuum between two calendar times, such as the hour between 9:00 and 10:00 on July 4, 1980.

An elapsed time is the length of an interval, such as 35 minutes.  People sometimes sloppily use the word interval to refer to the elapsed time of some interval.

An amount of time is a sum of elapsed times, which need not be of any specific intervals.  For example, the amount of time it takes to read a book might be 9 hours, independently of when and in how many sittings it is read.

A period is the elapsed time of an interval between two events, especially when they are part of a sequence of regularly repeating events.

CPU time is like calendar time, except that it is based on the subset of the time continuum when a particular process is actively using a CPU.  CPU time is therefore relative to a process.

Processor time is an amount of time that a CPU is in use. In fact, it's a basic system resource, since there is a limit to how much can exist in any given interval (that limit is the elapsed time of the interval multiplied by the number of CPUs in the processor).  People often call this CPU time, but we reserve the latter term in this manual for the definition above.

Elapsed Time
============

One way to represent an elapsed time is with a simple arithmetic data type, as with the following function to compute the elapsed time between two calendar times.  This function is declared in time.h:

 -- Function: double difftime (time_t TIME1, time_t TIME0)
     The difftime function returns the number of seconds of elapsed time between calendar time TIME1 and calendar time TIME0, as a value of type double.  The difference ignores leap seconds unless leap second support is enabled.

     In the GNU system, you can simply subtract time_t values.  But on other systems, the time_t data type might use some other encoding where subtraction doesn't work directly.

The GNU C Library provides two data types specifically for representing an elapsed time.  They are used by various GNU C Library functions, and you can use them for your own purposes too. They're exactly the same, except that one has a resolution in microseconds, and the other, newer one, has a resolution in nanoseconds.

 -- Data Type: struct timeval
     The struct timeval structure represents an elapsed time.  It is declared in sys/time.h and has the following members:

     long int tv_sec
          This represents the number of whole seconds of elapsed time.

     long int tv_usec
          This is the rest of the elapsed time (a fraction of a second), represented as the number of microseconds.  It is always less than one million.

 -- Data Type: struct timespec
     The struct timespec structure represents an elapsed time.  It is declared in time.h and has the following members:

     long int tv_sec
          This represents the number of whole seconds of elapsed time.

     long int tv_nsec
          This is the rest of the elapsed time (a fraction of a second), represented as the number of nanoseconds.  It is always less than one billion.

It is often necessary to subtract two values of type struct timeval or struct timespec.  Here is the best way to do this (it works even on some peculiar operating systems where the tv_sec member has an unsigned type):

     /* Subtract the `struct timeval' values X and Y,
        storing the result in RESULT.
        Return 1 if the difference is negative, otherwise 0.  */

     int
     timeval_subtract (result, x, y)
          struct timeval *result, *x, *y;
     {
       /* Perform the carry for the later subtraction by updating Y. */
       if (x->tv_usec < y->tv_usec) {
         int nsec = (y->tv_usec - x->tv_usec) / 1000000 + 1;
         y->tv_usec -= 1000000 * nsec;
         y->tv_sec += nsec;
       }
       if (x->tv_usec - y->tv_usec > 1000000) {
         int nsec = (x->tv_usec - y->tv_usec) / 1000000;
         y->tv_usec += 1000000 * nsec;
         y->tv_sec -= nsec;
       }

       /* Compute the time remaining to wait.
          tv_usec is certainly positive. */
       result->tv_sec = x->tv_sec - y->tv_sec;
       result->tv_usec = x->tv_usec - y->tv_usec;

       /* Return 1 if result is negative. */
       return x->tv_sec < y->tv_sec;
     }

Common functions that use struct timeval are gettimeofday and settimeofday.

There are no GNU C Library functions specifically oriented toward dealing with elapsed times, but the calendar time, processor time and alarm and sleeping functions have a lot to do with them.

Processor and CPU Time
======================

If you are trying to optimize your program or measure its efficiency, it's very useful to know how much processor time it uses.  For that, calendar time and elapsed times are useless because a process may spend time waiting for I/O or for other processes to use the CPU. However, you can get the information with the functions in this section.

CPU time (see Time Basics) is represented by the data type clock_t, which is a number of clock ticks.  It gives the total amount of time a process has actively used a CPU since some arbitrary event.  On the GNU system, that event is the creation of the process.  While arbitrary in general, the event is always the same event for any particular process, so you can always measure how much time on the CPU a particular computation takes by examinining the process' CPU time before and after the computation.

In the GNU system, clock_t is equivalent to long int and CLOCKS_PER_SEC is an integer value.  But in other systems, both clock_t and the macro CLOCKS_PER_SEC can be either integer or floating-point types.  Casting CPU time values to double, as in the example above, makes sure that operations such as arithmetic and printing work properly and consistently no matter what the underlying representation is.

Note that the clock can wrap around.  On a 32-bit system with CLOCKS_PER_SEC set to one million, this function will return the same value approximately every 72 minutes. (See Loosemore et al., ``Resource Usage and Limitation'', for additional functions to examine a process' use of processor time and to control it.)

CPU Time Inquiry
----------------

To get a process' CPU time, you can use the clock function.  This facility is declared in the header file time.h.

In typical usage, you call the clock function at the beginning and end of the interval you want to time, subtract the values and then divide by CLOCKS_PER_SEC (the number of clock ticks per second) to get processor time, like this:

     #include <time.h>

     clock_t start, end;
     double cpu_time_used;

     start = clock();
     ... /* Do the work. */
     end = clock();
     cpu_time_used = ((double) (end - start)) / CLOCKS_PER_SEC;

Do not use a single CPU time as an amount of time; it doesn't work that way.  Either do a subtraction as shown above or query processor time directly (see Processor Time).

Different computers and operating systems vary wildly in how they keep track of CPU time.  It's common for the internal processor clock to have a resolution somewhere between a hundredth and millionth of a second.

 -- Macro: int CLOCKS_PER_SEC
     The value of this macro is the number of clock ticks per second measured by the clock function.  POSIX requires that this value be one million independent of the actual resolution.

 -- Macro: int CLK_TCK
     This is an obsolete name for CLOCKS_PER_SEC.

 -- Data Type: clock_t
     This is the type of the value returned by the clock function. Values of type clock_t are numbers of clock ticks.

 -- Function: clock_t clock (void)
     This function returns the calling process' current CPU time.  If the CPU time is not available or cannot be represented, clock returns the value (clock_t)(-1).

Processor Time Inquiry
----------------------

The times function returns information about a process' consumption of processor time in a struct tms object, in addition to the process' CPU time (see Time Basics).  You should include the header file sys/times.h to use this facility.

 -- Data Type: struct tms
     The tms structure is used to return information about process times.  It contains at least the following members:

     clock_t tms_utime
          This is the total processor time the calling process has used in executing the instructions of its program.

     clock_t tms_stime
          This is the processor time the system has used on behalf of the calling process.

     clock_t tms_cutime
          This is the sum of the tms_utime values and the tms_cutime values of all terminated child processes of the calling process, whose status has been reported to the parent process by wait or waitpid. (Ibid., ``Process Completion''.)  In other words, it represents the total processor time used in executing the instructions of all the terminated child processes of the calling process, excluding child processes that have not yet been reported by wait or waitpid.

     clock_t tms_cstime
          This is similar to tms_cutime, but represents the total processor time the system has used on behalf of all the terminated child processes of the calling process.

     All of the times are given in numbers of clock ticks.  Unlike CPU time, these are the actual amounts of time; they are not relative to any event. (Ibid., ``Creating a Process''.)

 -- Function: clock_t times (struct tms *BUFFER)
     The times function stores the processor time information for the calling process in BUFFER.

     The return value is the calling process' CPU time (the same value you get from clock()).  times returns (clock_t)(-1) to indicate failure.

Portability Note: The clock function described in CPU Time, is specified by the ISO C standard.  The times function is a feature of POSIX.1.  In the GNU system, the CPU time is defined to be equivalent to the sum of the tms_utime and tms_stime fields returned by times.

Calendar Time
=============

This section describes facilities for keeping track of calendar time (see Time Basics).

The GNU C Library represents calendar time three ways:

  * Simple time (the time_t data type) is a compact representation, typically giving the number of seconds of elapsed time since some implementation-specific base time.

  * There is also a high-resolution time representation.  Like simple time, this represents a calendar time as an elapsed time since a base time, but instead of measuring in whole seconds, it uses a struct timeval data type, which includes fractions of a second.  Use this time representation instead of simple time when you need greater precision.

  * Local time or broken-down time (the struct tm data type) represents a calendar time as a set of components specifying the year, month, etc., in the Gregorian calendar for a specific time zone.  This calendar time representation is usually used only to communicate with people.

Simple Calendar Time
--------------------

This section describes the time_t data type for representing calendar time as simple time, and the functions that operate on simple time objects.  These facilities are declared in the header file time.h.

 -- Data Type: time_t
     This is the data type used to represent simple time.  Sometimes, it also represents an elapsed time.  When interpreted as a calendar time value, it represents the number of seconds elapsed since 00:00:00 on January 1, 1970, Coordinated Universal Time (this calendar time is sometimes referred to as the epoch).  POSIX requires that this count not include leap seconds, but on some systems this count includes leap seconds if you set TZ to certain values (see TZ Variable).

     A simple time has no concept of local time zone.  Calendar Time T is the same instant in time regardless of where on the globe the computer is.

     In the GNU C Library, time_t is equivalent to long int. In other systems, time_t might be either an integer or floating-point type.

The function difftime tells you the elapsed time between two simple calendar times, which is not always as easy to compute as just subtracting (see Elapsed Time).

 -- Function: time_t time (time_t *RESULT)
     The time function returns the current calendar time as a value of type time_t.  If the argument RESULT is not a null pointer, the calendar time value is also stored in *RESULT.  If the current calendar time is not available, the value (time_t)(-1) is returned.

 -- Function: int stime (time_t *NEWTIME)
     stime sets the system clock, i.e., it tells the system that the current calendar time is NEWTIME, where newtime is interpreted as described in the above definition of time_t.

     settimeofday is a newer function that sets the system clock to better than one second precision.  settimeofday is generally a better choice than stime (see High-Resolution Calendar).

     Only the superuser can set the system clock.

     If the function succeeds, the return value is zero.  Otherwise, it is -1 and errno is set accordingly:

     EPERM
          The process is not superuser.

High-Resolution Calendar
------------------------

The time_t data type used to represent simple times has a resolution of only 1 second.  Some applications need more precision.

So, the GNU C Library also contains functions that are capable of representing calendar times to a higher resolution than 1 second.  The functions and the associated data types described in this section are declared in sys/time.h.

 -- Data Type: struct timezone
     The struct timezone structure is used to hold minimal information about the local time zone.  It has the following members:

     int tz_minuteswest
          This is the number of minutes west of UTC.

     int tz_dsttime
          If nonzero, daylight saving time applies during some part of the year.

     The struct timezone type is obsolete and should never be used. Instead, use the facilities described in Time Zone Functions.

 -- Function: int gettimeofday (struct timeval *TP, struct timezone *TZP)
     The gettimeofday function returns the current calendar time as the elapsed time since the epoch in the struct timeval structure indicated by TP.  (see Elapsed Time for a description of struct timeval).  Information about the time zone is returned in the structure pointed at TZP.  If the TZP argument is a null pointer, time zone information is ignored.

     The return value is 0 on success and -1 on failure.  The following errno error condition is defined for this function:

     ENOSYS
          The operating system does not support getting time zone information, and TZP is not a null pointer.  The GNU operating system does not support using struct timezone to represent time zone information; that is an obsolete feature of 4.3 BSD. Instead, use the facilities described in Time Zone Functions.

 -- Function: int settimeofday (const struct timeval *TP, const struct timezone *TZP)
     The settimeofday function sets the current calendar time in the system clock according to the arguments.  As for gettimeofday, the calendar time is represented as the elapsed time since the epoch. As for gettimeofday, time zone information is ignored if TZP is a null pointer.

     You must be a privileged user in order to use settimeofday.

     Some kernels automatically set the system clock from some source such as a hardware clock when they start up.  Others, including Linux, place the system clock in an invalid state (in which attempts to read the clock fail).  A call of stime removes the system clock from an invalid state, and system start-up scripts typically run a program that calls stime.

     settimeofday causes a sudden jump forward or backward, which can cause a variety of problems in a system.  Use adjtime (below) to make a smooth transition from one time to another by temporarily speeding up or slowing down the clock.

     With a Linux kernel, adjtimex does the same thing and can also make permanent changes to the speed of the system clock so that it doesn't need to be corrected as often.

     The return value is 0 on success and -1 on failure.  The following errno error conditions are defined for this function:

     EPERM
          This process cannot set the clock because it is not privileged.

     ENOSYS
          The operating system does not support setting time zone information, and TZP is not a null pointer.

 -- Function: int adjtime (const struct timeval *DELTA, struct timeval *OLDDELTA)
     This function speeds up or slows down the system clock in order to make a gradual adjustment.  This ensures that the calendar time reported by the system clock is always monotonically increasing, which might not happen if you simply set the clock.

     The DELTA argument specifies a relative adjustment to be made to the clock time.  If negative, the system clock is slowed down for a while until it has lost this much elapsed time.  If positive, the system clock is speeded up for a while.

     If the OLDDELTA argument is not a null pointer, the adjtime function returns information about any previous time adjustment that has not yet been completed.

     This function is typically used to synchronize the clocks of computers in a local network.  You must be a privileged user to use it.

     With a Linux kernel, you can use the adjtimex function to permanently change the clock speed.

     The return value is 0 on success and -1 on failure.  The following errno error condition is defined for this function:

     EPERM
          You do not have privilege to set the time.

Portability Note: The gettimeofday, settimeofday and adjtime functions are derived from BSD.

Symbols for the following function are declared in sys/timex.h:

 -- Function: int adjtimex (struct timex *TIMEX)

     adjtimex is functionally identical to ntp_adjtime (see High-Accuracy Clock).

     This function is present only with a Linux kernel.

Broken-Down Time
----------------

Calendar time is represented by the usual GNU C Library functions as an elapsed time since a fixed base calendar time.  This is convenient for computation, but has no relation to the way people normally think of calendar time.  By contrast, broken-down time is a binary representation of calendar time separated into year, month, day and so on.  Broken-down time values are not useful for calculations, but they are useful for printing human-readable time information.

A broken-down time value is always relative to a choice of time zone, and it also indicates which time zone that is.

The symbols in this section are declared in the header file time.h.

 -- Data Type: struct tm
     This is the data type used to represent a broken-down time.  The structure contains at least the following members, which can appear in any order:

     int tm_sec
          This is the number of full seconds since the top of the minute (normally in the range 0 through 59, but the actual upper limit is 60, to allow for leap seconds if leap second support is available).

     int tm_min
          This is the number of full minutes since the top of the hour (in the range 0 through 59).

     int tm_hour
          This is the number of full hours past midnight (in the range 0 through 23).

     int tm_mday
          This is the ordinal day of the month (in the range 1 through 31). Watch out for this one!  As the only ordinal number in the structure, it is inconsistent with the rest of the structure.

     int tm_mon
          This is the number of full calendar months since the beginning of the year (in the range 0 through 11).  Watch out for this one! People usually use ordinal numbers for month-of-year (where January = 1).

     int tm_year
          This is the number of full calendar years since 1900.

     int tm_wday
          This is the number of full days since Sunday (in the range 0 through 6).

     int tm_yday
          This is the number of full days since the beginning of the year (in the range 0 through 365).

     int tm_isdst
          This is a flag that indicates whether daylight saving time is (or was, or will be) in effect at the time described.  The value is positive if daylight saving time is in effect, zero if it is not and negative if the information is not available.

     long int tm_gmtoff
          This field describes the time zone that was used to compute this broken-down time value, including any adjustment for daylight saving; it is the number of seconds that you must add to UTC to get local time. You can also think of this as the number of seconds east of UTC.  For example, for US eastern standard time, the value is -5*60*60. The tm_gmtoff field is derived from BSD and is a GNU library extension; it is not visible in a strict ISO C environment.

     const char *tm_zone
          This field is the name for the time zone that was used to compute this broken-down time value.  Like tm_gmtoff, this field is a BSD and GNU extension, and is not visible in a strict ISO C environment.

 -- Function: struct tm * localtime (const time_t *TIME)
     The localtime function converts the simple time pointed to by TIME to broken-down time representation, expressed relative to the user's specified time zone.

     The return value is a pointer to a static broken-down time structure, which might be overwritten by subsequent calls to ctime, gmtime, or localtime (but no other library function overwrites the contents of this object).

     The return value is the null pointer if TIME cannot be represented as a broken-down time; typically this is because the year cannot fit into an int.

     Calling localtime has one other effect: it sets the variable tzname with information about the current time zone (see Time Zone Functions).

Using the localtime function is a big problem in multithreaded programs.  The result is returned in a static buffer and this is used in all threads.  POSIX.1c introduced a variant of this function.

 -- Function: struct tm * localtime_r (const time_t *TIME, struct tm *RESULTP)
     The localtime_r function works just like the localtime function.  It takes a pointer to a variable containing a simple time and converts it to the broken-down time format.

     But the result is not placed in a static buffer.  Instead it is placed in the object of type struct tm to which the parameter RESULTP points.

     If the conversion is successful the function returns a pointer to the object the result was written into, i.e., it returns RESULTP.

 -- Function: struct tm * gmtime (const time_t *TIME)
     This function is similar to localtime, except that the broken-down time is expressed as Coordinated Universal Time (UTC) (formerly called Greenwich mean time (GMT)) rather than relative to a local time zone.

As for the localtime function, we have the problem that the result is placed in a static variable.  POSIX.1c also provides a replacement for gmtime.

 -- Function: struct tm * gmtime_r (const time_t *TIME, struct tm *RESULTP)
     This function is similar to localtime_r, except that it converts, just like gmtime, the given time as Coordinated Universal Time.

     If the conversion is successful, the function returns a pointer to the object the result was written into, i.e., it returns RESULTP.

 -- Function: time_t mktime (struct tm *BROKENTIME)
     The mktime function is used to convert a broken-down time structure to a simple time representation.  It also normalizes the contents of the broken-down time structure, by filling in the day of week and day of year based on the other date and time components.

     The mktime function ignores the specified contents of the tm_wday and tm_yday members of the broken-down time structure.  It uses the values of the other components to determine the calendar time; it's permissible for these components to have unnormalized values outside their normal ranges.  The last thing that mktime does is adjust the components of the BROKENTIME structure (including the tm_wday and tm_yday).

     If the specified broken-down time cannot be represented as a simple time, mktime returns a value of (time_t)(-1) and does not modify the contents of BROKENTIME.

     Calling mktime also sets the variable tzname with information about the current time zone (see Time Zone Functions).

 -- Function: time_t timelocal (struct tm *BROKENTIME)

     timelocal is functionally identical to mktime, but more mnemonically named.  It is the inverse of the localtime function.

     Portability Note:  mktime is essentially universally available.  timelocal is rather rare.

 -- Function: time_t timegm (struct tm *BROKENTIME)

     timegm is functionally identical to mktime, except that it always takes the input values to be Coordinated Universal Time (UTC) regardless of any local time zone setting.

     timegm is the inverse of gmtime.

     Portability Note:  mktime is essentially universally available.  timegm is rather rare.  For the most portable conversion from a UTC broken-down time to a simple time, set the TZ environment variable to UTC, call mktime, then set TZ back.

High-Accuracy Clock
-------------------

The ntp_gettime and ntp_adjtime functions provide an interface to monitor and manipulate the system clock to maintain high-accuracy time.  For example, you can fine-tune the speed of the clock or synchronize it with another time source.

A typical use of these functions is by a server implementing the Network Time Protocol to synchronize the clocks of multiple systems and high-precision clocks.

These functions are declared in sys/timex.h.

 -- Data Type: struct ntptimeval
     This structure is used for information about the system clock.  It contains the following members:

     struct timeval time
          This is the current calendar time, expressed as the elapsed time since the epoch.  The struct timeval data type is described in Elapsed Time.

     long int maxerror
          This is the maximum error, measured in microseconds.  Unless updated via ntp_adjtime periodically, this value will reach some platform-specific maximum value.

     long int esterror
          This is the estimated error, measured in microseconds.  This value can be set by ntp_adjtime to indicate the estimated offset of the system clock from the true calendar time.

 -- Function: int ntp_gettime (struct ntptimeval *TPTR)
     The ntp_gettime function sets the structure pointed to by TPTR to current values.  The elements of the structure afterwards contain the values the timer implementation in the kernel assumes.  They might or might not be correct.  If they are not, an ntp_adjtime call is necessary.

     The return value is 0 on success and other values on failure.  The following errno error conditions are defined for this function:

     TIME_ERROR
          The precision clock model is not properly set up at the moment, thus the clock must be considered unsynchronized, and the values should be treated with care.

 -- Data Type: struct timex
     This structure is used to control and monitor the system clock.  It contains the following members:

     unsigned int modes
          This variable controls whether and which values are set.  Several symbolic constants have to be combined with binary or to specify the effective mode.  These constants start with MOD_.

     long int offset
          This value indicates the current offset of the system clock from the true calendar time.  The value is given in microseconds.  If bit MOD_OFFSET is set in modes, the offset (and possibly other dependent values) can be set.  The offset's absolute value must not exceed MAXPHASE.

     long int frequency
          This value indicates the difference in frequency between the true calendar time and the system clock.  The value is expressed as scaled PPM (parts per million, 0.0001%).  The scaling is 1 << SHIFT_USEC.  The value can be set with bit MOD_FREQUENCY, but the absolute value must not exceed MAXFREQ.

     long int maxerror
          This is the maximum error, measured in microseconds.  A new value can be set using bit MOD_MAXERROR.  Unless updated via ntp_adjtime periodically, this value will increase steadily and reach some platform-specific maximum value.

     long int esterror
          This is the estimated error, measured in microseconds.  This value can be set using bit MOD_ESTERROR.

     int status
          This variable reflects the various states of the clock machinery.  There are symbolic constants for the significant bits, starting with STA_.  Some of these flags can be updated using the MOD_STATUS bit.

     long int constant
          This value represents the bandwidth or stiffness of the PLL (phase locked loop) implemented in the kernel.  The value can be changed using bit MOD_TIMECONST.

     long int precision
          This value represents the accuracy or the maximum error when reading the system clock.  The value is expressed in microseconds.

     long int tolerance
          This value represents the maximum frequency error of the system clock in scaled PPM.  This value is used to increase the maxerror every second.

     struct timeval time
          This is the current calendar time.

     long int tick
          This is the elapsed time between clock ticks in microseconds.  A clock tick is a periodic timer interrupt on which the system clock is based.

     long int ppsfreq
          This is the first of a few optional variables that are present only if the system clock can use a PPS (pulse per second) signal to discipline the system clock.  The value is expressed in scaled PPM and it denotes the difference in frequency between the system clock and the PPS signal.

     long int jitter
          This value expresses a median filtered average of the PPS signal's dispersion in microseconds.

     int shift
          This value is a binary exponent for the duration of the PPS calibration interval, ranging from PPS_SHIFT to PPS_SHIFTMAX.

     long int stabil
          This value represents the median filtered dispersion of the PPS frequency in scaled PPM.

     long int jitcnt
          This counter represents the number of pulses where the jitter exceeded the allowed maximum MAXTIME.

     long int calcnt
          This counter reflects the number of successful calibration intervals.

     long int errcnt
          This counter represents the number of calibration errors (caused by large offsets or jitter).

     long int stbcnt
          This counter denotes the number of calibrations where the stability exceeded the threshold.

 -- Function: int ntp_adjtime (struct timex *TPTR)
     The ntp_adjtime function sets the structure specified by TPTR to current values.

     In addition, ntp_adjtime updates some settings to match what you pass to it in *TPTR.  Use the modes element of *TPTR to select what settings to update.  You can set offset, freq, maxerror, esterror, status, constant and tick.

     modes = zero means set nothing.

     Only the superuser can update settings.

     The return value is 0 on success and other values on failure.  The following errno error conditions are defined for this function:

     TIME_ERROR
          The high-accuracy clock model is not properly set up at the moment, thus the clock must be considered unsynchronized, and the values should be treated with care.  Another reason could be that the specified new values are not allowed.

     EPERM
          The process specified a settings update but is not superuser.

     For more details see RFC1305 (Network Time Protocol, Version 3) and related documents.

     Portability Note: Early versions of the GNU C Library did not have this function but did have the synonymous adjtimex.

Formatting Calendar Time
------------------------

The functions described in this section format calendar time values as strings.  These functions are declared in the header file time.h.

 -- Function: char * asctime (const struct tm *BROKENTIME)
     The asctime function converts the broken-down time value that BROKENTIME points to into a string in a standard format:

          "Tue May 21 13:46:22 1991\n"

     The abbreviations for the days of week are: Sun, Mon, Tue, Wed, Thu, Fri and Sat.

     The abbreviations for the months are: Jan, Feb, Mar, Apr, May, Jun, Jul, Aug, Sep, Oct, Nov and Dec.

     The return value points to a statically allocated string, which might be overwritten by subsequent calls to asctime or ctime (but no other library function overwrites the contents of this string).

 -- Function: char * asctime_r (const struct tm *BROKENTIME, char *BUFFER)
     This function is similar to asctime, but instead of placing the result in a static buffer it writes the string in the buffer pointed to by the parameter BUFFER.  This buffer should have room for at least 26 bytes, including the terminating null.

     If no error occurred, the function returns a pointer to the string the result was written into, i.e., it returns BUFFER.  Otherwise, it returns NULL.

 -- Function: char * ctime (const time_t *TIME)
     The ctime function is similar to asctime, except that you specify the calendar time argument as a time_t simple time value rather than in broken-down local time format.  It is equivalent to:

          asctime (localtime (TIME))

     ctime sets the variable tzname, because localtime does so (see Time Zone Functions).

 -- Function: char * ctime_r (const time_t *TIME, char *BUFFER)
     This function is similar to ctime but places the result in the string pointed to by BUFFER.  It is equivalent to: (This was written using GCC extensions. See Richard M. Stallman, ``Statements and Declarations in Expressions'' in Using and Porting GCC (Boston, MA: GNU Press, July 1999), http:// gcc.gnu.org/ onlinedocs/ gcc-2.95.3/ gcc.htm.)

          ({ struct tm tm; asctime_r (localtime_r (time, &tm), buf); })

     If no error occurred the function returns a pointer to the string the result was written into, i.e., it returns BUFFER.  Otherwise, it returns a NULL.

 -- Function: size_t strftime (char *S, size_t SIZE, const char *TEMPLATE, const struct tm *BROKENTIME)
     This function is similar to the sprintf function (see Formatted Input), but the conversion specifications that can appear in the format template TEMPLATE are specialized for printing components of the date and time BROKENTIME according to the locale currently specified for time conversion (see Locales).

     Ordinary characters appearing in the TEMPLATE are copied to the output string S; this can include multibyte-character sequences. Conversion specifiers are introduced by a % character, followed by an optional flag, which can be one of the following.  These flags are all GNU extensions. The first three affect only the output of numbers:

     _
          The number is padded with spaces.

     -
          The number is not padded at all.

     0
          The number is padded with zeros even if the format specifies padding with spaces.

     ^
          The output uses uppercase characters, but only if this is possible (see Case Conversion).

     The default action is to pad the number with zeros to keep it a constant width.  Numbers that do not have a range indicated below are never padded, since there is no natural width for them.

     Following the flag, an optional specification of the width is possible.  This is specified in decimal notation.  If the natural size of the output of the field has less than the specified number of characters, the result is written right-adjusted and space padded to the given size.

     An optional modifier can follow the optional flag and width specification.  The modifiers, which were first standardized by POSIX.2-1992 and by ISO C99, are

     E
          Use the locale's alternate representation for date and time.  This modifier applies to the %c, %C, %x, %X, %y and %Y format specifiers.  In a Japanese locale, for example, %Ex might yield a date format based on the Japanese Emperors' reigns.

     O
          Use the locale's alternate numeric symbols for numbers.  This modifier applies only to numeric format specifiers.

     If the format supports the modifier but no alternate representation is available, it is ignored.

     The conversion specifier ends with a format specifier taken from the following list.  The whole % sequence is replaced in the output string as follows:

     %a
          This is the abbreviated weekday name according to the current locale.

     %A
          This is the full weekday name according to the current locale.

     %b
          This is the abbreviated month name according to the current locale.

     %B
          This is the full month name according to the current locale.

     %c
          This is the preferred calendar time representation for the current locale.

     %C
          This is the century of the year.  This is equivalent to the greatest integer not greater than the year divided by 100.  This format was first standardized by POSIX.2-1992 and by ISO C99.

     %d
          This is the day of the month as a decimal number (range 01 through 31).

     %D
          This is the date using the format %m/%d/%y.  This format was first standardized by POSIX.2-1992 and by ISO C99.

     %e
          This is the day of the month like with %d, but padded with blank (range 1 through 31).

          This format was first standardized by POSIX.2-1992 and by ISO C99.

     %F
          This is the date using the format %Y-%m-%d.  This is the form specified in the ISO 8601 standard and is the preferred form for all uses.  This format was first standardized by ISO C99 and by POSIX.1-2001.

     %g
          This is the year corresponding to the ISO week number, but without the century (range 00 through 99).  This has the same format and value as %y, except that if the ISO week number (see %V) belongs to the previous or next year, that year is used instead.  This format was first standardized by ISO C99 and by POSIX.1-2001.

     %G
          This is the year corresponding to the ISO week number.  This has the same format and value as %Y, except that if the ISO week number (see %V) belongs to the previous or next year, that year is used instead.  This format was first standardized by ISO C99 and by POSIX.1-2001 but was previously available as a GNU extension.

     %h
          This is the abbreviated month name according to the current locale. The action is the same as for %b.  This format was first standardized by POSIX.2-1992 and by ISO C99.

     %H
          This is the hour as a decimal number, using a 24-hour clock (range 00 through 23).

     %I
          This is the hour as a decimal number, using a 12-hour clock (range 01 through 12).

     %j
          This is the day of the year as a decimal number (range 001 through 366).

     %k
          This is the hour as a decimal number, using a 24-hour clock like %H, but padded with blank (range  0 through 23). This format is a GNU extension.

     %l
          This is the hour as a decimal number, using a 12-hour clock like %I, but padded with blank (range  1 through 12). This format is a GNU extension.

     %m
          This is the month as a decimal number (range 01 through 12).

     %M
          This is the minute as a decimal number (range 00 through 59).

     %n
          This is a single \n (newline) character.  This format was first standardized by POSIX.2-1992 and by ISO C99.

     %p
          This is either AM or PM, according to the given time value; or the corresponding strings for the current locale.  Noon is treated as PM and midnight as AM.  In most locales AM/PM format is not supported; in such cases, "%p" yields an empty string.

     %P
          This is either am or pm, according to the given time value; or the corresponding strings for the current locale, printed in lowercase characters.  Noon is treated as pm and midnight as am.  In most locales AM/PM format is not supported; in such cases, "%P" yields an empty string.  This format is a GNU extension.

     %r
          This is the complete calendar time using the a.m./p.m. format of the current locale.  This format was first standardized by POSIX.2-1992 and by ISO C99.  In the POSIX locale, this format is equivalent to %I:%M:%S %p.

     %R
          This is the hour and minute in decimal numbers using the format %H:%M. This format was first standardized by ISO C99 and by POSIX.1-2001 but was previously available as a GNU extension.

     %s
          This is the number of seconds since the epoch, i.e., since 1970-01-01 00:00:00 UTC.  Leap seconds are not counted unless leap second support is available.  This format is a GNU extension.

     %S
          This is the seconds as a decimal number (range 00 through 60).

     %t
          This is a single \t (tabulator) character.  This format was first standardized by POSIX.2-1992 and by ISO C99.

     %T
          This is the time of day using decimal numbers, in the format %H:%M:%S.  This format was first standardized by POSIX.2-1992 and by ISO C99.

     %u
          This is the day of the week as a decimal number (range 1 through 7), Monday being 1.  This format was first standardized by POSIX.2-1992 and by ISO C99.

     %U
          This is the week number of the current year as a decimal number (range 00 through 53), starting with the first Sunday as the first day of the first week.  Days preceding the first Sunday in the year are considered to be in week 00.

     %V
          This is the ISO 8601:1988 week number as a decimal number (range 01 through 53).  ISO weeks start with Monday and end with Sunday.  Week 01 of a year is the first week that has the majority of its days in that year; this is equivalent to the week containing the year's first Thursday, and it is also equivalent to the week containing January 4.  Week 01 of a year can contain days from the previous year.  The week before week 01 of a year is the last week (52 or 53) of the previous year even if it contains days from the new year.  This format was first standardized by POSIX.2-1992 and by ISO C99.

     %w
          This is the day of the week as a decimal number (range 0 through 6), Sunday being 0.

     %W
          This is the week number of the current year as a decimal number (range 00 through 53), starting with the first Monday as the first day of the first week.  All days preceding the first Monday in the year are considered to be in week 00.

     %x
          This is the preferred date representation for the current locale.

     %X
          This is the preferred time of day representation for the current locale.

     %y
          This is the year without a century as a decimal number (range 00 through 99).  This is equivalent to the year modulo 100.

     %Y
          This is the year as a decimal number, using the Gregorian calendar.  Years before the year 1 are numbered 0, -1, and so on.

     %z
          This is the RFC 822/ISO 8601:1988 numeric time zone (e.g., -0600 or +0100), or nothing if no time zone is determinable.  This format was first standardized by ISO C99 and by POSIX.1-2001 but was previously available as a GNU extension.

          In the POSIX locale, a full RFC 822 time stamp is generated by the format "%a, %d %b %Y %H:%M:%S %z" (or the equivalent "%a, %d %b %Y %T %z").

     %Z
          This is the time zone abbreviation (empty if the time zone can't be determined).

     %%
          This is a literal % character.

     The SIZE parameter can be used to specify the maximum number of characters to be stored in the array S, including the terminating null character.  If the formatted time requires more than SIZE characters, strftime returns zero and the contents of the array S are undefined.  Otherwise, the return value indicates the number of characters placed in the array S, not including the terminating null character.

     Warning: This convention for the return value, prescribed in ISO C, can lead to problems in some situations.  For certain format strings and certain locales, the output really can be the empty string, and this cannot be discovered by testing the return value only.  For example, in most locales the a.m./p.m. time format is not supported (most of the world uses the 24-hour time representation). In such locales, "%p" will return the empty string, i.e., the return value is zero.  To detect situations like this, something similar to the following code should be used:

          buf[0] = '\1';
          len = strftime (buf, bufsize, format, tp);
          if (len == 0 && buf[0] != '\0')
            {
              /* Something went wrong in the strftime call.  */
              ...
            }

     If S is a null pointer, strftime does not actually write anything, but instead returns the number of characters it would have written.

     According to POSIX.1, every call to strftime implies a call to tzset.  So the contents of the environment variable TZ are examined before any output is produced.

     For an example of strftime, see Time Functions Example.

 -- Function: size_t wcsftime (wchar_t *S, size_t SIZE, const wchar_t *TEMPLATE, const struct tm *BROKENTIME)
     The wcsftime function is equivalent to the strftime function with the difference that it operates on wide-character strings.  The buffer where the result is stored, pointed to by S, must be an array of wide characters.  The parameter SIZE, which specifies the size of the output buffer, gives the number of wide characters, not the number of bytes.

     Also, the format string TEMPLATE is a wide-character string. Since all characters needed to specify the format string are in the basic character set, it is portably possible to write format strings in the C source code using the L"..." notation.  The parameter BROKENTIME has the same meaning as in the strftime call.

     The wcsftime function supports the same flags, modifiers, and format specifiers as the strftime function.

     The return value of wcsftime is the number of wide characters stored in s.  When more characters would have to be written than can be placed in the buffer S the return value is zero, with the same problems indicated in the strftime documentation.

Convert Textual Time and Date Information Back
----------------------------------------------

The ISO C standard does not specify any functions that can convert the output of the strftime function back into a binary format.  This led to a variety of more-or-less successful implementations with different interfaces over the years.  Then the Unix standard was extended by the addition of two functions: strptime and getdate.  Both have strange interfaces, but at least they are widely available.

Interpret String According to Given Format
..........................................

The first function is rather low-level.  It is nevertheless frequently used in software since it is better known.  Its interface and implementation are heavily influenced by the getdate function, which is defined and implemented in terms of calls to strptime.

 -- Function: char * strptime (const char *S, const char *FMT, struct tm *TP)
     The strptime function parses the input string S according to the format string FMT and stores its results in the structure TP.

     The input string could be generated by a strftime call or obtained any other way.  It does not need to be in a human-recognizable format; e.g. a date passed as "02:1999:9" is acceptable, even though it is ambiguous without context.  As long as the format string FMT matches the input string, the function will succeed.

     The user has to make sure, though, that the input can be parsed in a unambiguous way.  The string "1999112" can be parsed using the format "%Y%m%d" as 1999-1-12, 1999-11-2, or even 19991-1-2.  It is necessary to add appropriate separators to reliably get results.

     The format string consists of the same components as the format string of the strftime function.  The only difference is that the flags _, -, 0 and ^ are not allowed. Several of the distinct formats of strftime do the same work in strptime, since differences like case of the input do not matter.  For reasons of symmetry, though, all formats are supported.

     The modifiers E and O are also allowed everywhere the strftime function allows them.

     The formats are

     %a
     %A
          This is the weekday name according to the current locale, in abbreviated form or the full name.

     %b
     %B
     %h
          This is the month name according to the current locale, in abbreviated form or the full name.

     %c
          This is the date and time representation for the current locale.

     %Ec
          This is like %c, but the locale's alternative date and time format is used.

     %C
          This is the century of the year.

          It makes sense to use this format only if the format string also contains the %y format.

     %EC
          This is the locale's representation of the period.

          Unlike %C, it sometimes makes sense to use this format since some cultures represent years relative to the beginning of eras instead of using the Gregorian years.

     %d

     %e
          This is the day of the month as a decimal number (range 1 through 31). Leading zeros are permitted but not required.

     %Od
     %Oe
          This is same as %d but using the locale's alternative numeric symbols.

          Leading zeros are permitted but not required.

     %D
          Equivalent to %m/%d/%y.

     %F
          This is equivalent to %Y-%m-%d, which is the ISO 8601 date format.

          This is a GNU extension following an ISO C99 extension to strftime.

     %g
          This is the year corresponding to the ISO week number, but without the century (range 00 through 99).

          Currently, this is not fully implemented.  The format is recognized, input is consumed, but no field in TM is set.

          This format is a GNU extension following a GNU extension of strftime.

     %G
          This is the year corresponding to the ISO week number.

          Currently, this is not fully implemented.  The format is recognized, input is consumed, but no field in TM is set.

          This format is a GNU extension following a GNU extension of strftime.

     %H
     %k
          This is the hour as a decimal number, using a 24-hour clock (range 00 through 23).

          %k is a GNU extension following a GNU extension of strftime.

     %OH
          This is the same as %H but using the locale's alternative numeric symbols.

     %I
     %l
          This is the hour as a decimal number, using a 12-hour clock (range 01 through 12).

          %l is a GNU extension following a GNU extension of strftime.

     %OI
          This is the same as %I but using the locale's alternative numeric symbols.

     %j
          This is the day of the year as a decimal number (range 1 through 366).

          Leading zeros are permitted but not required.

     %m
          This is the month as a decimal number (range 1 through 12).

          Leading zeros are permitted but not required.

     %Om
          This is the same as %m but using the locale's alternative numeric symbols.

     %M
          This is the minute as a decimal number (range 0 through 59).

          Leading zeros are permitted but not required.

     %OM
          This is the same as %M but using the locale's alternative numeric symbols.

     %n
     %t
          This matches any white space.

     %p

     %P
          This is the locale-dependent equivalent to AM or PM.

          This format is not useful unless %I or %l is also used. Another complication is that the locale might not define these values at all, and therefore the conversion fails.

          %P is a GNU extension following a GNU extension to strftime.

     %r
          This is the complete time using the a.m./p.m. format of the current locale.

          A complication is that the locale might not define this format at all, and therefore the conversion fails.

     %R
          This is the hour and minute in decimal numbers using the format %H:%M.

          %R is a GNU extension following a GNU extension to strftime.

     %s
          This is the number of seconds since the epoch, i.e., since 1970-01-01 00:00:00 UTC. Leap seconds are not counted unless leap second support is available.

          %s is a GNU extension following a GNU extension to strftime.

     %S
          This is the seconds as a decimal number (range 0 through 60).

          Leading zeros are permitted but not required.

          The Unix specification says the upper bound on this value is 61, a result of a decision to allow double leap seconds.  You will not see the value 61, because no minute has more than one leap second, but the myth persists.

     %OS
          This is the same as %S but using the locale's alternative numeric symbols.

     %T
          This is equivalent to the use of %H:%M:%S in this place.

     %u
          This is the day of the week as a decimal number (range 1 through 7), Monday being 1.

          Leading zeros are permitted but not required.

          Currently, this is not fully implemented.  The format is recognized, input is consumed, but no field in TM is set.

     %U
          This is the week number of the current year as a decimal number (range 0 through 53).

          Leading zeros are permitted but not required.

     %OU
          This is the same as %U but using the locale's alternative numeric symbols.

     %V
          This is the ISO 8601:1988 week number as a decimal number (range 1 through 53).

          Leading zeros are permitted but not required.

          Currently, this is not fully implemented.  The format is recognized, input is consumed, but no field in TM is set.

     %w
          This is the day of the week as a decimal number (range 0 through 6), Sunday being 0.

          Leading zeros are permitted but not required.

          Currently, this is not fully implemented.  The format is recognized, input is consumed but no field in TM is set.

     %Ow
          This is the same as %w but using the locale's alternative numeric symbols.

     %W
          This is the week number of the current year as a decimal number (range 0 through 53).

          Leading zeros are permitted but not required.

          Currently, this is not fully implemented.  The format is recognized, input is consumed, but no field in TM is set.

     %OW
          This is the same as %W but using the locale's alternative numeric symbols.

     %x
          This is the date using the locale's date format.

     %Ex
          This is like %x, but the locale's alternative data representation is used.

     %X
          This is the time using the locale's time format.

     %EX
          This is like %X, but the locale's alternative time representation is used.

     %y
          This is the year without a century as a decimal number (range 0 through 99).

          Leading zeros are permitted but not required.

          It is questionable to use this format without the %C format. The strptime function does regard input values in the range 68 to 99 as the years 1969 to 1999 and the values 0 to 68 as the years 2000 to 2068. But maybe this heuristic fails for some input data.

          Therefore, it is best to avoid %y completely and use %Y instead.

     %Ey
          This is the offset from %EC in the locale's alternative representation.

     %Oy
          This is the offset of the year (from %C) using the locale's alternative numeric symbols.

     %Y
          This is the year as a decimal number, using the Gregorian calendar.

     %EY
          This is the full alternative year representation.

     %z
          This is the offset from GMT in ISO 8601/RFC822 format.

     %Z
          This is the time zone name.

          Currently, this is not fully implemented.  The format is recognized, input is consumed, but no field in TM is set.

     %%
          This is a literal % character.

     All other characters in the format string must have a matching character in the input string.  Exceptions are white spaces in the input string that can match zero or more white-space characters in the format string.

     Portability Note: The XPG standard advises applications to use at least one white-space character (as specified by isspace) or other nonalphanumeric characters between any two conversion specifications.  The GNU C Library does not have this limitation, but other libraries might have trouble parsing formats like "%d%m%Y%H%M%S".

     The strptime function processes the input string from right to left.  Each of the three possible input elements (white space, literal, or format) are handled one after the other.  If the input cannot be matched to the format string, the function stops.  The remainder of the format and input strings are not processed.

     The function returns a pointer to the first character it was unable to process.  If the input string contains more characters than required by the format string, the return value points right after the last consumed input character.  If the whole input string is consumed, the return value points to the NULL byte at the end of the string. If an error occurs, i.e., strptime fails to match all of the format string, the function returns NULL.

The specification of the function in the XPG standard is rather vague, leaving out a few important pieces of information.  Most importantly, it does not specify what happens to those elements of TM that are not directly initialized by the different formats. The implementations on different Unix systems vary here.

The GNU libc implementation does not touch those fields that are not directly initialized.  Exceptions are the tm_wday and tm_yday elements, which are recomputed if any of the year, month, or date elements changed.  This has two implications:

  * Before calling the strptime function for a new input string, you should prepare the TM structure you pass.  Normally this will mean initializing all values to zero.  Alternatively, you can set all fields to values like INT_MAX, allowing you to determine which elements were set by the function call.  Zero does not work here, since it is a valid value for many of the fields.

     Careful initialization is necessary if you want to find out whether a certain field in TM was initialized by the function call.

  * You can construct a struct tm value with several consecutive strptime calls.  A useful application of this is the parsing of two separate strings, one containing date information and the other time information.  By parsing one after the other without clearing the structure in between, you can construct a complete broken-down time.

The following example shows a function that parses a string that contains the date information in either US style or ISO 8601 form:

     const char *
     parse_date (const char *input, struct tm *tm)
     {
       const char *cp;

       /* First clear the result structure.  */
       memset (tm, '\0', sizeof (*tm));

       /* Try the ISO format first.  */
       cp = strptime (input, "%F", tm);
       if (cp == NULL)
         {
           /* Does not match.  Try the US form.  */
           cp = strptime (input, "%D", tm);
         }

       return cp;
     }

A More User-Friendly Way to Parse Times and Dates
.................................................

The Unix standard defines another function for parsing date strings. The interface is weird, but if the function happens to suit your application, it is just fine.  It is problematic to use this function in multithreaded programs or libraries, since it returns a pointer to a static variable and uses a global variable and global state (an environment variable).

 -- Variable: getdate_err
     This variable of type int contains the error code of the last unsuccessful call to getdate.  Defined values are:

     1
          The environment variable DATEMSK is not defined or is null.

     2
          The template file denoted by the DATEMSK environment variable cannot be opened.

     3
          Information about the template file cannot retrieved.

     4
          The template file is not a regular file.

     5
          An I/O error occurred while reading the template file.

     6
          Not enough memory is available to execute the function.

     7
          The template file contains no matching template.

     8
          The input date is invalid, but would match a template otherwise.  This includes dates like February 31st, and dates that cannot be represented in a time_t variable.

 -- Function: struct tm * getdate (const char *STRING)
     The interface to getdate is the simplest possible way for a function to parse a string and return the value.  STRING is the input string, and the result is returned in a statically allocated variable.

     The details about how the string is processed are hidden from the user.  In fact, they can be outside the control of the program.  Which formats are recognized is controlled by the file named by the environment variable DATEMSK.  This file should contain lines of valid format strings that could be passed to strptime.

     The getdate function reads these format strings one after the other and tries to match the input string.  The first line that completely matches the input string is used.

     Elements not initialized through the format string retain the values present at the time of the getdate function call.

     The formats recognized by getdate are the same as for strptime.  See above for an explanation.  There are only a few extensions to the strptime behavior:

       * If the %Z format is given, the broken-down time is based on the current time of the time zone matched, not of the current time zone of the run-time environment.

          Currently, this is not implemented.  The problem is that time zone names are not unique.  If a fixed time zone is assumed for a given string (say EST meaning US East Coast time), then uses for countries other than the United States will fail.  So far, we have found no good solution to this.

       * If only the weekday is specified, the selected day depends on the current date.  If the current weekday is greater or equal to the tm_wday value, the current week's day is chosen, otherwise the day next week is chosen.

       * A similar heuristic is used when only the month is given and not the year.  If the month is greater than or equal to the current month, then the current year is used.  Otherwise, it wraps to next year.  The first day of the month is assumed if one is not explicitly specified.

       * The current hour, minute and second are used if the appropriate value is not set through the format.

       * If no date is given, tomorrow's date is used if the time is smaller than the current time.  Otherwise, today's date is taken.

     The format in the template file need not contain only format elements. The following is a list of possible format strings (taken from the Unix standard):

          %m
          %A %B %d, %Y %H:%M:%S
          %A
          %B
          %m/%d/%y %I %p
          %d,%m,%Y %H:%M
          at %A the %dst of %B in %Y
          run job at %I %p,%B %dnd
          %A den %d. %B %Y %H.%M Uhr

     As you can see, the template list can contain very specific strings like run job at %I %p,%B %dnd.  Using the above list of templates and assuming the current time is Mon Sep 22 12:19:47 EDT 1986, we can obtain the following results for the given input.

     Input       Match   Result
     Mon         %A      Mon Sep 22 12:19:47 EDT 1986
     Sun         %A      Sun Sep 28 12:19:47 EDT 1986
     Fri         %A      Fri Sep 26 12:19:47 EDT 1986
     September   %B      Mon Sep 1 12:19:47 EDT 1986
     January     %B      Thu Jan 1 12:19:47 EST 1987
     December    %B      Mon Dec 1 12:19:47 EST 1986
     Sep Mon     %B %A   Mon Sep 1 12:19:47 EDT 1986
     Jan Fri     %B %A   Fri Jan 2 12:19:47 EST 1987
     Dec Mon     %B %A   Mon Dec 1 12:19:47 EST 1986
     Jan Wed 1989    %B %A %Y   Wed Jan 4 12:19:47 EST 1989
     Fri 9       %A %H   Fri Sep 26 09:00:00 EDT 1986
     Feb 10:30   %B %H:%S   Sun Feb 1 10:00:30 EST 1987
     10:30       %H:%M   Tue Sep 23 10:30:00 EDT 1986
     13:30       %H:%M   Mon Sep 22 13:30:00 EDT 1986

     The return value of the function is a pointer to a static variable of type struct tm, or a null pointer if an error occurred.  The result is only valid until the next getdate call, making this function unusable in multithreaded applications.

     The errno variable is not changed.  Error conditions are stored in the global variable getdate_err.  See the description above for a list of the possible error values.

     Warning: The getdate function should never be used in SUID-programs.  Using the DATEMSK environment variable, you can get the function to open any arbitrary file, and chances are high that with some bogus input (such as a binary file), the program will crash.

 -- Function: int getdate_r (const char *STRING, struct tm *TP)
     The getdate_r function is the reentrant counterpart of getdate.  It does not use the global variable getdate_err to signal an error, but instead returns an error code.  The same error codes as described in the getdate_err documentation above are used, with 0 meaning success.

     Moreover, getdate_r stores the broken-down time in the variable of type struct tm pointed to by the second argument, rather than in a static variable.

     This function is not defined in the Unix standard.  Nevertheless, it is available on some other Unix systems as well.

     The warning against using getdate in SUID-programs applies to getdate_r.

Specifying the Time Zone with TZ
--------------------------------

In POSIX systems, a user can specify the time zone by means of the TZ environment variable.  For information about how to set environment variables, see Environment Variables.  The functions for accessing the time zone are declared in time.h.

You should not normally need to set TZ.  If the system is configured properly, the default time zone will be correct.  You might set TZ if you are using a computer over a network from a different time zone, and would like times reported to you in the time zone local to you, rather than what is local to the computer.

In POSIX.1 systems the value of the TZ variable can be in one of three formats.  With the GNU C Library, the most common format is the last one, which can specify a selection from a large database of time zone information for many regions of the world.  The first two formats are used to describe the time zone information directly, which is both more cumbersome and less precise.  But the POSIX.1 standard only specifies the details of the first two formats, so it is good to be familiar with them in case you come across a POSIX.1 system that doesn't support a time zone information database.

The first format is used when there is no daylight saving time (or summer time) in the local time zone:

     STD OFFSET

The STD string specifies the name of the time zone.  It must be three or more characters long and must not contain a leading colon, embedded digits, commas, or plus or minus signs.  There is no space character separating the time zone name from the OFFSET, so these restrictions are necessary to parse the specification correctly.

The OFFSET specifies the time value you must add to the local time to get a Coordinated Universal Time value.  It has syntax like [+|-]HH[:MM[:SS]].  This is positive if the local time zone is west of the prime meridian and negative if it is east.  The hour must be between 0 and 23 and the minute and seconds between 0 and 59.

For example, here is how we would specify eastern standard time, but without any daylight saving time alternative:

     EST+5

The second format is used when there is daylight saving time:

     STD OFFSET DST [OFFSET],START[/TIME],END[/TIME]

The initial STD and OFFSET specify the standard time zone, as described above.  The DST string and OFFSET specify the name and offset for the corresponding daylight saving time zone; if the OFFSET is omitted, it defaults to one hour ahead of standard time.

The remainder of the specification describes when daylight saving time is in effect.  The START field is when daylight saving time goes into effect, and the END field is when the change is made back to standard time.  The following formats are recognized for these fields:

JN
     This specifies the Julian day, with N between 1 and 365. February 29 is never counted, even in leap years.

N
     This specifies the Julian day, with N between 0 and 365. February 29 is counted in leap years.

MM.W.D
     This specifies day D of week W of month M.  The day D must be between 0 (Sunday) and 6.  The week W must be between 1 and 5; week 1 is the first week in which day D occurs, and week 5 specifies the last D day in the month.  The month M should be between 1 and 12.

The TIME fields specify when, in the local time currently in effect, the change to the other time occurs.  If omitted, the default is 02:00:00.

For example, here is how you would specify the eastern time zone in the United States, including the appropriate daylight saving time and its dates of applicability.  The normal offset from UTC is five hours; since this is west of the prime meridian, the sign is positive. Summer time begins on the first Sunday in April at 2:00 a.m., and ends on the last Sunday in October at 2:00 a.m.

     EST+5EDT,M4.1.0/2,M10.5.0/2

The schedule of daylight saving time in any particular jurisdiction has changed over the years.  To be strictly correct, the conversion of dates and times in the past should be based on the schedule that was in effect then.  However, this format has no facilities to let you specify how the schedule has changed from year to year.  The most you can do is specify one particular schedule---usually the present day schedule---and this is used to convert any date, no matter when.  For precise time zone specifications, it is best to use the time zone information database (see below).

The third format looks like this:

     :CHARACTERS

Each operating system interprets this format differently; in the GNU C Library, CHARACTERS is the name of a file that describes the time zone.

If the TZ environment variable does not have a value, the operation chooses a time zone by default.  In the GNU C Library, the default time zone is like the specification TZ=:/etc/localtime (or TZ=:/usr/local/etc/localtime, depending on how GNU C Library was configured. (Ibid., ``Installing the GNU C Library''.)  Other C libraries use their own rule for choosing the default time zone, so there is little we can say about them.

If CHARACTERS begins with a slash, it is an absolute file name; otherwise the library looks for the file /share/lib/zoneinfo/CHARACTERS.  The zoneinfo directory contains data files describing local time zones in many different parts of the world.  The names represent major cities, with subdirectories for geographical areas; for example, America/New_York, Europe/London, Asia/Hong_Kong. These data files are installed by the system administrator, who also sets /etc/localtime to point to the data file for the local time zone.  The GNU C Library comes with a large database of time zone information for most regions of the world, which is maintained by a community of volunteers and put in the public domain.

Functions and Variables for Time Zones
--------------------------------------

 -- Variable: char * tzname [2]
     The array tzname contains two strings, which are the standard names of the pair of time zones (standard and daylight saving) that the user has selected.  tzname[0] is the name of the standard time zone (for example, "EST"), and tzname[1] is the name for the time zone when daylight saving time is in use (for example, "EDT").  These correspond to the STD and DST strings (respectively) from the TZ environment variable.  If daylight saving time is never used, tzname[1] is the empty string.

     The tzname array is initialized from the TZ environment variable whenever tzset, ctime, strftime, mktime or localtime is called.  If multiple abbreviations have been used (e.g., "EWT" and "EDT" for US eastern war time and eastern daylight time), the array contains the most recent abbreviation.

     The tzname array is required for POSIX.1 compatibility, but in GNU programs it is better to use the tm_zone member of the broken-down time structure, since tm_zone reports the correct abbreviation even when it is not the latest one.

     Though the strings are declared as char *, the user must refrain from modifying these strings.  Modifying the strings will almost certainly lead to trouble.

 -- Function: void tzset (void)
     The tzset function initializes the tzname variable from the value of the TZ environment variable.  It is not usually necessary for your program to call this function, because it is called automatically when you use the other time conversion functions that depend on the time zone.

The following variables are defined for compatibility with System V Unix.  Like tzname, these variables are set by calling tzset or the other time conversion functions.

 -- Variable: long int timezone
     This contains the difference between UTC and the latest local standard time, in seconds west of UTC.  For example, in the US eastern time zone, the value is 5*60*60.  Unlike the tm_gmtoff member of the broken-down time structure, this value is not adjusted for daylight saving, and its sign is reversed.  In GNU programs it is better to use tm_gmtoff, since it contains the correct offset even when it is not the latest one.

 -- Variable: int daylight
     This variable has a nonzero value if daylight saving time rules apply. A nonzero value does not necessarily mean that daylight saving time is now in effect; it means only that daylight saving time is sometimes in effect.

Time Functions Example
----------------------

Here is an example program showing the use of some of the calendar time functions:

     #include <time.h>
     #include <stdio.h>

     #define SIZE 256

     int
     main (void)
     {
       char buffer[SIZE];
       time_t curtime;
       struct tm *loctime;

       /* Get the current time. */
       curtime = time (NULL);

       /* Convert it to local time representation. */
       loctime = localtime (&curtime);

       /* Print out the date and time in the standard format. */
       fputs (asctime (loctime), stdout);

       /* Print it out in a nice format. */
       strftime (buffer, SIZE, "Today is %A, %B %d.\n", loctime);
       fputs (buffer, stdout);
       strftime (buffer, SIZE, "The time is %I:%M %p.\n", loctime);
       fputs (buffer, stdout);

       return 0;
     }

It produces output like this:

     Wed Jul 31 13:02:36 1991
     Today is Wednesday, July 31.
     The time is 01:02 PM.

Setting an Alarm
================

The alarm and setitimer functions provide a mechanism for a process to interrupt itself in the future.  They do this by setting a timer; when the timer expires, the process receives a signal.

Each process has three independent interval timers available:

  * A real-time timer that counts elapsed time.  This timer sends a SIGALRM signal to the process when it expires.

  * A virtual timer that counts processor time used by the process.  This timer sends a SIGVTALRM signal to the process when it expires.

  * A profiling timer that counts both processor time used by the process and processor time spent in system calls on behalf of the process. This timer sends a SIGPROF signal to the process when it expires.

     This timer is useful for profiling in interpreters.  The interval timer mechanism does not have the fine granularity necessary for profiling native code.

You can only have one timer of each kind set at any given time.  If you set a timer that has not yet expired, that timer is simply reset to the new value.

You should establish a handler for the appropriate alarm signal using signal or sigaction before issuing a call to setitimer or alarm.  Otherwise, an unusual chain of events could cause the timer to expire before your program establishes the handler.  In this case it would be terminated, since termination is the default action for the alarm signals. (Ibid., ``Signal Handling''.)

To be able to use the alarm function to interrupt a system call that might otherwise block indefinitely, it is important to not set the SA_RESTART flag when registering the signal handler using sigaction.  When not using sigaction, things get even uglier---the signal function has to fix semantics with respect to restarts.  The BSD semantics for this function is to set the flag.  Therefore, if sigaction for whatever reason cannot be used, it is necessary to use sysv_signal and not signal.

The setitimer function is the primary means for setting an alarm. This facility is declared in the header file sys/time.h.  The alarm function, declared in unistd.h, provides a somewhat simpler interface for setting the real-time timer.

 -- Data Type: struct itimerval
     This structure is used to specify when a timer should expire.  It contains the following members:

     struct timeval it_interval
          This is the period between successive timer interrupts.  If zero, the alarm will only be sent once.

     struct timeval it_value
          This is the period between now and the first timer interrupt.  If zero, the alarm is disabled.

     The struct timeval data type is described in Elapsed Time.

 -- Function: int setitimer (int WHICH, struct itimerval *NEW, struct itimerval *OLD)
     The setitimer function sets the timer specified by WHICH according to NEW.  The WHICH argument can have a value of ITIMER_REAL, ITIMER_VIRTUAL, or ITIMER_PROF.

     If OLD is not a null pointer, setitimer returns information about any previous unexpired timer of the same kind in the structure it points to.

     The return value is 0 on success and -1 on failure.  The following errno error conditions are defined for this function:

     EINVAL
          The timer period is too large.

 -- Function: int getitimer (int WHICH, struct itimerval *OLD)
     The getitimer function stores information about the timer specified by WHICH in the structure pointed at by OLD.

     The return value and error conditions are the same as for setitimer.

ITIMER_REAL
     This constant can be used as the WHICH argument to the setitimer and getitimer functions to specify the real-time timer.

ITIMER_VIRTUAL
     This constant can be used as the WHICH argument to the setitimer and getitimer functions to specify the virtual timer.

ITIMER_PROF
     This constant can be used as the WHICH argument to the setitimer and getitimer functions to specify the profiling timer.

 -- Function: unsigned int alarm (unsigned int SECONDS)
     The alarm function sets the real-time timer to expire in SECONDS seconds. (See Loosemore et al., ``Signal Handlers That Return'', for an example showing the use of the alarm function.)  If you want to cancel any existing alarm, you can do this by calling alarm with a SECONDS argument of zero.

     The return value indicates how many seconds remain before the previous alarm would have been sent.  If there is no previous alarm, alarm returns zero.

The alarm function could be defined in terms of setitimer like this:

     unsigned int
     alarm (unsigned int seconds)
     {
       struct itimerval old, new;
       new.it_interval.tv_usec = 0;
       new.it_interval.tv_sec = 0;
       new.it_value.tv_usec = 0;
       new.it_value.tv_sec = (long int) seconds;
       if (setitimer (ITIMER_REAL, &new, &old) < 0)
         return 0;
       else
         return old.it_value.tv_sec;
     }

If you simply want your process to wait for a given number of seconds, you should use the sleep function (see Sleeping).

You shouldn't count on the signal arriving precisely when the timer expires.  In a multiprocessing environment, there is typically some amount of delay involved.

Portability Note: The setitimer and getitimer functions are derived from BSD Unix, while the alarm function is specified by the POSIX.1 standard.  setitimer is more powerful than alarm, but alarm is more widely used.

Sleeping
========

The function sleep gives a simple way to make the program wait for a short interval.  If your program doesn't use signals (except to terminate), then you can expect sleep to wait reliably throughout the specified interval.  Otherwise, sleep can return sooner if a signal arrives; if you want to wait for a given interval regardless of signals, use select and don't specify any descriptors to wait for. (Ibid., ``Waiting for Input or Output''.)

 -- Function: unsigned int sleep (unsigned int SECONDS)
     The sleep function waits for SECONDS or until a signal is delivered, whichever happens first.

     If the sleep function returns because the requested interval is over, it returns a value of zero.  If it returns because of delivery of a signal, its return value is the remaining time in the sleep interval.

     The sleep function is declared in unistd.h.

Resist the temptation to implement a sleep for a fixed amount of time by using the return value of sleep, when nonzero, to call sleep again.  This will work with a certain amount of accuracy as long as signals arrive infrequently.  But each signal can cause the eventual wake-up time to be off by an additional second or so. Suppose a few signals happen to arrive in rapid succession by bad luck---there is no limit on how much this could shorten or lengthen the wait.

Instead, compute the calendar time at which the program should stop waiting, and keep trying to wait until that calendar time.  This won't be off by more than a second.  With just a little more work, you can use select and make the waiting period quite accurate.  Of course, heavy system load can cause additional unavoidable delays---unless the machine is dedicated to one application, there is no way you can avoid this.

On some systems, sleep can do strange things if your program uses SIGALRM explicitly.  Even if SIGALRM signals are being ignored or blocked when sleep is called, sleep might return prematurely on delivery of a SIGALRM signal.  If you have established a handler for SIGALRM signals and a SIGALRM signal is delivered while the process is sleeping, the action taken might be just to cause sleep to return instead of to invoke your handler.  And, if sleep is interrupted by delivery of a signal whose handler requests an alarm or alters the handling of SIGALRM, this handler and sleep will interfere.

On the GNU system, it is safe to use sleep and SIGALRM in the same program, because sleep does not work by means of SIGALRM.

 -- Function: int nanosleep (const struct timespec *REQUESTED_TIME, struct timespec *REMAINING)
     If resolution to seconds is not enough, the nanosleep function can be used.  As the name suggests, the sleep interval can be specified in nanoseconds.  The actual elapsed time of the sleep interval might be longer, since the system rounds the elapsed time you request up to the next integer multiple of the actual resolution the system can deliver.

     *requested_time is the elapsed time of the interval you want to sleep.

     The function returns as *remaining the elapsed time left in the interval for which you requested to sleep.  If the interval completed without getting interrupted by a signal, this is zero.

     struct timespec is described in Elapsed Time.

     If the function returns because the interval is over, the return value is zero.  If the function returns -1, the global variable ERRNO is set to the following values:

     EINTR
          The call was interrupted because a signal was delivered to the thread. If the REMAINING parameter is not the null pointer, the structure pointed to by REMAINING is updated to contain the remaining elapsed time.

     EINVAL
          The nanosecond value in the REQUESTED_TIME parameter contains an illegal value.  Either the value is negative or greater than or equal to 1000 million.

     This function is a cancellation point in multithreaded programs.  This is a problem if the thread allocates some resources (like memory, file descriptors, semaphores or whatever) at the time nanosleep is called.  If the thread gets canceled, these resources stay allocated until the program ends.  To avoid this, calls to nanosleep should be protected using cancellation handlers.

     The nanosleep function is declared in time.h.

Message Translation
*******************

The program's interface with a user should be designed to make tasks easier for the user.  One way it can do this is to use messages in whichever language the user prefers.

Printing messages in different languages can be implemented in different ways.  You could add all the different languages in the source code and add among the variants every time a message has to be printed.  This is certainly not a good solution, since extending the set of languages is difficult (the code must be changed), and the code itself can become really big with dozens of message sets.

A better solution is to keep the message sets for each language in separate files that are loaded at run time depending on the language selection of the user.

The GNU C Library provides two different sets of functions to support message translation.  The problem is that neither of the interfaces is officially defined by the POSIX standard.  The catgets family of functions is defined in the X/Open standard, but this is derived from industry decisions and therefore not necessarily based on reasonable decisions.

As mentioned above, the message catalog handling provides easy extendability by using external data files that contain the message translations.  These files contain for each of the messages used in the program a translation for the appropriate language.  So the tasks of the message handling functions are

  * Locate the external data file with the appropriate translations.

  * Load the data and make it possible to address the messages.

  * Map a given key to the translated message.

The two approaches mainly differ in the implementation of this last step.  The design decisions made for this step influence the rest.

X/Open Message Catalog Handling
===============================

The catgets functions are based on the simple scheme:

     Associate every message to translate in the source code with a unique identifier.  To retrieve a message from a catalog file, just the identifier is used.

This means for the author of the program that he will have to make sure the meaning of the identifier in the program code and in the message catalogs are always the same.

Before a message can be translated, the catalog file must be located. The user of the program must be able to guide the responsible function to find whichever catalog she wants.

All the types, constants and functions for the catgets functions are defined/declared in the nl_types.h header file.

The catgets Function Family
---------------------------

 -- Function: nl_catd catopen (const char *CAT_NAME, int FLAG)
     The catgets function tries to locate the message data file name CAT_NAME and loads it when found.  The return value is of an opaque type and can be used in calls to the other functions to refer to this loaded catalog.

     The return value is (nl_catd) -1 in case the function failed and no catalog was loaded.  The global variable ERRNO contains a code for the error causing the failure.  But even if the function call succeeded, this does not mean that all messages can be translated.

     Locating the catalog file must happen in a way that lets the user of the program influence the decision.  It is up to the user to decide about the language to use, and sometimes it is useful to use alternate catalog files.  All this can be specified by the user by setting some environment variables.

     The first problem is to find out where all the message catalogs are stored.  Every program could have its own place to keep all the different files, but usually the catalog files are grouped by languages and the catalogs for all programs are kept in the same place.

     To tell the catopen function where the catalog for the program can be found, the user can set the environment variable NLSPATH to a value that describes her choice.  Since this value must be usable for different languages and locales, it cannot be a simple string. Instead, it is a format string (similar to printf's).  An example is

          /usr/share/locale/%L/%N:/usr/share/locale/%L/LC_MESSAGES/%N

     First, one can see that more than one directory can be specified (with the usual syntax of separating them by colons).  The next things to observe are the format strings, %L and %N in this case. The catopen function knows about several of them, and the replacement for all of them is of course different.

     %N
          This format element is substituted with the name of the catalog file. This is the value of the CAT_NAME argument given to catgets.

     %L
          This format element is substituted with the name of the currently selected locale for translating messages.  How this is determined is explained below.

     %l
          This is the lowercase ell. This format element is substituted with the language element of the locale name.  The string describing the selected locale is expected to have the form LANG[_TERR[.CODESET]], and this format uses the first part LANG.

     %t
          This format element is substituted by the territory part TERR of the name of the currently selected locale.  See the explanation of the format above.

     %c
          This format element is substituted by the codeset part CODESET of the name of the currently selected locale.  See the explanation of the format above.

     %%
          Since % is used in a meta character, there must be a way to express the % character in the result itself.  Using %% does this just like it works for printf.

     Using NLSPATH allows arbitrary directories to be searched for message catalogs while still allowing different languages to be used. If the NLSPATH environment variable is not set, the default value is

          PREFIX/share/locale/%L/%N:PREFIX/share/locale/%L/LC_MESSAGES/%N

     where PREFIX is given to configure while installing the GNU C Library (this value is in many cases /usr or the empty string).

     The remaining problem is to decide which must be used.  The value decides about the substitution of the format elements mentioned above. First of all, the user can specify a path in the message catalog name (i.e., the name contains a slash character).  In this situation the NLSPATH environment variable is not used.  The catalog must exist as specified in the program, perhaps relative to the current working directory.  This situation in not desirable and catalog names should never be written this way.  Besides this, this behavior is not portable to all other platforms providing the catgets interface.

     Otherwise, the values of environment variables from the standard environment are examined (see Standard Environment).  Which variables are examined is decided by the FLAG parameter of catopen.  If the value is NL_CAT_LOCALE (which is defined in nl_types.h), then the catopen function uses the name of the locale currently selected for the LC_MESSAGES category.

     If FLAG is zero, the LANG environment variable is examined.  This is a leftover from the early days where the concept of the locales had not even reached the level of POSIX locales.

     The environment variable and the locale name should have a value of the form LANG[_TERR[.CODESET]] as explained above.  If no environment variable is set, the "C" locale is used, which prevents any translation.

     The return value of the function is in any case a valid string.  Either it is a translation from a message catalog or it is the same as the STRING parameter.  So a piece of code to decide whether a translation actually happened must look like this:

          {
            char *trans = catgets (desc, set, msg, input_string);
            if (trans == input_string)
              {
                /* Something went wrong.  */
              }
          }

     When an error occurred the global variable ERRNO is set to:

     EBADF
          The catalog does not exist.

     ENOMSG
          The set/message tuple does not name an existing element in the message catalog.

     While it sometimes can be useful to test for errors, programs normally will avoid any test.  If the translation is not available, it is not a big problem if the original, untranslated message is printed.  Either the user understands this as well, or he will look for the reason why the messages are not translated.

The currently selected locale does not depend on a call to the setlocale function.  It is not necessary for the locale data files for this locale to exist and for calling setlocale to succeed.  The catopen function directly reads the values of the environment variables.

 -- Function: char * catgets (nl_catd CATALOG_DESC, int SET, int MESSAGE, const char *STRING)
     The function catgets has to be used to access the massage catalog previously opened using the catopen function.  The CATALOG_DESC parameter must be a value previously returned by catopen.

     The next two parameters, SET and MESSAGE, reflect the internal organization of the message catalog files.  This will be explained in detail below.  For now it is interesting to know that a catalog can consist of several sets and the messages in each thread are individually numbered using numbers.  Neither the set number nor the message number must be consecutive.  They can be arbitrarily chosen.  But each message (unless equal to another one) must have its own unique pair of set and message number.

     Since it is not guaranteed that the message catalog for the language selected by the user exists, the last parameter STRING helps to handle this case gracefully.  If no matching string can be found, STRING is returned.  This means for the programmer that:

       * The STRING parameters should contain reasonable text (this also helps to understand the program, since otherwise there would be no hint as to the string that is expected to be returned.

       * All STRING arguments should be written in the same language.

It is somewhat uncomfortable to write a program using the catgets functions if no supporting functionality is available. Since each set/message number tuple must be unique, the programmer must keep lists of the messages at the same time the code is written. And the work between several people working on the same project must be coordinated.  We will see how these problems can be relaxed a bit (see Common Usage).

 -- Function: int catclose (nl_catd CATALOG_DESC)
     The catclose function can be used to free the resources associated with a message catalog that previously was opened by a call to catopen.  If the resources can be successfully freed, the function returns 0.  Otherwise, it returns -1 and the global variable ERRNO is set.  Errors can occur if the catalog descriptor CATALOG_DESC is not valid, in which case ERRNO is set to EBADF.

Format of the Message Catalog Files
-----------------------------------

The only reasonable way to translate all the messages of a function and store the result in a message catalog file that can be read by the catopen function is to write all the message text to the translator and let her translate them all; we must have a file with entries that associate the set/message tuple with a specific translation.  This file format is specified in the X/Open standard and is as follows:

  * Lines containing only white-space characters or empty lines are ignored.

  * Lines that contain as the first non-white-space character a $ followed by a white-space character are comment and are also ignored.

  * If a line contains as the first non-white-space characters the sequence $set followed by a white-space character, an additional argument is required to follow.  This argument can either be

       - A number; in this case, the value of this number determines the set to which the following messages are added.

       - An identifier consisting of alphanumeric characters plus the underscore character; in this case, the set automatically gets a number assigned.  This value is one added to the largest set number that has appeared so far.

          How to use the symbolic names is explained in Common Usage.

          It is an error if a symbol name appears more than once.  All following messages are placed in a set with this number.

  * If a line contains as the first non-white-space characters the sequence $delset followed by a white-space character, an additional argument is required to follow.  This argument can either be

       - A number; in this case, the value of this number determines the set that will be deleted.

       - An identifier consisting of alphanumeric characters plus the underscore character; this symbolic identifier must match a name for a set that was previously defined.  It is an error if the name is unknown.

     In both cases, all messages in the specified set will be removed. They will not appear in the output.  But if this set is selected again later with a $set command, messages could be added again, and these messages will appear in the output.

  * If a line contains after leading white spaces the sequence $quote, the quoting character used for this input file is changed to the first non-white-space character following the $quote.  If no non-white-space character is present before the line ends, quoting is disabled.

     By default, no quoting character is used.  In this mode, strings are terminated with the first unescaped line break.  If there is a $quote sequence present, newline need not be escaped.  Instead, a string is terminated with the first unescaped appearance of the quote character.

     A common usage of this feature would be to set the quote character to ".  Then any appearance of the " in the strings must be escaped using the backslash (i.e., \" must be written).

  * Any other line must start with a number or an alphanumeric identifier (with the underscore character included).  The following characters (starting after the first white-space character) will form the string that gets associated with the currently selected set and the message number represented by the number and identifier respectively.

     If the start of the line is a number, the message number is obvious. It is an error if the same message number already appeared for this set.

     If the leading token was an identifier, the message number gets automatically assigned.  The value is the current maximum messages number for this set plus one.  It is an error if the identifier was already used for a message in this set.  It is OK to reuse the identifier for a message in another thread.  How to use the symbolic identifiers will be explained below (see Common Usage).  There is one limitation with the identifier: it must not be Set.  The reason will be explained below.

     The text of the messages can contain escape characters.  The usual bunch of characters known from the ISO C language are recognized (\n, \t, \v, \b, \r, \f, \\, and \NNN, where NNN is the octal coding of a character code).

Important: The handling of identifiers instead of numbers for the set and messages is a GNU extension.  Systems strictly following the X/Open specification do not have this feature.  An example for a message catalog file is this:

     $ This is a leading comment.
     $quote "

     $set SetOne
     1 Message with ID 1.
     two "   Message with ID \"two\", which gets the value 2 assigned"

     $set SetTwo
     $ Since the last set got the number 1 assigned this set has number 2.
     4000 "The numbers can be arbitrary, they need not start at one."

This small example shows various aspects:

  * Lines 1 and 9 are comments, since they start with $ followed by a white space.

  * The quoting character is set to ".  Otherwise, the quotes in the message definition would have to be left away, and in this case, the message with the identifier two would lose its leading white space.

  * Mixing numbered messages with messages having symbolic names is no problem, and the numbering happens automatically.

While this file format is pretty easy, it is not the best possible for use in a running program.  The catopen function would have to parse the file and handle syntactic errors gracefully.  This is not so easy, and the whole process is pretty slow.  Therefore, the catgets functions expect the data in another more compact and ready-to-use file format.  There is a special program gencat, which is explained in detail in the next section.

Files in this other format are not human readable.  To be easy to use for programs, it is a binary file.  But the format is byte-order independent, so translation files can be shared by systems of arbitrary architecture (as long as they use the GNU C Library).

Details about the binary file format are not important to know, since these files are always created by the gencat program.  The sources of the GNU C Library also provide the sources for the gencat program, so the interested reader can look through these source files to learn about the file format.

Generate Message Catalogs Files
-------------------------------

The gencat program is specified in the X/Open standard, and the GNU implementation follows this specification, so it processes all correctly formed input files.  Additionally, some extension are implemented that help to work in a more reasonable way with the catgets functions.

The gencat program can be invoked in two ways:

     `gencat [OPTION]... [OUTPUT-FILE [INPUT-FILE]...]`

This is the interface defined in the X/Open standard.  If no INPUT-FILE parameter is given, input will be read from standard input.  Multiple input files will be read as if they are concatenated. If OUTPUT-FILE is also missing, the output will be written to standard output.  To provide the interface one is used to from other programs, a second interface is provided.

     `gencat [OPTION]... -o OUTPUT-FILE [INPUT-FILE]...`

The option -o is used to specify the output file, and all file arguments are used as input files.

Besides this, you can use - or /dev/stdin for INPUT-FILE to denote the standard input.  Correspondingly, one can use - and /dev/stdout for OUTPUT-FILE to denote standard output.  Using - as a file name is allowed in X/Open, while using the device names is a GNU extension.

The gencat program works by concatenating all input files and then merging the resulting collection of message sets with a possibly existing output file.  This is done by removing all messages with set/message number tuples matching any of the generated messages from the output file, and then adding all the new messages.  To regenerate a catalog file while ignoring the old contents therefore requires you to remove the output file if it exists.  If the output is written to standard output, no merging takes place.

The following table shows the options understood by the gencat program; the X/Open standard does not specify any option for the program, so all of these are GNU extensions:

-V
--version
     Print the version information and exit.

-h
--help
     Print a usage message listing all available options, then exit successfully.

--new
     Never merge the new messages from the input files with the old content of the output files.  The old content of the output files is discarded.

-H
--header=name
     This option is used to emit the symbolic names given to sets and messages in the input files for use in the program.  Details about how to use this are given in the next section.  The NAME parameter to this option specifies the name of the output file.  It will contain a number of C preprocessor #defines to associate a name with a number.

     The generated file only contains the symbols from the input files.  If the output is merged with the previous content of the output file, the possibly existing symbols from the file(s) that generated the old output files are not in the generated header file.

How to Use the catgets Interface
--------------------------------

The catgets functions can be used in two different ways: by slavishly following the X/Open specs and not relying on the extensions or by using the GNU extensions.  We will take a look at the former method first to understand the benefits of extensions.

Not Using Symbolic Names
........................

Since the X/Open format of the message catalog files does not allow symbol names, we have to work with numbers all the time.  When we start writing a program, we have to replace all appearances of translatable strings with something like:

     catgets (catdesc, set, msg, "string")

CATGETS is retrieved from a call to catopen, which is normally done once at the program start.  The "string" is the string we want to translate.  The problems start with the set and message numbers.

In a bigger program, several programmers usually work at the same time on the program, so coordinating the number allocation is crucial. Though no two different strings must be indexed by the same tuple of numbers, it is highly desirable to reuse the numbers for equal strings with equal translations (there might be strings that are equal in one language but have different translations due to different contexts).

The allocation process can be relaxed a bit by different set numbers for different parts of the program.  So the number of developers who have to coordinate the allocation can be reduced.  But still, lists must be made to keep track of the allocation, and errors can easily happen.  These errors cannot be discovered by the compiler or the catgets functions.  Only the user of the program might see wrong messages printed.  In the worst cases, the messages are so irritating that they cannot be recognized as wrong.  Think about the translations for "true" and "false" being exchanged. This could result in a disaster.

Using Symbolic Names
....................

The problems mentioned in the last section derive from the fact that:

 1. The numbers are allocated once, and due to the possibly frequent use of them, it is difficult to change a number later.

 2. The numbers do not allow you to guess anything about the string, and therefore collisions can easily happen.

By constantly using symbolic names and by providing a method that maps the string content to a symbolic name (however this will happen), you can prevent both of the above problems.  The cost of this is that the programmer has to write a complete message catalog file while she is writing the program itself.

This is necessary since the symbolic names must be mapped to numbers before the program sources can be compiled.  In the last section, it was described how to generate a header containing the mapping of the names.  For the example message file given in the last section, we could call the gencat program as follows (assume ex.msg contains the sources):

     gencat -H ex.h -o ex.cat ex.msg

This generates a header file with the following content:

     #define SetTwoSet 0x2   /* ex.msg:8 */

     #define SetOneSet 0x1   /* ex.msg:4 */
     #define SetOnetwo 0x2   /* ex.msg:6 */

As can be seen, the various symbols given in the source file are mangled to generate unique identifiers and these identifiers get numbers assigned.  Reading the source file and knowing about the rules will allow you to predict the content of the header file (it is deterministic), but this is not necessary.  The gencat program can take care of everything.  All the programmer has to do is put the generated header file in the dependency list of the source files of his project and add a rule to regenerate the header of any of the input files change.

One word about the symbol mangling.  Every symbol consists of two parts: the name of the message set plus the name of the message or the special string Set.  So SetOnetwo means this macro can be used to access the translation with identifier two in the message set SetOne.

The other names denote the names of the message sets.  The special string Set is used in the place of the message identifier.

If in the code the second string of the set SetOne is used, the C code should look like this:

     catgets (catdesc, SetOneSet, SetOnetwo,
              "   Message with ID \"two\", which gets the value 2 assigned")

Writing the function this way will allow you to change the message number and even the set number without any change in the C source code (the text of the string is normally not the same; this is only for this example).

Using Symbolic Version Numbers
..............................

To illustrate the usual way to work with the symbolic version numbers, here is a little example.  Assume we want to write the very complex and famous greeting program.  We start by writing the code as usual:

     #include <stdio.h>
     int
     main (void)
     {
       printf ("Hello, world!\n");
       return 0;
     }

Now we want to internationalize the message and therefore replace the message with whatever the user wants:

     #include <nl_types.h>
     #include <stdio.h>
     #include "msgnrs.h"
     int
     main (void)
     {
       nl_catd catdesc = catopen ("hello.cat", NL_CAT_LOCALE);
       printf (catgets (catdesc, SetMainSet, SetMainHello,
                        "Hello, world!\n"));
       catclose (catdesc);
       return 0;
     }

We see how the catalog object is opened and the returned descriptor used in the other function calls.  It is not really necessary to check for failure of any of the functions, since even in these situations, the functions will behave reasonably.  They simply will be return a translation.

What remains unspecified here are the constants SetMainSet and SetMainHello.  These are the symbolic names describing the message.  To get the actual definitions that match the information in the catalog file, we have to create the message catalog source file and process it using the gencat program:

     $ Messages for the famous greeting program.
     $quote "

     $set Main
     Hello "Hallo, Welt!\n"

Now we can start building the program (assume the message catalog source file is named hello.msg and the program source file hello.c):

     % gencat -H msgnrs.h -o hello.cat hello.msg
     % cat msgnrs.h
     #define MainSet 0x1     /* hello.msg:4 */
     #define MainHello 0x1   /* hello.msg:5 */
     % gcc -o hello hello.c -I.
     % cp hello.cat /usr/share/locale/de/LC_MESSAGES
     % echo $LC_ALL
     de
     % ./hello
     Hallo, Welt!
     %

The call of the gencat program creates the missing header file msgnrs.h as well as the message catalog binary.  The former is used in the compilation of hello.c while the latter is placed in a directory where the catopen function will try to locate it.  Please check the LC_ALL environment variable and the default path for catopen presented in the description above.

The Uniforum Approach to Message Translation
============================================

Sun Microsystems tried to standardize a different approach to message translation in the Uniforum group.  There never was a real standard defined, but the interface was used in Sun's operation systems.  Since this approach fits better in the development process of free software, it is also used throughout the GNU project and the GNU gettext package provides support for this outside the GNU C Library.

The code of the libintl from GNU gettext is the same as the code in the GNU C Library.  So the documentation in the GNU gettext manual is also valid for the functionality here.  The following text will describe the library functions in detail.  But the numerous helper programs are not described in this manual.  Instead, people should read the GNU gettext manual. (The GNU Project, GNU gettext Utilities (Free Software Foundation, May 6, 2003), http:// www.gnu.org/ software/ gettext/ manual/ gettext.)  We will only give a short overview.

Though the catgets functions are available by default on more systems, the gettext interface is at least as portable as the former.  The GNU gettext package can be used wherever the functions are not available.

The gettext Family of Functions
-------------------------------

The paradigm underlying the gettext approach to message translation is different from that of the catgets functions, though the basic functionally is equivalent.

What Has to Be Done to Translate a Message?
...........................................

The gettext functions have a very simple interface.  The most basic function just takes the string that will be translated as the argument and returns the translation.  This is fundamentally different from the catgets approach, where an extra key is necessary and the original string is only used for the error case.

If the string that has to be translated is the only argument, this of course means the string itself is the key---the translation will be selected based on the original string.  The message catalogs must therefore contain the original strings plus one translation for any such string.  The task of the gettext function is it to compare the argument string with the available strings in the catalog and return the appropriate translation.  Of course this process is optimized so that it is not more expensive than an access using an atomic key like in catgets.

The gettext approach has some advantages but also some disadvantages.  Please see the GNU gettext manual for a detailed discussion of the pros and cons. (Ibid.)

All the definitions and declarations for gettext can be found in the libintl.h header file.  On systems where these functions are not part of the C library, they can be found in a separate library named libintl.a (or accordingly different for shared libraries).

 -- Function: char * gettext (const char *MSGID)
     The gettext function searches the currently selected message catalogs for a string that is equal to MSGID.  If there is such a string available, it is returned.  Otherwise, the argument string MSGID is returned.

     Although the return value is char *, the returned string must not be changed.  This broken type results from the history of the function and does not reflect the way the function should be used.

     Above, we referred to ``message catalogs'' (plural).  This is a specialty of the GNU implementation of these functions, and we will say more about this when we talk about the ways message catalogs are selected (see Locating gettext catalog).

     The gettext function does not modify the value of the global ERRNO variable.  This is necessary to make it possible to write something like:

            printf (gettext ("Operation failed: %m\n"));

     Here the ERRNO value is used in the printf function while processing the %m format element, and if the gettext function would change this value (it is called before printf is called), we would get a wrong message.

     So there is no easy way to detect a missing message catalog besides comparing the argument string with the result.  But it is normally the task of the user to react on missing catalogs.  The program cannot guess when a message catalog is really necessary, since no translation is necessary for a user who speaks the language the program was developed in.

The remaining two functions to access the message catalog add some functionality to select a message catalog that is not the default one. This is important if parts of the program are developed independently. Every part can have its own message catalog and all of them can be used at the same time.  The C library itself is an example; internally it uses the gettext functions, but since it must not depend on a currently selected default message catalog, it must specify all ambiguous information.

 -- Function: char * dgettext (const char *DOMAINNAME, const char *MSGID)
     The dgettext function acts just like the gettext function.  It only takes an additional first argument DOMAINNAME, which guides the selection of the message catalogs that are searched for the translation.  If the DOMAINNAME parameter is the null pointer, the dgettext function is exactly equivalent to gettext, since the default value for the domain name is used.

     As for gettext, the return value type is char *, which is an anachronism.  The returned string must never be modified.

 -- Function: char * dcgettext (const char *DOMAINNAME, const char *MSGID, int CATEGORY)
     The dcgettext adds another argument to those that dgettext takes.  This argument CATEGORY specifies the last piece of information needed to localize the message catalog---the domain name and the locale category specify exactly which message catalog has to be used (relative to a given directory, see below).

     The dgettext function can be expressed in terms of dcgettext by using:

          dcgettext (domain, string, LC_MESSAGES)

     instead of:

          dgettext (domain, string)

     This also shows which values are expected for the third parameter. You have to use the available selectors for the categories available in locale.h.  Normally the available values are LC_CTYPE, LC_COLLATE, LC_MESSAGES, LC_MONETARY, LC_NUMERIC and LC_TIME. LC_ALL must not be used, and even though the names might suggest this, there is no relation to the environment variables with the same names.

     The dcgettext function is only implemented for compatibility with other systems that have gettext functions.  There is not really any situation where it is necessary (or useful) to use a value other than LC_MESSAGES for the CATEGORY parameter.  We are dealing with messages here, and any other choice can only be irritating.

     As for gettext, the return value type is char *, which is an anachronism.  The returned string must never be modified.

When using the three functions above in a program, it is a frequent case that the MSGID argument is a constant string, so it is worthwhile to optimize this case.  As long as no new message catalog is loaded, the translation of a message will not change.  This optimization is actually implemented by the gettext, dgettext and dcgettext functions.

How to Determine Which Catalog to Use
.....................................

The functions to retrieve the translations for a given message have a remarkably simple interface.  But to provide the user of the program with the opportunity to select exactly the translation she wants and also to provide the programmer the ability to influence the way to locate the search for catalog files, there is a quite complicated underlying control mechanism.  The code is complicated, the use is easy.

Basically, we have two different tasks to perform that can also be performed by the catgets functions:

 1. Locate the set of message catalogs.  There are a number of files for different languages and that all belong to the package.  Usually they are all stored in the file system below a certain directory.

     There can be an arbitrary number of packages installed, and they can follow different guidelines for the placement of their files.

 2. Relative to the location specified by the package, the actual translation files must be searched, based on the wishes of the user---for each language the user selects, the program should be able to locate the appropriate file.

This is the functionality required by the specifications for gettext, and this is also what the catgets functions are able to do.  But there are some unresolved problems:

  * The language to be used can be specified in several different ways. There is no generally accepted standard for this, and the user always expects the program to understand what he means.  For example, to select the German translation, one could write de, german or deutsch, and the program should always react the same.

  * Sometimes the user's specification is too detailed.  If he for example, specifies de_DE.ISO-8859-1, which means German, spoken in Germany, coded using the ISO 8859-1 character set, there is the possibility that an exact message catalog match is not available.  But there could be a catalog matching de, and if the character set used on the machine is always ISO 8859-1, there is no reason why this latter message catalog should not be used (we call this message inheritance).

  * If a catalog for a desired language is not available, it is not always the second-best choice to fall back on the language of the developer and simply not translate any message.  Instead, a user might be better able to read the messages in another language, and so the user of the program should be able to define a precedence order of languages.

We can divide the configuration actions into two parts: one is performed by the programmer and the other by the user.  We will start with the functions the programmer can use since the user configuration will be based on this.

As the description of the functions in the previous sections already mentioned, separate sets of messages can be selected by a domain name.  This is a simple string that should be unique for each program part that uses a separate domain.  It is possible to use in one program an arbitrary number of domains at the same time.  For example, the GNU C Library itself uses a domain named libc while the program using the C Library could use a domain named foo. The important point is that at any given time, exactly one domain is active.  This is controlled with the following function:

 -- Function: char * textdomain (const char *DOMAINNAME)
     The textdomain function sets the default domain, which is used in all future gettext calls, to DOMAINNAME. dgettext and dcgettext calls are not influenced if the DOMAINNAME parameter of these functions is not the null pointer.

     Before the first call to textdomain, the default domain is messages.  This is the name specified in the specification of the gettext API.  This name is as good as any other name.  No program should ever really use a domain with this name, since this can only lead to problems.

     The function returns the value that is from now on taken as the default domain.  If the system ran out of memory, the returned value is NULL and the global variable ERRNO is set to ENOMEM.  Despite the return value type being char *, the return string must not be changed.  It is allocated internally by the textdomain function.

     If the DOMAINNAME parameter is the null pointer, no new default domain is set.  Instead, the currently selected default domain is returned.

     If the DOMAINNAME parameter is an empty string, the default domain is reset to its initial value---the domain with the name messages.  Note, though, that the domain messages really never should be used.

 -- Function: char * bindtextdomain (const char *DOMAINNAME, const char *DIRNAME)
     The bindtextdomain function can be used to specify the directory that contains the message catalogs for domain DOMAINNAME for the different languages.  This is the directory where the hierarchy of directories is expected.  Details are explained below.

     For the programmer, it is important to note that the translations that come with the program have be placed in a directory hierarchy starting at, say, /foo/bar.  Then the program should make a bindtextdomain call to bind the domain for the current program to this directory.  So it is ensured that the catalogs will be found. A correctly running program does not depend on the user setting an environment variable.

     The bindtextdomain function can be used several times and if the DOMAINNAME argument is different, the previously bound domains will not be overwritten.

     If a program that wishes to use bindtextdomain at some point uses the chdir function to change the current working directory, it is important that the DIRNAME strings be an absolute pathname.  Otherwise, the addressed directory might vary with the time.

     If the DIRNAME parameter is the null pointer, bindtextdomain returns the currently selected directory for the domain with the name DOMAINNAME.

     The bindtextdomain function returns a pointer to a string containing the name of the selected directory name.  The string is allocated internally in the function and must not be changed by the user.  If the system runs out of memory during the execution of bindtextdomain, the return value is NULL, and the global variable ERRNO is set accordingly.

Additional Functions for More Complicated Situations
....................................................

The functions of the gettext family described so far (and all the catgets functions as well) have one problem in the real world that has been neglected completely in all existing approaches---the handling of plural forms.

Looking through Unix source code written before the time anybody thought about internationalization (and, sadly, even afterwards), you can often find code similar to the following:

        printf ("%d file%s deleted", n, n == 1 ? "" : "s");

After the first complaints from people internationalizing the code, people either completely avoided formulations like this, or used strings like "file(s)".  Both look unnatural and should be avoided.  First attempts to solve the problem correctly looked like this:

        if (n == 1)
          printf ("%d file deleted", n);
        else
          printf ("%d files deleted", n);

But this does not solve the problem.  It helps languages where the plural form of a noun is not simply constructed by adding an s, but that is all.  Once again, people fell into the trap of believing the rules their language is using are universal.  But the handling of plural forms differs widely between the language families (and even inside language families).  There are two things we can differentiate between:

  * The form for how plural forms are built differs.  This is a problem with language that have many irregularities.  German, for instance, is a drastic case.  Though English and German are part of the same language family (Germanic), the almost regular forming of plural noun forms (appending an s) is hardly found in German.

  * The number of plural forms differ.  This is somewhat surprising for those who only have experiences with Romanic and Germanic languages since here the number is the same (there are two).

     Other language families have only one form or many forms.

The consequence of this is that application writers should not try to solve the problem in their code.  This would be localization, since it is only usable for certain, hard-coded language environments. Instead, the extended gettext interface should be used.

These extra functions are taking two strings and a numerical argument instead of the one key string.  The idea behind this is that using the numerical argument and the first string as a key, the implementation can select, using rules specified by the translator, the right plural form.  The two string arguments then will be used to provide a return value in case no message catalog is found (similar to the normal gettext behavior).  In this case, the rules for Germanic language are used, and it is assumed that the first string argument is the singular form, the second the plural form.

This has the consequence that programs without language catalogs can display the correct strings only if the program itself is written using a Germanic language.  This is a limitation, but since the GNU C Library (as well as the GNU gettext package) are written as part of the GNU package and the coding standards for the GNU project require programs to be written in English, this solution nevertheless fulfills its purpose.

 -- Function: char * ngettext (const char *MSGID1, const char *MSGID2, unsigned long int N)
     The ngettext function is similar to the gettext function, since it finds the message catalogs in the same way.  But it takes two extra arguments.  The MSGID1 parameter must contain the singular form of the string to be converted.  It is also used as the key for the search in the catalog.  The MSGID2 parameter is the plural form.  The parameter N is used to determine the plural form.  If no message catalog is found, MSGID1 is returned if n == 1, otherwise msgid2 is returned.

     An example for the us of this function is

            printf (ngettext ("%d file removed", "%d files removed", n), n);

     Please note that the numeric value N has to be passed to the printf function as well.  It is not sufficient to pass it only to ngettext.

 -- Function: char * dngettext (const char *DOMAIN, const char *MSGID1, const char *MSGID2, unsigned long int N)
     The dngettext is similar to the dgettext function in the way the message catalog is selected.  The difference is that it takes two extra parameters to provide the correct plural form.  These two parameters are handled in the same way ngettext handles them.

 -- Function: char * dcngettext (const char *DOMAIN, const char *MSGID1, const char *MSGID2, unsigned long int N, int CATEGORY)
     The dcngettext is similar to the dcgettext function in the way the message catalog is selected.  The difference is that it takes two extra parameter to provide the correct plural form.  These two parameters are handled in the same way ngettext handles them.

The Problem of Plural Forms
...........................

A description of the problem can be found at the beginning of the last section.  Now there is the question how to solve it.  Without the input of linguists (which was not available), it was not possible to determine whether there are only a few different ways in which plurals are formed or whether the number can increase with every new supported language.

Therefore, the solution implemented is to allow the translator to specify the rules for how to select the plural form.  Since the formula varies with every language, this is the only viable solution except for hard-coding the information in the code (which still would require the possibility of extensions to not prevent the use of new languages).  The details are explained in the GNU gettext manual. (The GNU Project, GNU gettext Utilities (Free Software Foundation, May 6, 2003), http:// www.gnu.org/ software/ gettext/ manual/ gettext.)  Here, only a a bit of information is provided.

The information about the plural form selection has to be stored in the header entry (the one with the empty msgid string).  It looks like this:

     Plural-Forms: nplurals=2; plural=n == 1 ? 0 : 1;

The nplurals value must be a decimal number that specifies how many different plural forms exist for this language.  The string following plural is an expression that is using the C language syntax.  Exceptions are that nonnegative numbers are allowed, numbers must be decimal and the only variable allowed is n.  This expression will be evaluated whenever one of the functions ngettext, dngettext or dcngettext is called.  The numeric value passed to these functions is then substituted for all uses of the variable n in the expression.  The resulting value then must be greater than or equal to zero and smaller than the value given as the value of nplurals.

The following rules are known at this point.  The languages with their families are listed.  But this does not necessarily mean that the information can be generalized for the whole family (as can be easily seen in the table below). (Additions are welcome.  Send appropriate information to bug-glibc-manual@gnu.org.)

  * Only one form:

       - Some languages require only one single form.  There is no distinction between the singular and plural form.  An appropriate header entry would look like this:

               Plural-Forms: nplurals=1; plural=0;

       - Languages with this property include:

            * Finno-Ugric family

                 - Hungarian

            * Asian family

                 - Japanese

                 - Korean

            * Turkic/Altaic family

                 - Turkish

  * Two forms, singular used for one only:

       - This is the form used in most existing programs since it is what English is using.  A header entry would look like this:

               Plural-Forms: nplurals=2; plural=n != 1;

          (Note: this uses the feature of C expressions that Boolean expressions have to value zero or one.)

       - Languages with this property include:

            * Germanic family

                 - Danish

                 - Dutch

                 - English

                 - German

                 - Norwegian

                 - Swedish

            * Finno-Ugric family

                 - Estonian

                 - Finnish

            * Latin/Greek family

                 - Greek

            * Semitic family

                 - Hebrew

            * Romance family

                 - Italian

                 - Portuguese

                 - Spanish

            * Artificial

                 - Esperanto

  * Two forms, singular used for zero and one:

       - This is an exceptional case in the language family.  The header entry would be

               Plural-Forms: nplurals=2; plural=n>1;

       - Languages with this property include:

            * Romanic family

                 - French

                 - Brazilian

                 - Portuguese

  * Three forms, special case for zero:

       - The header entry would be

               Plural-Forms: nplurals=3; \
               plural=n%10==1 && n%100!=11 ? 0 : n != 0 ? 1 : 2;

       - Languages with this property include:

            * Baltic family

                 - Latvian

  * Three forms, special cases for one and two:

       - The header entry would be

               Plural-Forms: nplurals=3; plural=n==1 ? 0 : n==2 ? 1 : 2;

       - Languages with this property include:

            * Celtic

                 - Gaeilge (Irish)

  * Three forms, special case for numbers ending in 1[2-9]:

       - The header entry would look like this:

               Plural-Forms: nplurals=3; \
                   plural=n%10==1 && n%100!=11 ? 0 : \
                          n%10>=2 && (n%100<10 || n%100>=20) ? 1 : 2;

       - Languages with this property include:

            * Baltic family

                 - Lithuanian

  * Three forms, special cases for numbers ending in 1 and 2, 3, 4, except those ending in 1[1-4]:

       - The header entry would look like this:

               Plural-Forms: nplurals=3; \
                   plural=n%100/10==1 ? 2 : n%10==1 ? 0 : (n+9)%10>3 ? 2 : 1;

       - Languages with this property include:

            * Slavic family

                 - Croatian

                 - Czech

                 - Russian

                 - Ukrainian

  * Three forms, special cases for 1 and 2, 3, 4:

       - The header entry would look like this:

               Plural-Forms: nplurals=3; \
                   plural=(n==1) ? 1 : (n>=2 && n<=4) ? 2 : 0;

       - Languages with this property include:

            * Slavic family

                 - Slovak

  * Three forms, special case for one and some numbers ending in 2, 3, or 4:

       - The header entry would look like this:

               Plural-Forms: nplurals=3; \
                 plural= n ==1 ? 0 : \
                 n%10>=2 && n%10<=4 && (n%100<10 || n%100>=20) ? 1 : 2;

       - Languages with this property include:

            * Slavic family

                 - Polish

  * Four forms, special case for one and all numbers ending in 02, 03, or 04:

       - The header entry would look like this:

               Plural-Forms: nplurals=4; \
                 plural= n%100 == 1 ? 0 : n%100==2 ? 1 : n%100 == 3
                        || n%100 == 4 ? 2 : 3;

       - Languages with this property include:

            * Slavic family

                 - Slovenian

How to Specify the Output Character Set That gettext Uses
.........................................................

gettext not only looks up a translation in a message catalog, it also converts the translation on the fly to the desired output character set.  This is useful if the user is working in a different character set than the translator who created the message catalog, because it avoids distributing variants of message catalogs that differ only in the character set.

The output character set is, by default, the value of nl_langinfo (CODESET), which depends on the LC_CTYPE part of the current locale.  But programs that store strings in a locale-independent way (e.g. UTF-8) can request that gettext and related functions return the translations in that encoding, by use of the bind_textdomain_codeset function.

The MSGID argument to gettext is not subject to character-set conversion.  Also, when gettext does not find a translation for MSGID, it returns MSGID unchanged---independently of the current output character set.  It is therefore recommended that all MSGIDs be US-ASCII strings.

 -- Function: char * bind_textdomain_codeset (const char *DOMAINNAME, const char *CODESET)
     The bind_textdomain_codeset function can be used to specify the output character set for message catalogs for domain DOMAINNAME. The CODESET argument must be a valid codeset name that can be used for the iconv_open function, or a null pointer.

     If the CODESET parameter is the null pointer, bind_textdomain_codeset returns the currently selected codeset for the domain with the name DOMAINNAME. It returns NULL if no codeset has yet been selected.

     The bind_textdomain_codeset function can be used several times. If used multiple times with the same DOMAINNAME argument, the later call overrides the settings made by the earlier one.

     The bind_textdomain_codeset function returns a pointer to a string containing the name of the selected codeset.  The string is allocated internally in the function and must not be changed by the user.  If the system runs out of memory during the execution of bind_textdomain_codeset, the return value is NULL and the global variable ERRNO is set accordingly. deftypefun

How to Use gettext in GUI Programs
..................................

One place where the gettext functions, if used normally, have big problems is within programs with graphical user interfaces (GUIs). The problem is that many of the strings that have to be translated are very short.  They have to appear in pull-down menus, which restricts their length.  But strings that do not contain entire sentences or at least large fragments of a sentence may appear in more than one situation in the program with different translations.  This is especially true for the one-word strings that are frequently used in GUI programs.

As a consequence, many people say that the gettext approach is wrong and that instead catgets, which indeed does not have this problem, should be used.  But there is a very simple and powerful method to handle this problem with the gettext functions.

As as example, consider the following fictional situation.  A GUI program has a menu bar with the following entries:

     +------------+------------+--------------------------------------+
     | File       | Printer    |                                      |
     +------------+------------+--------------------------------------+
     | Open     | | Select   |
     | New      | | Open     |
     +----------+ | Connect  |
                  +----------+

To have the strings File, Printer, Open, New, Select and Connect translated, there has to be at some point in the code a call to a function of the gettext family.  But in two places the string passed into the function would be Open.  The translations might not be the same and therefore we are in the dilemma described above.

One solution to this problem is to artificially lengthen the strings to make them unambiguous.  But what would the program do if no translation were available?  The lengthened string is not what should be printed.  So we should use a modified version of the functions.

To lengthen the strings, a uniform method should be used.  In the example above, the strings could be chosen as:

     Menu|File
     Menu|Printer
     Menu|File|Open
     Menu|File|New
     Menu|Printer|Select
     Menu|Printer|Open
     Menu|Printer|Connect

Now all the strings are different, and if now instead of gettext the following little wrapper function is used, everything works just fine:

       char *
       sgettext (const char *msgid)
       {
         char *msgval = gettext (msgid);
         if (msgval == msgid)
           msgval = strrchr (msgid, '|') + 1;
         return msgval;
       }

What this little function does is to recognize the case when no translation is available.  This can be done very efficiently by a pointer comparison since the return value is the input value.  If there is no translation, we know that the input string is in the format we used for the Menu entries and therefore contains a | character.  We simply search for the last occurrence of this character and return a pointer to the character following it.  That's it!

If one now consistently uses the lengthened-string form and replaces the gettext calls with calls to sgettext (this is normally limited to very few places in the GUI implementation), then it is possible to produce a program that can be internationalized.

With advanced compilers (such as GNU C), one can write the sgettext functions as an in-line function or as a macro like this:

     #define sgettext(msgid) \
       ({ const char *__msgid = (msgid);            \
          char *__msgstr = gettext (__msgid);       \
          if (__msgval == __msgid)                  \
            __msgval = strrchr (__msgid, '|') + 1;  \
          __msgval; })

The other gettext functions (dgettext, dcgettext and the ngettext equivalents) can and should have corresponding functions as well that look almost identical, except for the parameters and the call to the underlying function.

Now, why do such functions not exist in the GNU C Library?  There are two parts to the answer for this question:

  * They are easy to write and therefore can be provided by the project they are used in.  This is not an answer by itself and must be seen together with the second part, which is

  * There is no way the C library can contain a version that can work everywhere.  The problem is the selection of the character to separate the prefix from the actual string in the lenghtened string.  The examples above used |, which is a good choice because it resembles a notation frequently used in this context and it also is a character not often used in message strings. But, what if the character is used in message strings?  Or what if the chosen character is not available in the character set on the machine one compiles on (e.g., | is not required to exist for ISO C; this is why the iso646.h file exists in ISO C programming environments)? So, even this method is not perfect.

There is only one more comment left to make.  The wrapper function above requires that the translation strings themselves not be lengthened.  This is only logical.  There is no need to disambiguate the strings (since they are never used as keys for a search), and one also saves some memory and disk space by doing this.

User Influence on gettext
.........................

The last sections described what the programmer can do to internationalize the messages of the program.  But it is up to the user in the end to select the message she wants to see.  She must understand them.

The POSIX locale model uses the environment variables LC_COLLATE, LC_CTYPE, LC_MESSAGES, LC_MONETARY, NUMERIC and LC_TIME to select the locale that is to be used.  This way the user can influence lots of functions.  As we mentioned above, the gettext functions also take advantage of this.

To understand how this happens, it is necessary to take a look at the various components of the file name that get computed to locate a message catalog.  It is composed as follows:

     DIR_NAME/LOCALE/LC_CATEGORY/DOMAIN_NAME.mo

The default value for DIR_NAME is system specific.  It is computed from the value given as the prefix while configuring the C library.  This value normally is /usr or /.  For the former, the complete DIR_NAME is

     /usr/share/locale

We can use /usr/share since the .mo files containing the message catalogs are system independent, so all systems can use the same files.  If the program executed the bindtextdomain function for the message domain that is currently handled, the dir_name component is exactly the value that was given to the function as the second parameter.  bindtextdomain allows overwriting the only system-dependent and fixed value to make it possible to address files anywhere in the file system.

The CATEGORY is the name of the locale category that was selected in the program code.  For gettext and dgettext, this is always LC_MESSAGES; for dcgettext, this is selected by the value of the third parameter.  As said above, using a category other than LC_MESSAGES should be avoided.

The LOCALE component is computed based on the category used. Just like for the setlocale function, user selection comes into play.  Some environment variables are examined in a fixed order, and the first environment variable set determines the return value of the lookup process.  In detail, for the category LC_xxx the following variables are examined in this order:

  * LANGUAGE

  * LC_ALL

  * LC_xxx

  * LANG

This looks very familiar.  With the exception of the LANGUAGE environment variable, this is exactly the lookup order the setlocale function uses.  But why introduce the LANGUAGE variable?

The reason is that the syntax of the values these variables can have is different from what is expected by the setlocale function. If we would set LC_ALL to a value following the extended syntax, that would mean the setlocale function would never be able to use the value of this variable.  An additional variable removes this problem, plus we can select the language independently of the locale setting, which is sometimes useful.

While for the LC_xxx variables, the value should consist of exactly one specification of a locale, the LANGUAGE variable's value can consist of a colon-separated list of locale names.  This is the way we manage to implement one of our additional demands above, to be able to specify an ordered list of languages.

Back to the constructed file name.  We have only one component missing.  The DOMAIN_NAME part is the name that was either registered using the textdomain function or that was given to dgettext or dcgettext as the first parameter.  Now it becomes obvious that a good choice for the domain name in the program code is a string that is closely related to the program or package name.  For example, the domain name for the GNU C Library is libc.

A limited piece of example code should show how the programmer is supposed to work:

     {
       setlocale (LC_ALL, "");
       textdomain ("test-package");
       bindtextdomain ("test-package", "/usr/local/share/locale");
       puts (gettext ("Hello, world!"));
     }

At the program start, the default domain is messages, and the default locale is C.  The setlocale call sets the locale according to the user's environment variables; remember that correct functioning of gettext relies on the correct setting of the LC_MESSAGES locale (for looking up the message catalog) and of the LC_CTYPE locale (for the character-set conversion).  The textdomain call changes the default domain to test-package.  The bindtextdomain call specifies that the message catalogs for the domain test-package can be found below the directory /usr/local/share/locale.

If now the user set in her environment the variable LANGUAGE to de, the gettext function will try to use the translations from the file:

     /usr/local/share/locale/de/LC_MESSAGES/test-package.mo

From the above descriptions, it should be clear which component of this file name is determined by which source.

In the above example, we assumed that the LANGUAGE environment variable was set to de.  This might be an appropriate selection, but what happens if the user wants to use LC_ALL because of the wider usability, and here the required value is de_DE.ISO-8859-1?  We already mentioned above that a situation like this is not infrequent.  For example, a person might prefer reading a dialect and, if this is not available, falling back on the standard language.

The gettext functions know about situations like this and can handle them gracefully.  The functions recognize the format of the value of the environment variable.  It can split the value into different pieces and, by leaving out one or the other part, it can construct new values.  This happens of course in a predictable way. To understand this, one must know the format of the environment variable value.  There is one more or less standardized form, originally from the X/Open specification:

language[_territory[.codeset]][@modifier]

Less specific locale names will be stripped off in the order of the following list:

 1. codeset

 2. normalized codeset

 3. territory

 4. modifier

The language field will never be dropped for obvious reasons.

The only new thing is the normalized codeset entry.  This is another goodie, which is introduced to help reduce the chaos that derives from the inability of people to standardize the names of character sets.  Instead of ISO-8859-1, one can often see 8859-1, 88591, iso8859-1 or iso_8859-1.  The normalized codeset value is generated from the user-provided character-set name by applying the following rules:

 1. Remove all characters besides numbers and letters.

 2. Fold letters to lowercase.

 3. If the name only contains digits, prepend the string iso.

So all of the above names will be normalized to iso88591.  This allows the program user to more freely choose the locale name.

Even this extended functionality still does not help solve the problem that completely different names can be used to denote the same locale (e.g., de and german).  To be of help in this situation, the locale implementation and the gettext functions know about aliases.

The file /usr/share/locale/locale.alias (replace /usr with whatever prefix you used for configuring the C library) contains a mapping of alternative names to more regular names.  The system manager is free to add new entries to fill his own needs.  The selected locale from the environment is compared with the entries in the first column of this file, ignoring the case.  If they match, the value of the second column is used instead for further handling.

In the description of the format of the environment variables we already mentioned the character set as a factor in the selection of the message catalog.  In fact, only catalogs that contain text written using the character set of the system or program can be used (directly; a solution for this will come some day).  The user will always have to be careful about this.  Also, if in the collection of message catalogs there are files for the same language that are coded using different character sets, the user has to be careful.

Programs to Handle Message Catalogs for gettext
-----------------------------------------------

The GNU C Library does not contain the source code for the programs to handle message catalogs for the gettext functions. As part of the GNU project, the GNU gettext package contains everything the developer needs.  The functionality provided by the tools in this package by far exceeds the abilities of the gencat program described above for the catgets functions.

There is a program msgfmt, which is equivalent to the gencat program.  It generates from the human-readable and human-editable form of the message catalog a binary file that can be used by the gettext functions.  But there are several more programs available.

The xgettext program can be used to automatically extract the translatable messages from a source file, so the programmer need not be careful of the translations and the list of messages that have to be translated.  She will simply wrap the translatable string in calls to gettext and the others, and the rest will be done by xgettext.  This program has a lot of options that help customize the output or help understand the input.

Other programs help to manage the development cycle when new messages appear in the source files or when a new translation of the messages appear.  Here it should only be noted that using all the tools in GNU gettext, it is possible to completely automatize the handling of message catalogs.  Besides marking the translatable string in the source code and generating the translations, the developers themselves do not have to do anything.

Searching and Sorting
*********************

This chapter describes functions for searching and sorting arrays of arbitrary objects.  You pass the appropriate comparison function to be applied as an argument, along with the size of the objects in the array and the total number of elements.

Defining the Comparison Function
================================

In order to use the sorted array library functions, you have to describe how to compare the elements of the array.

To do this, you supply a comparison function to compare two elements of the array.  The library will call this function, passing as arguments pointers to two array elements to be compared.  Your comparison function should return a value the way strcmp (see String/Array Comparison) does: negative if the first argument is ``less'' than the second, zero if they are ``equal'' and positive if the first argument is ``greater''.

Here is an example of a comparison function that works with an array of numbers of type double:

     int
     compare_doubles (const void *a, const void *b)
     {
       const double *da = (const double *) a;
       const double *db = (const double *) b;

       return (*da > *db) - (*da < *db);
     }

The header file stdlib.h defines a name for the data type of comparison functions.  This type is a GNU extension.

     int comparison_fn_t (const void *, const void *);

Array Search Function
=====================

Generally, searching for a specific element in an array means that potentially all elements must be checked.  The GNU C Library contains functions to perform linear searches.  The prototypes for the following two functions can be found in search.h.

 -- Function: void * lfind (const void *KEY, void *BASE, size_t *NMEMB, size_t SIZE, comparison_fn_t COMPAR)
     The lfind function searches in the array with *NMEMB elements of SIZE bytes pointed to by BASE for an element that matches the one pointed to by KEY.  The function pointed to by COMPAR is used to decide whether two elements match.

     The return value is a pointer to the matching element in the array starting at BASE if it is found.  If no matching element is available, NULL is returned.

     The mean run time of this function is *NMEMB/2.  This function should only be used if elements often get added to or deleted from the array, in which case it might not be useful to sort the array before searching.

 -- Function: void * lsearch (const void *KEY, void *BASE, size_t *NMEMB, size_t SIZE, comparison_fn_t COMPAR)
     The lsearch function is similar to the lfind function. It searches the given array for an element and returns it if found. The difference is that if no matching element is found, the lsearch function adds the object pointed to by KEY (with a size of SIZE bytes) at the end of the array and it increments the value of *NMEMB to reflect this addition.

     This means that if the caller is not sure that the array contains the element you are searching for, the memory allocated for the array starting at BASE must have room for at least SIZE more bytes.  If you are sure the element is in the array, it is better to use lfind, so having more room in the array is always necessary when calling lsearch.

To search a sorted array for an element matching the key, use the bsearch function.  The prototype for this function is in the header file stdlib.h.

 -- Function: void * bsearch (const void *KEY, const void *ARRAY, size_t COUNT, size_t SIZE, comparison_fn_t COMPARE)
     The bsearch function searches the sorted array ARRAY for an object that is equivalent to KEY.  The array contains COUNT elements, each of which is of size SIZE bytes.

     The COMPARE function is used to perform the comparison.  This function is called with two pointer arguments and should return an integer less than, equal to or greater than zero corresponding to whether its first argument is considered less than, equal to or greater than its second argument.  The elements of the ARRAY must already be sorted in ascending order according to this comparison function.

     The return value is a pointer to the matching array element, or a null pointer if no match is found.  If the array contains more than one element that matches, the one that is returned is unspecified.

     This function derives its name from the fact that it is implemented using the binary search algorithm.

Array Sort Function
===================

To sort an array using an arbitrary comparison function, use the qsort function.  The prototype for this function is in stdlib.h.

 -- Function: void qsort (void *ARRAY, size_t COUNT, size_t SIZE, comparison_fn_t COMPARE)
     The QSORT function sorts the array ARRAY.  The array contains COUNT elements, each of which is of size SIZE.

     The COMPARE function is used to perform the comparison on the array elements.  This function is called with two pointer arguments and should return an integer less than, equal to or greater than zero corresponding to whether its first argument is considered less than, equal to or greater than its second argument.

     Warning: If two objects compare as equal, their order after sorting is unpredictable.  That is to say, the sorting is not stable. This can make a difference when the comparison considers only part of the elements.  Two elements with the same sort key may differ in other respects.

     If you want the effect of a stable sort, you can get this result by writing the comparison function so that, lacking other reasons to distinguish between two elements, it compares them by their addresses. Doing this may make the sorting algorithm less efficient, so do it only if necessary.

     Here is a simple example of sorting an array of doubles in numerical order, using the comparison function defined above (see Comparison Functions):

          {
            double *array;
            int size;
            ...
            qsort (array, size, sizeof (double), compare_doubles);
          }

     The qsort function derives its name from the fact that it was originally implemented using the ``quick sort'' algorithm.

     The implementation of qsort in this library might not be an in-place sort and might thereby use an extra amount of memory to store the array.

Searching and Sorting Example
=============================

Here is an example showing the use of qsort and bsearch with an array of structures.  The objects in the array are sorted by comparing their name fields with the strcmp function. Then, we can look up individual objects based on their names.

     #include <stdlib.h>
     #include <stdio.h>
     #include <string.h>

     /* Define an array of critters to sort. */

     struct critter
       {
         const char *name;
         const char *species;
       };

     struct critter muppets[] =
       {
         {"Kermit", "frog"},
         {"Piggy", "pig"},
         {"Gonzo", "whatever"},
         {"Fozzie", "bear"},
         {"Sam", "eagle"},
         {"Robin", "frog"},
         {"Animal", "animal"},
         {"Camilla", "chicken"},
         {"Sweetums", "monster"},
         {"Dr. Strangepork", "pig"},
         {"Link Hogthrob", "pig"},
         {"Zoot", "human"},
         {"Dr. Bunsen Honeydew", "human"},
         {"Beaker", "human"},
         {"Swedish Chef", "human"}
       };

     int count = sizeof (muppets) / sizeof (struct critter);

     /* This is the comparison function used for sorting and searching. */

     int
     critter_cmp (const struct critter *c1, const struct critter *c2)
     {
       return strcmp (c1->name, c2->name);
     }

     /* Print information about a critter. */

     void
     print_critter (const struct critter *c)
     {
       printf ("%s, the %s\n", c->name, c->species);
     }

     /* Do the lookup into the sorted array. */

     void
     find_critter (const char *name)
     {
       struct critter target, *result;
       target.name = name;
       result = bsearch (&target, muppets, count, sizeof (struct critter),
                         critter_cmp);
       if (result)
         print_critter (result);
       else
         printf ("Couldn't find %s.\n", name);
     }

     /* Main program. */

     int
     main (void)
     {
       int i;

       for (i = 0; i < count; i++)
         print_critter (&muppets[i]);
       printf ("\n");

       qsort (muppets, count, sizeof (struct critter), critter_cmp);

       for (i = 0; i < count; i++)
         print_critter (&muppets[i]);
       printf ("\n");

       find_critter ("Kermit");
       find_critter ("Gonzo");
       find_critter ("Janice");

       return 0;
     }

The output from this program looks like:

     Kermit, the frog
     Piggy, the pig
     Gonzo, the whatever
     Fozzie, the bear
     Sam, the eagle
     Robin, the frog
     Animal, the animal
     Camilla, the chicken
     Sweetums, the monster
     Dr. Strangepork, the pig
     Link Hogthrob, the pig
     Zoot, the human
     Dr. Bunsen Honeydew, the human
     Beaker, the human
     Swedish Chef, the human

     Animal, the animal
     Beaker, the human
     Camilla, the chicken
     Dr. Bunsen Honeydew, the human
     Dr. Strangepork, the pig
     Fozzie, the bear
     Gonzo, the whatever
     Kermit, the frog
     Link Hogthrob, the pig
     Piggy, the pig
     Robin, the frog
     Sam, the eagle
     Swedish Chef, the human
     Sweetums, the monster
     Zoot, the human

     Kermit, the frog
     Gonzo, the whatever
     Couldn't find Janice.

The hsearch Function
====================

The functions mentioned so far in this chapter are for searching in a sorted or an unsorted array.  There are functions available that can organize information to make searching easier.  The costs of insert, delete and search differ.  One possible implementation is using hashing tables.  The following functions are declared in the the header file search.h.

 -- Function: int hcreate (size_t NEL)
     The hcreate function creates a hashing table that can contain at least NEL elements.  There is no way to grow this table, so it is necessary to choose the value for NEL wisely.  The methods used to implement this function might make it necessary to make the number of elements in the hashing table larger than the expected maximum number of elements.  Hashing tables usually work inefficiently if they are filled 80% or more.  The constant access time guaranteed by hashing can only be achieved if few collisions exist. (See Donald E. Knuth, The Art of Computer Programming, vol. 3, Sorting and Searching, 2nd ed. (Reading, MA: Addison-Wesley, April 24, 1998).)

     The weakest aspect of this function is that there can be at most one hashing table used through the whole program.  The table is allocated in local memory out of control of the programmer.  As an extension, the GNU C Library provides an additional set of functions with a reentrant interface that provide a similar interface but that allow you to keep an arbitrary number of hashing tables.

     It is possible to use more than one hashing table in the program run if the former table is first destroyed by a call to hdestroy.

     The function returns a nonzero value if successful.  If it returns zero, something went wrong.  This could either mean there was already a hashing table in use, or the program has run out of memory.

 -- Function: void hdestroy (void)
     The hdestroy function can be used to free all the resources allocated in a previous call of hcreate.  After a call to this function, it is again possible to call hcreate and allocate a new table with a potentially different size.

     It is important to remember that the elements contained in the hashing table at the time hdestroy is called are not freed by this function.  It is the responsibility of the program code to free those strings (if necessary at all).  Freeing all the element memory is not possible without extra, separately kept information, since there is no function to iterate through all available elements in the hashing table.  If it is really necessary to free a table and all elements, the programmer has to keep a list of all table elements, and before calling hdestroy he has to free all element data using this list.  This is a very unpleasant mechanism and it also shows that this kind of hashing table is mainly meant for tables that are created once and used until the end of the program run.

Entries of the hashing table and keys for the search are defined using this type:

 -- Data type: struct ENTRY
     Both elements of this structure are pointers to zero-terminated strings.  This is a limiting restriction of the functionality of the hsearch functions.  They can only be used for data sets that use the NUL character always and solely to terminate the records.  It is not possible to handle general binary data.

     char *key
          This is a pointer to a zero-terminated string of characters describing the key for the search or the element in the hashing table.

     char *data
          This is a pointer to a zero-terminated string of characters describing the data.  If the functions will be called only for searching an existing entry, this element might stay undefined, since it is not used.

 -- Function: ENTRY * hsearch (ENTRY ITEM, ACTION ACTION)
     To search in a hashing table created using hcreate, the hsearch function must be used.  This function can perform a simple search for an element (if ACTION has the FIND) or alternatively it can insert the key element into the hashing table. Entries are never replaced.

     The key is denoted by a pointer to an object of type ENTRY. For locating the corresponding position in the hashing table, only the key element of the structure is used.

     If an entry with matching key is found, the ACTION parameter is irrelevant.  The found entry is returned.  If no matching entry is found and the ACTION parameter has the value FIND, the function returns a NULL pointer.  If no entry is found and the ACTION parameter has the value ENTER, a new entry is added to the hashing table that is initialized with the parameter ITEM.  A pointer to the newly added entry is returned.

As mentioned before, the hashing table used by the functions described so far is global and there can be at any time at most one hashing table in the program.  A solution is to use the following functions, which are a GNU extension.  All have in common that they operate on a hashing table that is described by the content of an object of the type struct hsearch_data.  This type should be treated as opaque; none of its members should be changed directly.

 -- Function: int hcreate_r (size_t NEL, struct hsearch_data *HTAB)
     The hcreate_r function initializes the object pointed to by HTAB to contain a hashing table with at least NEL elements.  This function is equivalent to the hcreate function except that the initialized data structure is controlled by the user.

     This allows having more than one hashing table at one time.  The memory necessary for the struct hsearch_data object can be allocated dynamically.  It must be initialized with zero before calling this function.

     The return value is nonzero if the operation was successful.  if the return value is zero, something went wrong, which probably means the program runs out of memory.

 -- Function: void hdestroy_r (struct hsearch_data *HTAB)
     The hdestroy_r function frees all resources allocated by the hcreate_r function for this very same object HTAB.  As for hdestroy, it is the program's responsibility to free the strings for the elements of the table.

 -- Function: int hsearch_r (ENTRY ITEM, ACTION ACTION, ENTRY **RETVAL, struct hsearch_data *HTAB)
     The hsearch_r function is equivalent to hsearch.  The meaning of the first two arguments is identical.  But instead of operating on a single global hashing table the function works on the table described by the object pointed to by HTAB (which is initialized by a call to hcreate_r).

     Another difference with hcreate is that the pointer to the found entry in the table is not the return value of the functions.  It is returned by storing it in a pointer variable pointed to by the RETVAL parameter.  The return value of the function is an integer value indicating success if it is nonzero and failure if it is zero.  In the latter case, the global variable ERRNO signals the reason for the failure.

     ENOMEM
          The table is filled, hsearch_r was called with a so far unknown key and ACTION was set to ENTER.

     ESRCH
          The ACTION parameter is FIND, and no corresponding element is found in the table.

The tsearch Function
====================

Another common way to organize data for efficient search is to use trees.  The tsearch function family provides a nice interface to functions to organize possibly large amounts of data by providing a mean access time proportional to the logarithm of the number of elements.  The GNU C Library implementation guarantees that this bound is never exceeded even for input data that cause problems for simple binary tree implementations.

The functions described in this chapter are all described in the System V and X/Open specifications and are therefore quite portable.

In contrast to the hsearch functions, the tsearch functions can be used with arbitrary data and not just zero-terminated strings.

The tsearch functions have the advantage that no function to initialize data structures is necessary.  A simple pointer of type void * initialized to NULL is a valid tree and can be extended or searched.  The prototypes for these functions can be found in the header file search.h.

 -- Function: void * tsearch (const void *KEY, void **ROOTP, comparison_fn_t COMPAR)
     The tsearch function searches in the tree pointed to by *ROOTP for an element matching KEY.  The function pointed to by COMPAR is used to determine whether two elements match.  See Comparison Functions, for a specification of the functions that can be used for the COMPAR parameter.

     If the tree does not contain a matching entry, the KEY value will be added to the tree.  tsearch does not make a copy of the object pointed to by KEY (how could it, since the size is unknown).  Instead, it adds a reference to this object, which means the object must be available as long as the tree data structure is used.

     The tree is represented by a pointer to a pointer, since it is sometimes necessary to change the root node of the tree.  It must not be assumed that the variable pointed to by ROOTP has the same value after the call.  This also shows that it is not safe to call the tsearch function more than once at the same time using the same tree.  It is no problem to run it more than once at a time on different trees.

     The return value is a pointer to the matching element in the tree.  If a new element was created, the pointer points to the new data (which is in fact KEY).  If an entry had to be created and the program ran out of space, NULL is returned.

 -- Function: void * tfind (const void *KEY, void *const *ROOTP, comparison_fn_t COMPAR)
     The tfind function is similar to the tsearch function. It locates an element matching the one pointed to by KEY and returns a pointer to this element.  But if no matching element is available, no new element is entered (note that the ROOTP parameter points to a constant pointer).  Instead the function returns NULL.

Another advantage of the tsearch function in contrast to the hsearch functions is that there is an easy way to remove elements.

 -- Function: void * tdelete (const void *KEY, void **ROOTP, comparison_fn_t COMPAR)
     To remove a specific element matching KEY from the tree, tdelete can be used.  It locates the matching element using the same method as tfind.  The corresponding element is then removed and a pointer to the parent of the deleted node is returned by the function.  If there is no matching entry in the tree, nothing can be deleted and the function returns NULL.  If the root of the tree is deleted, tdelete returns some unspecified value not equal to NULL.

 -- Function: void tdestroy (void *VROOT, __free_fn_t FREEFCT)
     If the complete search tree has to be removed, one can use tdestroy.  It frees all resources allocated by the tsearch function to generate the tree pointed to by VROOT.

     For the data in each tree node, the function FREEFCT is called. The pointer to the data is passed as the argument to the function.  If no such work is necessary, FREEFCT must point to a function doing nothing.  It is called in any case.

     This function is a GNU extension and is not covered by the System V or X/Open specifications.

In addition to the function to create and destroy the tree data structure, there is another function that allows you to apply a function to all elements of the tree.  The function must have this type:

     void __action_fn_t (const void *nodep, VISIT value, int level);

The NODEP is the data value of the current node (once given as the KEY argument to tsearch).  LEVEL is a numeric value that corresponds to the depth of the current node in the tree. The root node has the depth 0, its children have a depth of 1, and so on.  The VISIT type is an enumeration type.

 -- Data Type: VISIT
     The VISIT value indicates the status of the current node in the tree and how the function is called.  The status of a node is either `leaf' or `internal node'.  For each leaf node the function is called exactly once; for each internal node it is called three times: before the first child is processed, after the first child is processed and after both children are processed.  This makes it possible to handle all three methods of tree traversal (or even a combination of them).

     preorder
          The current node is an internal node and the function is called before the first child was processed.

     postorder
          The current node is an internal node and the function is called after the first child was processed.

     endorder
          The current node is an internal node and the function is called after the second child was processed.

     leaf
          The current node is a leaf.

 -- Function: void twalk (const void *ROOT, __action_fn_t ACTION)
     For each node in the tree with a node pointed to by ROOT, the twalk function calls the function provided by the parameter ACTION.  For leaf nodes, the function is called exactly once with VALUE set to leaf.  For internal nodes, the function is called three times, setting the VALUE parameter or ACTION to the appropriate value.  The LEVEL argument for the ACTION function is computed while descending the tree with increasing the value by one for the descend to a child, starting with the value 0 for the root node.

     Since the functions used for the ACTION parameter to twalk must not modify the tree data, it is safe to run twalk in more than one thread at the same time, working on the same tree.  It is also safe to call tfind in parallel. Functions that modify the tree must not be used, otherwise the behavior is undefined.

Pattern Matching
****************

The GNU C Library provides pattern-matching facilities for two kinds of patterns: regular expressions and file-name wildcards.  The library also provides a facility for expanding variable and command references and parsing text into words in the way the shell does.

Wildcard Matching
=================

This section describes how to match a wildcard pattern against a particular string.  The result is a yes or no answer---does the string fit the pattern or not.  The symbols described here are all declared in fnmatch.h.

 -- Function: int fnmatch (const char *PATTERN, const char *STRING, int FLAGS)
     This function tests whether the string STRING matches the pattern PATTERN.  It returns 0 if they do match; otherwise, it returns the nonzero value FNM_NOMATCH.  The arguments PATTERN and STRING are both strings.

     The argument FLAGS is a combination of flag bits that alter the details of matching.  See below for a list of the defined flags.

     In the GNU C Library, fnmatch cannot experience an ``error''---it always returns an answer for whether the match succeeds.  However, other implementations of fnmatch might sometimes report ``errors''. They would do so by returning nonzero values that are not equal to FNM_NOMATCH.

These are the available flags for the FLAGS argument:

FNM_FILE_NAME
     Treat the / character specially, for matching file names.  If this flag is set, wildcard constructs in PATTERN cannot match / in STRING.  Thus, the only way to match / is with an explicit / in PATTERN.

FNM_PATHNAME
     This is an alias for FNM_FILE_NAME; it comes from POSIX.2.  We don't recommend this name because we don't use the term ``pathname'' for file names.

FNM_PERIOD
     Treat the . character specially if it appears at the beginning of STRING.  If this flag is set, wildcard constructs in PATTERN cannot match . as the first character of STRING.

     If you set both FNM_PERIOD and FNM_FILE_NAME, then the special treatment applies to . following / as well as to . at the beginning of STRING.  (The shell uses the FNM_PERIOD and FNM_FILE_NAME flags together for matching file names.)

FNM_NOESCAPE
     Don't treat the \ character specially in patterns.  Normally, \ quotes the following character, turning off its special meaning (if any) so that it matches only itself.  When quoting is enabled, the pattern \? matches only the string ?, because the question mark in the pattern acts like an ordinary character.

     If you use FNM_NOESCAPE, then \ is an ordinary character.

FNM_LEADING_DIR
     Ignore a trailing sequence of characters starting with a / in STRING; that is to say, test whether STRING starts with a directory name that PATTERN matches.

     If this flag is set, either foo* or foobar as a pattern would match the string foobar/frobozz.

FNM_CASEFOLD
     Ignore case in comparing STRING to PATTERN.

FNM_EXTMATCH
     Recognize besides the normal patterns also the extended patterns introduced in ksh.  The patterns are written in the form explained in the following table where PATTERN-LIST is a |-separated list of patterns.

     ?(PATTERN-LIST)
          The pattern matches if zero or one occurrences of any of the patterns in the PATTERN-LIST allow matching the input string.

     *(PATTERN-LIST)
          The pattern matches if zero or more occurrences of any of the patterns in the PATTERN-LIST allow matching the input string.

     +(PATTERN-LIST)
          The pattern matches if one or more occurrences of any of the patterns in the PATTERN-LIST allow matching the input string.

     @(PATTERN-LIST)
          The pattern matches if exactly one occurrence of any of the patterns in the PATTERN-LIST allows matching the input string.

     !(PATTERN-LIST)
          The pattern matches if the input string cannot be matched with any of the patterns in the PATTERN-LIST.

Globbing
========

The archetypal use of wildcards is for matching against the files in a directory, and making a list of all the matches.  This is called globbing.

You could do this using fnmatch, by reading the directory entries one by one and testing each one with fnmatch.  But that would be slow (and complex, since you would have to handle subdirectories by hand).

The library provides a function glob to make this particular use of wildcards convenient.  glob and the other symbols in this section are declared in glob.h.

Calling glob
------------

The result of globbing is a vector of file names (strings).  To return this vector, glob uses a special data type, glob_t, which is a structure.  You pass glob the address of the structure, and it fills in the structure's fields to tell you about the results.

 -- Data Type: glob_t
     This data type holds a pointer to a word vector.  More precisely, it records both the address of the word vector and its size.  The GNU implementation contains some more fields that are nonstandard extensions.

     gl_pathc
          The number of elements in the vector, excluding the initial null entries if the GLOB_DOOFFS flag is used (see gl_offs below).

     gl_pathv
          The address of the vector; this field has type char **.

     gl_offs
          The offset of the first real element of the vector, from its nominal address in the gl_pathv field; unlike the other fields, this is always an input to glob, rather than an output from it.

          If you use a nonzero offset, then that many elements at the beginning of the vector are left empty (the glob function fills them with null pointers).

          The gl_offs field is meaningful only if you use the GLOB_DOOFFS flag.  Otherwise, the offset is always zero regardless of what is in this field, and the first real element comes at the beginning of the vector.

     gl_closedir
          The address of an alternative implementation of the closedir function; it is used if the GLOB_ALTDIRFUNC bit is set in the flag parameter.  The type of this field is void (*) (void *).

          This is a GNU extension.

     gl_readdir
          The address of an alternative implementation of the readdir function used to read the contents of a directory; it is used if the GLOB_ALTDIRFUNC bit is set in the flag parameter.  The type of this field is struct dirent *(*) (void *).

          This is a GNU extension.

     gl_opendir
          The address of an alternative implementation of the opendir function; it is used if the GLOB_ALTDIRFUNC bit is set in the flag parameter.  The type of this field is void *(*) (const char *).

          This is a GNU extension.

     gl_stat
          The address of an alternative implementation of the stat function to get information about an object in the file system; it is used if the GLOB_ALTDIRFUNC bit is set in the flag parameter. The type of this field is int (*) (const char *, struct stat *).

          This is a GNU extension.

     gl_lstat
          The address of an alternative implementation of the lstat function to get information about an object in the file systems, not following symbolic links; it is used if the GLOB_ALTDIRFUNC bit is set in the flag parameter.  The type of this field is int (*) (const char *, struct stat *).

          This is a GNU extension.

For use in the glob64 function, glob.h contains another definition for a very similar type.  glob64_t differs from glob_t only in the types of the members gl_readdir, gl_stat and gl_lstat.

 -- Data Type: glob64_t
     This data type holds a pointer to a word vector.  More precisely, it records both the address of the word vector and its size.  The GNU implementation contains some more fields that are nonstandard extensions.

     gl_pathc
          The number of elements in the vector, excluding the initial null entries if the GLOB_DOOFFS flag is used (see gl_offs below).

     gl_pathv
          The address of the vector; this field has type char **.

     gl_offs
          The offset of the first real element of the vector, from its nominal address in the gl_pathv field; unlike the other fields, this is always an input to glob, rather than an output from it.

          If you use a nonzero offset, then that many elements at the beginning of the vector are left empty.  (The glob function fills them with null pointers.)

          The gl_offs field is meaningful only if you use the GLOB_DOOFFS flag.  Otherwise, the offset is always zero regardless of what is in this field, and the first real element comes at the beginning of the vector.

     gl_closedir
          The address of an alternative implementation of the closedir function; it is used if the GLOB_ALTDIRFUNC bit is set in the flag parameter.  The type of this field is void (*) (void *).

          This is a GNU extension.

     gl_readdir
          The address of an alternative implementation of the readdir64 function used to read the contents of a directory; it is used if the GLOB_ALTDIRFUNC bit is set in the flag parameter.  The type of this field is struct dirent64 *(*) (void *).

          This is a GNU extension.

     gl_opendir
          The address of an alternative implementation of the opendir function; it is used if the GLOB_ALTDIRFUNC bit is set in the flag parameter.  The type of this field is void *(*) (const char *).

          This is a GNU extension.

     gl_stat
          The address of an alternative implementation of the stat64 function to get information about an object in the file system; it is used if the GLOB_ALTDIRFUNC bit is set in the flag parameter.  The type of this field is int (*) (const char *, struct stat64 *).

          This is a GNU extension.

     gl_lstat
          The address of an alternative implementation of the lstat64 function to get information about an object in the file systems, not following symbolic links; it is used if the GLOB_ALTDIRFUNC bit is set in the flag parameter.  The type of this field is int (*) (const char *, struct stat64 *).

          This is a GNU extension.

 -- Function: int glob (const char *PATTERN, int FLAGS, int (*ERRFUNC) (const char *FILENAME, int ERROR-CODE), glob_t *VECTOR-PTR)
     The function glob does globbing using the pattern PATTERN in the current directory.  It puts the result in a newly allocated vector, and stores the size and address of this vector into *VECTOR-PTR.  The argument FLAGS is a combination of bit flags (see Flags for Globbing).

     The result of globbing is a sequence of file names.  The function glob allocates a string for each resulting word, then allocates a vector of type char ** to store the addresses of these strings.  The last element of the vector is a null pointer. This vector is called the word vector.

     To return this vector, glob stores both its address and its length (number of elements, not counting the terminating null pointer) into *VECTOR-PTR.

     Normally, glob sorts the file names alphabetically before returning them.  You can turn this off with the flag GLOB_NOSORT if you want to get the information as fast as possible.  Usually it's a good idea to let glob sort them---if you process the files in alphabetical order, the users will have a feel for the rate of progress that your application is making.

     If glob succeeds, it returns 0.  Otherwise, it returns one of these error codes:

     GLOB_ABORTED
          There was an error opening a directory, and you used the flag GLOB_ERR or your specified ERRFUNC returned a nonzero value. See Flags for Globbing, for an explanation of the GLOB_ERR flag and ERRFUNC.

     GLOB_NOMATCH
          The pattern didn't match any existing files.  If you use the GLOB_NOCHECK flag, then you never get this error code, because that flag tells glob to pretend that the pattern matched at least one file.

     GLOB_NOSPACE
          It was impossible to allocate memory to hold the result.

     In the event of an error, glob stores information in *VECTOR-PTR about all the matches it has found so far.

     It is important to notice that the glob function will not fail if it encounters directories or files that cannot be handled without the LFS interfaces.  The implementation of glob is supposed to use these functions internally.  This at least is the assumption made by the Unix standard.  The GNU extension of allowing the user to provide her own directory handling and stat functions complicates things a bit.  If these callback functions are used and a large file or directory is encountered, glob can fail.

 -- Function: int glob64 (const char *PATTERN, int FLAGS, int (*ERRFUNC) (const char *FILENAME, int ERROR-CODE), glob64_t *VECTOR-PTR)
     The glob64 function was added as part of the Large File Summit extensions but is not part of the original LFS proposal.  The reason for this is simple---it is not necessary.  The necessity for a glob64 function is added by the extensions of the GNU glob implementation, which allows the user to provide his own directory handling and stat functions.  The readdir and stat functions do depend on the choice of _FILE_OFFSET_BITS, since the definition of the types struct dirent and struct stat will change depending on the choice.

     Besides this difference, the glob64 works just like glob in all aspects.

     This function is a GNU extension.

Flags for Globbing
------------------

This section describes the flags that you can specify in the FLAGS argument to glob.  Choose the flags you want, and combine them with the C bit-wise OR operator |.

GLOB_APPEND
     Append the words from this expansion to the vector of words produced by previous calls to glob.  This way you can effectively expand several words as if they were concatenated with spaces between them.

     In order for appending to work, you must not modify the contents of the word vector structure between calls to glob.  And, if you set GLOB_DOOFFS in the first call to glob, you must also set it when you append to the results.

     Note that the pointer stored in gl_pathv may no longer be valid after you call glob the second time, because glob might have relocated the vector.  So always fetch gl_pathv from the glob_t structure after each glob call; never save the pointer across calls.

GLOB_DOOFFS
     Leave blank slots at the beginning of the vector of words. The gl_offs field says how many slots to leave. The blank slots contain null pointers.

GLOB_ERR
     Give up right away and report an error if there is any difficulty reading the directories that must be read in order to expand PATTERN fully.  Such difficulties might include a directory in which you don't have the requisite access.  Normally, glob tries its best to keep on going despite any errors, reading whatever directories it can.

     You can exercise even more control than this by specifying an error-handler function ERRFUNC when you call glob.  If ERRFUNC is not a null pointer, then glob doesn't give up right away when it can't read a directory; instead, it calls ERRFUNC with two arguments, like this:

          (*ERRFUNC) (FILENAME, ERROR-CODE)

     The argument FILENAME is the name of the directory that glob couldn't open or couldn't read, and ERROR-CODE is the errno value that was reported to glob.

     If the error-handler function returns nonzero, then glob gives up right away.  Otherwise, it continues.

GLOB_MARK
     If the pattern matches the name of a directory, append / to the directory's name when returning it.

GLOB_NOCHECK
     If the pattern doesn't match any file names, return the pattern itself as if it were a file name that had been matched.  (Normally, when the pattern doesn't match anything, glob returns that there were no matches.)

GLOB_NOSORT
     Don't sort the file names; return them in no particular order. (In practice, the order will depend on the order of the entries in the directory.)  The only reason not to sort is to save time.

GLOB_NOESCAPE
     Don't treat the \ character specially in patterns.  Normally, \ quotes the following character, turning off its special meaning (if any) so that it matches only itself.  When quoting is enabled, the pattern \? matches only the string ?, because the question mark in the pattern acts like an ordinary character.

     If you use GLOB_NOESCAPE, then \ is an ordinary character.

     glob does its work by calling the function fnmatch repeatedly.  It handles the flag GLOB_NOESCAPE by turning on the FNM_NOESCAPE flag in calls to fnmatch.

More Flags for Globbing
-----------------------

Besides the flags described in the last section, the GNU implementation of glob allows a few more flags, which are also defined in the glob.h file.  Some of the extensions implement functionality that is available in modern shell implementations.

GLOB_PERIOD
     The . character (period) is treated special.  It cannot be matched by wildcards (see Wildcard Matching, FNM_PERIOD).

GLOB_MAGCHAR
     The GLOB_MAGCHAR value is not to be given to glob in the FLAGS parameter.  Instead, glob sets this bit in the GL_FLAGS element of the GLOB_T structure provided as the result if the pattern used for matching contains any wildcard character.

GLOB_ALTDIRFUNC
     Instead of using the normal functions for accessing the file system, the glob implementation uses the user-supplied functions specified in the structure pointed to by the PGLOB parameter. (For more information about the functions, see Loosemore et al., ``Accessing Directories'' and ``Reading the Attributes of a File'' (see chap. 1, n. 1).)

GLOB_BRACE
     If this flag is given, the handling of braces in the pattern is changed.  It is now required that braces appear correctly grouped---for each opening brace there must be a closing one.  Braces can be used recursively.  So it is possible to define one brace expression in another one.  It is important to note that the range of each brace expression is completely contained in the outer brace expression (if there is one).

     The string between the matching braces is separated into single expressions by splitting at , (comma) characters.  The commas themselves are discarded.  Please note what we said above about recursive brace expressions.  The commas used to separate the subexpressions must be at the same level.  Commas in brace subexpressions are not matched.  They are used during expansion of the brace expression of the deeper level.  The example below shows this:

          glob ("{foo/{,bar,biz},baz}", GLOB_BRACE, NULL, &result)

     is equivalent to the sequence:

          glob ("foo/", GLOB_BRACE, NULL, &result)
          glob ("foo/bar", GLOB_BRACE|GLOB_APPEND, NULL, &result)
          glob ("foo/biz", GLOB_BRACE|GLOB_APPEND, NULL, &result)
          glob ("baz", GLOB_BRACE|GLOB_APPEND, NULL, &result)

     if we leave aside error handling.

GLOB_NOMAGIC
     If the pattern contains no wildcard constructs (it is a literal file name), return it as the sole ``matching'' word, even if no file exists by that name.

GLOB_TILDE
     If this flag is used, the character ~ (tilde) is handled specially if it appears at the beginning of the pattern.  Instead of being taken verbatim, it is used to represent the home directory of a known user.

     If ~ is the only character in the pattern or it is followed by a / (slash), the home directory of the process owner is substituted.  Using getlogin and getpwnam, the information is read from the system databases.  As an example, take user bart with his home directory at /home/bart.  For him a call like:

          glob ("~/bin/*", GLOB_TILDE, NULL, &result)

     would return the contents of the directory /home/bart/bin. Instead of referring to your own home directory, it is also possible to name the home directory of other users.  To do so one has to append the user name after the tilde character.  So the contents of user homer's bin directory can be retrieved by:

          glob ("~homer/bin/*", GLOB_TILDE, NULL, &result)

     If the user name is not valid or the home directory cannot be determined for some reason, the pattern is left untouched and is itself used as the result. For example, if in the last example home is not available, the tilde expansion yields to "~homer/bin/*" and glob is not looking for a directory named ~homer.

     This functionality is equivalent to what is available in C shells if the nonomatch flag is set.

GLOB_TILDE_CHECK
     If this flag is used, glob behaves as if GLOB_TILDE is given.  The only difference is that if the user name is not available or the home directory cannot be determined for other reasons, this leads to an error.  glob will return GLOB_NOMATCH instead of using the pattern itself as the name.

     This functionality is equivalent to what is available in C shells if nonomatch flag is not set.

GLOB_ONLYDIR
     If this flag is used, the globbing function takes this as a hint that the caller is only interested in directories matching the pattern.  If the information about the type of the file is easily available, nondirectories will be rejected, but no extra work will be done to determine the information for each file---the caller must still be able to filter directories out.

     This functionality is only available with the GNU glob implementation.  It is mainly used internally to increase performance but might be useful for a user as well.

Calling glob will in most cases allocate resources, which are used to represent the result of the function call.  If the same object of type glob_t is used in multiple calls to glob, the resources are freed or reused so that no leaks appear.  But this does not include the time when all glob calls are done.

 -- Function: void globfree (glob_t *PGLOB)
     The globfree function frees all resources allocated by previous calls to glob associated with the object pointed to by PGLOB.  This function should be called whenever the currently used glob_t typed object isn't used anymore.

 -- Function: void globfree64 (glob64_t *PGLOB)
     This function is equivalent to globfree, but it frees records of type glob64_t that were allocated by glob64.

Regular Expression Matching
===========================

The GNU C Library supports two interfaces for matching regular expressions.  One is the standard POSIX.2 interface, and the other is what the GNU system has had for many years.

Both interfaces are declared in the header file regex.h.  If you define _POSIX_C_SOURCE, then only the POSIX.2 functions, structures and constants are declared.

POSIX Regular Expression Compilation
------------------------------------

Before you can actually match a regular expression, you must compile it.  This is not true compilation---it produces a special data structure, not machine instructions.  But it is like ordinary compilation in that its purpose is to enable you to ``execute'' the pattern quickly (see Matching POSIX Regexps, for how to use the compiled regular expression for matching).

There is a special data type for compiled regular expressions:

 -- Data Type: regex_t
     This type of object holds a compiled regular expression. It is actually a structure.  It has just one field that your programs should look at:

     re_nsub
          This field holds the number of parenthetical subexpressions in the regular expression that was compiled.

     There are several other fields, but we don't describe them here, because only the functions in the library should use them.

After you create a regex_t object, you can compile a regular expression into it by calling regcomp.

 -- Function: int regcomp (regex_t *COMPILED, const char *PATTERN, int CFLAGS)
     The function regcomp ``compiles'' a regular expression into a data structure that you can use with regexec to match against a string.  The compiled regular expression format is designed for efficient matching.  regcomp stores it into *COMPILED.

     It's up to you to allocate an object of type regex_t and pass its address to regcomp.

     The argument CFLAGS lets you specify various options that control the syntax and semantics of regular expressions (see Flags for POSIX Regexps).

     If you use the flag REG_NOSUB, then regcomp omits from the compiled regular expression the information necessary to record how subexpressions actually match.  In this case, you might as well pass 0 for the MATCHPTR and NMATCH arguments when you call regexec.

     If you don't use REG_NOSUB, then the compiled regular expression does have the capacity to record how subexpressions match. Also, regcomp tells you how many subexpressions PATTERN has, by storing the number in COMPILED->re_nsub.  You can use that value to decide how long of an array to allocate to hold information about subexpression matches.

     regcomp returns 0 if it succeeds in compiling the regular expression; otherwise, it returns a nonzero error code (see the table below).  You can use regerror to produce an error message string describing the reason for a nonzero value (see Regexp Clean-Up).

Here are the possible nonzero values that regcomp can return:

REG_BADBR
     There was an invalid \{...\} construct in the regular expression.  A valid \{...\} construct must contain either a single number, or two numbers in increasing order separated by a comma.

REG_BADPAT
     There was a syntax error in the regular expression.

REG_BADRPT
     A repetition operator such as ? or * appeared in a bad position (with no preceding subexpression to act on).

REG_ECOLLATE
     The regular expression referred to an invalid collating element (one not defined in the current locale for string collation) (see Locale Categories).

REG_ECTYPE
     The regular expression referred to an invalid character class name.

REG_EESCAPE
     The regular expression ended with \.

REG_ESUBREG
     There was an invalid number in the \DIGIT construct.

REG_EBRACK
     There were unbalanced square brackets in the regular expression.

REG_EPAREN
     An extended regular expression had unbalanced parentheses, or a basic regular expression had unbalanced \( and \).

REG_EBRACE
     The regular expression had unbalanced \{ and \}.

REG_ERANGE
     One of the endpoints in a range expression was invalid.

REG_ESPACE
     regcomp ran out of memory.

Flags for POSIX Regular Expressions
-----------------------------------

These are the bit flags that you can use in the CFLAGS operand when compiling a regular expression with regcomp.

REG_EXTENDED
     Treat the pattern as an extended regular expression, rather than as a basic regular expression.

REG_ICASE
     Ignore case when matching letters.

REG_NOSUB
     Don't bother storing the contents of the MATCHES-PTR array.

REG_NEWLINE
     Treat a newline in STRING as dividing STRING into multiple lines, so that $ can match before the newline and ^ can match after.  Also, don't permit . to match a newline, and don't permit [^...] to match a newline.

     Otherwise, newline acts like any other ordinary character.

Matching a Compiled POSIX Regular Expression
--------------------------------------------

Once you have compiled a regular expression, as described in POSIX Regexp Compilation, you can match it against strings using regexec.  A match anywhere inside the string counts as success, unless the regular expression contains anchor characters (^ or $).

 -- Function: int regexec (regex_t *COMPILED, char *STRING, size_t NMATCH, regmatch_t MATCHPTR [], int EFLAGS)
     This function tries to match the compiled regular expression *COMPILED against STRING.

     regexec returns 0 if the regular expression matches; otherwise, it returns a nonzero value.  See the table below for what nonzero values mean.  You can use regerror to produce an error message string describing the reason for a nonzero value (see Regexp Clean-Up).

     The argument EFLAGS is a word of bit flags that enable various options.

     If you want to get information about what part of STRING actually matched the regular expression or its subexpressions, use the arguments MATCHPTR and NMATCH.  Otherwise, pass 0 for NMATCH, and NULL for MATCHPTR (see Regexp Subexpressions).

You must match the regular expression with the same set of current locales that were in effect when you compiled the regular expression.

The function regexec accepts the following flags in the EFLAGS argument:

REG_NOTBOL
     Do not regard the beginning of the specified string as the beginning of a line; more generally, don't make any assumptions about what text might precede it.

REG_NOTEOL
     Do not regard the end of the specified string as the end of a line; more generally, don't make any assumptions about what text might follow it.

Here are the possible nonzero values that regexec can return:

REG_NOMATCH
     The pattern didn't match the string.  This isn't really an error.

REG_ESPACE
     regexec ran out of memory.

Match Results with Subexpressions
---------------------------------

When regexec matches parenthetical subexpressions of PATTERN, it records which parts of STRING they match.  It returns that information by storing the offsets into an array whose elements are structures of type regmatch_t.  The first element of the array (index 0) records the part of the string that matched the entire regular expression.  Each other element of the array records the beginning and end of the part that matched a single parenthetical subexpression.

 -- Data Type: regmatch_t
     This is the data type of the MATCHARRAY array that you pass to regexec.  It contains two structure fields, as follows:

     rm_so
          The offset in STRING of the beginning of a substring.  Add this value to STRING to get the address of that part.

     rm_eo
          The offset in STRING of the end of the substring.

 -- Data Type: regoff_t
     regoff_t is an alias for another signed integer type. The fields of regmatch_t have type regoff_t.

The regmatch_t elements correspond to subexpressions positionally; the first element (index 1) records where the first subexpression matched, the second element records the second subexpression, and so on.  The order of the subexpressions is the order in which they begin.

When you call regexec, you specify how long the MATCHPTR array is, with the NMATCH argument.  This tells regexec how many elements to store.  If the actual regular expression has more than NMATCH subexpressions, then you won't get offset information about the rest of them.  But this doesn't alter whether the pattern matches a particular string or not.

If you don't want regexec to return any information about where the subexpressions matched, you can either supply 0 for NMATCH, or use the flag REG_NOSUB when you compile the pattern with regcomp.

Complications in Subexpression Matching
---------------------------------------

Sometimes, a subexpression matches a substring of no characters.  This happens when f\(o*\) matches the string fum (it really matches just the f).  In this case, both of the offsets identify the point in the string where the null substring was found. In this example, the offsets are both 1.

Sometimes, the entire regular expression can match without using some of its subexpressions at all---for example, when ba\(na\)* matches the string ba, the parenthetical subexpression is not used.  When this happens, regexec stores -1 in both fields of the element for that subexpression.

Sometimes matching the entire regular expression can match a particular subexpression more than once---for example, when ba\(na\)* matches the string bananana, the parenthetical subexpression matches three times.  When this happens, regexec usually stores the offsets of the last part of the string that matched the subexpression.  In the case of bananana, these offsets are 6 and 8.

But the last match is not always the one that is chosen.  It's more accurate to say that the last opportunity to match is the one that takes precedence.  What this means is that when one subexpression appears within another, then the results reported for the inner subexpression reflect whatever happened on the last match of the outer subexpression.  For an example, consider \(ba\(na\)*s \)* matching the string bananas bas .  The last time the inner expression actually matches is near the end of the first word.  But it is considered again in the second word, and fails to match there. regexec reports nonuse of the na subexpression.

Another place where this rule applies is when the regular expression:

     \(ba\(na\)*s \|nefer\(ti\)* \)*

matches bananas nefertiti.  The na subexpression does match in the first word, but it doesn't match in the second word because the other alternative is used there.  Once again, the second repetition of the outer subexpression overrides the first, and within that second repetition, the na subexpression is not used.  So regexec reports nonuse of the na subexpression.

POSIX Regexp Matching Clean-Up
------------------------------

When you are finished using a compiled regular expression, you can free the storage it uses by calling regfree.

 -- Function: void regfree (regex_t *COMPILED)
     Calling regfree frees all the storage that *COMPILED points to.  This includes various internal fields of the regex_t structure that aren't documented in this manual.

     regfree does not free the object *COMPILED itself.

You should always free the space in a regex_t structure with regfree before using the structure to compile another regular expression.

When regcomp or regexec reports an error, you can use the function regerror to turn it into an error message string.

 -- Function: size_t regerror (int ERRCODE, regex_t *COMPILED, char *BUFFER, size_t LENGTH)
     This function produces an error message string for the error code ERRCODE, and stores the string in LENGTH bytes of memory starting at BUFFER.  For the COMPILED argument, supply the same compiled regular expression structure that regcomp or regexec was working with when it got the error.  Alternatively, you can supply NULL for COMPILED; you will still get a meaningful error message, but it might not be as detailed.

     If the error message can't fit in LENGTH bytes (including a terminating null character), then regerror truncates it. The string that regerror stores is always null-terminated even if it has been truncated.

     The return value of regerror is the minimum length needed to store the entire error message.  If this is less than LENGTH, then the error message was not truncated, and you can use it.  Otherwise, you should call regerror again with a larger buffer.

     Here is a function that uses regerror, but always dynamically allocates a buffer for the error message:

          char *get_regerror (int errcode, regex_t *compiled)
          {
            size_t length = regerror (errcode, compiled, NULL, 0);
            char *buffer = xmalloc (length);
            (void) regerror (errcode, compiled, buffer, length);
            return buffer;
          }

Shell-Style Word Expansion
==========================

Word expansion means the process of splitting a string into words and substituting for variables, commands, and wildcards just as the shell does.

For example, when you write ls -l foo.c, this string is split into three separate words---ls, -l and foo.c. This is the most basic function of word expansion.

When you write ls *.c, this can become many words, because the word *.c can be replaced with any number of file names. This is called wildcard expansion, and it is also a part of word expansion.

When you use echo $PATH to print your path, you are taking advantage of variable substitution, which is also part of word expansion.

Ordinary programs can perform word expansion just like the shell by calling the library function wordexp.

The Stages of Word Expansion
----------------------------

When word expansion is applied to a sequence of words, it performs the following transformations in the order shown here:

 1. Tilde expansion ~foo us replaced with the name of the home directory of foo.

 2. Next, three different transformations are applied in the same step, from left to right:

       * Variable substitution Environment variables are substituted for references such as $foo.

       * Command substitution Constructs such as `cat foo` and the equivalent $(cat foo) are replaced with the output from the inner command.

       * Arithmetic expansion Constructs such as $(($x-1)) are replaced with the result of the arithmetic computation.

 3. Field splitting The text is subdivided into words.

 4. Wildcard expansion A construct such as *.c is replaced with a list of .c file names.  Wildcard expansion applies to an entire word at a time, and replaces that word with 0 or more file names that are themselves words.

 5. Quote removal String-quotes are deleted, now that they have done their job by inhibiting the above transformations when appropriate.

For the details of these transformations, and how to write the constructs that use them, see The GNU Bash Reference Manual. (Chet Ramey and Brian Fox, The GNU Bash Reference Manual (Bristol, UK: Network Theory Ltd., January 2003), http:// www.gnu.org/ software/ bash/ manual/ bashref.html.)

Calling wordexp
---------------

All the functions, constants and data types for word expansion are declared in the header file wordexp.h.

Word expansion produces a vector of words (strings).  To return this vector, wordexp uses a special data type, wordexp_t, which is a structure.  You pass wordexp the address of the structure, and it fills in the structure's fields to tell you about the results.

 -- Data Type: wordexp_t
     This data type holds a pointer to a word vector.  More precisely, it records both the address of the word vector and its size.

     we_wordc
          This is the number of elements in the vector.

     we_wordv
          This is the address of the vector.  This field has type char **.

     we_offs
          This is the offset of the first real element of the vector, from its nominal address in the we_wordv field.  Unlike the other fields, this is always an input to wordexp, rather than an output from it.

          If you use a nonzero offset, then that many elements at the beginning of the vector are left empty (the wordexp function fills them with null pointers).

          The we_offs field is meaningful only if you use the WRDE_DOOFFS flag.  Otherwise, the offset is always zero regardless of what is in this field, and the first real element comes at the beginning of the vector.

 -- Function: int wordexp (const char *WORDS, wordexp_t *WORD-VECTOR-PTR, int FLAGS)
     Perform word expansion on the string WORDS, putting the result in a newly allocated vector, and store the size and address of this vector into *WORD-VECTOR-PTR.  The argument FLAGS is a combination of bit flags (see Flags for Wordexp).

     You shouldn't use any of the characters |&;<> in the string WORDS unless they are quoted; likewise for newline.  If you use these characters unquoted, you will get the WRDE_BADCHAR error code.  Don't use parentheses or braces unless they are quoted or part of a word expansion construct.  If you use quotation characters '"`, they should come in pairs that balance.

     The results of word expansion are a sequence of words.  The function wordexp allocates a string for each resulting word, then allocates a vector of type char ** to store the addresses of these strings.  The last element of the vector is a null pointer. This vector is called the word vector.

     To return this vector, wordexp stores both its address and its length (number of elements, not counting the terminating null pointer) into *WORD-VECTOR-PTR.

     If wordexp succeeds, it returns 0.  Otherwise, it returns one of these error codes:

     WRDE_BADCHAR
          The input string WORDS contains an unquoted invalid character such as |.

     WRDE_BADVAL
          The input string refers to an undefined shell variable, and you used the flag WRDE_UNDEF to forbid such references.

     WRDE_CMDSUB
          The input string uses command substitution, and you used the flag WRDE_NOCMD to forbid command substitution.

     WRDE_NOSPACE
          It was impossible to allocate memory to hold the result.  In this case, wordexp can store part of the results---as much as it could allocate room for.

     WRDE_SYNTAX
          There was a syntax error in the input string.  For example, an unmatched quoting character is a syntax error.

 -- Function: void wordfree (wordexp_t *WORD-VECTOR-PTR)
     Free the storage used for the word-strings and vector that *WORD-VECTOR-PTR points to.  This does not free the structure *WORD-VECTOR-PTR itself---only the other data it points to.

Flags for Word Expansion
------------------------

This section describes the flags that you can specify in the FLAGS argument to wordexp.  Choose the flags you want, and combine them with the C operator |.

WRDE_APPEND
     Append the words from this expansion to the vector of words produced by previous calls to wordexp.  This way you can effectively expand several words as if they were concatenated with spaces between them.

     In order for appending to work, you must not modify the contents of the word vector structure between calls to wordexp.  And, if you set WRDE_DOOFFS in the first call to wordexp, you must also set it when you append to the results.

WRDE_DOOFFS
     Leave blank slots at the beginning of the vector of words. The we_offs field says how many slots to leave. The blank slots contain null pointers.

WRDE_NOCMD
     Don't do command substitution; if the input requests command substitution, report an error.

WRDE_REUSE
     Reuse a word vector made by a previous call to wordexp. Instead of allocating a new vector of words, this call to wordexp will use the vector that already exists (making it larger if necessary).

     The vector may move, so it is not safe to save an old pointer and use it again after calling wordexp.  You must fetch we_pathv anew after each call.

WRDE_SHOWERR
     Do show any error messages printed by commands run by command substitution. More precisely, allow these commands to inherit the standard error output stream of the current process.  By default, wordexp gives these commands a standard error stream that discards all output.

WRDE_UNDEF
     If the input refers to a shell variable that is not defined, report an error.

wordexp Example
---------------

Here is an example of using wordexp to expand several strings and use the results to run a shell command.  It also shows the use of WRDE_APPEND to concatenate the expansions and of wordfree to free the space allocated by wordexp.

     int
     expand_and_execute (const char *program, const char **options)
     {
       wordexp_t result;
       pid_t pid
       int status, i;

       /* Expand the string for the program to run.  */
       switch (wordexp (program, &result, 0))
         {
         case 0:			/* Successful.  */
           break;
         case WRDE_NOSPACE:
           /* If the error was WRDE_NOSPACE,
              then perhaps part of the result was allocated.  */
           wordfree (&result);
         default:                    /* Some other error.  */
           return -1;
         }

       /* Expand the strings specified for the arguments.  */
       for (i = 0; options[i] != NULL; i++)
         {
           if (wordexp (options[i], &result, WRDE_APPEND))
             {
               wordfree (&result);
               return -1;
             }
         }

       pid = fork ();
       if (pid == 0)
         {
           /* This is the child process.  Execute the command. */
           execv (result.we_wordv[0], result.we_wordv);
           exit (EXIT_FAILURE);
         }
       else if (pid < 0)
         /* The fork failed.  Report failure.  */
         status = -1;
       else
         /* This is the parent process.  Wait for the child to complete.  */
         if (waitpid (pid, &status, 0) != pid)
           status = -1;

       wordfree (&result);
       return status;
     }

Details of Tilde Expansion
--------------------------

It's a standard part of shell syntax that you can use ~ at the beginning of a file name to stand for your own home directory.  You can use ~USER to stand for USER's home directory.

Tilde expansion is the process of converting these abbreviations to the directory names that they stand for.

Tilde expansion applies to the ~ plus all following characters up to white space or a slash.  It takes place only at the beginning of a word, and only if none of the characters to be transformed is quoted in any way.

Plain ~ uses the value of the environment variable HOME as the proper home directory name.  ~ followed by a user name uses getpwname to look up that user in the user database, and uses whatever directory is recorded there.  Thus, ~ followed by your own name can give different results from plain ~, if the value of HOME is not really your home directory.

Details of Variable Substitution
--------------------------------

Part of ordinary shell syntax is the use of $VARIABLE to substitute the value of a shell variable into a command.  This is called variable substitution, and it is one part of doing word expansion.

There are two basic ways you can write a variable reference for substitution:

${VARIABLE}
     If you write braces around the variable name, then it is completely unambiguous where the variable name ends.  You can concatenate additional letters onto the end of the variable value by writing them immediately after the close brace.  For example, ${foo}s expands into tractors.

$VARIABLE
     If you do not put braces around the variable name, then the variable name consists of all the alphanumeric characters and underscores that follow the $.  The next punctuation character ends the variable name.  Thus, $foo-bar refers to the variable foo and expands into tractor-bar.

When you use braces, you can also use various constructs to modify the value that is substituted, or test it in various ways:

${VARIABLE:-DEFAULT}
     Substitute the value of VARIABLE, but if that is empty or undefined, use DEFAULT instead.

${VARIABLE:=DEFAULT}
     Substitute the value of VARIABLE, but if that is empty or undefined, use DEFAULT instead and set the variable to DEFAULT.

${VARIABLE:?MESSAGE}
     If VARIABLE is defined and not empty, substitute its value.

     Otherwise, print MESSAGE as an error message on the standard error stream, and consider word expansion a failure.

${VARIABLE:+REPLACEMENT}
     Substitute REPLACEMENT, but only if VARIABLE is defined and nonempty.  Otherwise, substitute nothing for this construct.

${#VARIABLE}
     Substitute a numeral that expresses in base-10 the number of characters in the value of VARIABLE.  ${#foo} stands for 7, because tractor is seven characters.

These variants of variable substitution let you remove part of the variable's value before substituting it.  The PREFIX and SUFFIX are not mere strings; they are wildcard patterns, just like the patterns that you use to match multiple file names.  But in this context, they match against parts of the variable value rather than against file names.

${VARIABLE%%SUFFIX}
     Substitute the value of VARIABLE, but first discard from that variable any portion at the end that matches the pattern SUFFIX.

     If there is more than one alternative for how to match against SUFFIX, this construct uses the longest possible match.

     Thus, ${foo%%r*} substitutes t, because the largest match for r* at the end of tractor is ractor.

${VARIABLE%SUFFIX}
     Substitute the value of VARIABLE, but first discard from that variable any portion at the end that matches the pattern SUFFIX.

     If there is more than one alternative for how to match against SUFFIX, this construct uses the shortest possible alternative.

     Thus, ${foo%r*} substitutes tracto, because the shortest match for r* at the end of tractor is just r.

${VARIABLE##PREFIX}
     Substitute the value of VARIABLE, but first discard from that variable any portion at the beginning that matches the pattern PREFIX.

     If there is more than one alternative for how to match against PREFIX, this construct uses the longest possible match.

     Thus, ${foo##*t} substitutes or, because the largest match for *t at the beginning of tractor is tract.

${VARIABLE#PREFIX}
     Substitute the value of VARIABLE, but first discard from that variable any portion at the beginning that matches the pattern PREFIX.

     If there is more than one alternative for how to match against PREFIX, this construct uses the shortest possible alternative.

     Thus, ${foo#*t} substitutes ractor, because the shortest match for *t at the beginning of tractor is just t.

The Basic Program/System Interface
**********************************

Processes are the primitive units for allocation of system resources.  Each process has its own address space and (usually) one thread of control.  A process executes a program; you can have multiple processes executing the same program, but each process has its own copy of the program within its own address space and executes it independently of the other copies.  Though it may have multiple threads of control within the same program and a program may be composed of multiple logically separate modules, a process always executes exactly one program.

We are using a specific definition of program for the purposes of this manual, which corresponds to a common definition in the context of Unix system.  In popular usage, program enjoys a much broader definition; it can refer for example to a system's kernel, an editor macro, a complex package of software, or a discrete section of code executing within a process.

Writing the program is what this manual is all about.  This chapter explains the most basic interface between your program and the system that runs, or calls, it.  This includes passing of parameters (arguments and environment) from the system, requesting basic services from the system, and telling the system the program is done.

A program starts another program with the exec family of system calls.  This chapter looks at program start-up from the execee's point of view. (See Loosemore et al., ``Executing a File'' (see chap. 1, n. 1), to see the event from the execor's point of view.)

Program Arguments
=================

The system starts a C program by calling the function main.  It is up to you to write a function named main---otherwise, you won't even be able to link your program without errors.

In ISO C you can define main either to take no arguments, or to take two arguments that represent the command-line arguments to the program, like this:

     int main (int ARGC, char *ARGV[])

The command-line arguments are the white-space-separated tokens given in the shell command used to invoke the program; thus, in cat foo bar, the arguments are foo and bar.  The only way a program can look at its command-line arguments is via the arguments of main.  If main doesn't take arguments, then you cannot get at the command line.

The value of the ARGC argument is the number of command-line arguments.  The ARGV argument is a vector of C strings; its elements are the individual command-line argument strings.  The file name of the program being run is also included in the vector as the first element; the value of ARGC counts this element.  A null pointer always follows the last element: ARGV[ARGC] is this null pointer.

For the command cat foo bar, ARGC is 3 and ARGV has three elements, cat, foo and bar.

In Unix systems you can define main a third way, using three arguments:

     int main (int ARGC, char *ARGV[], char *ENVP[])

The first two arguments are just the same.  The third argument ENVP gives the program's environment; it is the same as the value of environ (see Environment Variables).  POSIX.1 does not allow this three-argument form, so to be portable it is best to write main to take two arguments, and use the value of environ.

Program Argument Syntax Conventions
-----------------------------------

POSIX recommends these conventions for command-line arguments. getopt (see Getopt) and argp_parse (see Argp) make it easy to implement them.

  * Arguments are options if they begin with a hyphen delimiter (-).

  * Multiple options may follow a hyphen delimiter in a single token if the options do not take arguments.  Thus, -abc is equivalent to -a -b -c.

  * Option names are single alphanumeric characters (as for isalnum; see Classification of Characters).

  * Certain options require an argument.  For example, the -o command of the ld command requires an argument---an output file name.

  * An option and its argument may or may not appear as separate tokens.  (In other words, the white space separating them is optional.)  Thus, -o foo and -ofoo are equivalent.

  * Options typically precede other non-option arguments.

     The implementations of getopt and argp_parse in the GNU C Library normally make it appear as if all the option arguments were specified before all the non-option arguments for the purposes of parsing, even if the user of your program intermixed option and non-option arguments.  They do this by reordering the elements of the ARGV array.  This behavior is nonstandard; if you want to suppress it, define the _POSIX_OPTION_ORDER environment variable (see Standard Environment).

  * The argument -- terminates all options; any following arguments are treated as non-option arguments, even if they begin with a hyphen.

  * A token consisting of a single hyphen character is interpreted as an ordinary non-option argument.  By convention, it is used to specify input from or output to the standard input and output streams.

  * Options may be supplied in any order, or appear multiple times.  The interpretation is left up to the particular application program.

GNU adds long options to these conventions.  Long options consist of -- followed by a name made of alphanumeric characters and dashes.  Option names are typically one to three words long, with hyphens to separate words.  Users can abbreviate the option names as long as the abbreviations are unique.

To specify an argument for a long option, write --NAME=VALUE.  This syntax enables a long option to accept an argument that is itself optional.

Eventually, the GNU system will provide completion for long option names in the shell.

Parsing Program Arguments
-------------------------

If the syntax for the command-line arguments to your program is simple enough, you can pick the arguments off from ARGV by hand.  But unless your program takes a fixed number of arguments, or all of the arguments are interpreted in the same way (as file names, for example), you are usually better off using getopt (see Getopt) or argp_parse (see Argp) to do the parsing.

getopt is more standard (the short-option-only version of it is a part of the POSIX standard), but using argp_parse is often easier, both for very simple and very complex option structures, because it does more of the dirty work for you.

Parsing Program Options Using getopt
====================================

The getopt and getopt_long functions automate some of the chore involved in parsing typical Unix command-line options.

Using the getopt Function
-------------------------

Here are the details about how to call the getopt function.  To use this facility, your program must include the header file unistd.h.

 -- Variable: int opterr
     If the value of this variable is nonzero, then getopt prints an error message to the standard error stream if it encounters an unknown option character or an option with a missing required argument.  This is the default behavior.  If you set this variable to zero, getopt does not print any messages, but it still returns the character ?  to indicate an error.

 -- Variable: int optopt
     When getopt encounters an unknown option character or an option with a missing required argument, it stores that option character in this variable.  You can use this for providing your own diagnostic messages.

 -- Variable: int optind
     This variable is set by getopt to the index of the next element of the ARGV array to be processed.  Once getopt has found all of the option arguments, you can use this variable to determine where the remaining non-option arguments begin.  The initial value of this variable is 1.

 -- Variable: char * optarg
     This variable is set by getopt to point at the value of the option argument, for those options that accept arguments.

 -- Function: int getopt (int ARGC, char **ARGV, const char *OPTIONS)
     The getopt function gets the next option argument from the argument list specified by the ARGV and ARGC arguments. Normally these values come directly from the arguments received by main.

     The OPTIONS argument is a string that specifies the option characters that are valid for this program.  An option character in this string can be followed by a colon (:) to indicate that it takes a required argument.  If an option character is followed by two colons (::), its argument is optional; this is a GNU extension.

     getopt has three ways to deal with options that follow non-options ARGV elements.  The special argument -- forces in all cases the end of option scanning.

       * The default is to permute the contents of ARGV while scanning it so that eventually all the non-options are at the end.  This allows options to be given in any order, even with programs that were not written to expect this.

       * If the OPTIONS argument string begins with a hyphen (-), this is treated specially.  It permits arguments that are not options to be returned as if they were associated with option character \1.

       * POSIX demands the following behavior: The first non-option stops option processing.  This mode is selected by either setting the environment variable POSIXLY_CORRECT or beginning the OPTIONS argument string with a plus sign (+).

     The getopt function returns the option character for the next command-line option.  When no more option arguments are available, it returns -1.  There may still be more non-option arguments; you must compare the external variable optind against the ARGC parameter to check this.

     If the option has an argument, getopt returns the argument by storing it in the variable OPTARG.  You don't ordinarily need to copy the optarg string, since it is a pointer into the original ARGV array, not into a static area that might be overwritten.

     If getopt finds an option character in ARGV that was not included in OPTIONS, or a missing option argument, it returns ? and sets the external variable optopt to the actual option character.  If the first character of OPTIONS is a colon (:), then getopt returns : instead of ? to indicate a missing option argument.  In addition, if the external variable opterr is nonzero (which is the default), getopt prints an error message.

Example of Parsing Arguments with getopt
----------------------------------------

Here is an example showing how getopt is typically used.  The key points to notice are

  * Normally, getopt is called in a loop.  When getopt returns -1, indicating no more options are present, the loop terminates.

  * A switch statement is used to dispatch on the return value from getopt.  In typical use, each case just sets a variable that is used later in the program.

  * A second loop is used to process the remaining non-option arguments.

     #include <unistd.h>
     #include <stdio.h>

     int
     main (int argc, char **argv)
     {
       int aflag = 0;
       int bflag = 0;
       char *cvalue = NULL;
       int index;
       int c;

       opterr = 0;

       while ((c = getopt (argc, argv, "abc:")) != -1)
         switch (c)
           {
           case 'a':
             aflag = 1;
             break;
           case 'b':
             bflag = 1;
             break;
           case 'c':
             cvalue = optarg;
             break;
           case '?':
             if (isprint (optopt))
               fprintf (stderr, "Unknown option `-%c'.\n", optopt);
             else
               fprintf (stderr,
                        "Unknown option character `\\x%x'.\n",
                        optopt);
             return 1;
           default:
             abort ();
           }

       printf ("aflag = %d, bflag = %d, cvalue = %s\n",
               aflag, bflag, cvalue);

       for (index = optind; index < argc; index++)
         printf ("Non-option argument %s\n", argv[index]);
       return 0;
     }

Here are some examples showing what this program prints with different combinations of arguments:

     % testopt
     aflag = 0, bflag = 0, cvalue = (null)

     % testopt -a -b
     aflag = 1, bflag = 1, cvalue = (null)

     % testopt -ab
     aflag = 1, bflag = 1, cvalue = (null)

     % testopt -c foo
     aflag = 0, bflag = 0, cvalue = foo

     % testopt -cfoo
     aflag = 0, bflag = 0, cvalue = foo

     % testopt arg1
     aflag = 0, bflag = 0, cvalue = (null)
     Non-option argument arg1

     % testopt -a arg1
     aflag = 1, bflag = 0, cvalue = (null)
     Non-option argument arg1

     % testopt -c foo arg1
     aflag = 0, bflag = 0, cvalue = foo
     Non-option argument arg1

     % testopt -a -- -b
     aflag = 1, bflag = 0, cvalue = (null)
     Non-option argument -b

     % testopt -a -
     aflag = 1, bflag = 0, cvalue = (null)
     Non-option argument -

Parsing Long Options with getopt_long
-------------------------------------

To accept GNU-style long options as well as single-character options, use getopt_long instead of getopt.  This function is declared in getopt.h, not unistd.h.  You should make every program accept long options if it uses any options, because this takes little extra work and helps beginners remember how to use the program.

 -- Data Type: struct option
     This structure describes a single long option name for the sake of getopt_long.  The argument LONGOPTS must be an array of these structures, one for each long option.  Terminate the array with an element containing all zeros.

     The struct option structure has these fields:

     const char *name
          This field is the name of the option.  It is a string.

     int has_arg
          This field says whether the option takes an argument.  It is an integer, and there are three legitimate values: no_argument, required_argument and optional_argument.

     int *flag
     int val
          These fields control how to report or act on the option when it occurs.

          If flag is a null pointer, then the val is a value that identifies this option.  Often these values are chosen to uniquely identify particular long options.

          If flag is not a null pointer, it should be the address of an int variable that is the flag for this option.  The value in val is the value to store in the flag to indicate that the option was seen.

 -- Function: int getopt_long (int ARGC, char *const *ARGV, const char *SHORTOPTS, const struct option *LONGOPTS, int *INDEXPTR)
     Decode options from the vector ARGV (whose length is ARGC). The argument SHORTOPTS describes the short options to accept, just as it does in getopt.  The argument LONGOPTS describes the long options to accept (see above).

     When getopt_long encounters a short option, it does the same thing that getopt would do---it returns the character code for the option, and stores the options argument (if it has one) in optarg.

     When getopt_long encounters a long option, it takes actions based on the flag and val fields of the definition of that option.

     If flag is a null pointer, then getopt_long returns the contents of val to indicate which option it found.  You should arrange distinct values in the val field for options with different meanings, so you can decode these values after getopt_long returns.  If the long option is equivalent to a short option, you can use the short option's character code in val.

     If flag is not a null pointer, that means this option should just set a flag in the program.  The flag is a variable of type int that you define.  Put the address of the flag in the flag field. Put in the val field the value you would like this option to store in the flag.  In this case, getopt_long returns 0.

     For any long option, getopt_long tells you the index in the array LONGOPTS of the options definition, by storing it into *INDEXPTR.  You can get the name of the option with LONGOPTS[*INDEXPTR].name.  So you can distinguish among long options either by the values in their val fields or by their indices.  You can also distinguish in this way among long options that set flags.

     When a long option has an argument, getopt_long puts the argument value in the variable optarg before returning.  When the option has no argument, the value in optarg is a null pointer.  This is how you can tell whether an optional argument was supplied.

     When getopt_long has no more options to handle, it returns -1 and leaves, in the variable optind, the index in ARGV of the next remaining argument.

Since long option names were used before before the getopt_long options were invented, there are program interfaces that require programs to recognize options like -option value instead of --option value.  To enable these programs to use the GNU getopt functionality, there is one more function available.

 -- Function: int getopt_long_only (int ARGC, char *const *ARGV, const char *SHORTOPTS, const struct option *LONGOPTS, int *INDEXPTR)

     The getopt_long_only function is equivalent to the getopt_long function, but it allows the user of the application to specify passing long options with only - instead of --.  The -- prefix is still recognized, but instead of looking through the short options if a - is seen, it first determines whether this parameter names a long option.  If not, it is parsed as a short option.

     Assuming getopt_long_only is used starting an application with:

            app -foo

     the getopt_long_only will first look for a long option named foo.  If this is not found, the short options f, o, and again o are recognized.

Example of Parsing Long Options with getopt_long
------------------------------------------------

     #include <stdio.h>
     #include <stdlib.h>
     #include <getopt.h>

     /* Flag set by --verbose. */
     static int verbose_flag;

     int
     main (argc, argv)
          int argc;
          char **argv;
     {
       int c;

       while (1)
         {
           static struct option long_options[] =
             {
               /* These options set a flag. */
               {"verbose", no_argument,       &verbose_flag, 1},
               {"brief",   no_argument,       &verbose_flag, 0},
               /* These options don't set a flag.
                  We distinguish them by their indices. */
               {"add",     no_argument,       0, 'a'},
               {"append",  no_argument,       0, 'b'},
               {"delete",  required_argument, 0, 'd'},
               {"create",  required_argument, 0, 'c'},
               {"file",    required_argument, 0, 'f'},
               {0, 0, 0, 0}
             };
           /* getopt_long stores the option index here. */
           int option_index = 0;

           c = getopt_long (argc, argv, "abc:d:f:",
                            long_options, &option_index);

           /* Detect the end of the options. */
           if (c == -1)
             break;

           switch (c)
             {
             case 0:
               /* If this option set a flag, do nothing else now. */
               if (long_options[option_index].flag != 0)
                 break;
               printf ("option %s", long_options[option_index].name);
               if (optarg)
                 printf (" with arg %s", optarg);
               printf ("\n");
               break;

             case 'a':
               puts ("option -a\n");
               break;

             case 'b':
               puts ("option -b\n");
               break;

             case 'c':
               printf ("option -c with value `%s'\n", optarg);
               break;

             case 'd':
               printf ("option -d with value `%s'\n", optarg);
               break;

             case 'f':
               printf ("option -f with value `%s'\n", optarg);
               break;

             case '?':
               /* getopt_long already printed an error message. */
               break;

             default:
               abort ();
             }
         }

       /* Instead of reporting --verbose
          and --brief as they are encountered,
          we report the final status resulting from them. */
       if (verbose_flag)
         puts ("verbose flag is set");

       /* Print any remaining command-line arguments (not options). */
       if (optind < argc)
         {
           printf ("non-option ARGV-elements: ");
           while (optind < argc)
             printf ("%s ", argv[optind++]);
           putchar ('\n');
         }

       exit (0);
     }

Parsing Program Options with Argp
=================================

Argp is an interface for parsing Unix-style argument vectors (see Program Arguments).

Argp provides features unavailable in the more commonly used getopt interface.  These features include automatically producing output in response to the --help and --version options, as described in the GNU coding standards.  Using argp makes it less likely that programmers will neglect to implement these additional options or keep them up to date.

Argp also provides the ability to merge several independently defined option parsers into one, mediating conflicts between them and making the result appear seamless.  A library can export an argp option parser that user programs might employ in conjunction with their own option parsers, resulting in less work for the user programs.  Some programs may use only argument parsers exported by libraries, thereby achieving consistent and efficient option-parsing for abstractions implemented by the libraries.

The header file <argp.h> should be included to use argp.

The argp_parse Function
-----------------------

The main interface to argp is the argp_parse function.  In many cases, calling argp_parse is the only argument-parsing code needed in main (see Program Arguments).

 -- Function: error_t argp_parse (const struct argp *ARGP, int ARGC, char **ARGV, unsigned FLAGS, int *ARG_INDEX, void *INPUT)
     The argp_parse function parses the arguments in ARGV, of length ARGC, using the argp parser ARGP (see Argp Parsers).

     A value of zero is the same as a struct argp containing all zeros.  FLAGS is a set of flag bits that modify the parsing behavior (see Argp Flags).  INPUT is passed through to the argp parser ARGP, and has meaning defined by ARGP.  A typical usage is to pass a pointer to a structure that is used for specifying parameters to the parser and passing back the results.

     Unless the ARGP_NO_EXIT or ARGP_NO_HELP flags are included in FLAGS, calling argp_parse may result in the program exiting.  This behavior is true if an error is detected, or when an unknown option is encountered (see Program Termination).

     If ARG_INDEX is nonnull, the index of the first unparsed option in ARGV is returned as a value.

     The return value is zero for successful parsing, or an error code (see Error Codes) if an error is detected.  Different argp parsers may return arbitrary error codes, but the standard error codes are: ENOMEM if a memory allocation error occurred, or EINVAL if an unknown option or option argument is encountered.

Argp Global Variables
---------------------

These variables make it easy for user programs to implement the --version option and provide a bug-reporting address in the --help output.  These are implemented in argp by default.

 -- Variable: const char * argp_program_version
     If defined or set by the user program to a nonzero value, then a --version option is added when parsing with argp_parse, which will print the --version string followed by a newline and exit.  The exception to this is if the ARGP_NO_EXIT flag is used.

 -- Variable: const char * argp_program_bug_address
     If defined or set by the user program to a nonzero value, argp_program_bug_address should point to a string that will be printed at the end of the standard output for the --help option, embedded in a sentence that says Report bugs to ADDRESS..

 -- Variable: argp_program_version_hook
     If defined or set by the user program to a nonzero value, a --version option is added when parsing with arg_parse, which prints the program version and exits with a status of zero.  This is not the case if the ARGP_NO_HELP flag is used.  If the ARGP_NO_EXIT flag is set, the exit behavior of the program is suppressed or modified, as when the argp parser is going to be used by other programs.

     It should point to a function with this type of signature:

          void PRINT-VERSION (FILE *STREAM, struct argp_state *STATE)

     See Argp Parsing State, for an explanation of STATE.

     This variable takes precedence over argp_program_version, and is useful if a program has version information not easily expressed in a simple string.

 -- Variable: error_t argp_err_exit_status
     This is the exit status used when argp exits due to a parsing error. If not defined or set by the user program, this defaults to EX_USAGE from <sysexits.h>.

Specifying Argp Parsers
-----------------------

The first argument to the argp_parse function is a pointer to a struct argp, which is known as an argp parser:

 -- Data Type: struct argp
     This structure specifies how to parse a given set of options and arguments, perhaps in conjunction with other argp parsers.  It has the following fields:

     const struct argp_option *options
          This is a pointer to a vector of argp_option structures specifying which options this argp parser understands; it may be zero if there are no options at all (see Argp Option Vectors).

     argp_parser_t parser
          This is a pointer to a function that defines actions for this parser; it is called for each option parsed, and at other well-defined points in the parsing process.  A value of zero is the same as a pointer to a function that always returns ARGP_ERR_UNKNOWN (see Argp Parser Functions).

     const char *args_doc
          If nonzero, this is a string describing what non-option arguments are called by this parser.  This is only used to print the Usage: message.  If it contains newlines, the strings separated by them are considered alternative usage patterns and printed on separate lines. Lines after the first are prefixed by  or:  instead of Usage:.

     const char *doc
          If nonzero, this is a string containing extra text to be printed before and after the options in a long help message, with the two sections separated by a vertical tab ('\v', '\013') character.  By convention, the documentation before the options is just a short string explaining what the program does.  Documentation printed after the options describe behavior in more detail.

     const struct argp_child *children
          This is a pointer to a vector of argp_children structures. This pointer specifies which additional argp parsers should be combined with this one (see Argp Children).

     char *(*help_filter)(int KEY, const char *TEXT, void *INPUT)
          If nonzero, this is a pointer to a function that filters the output of help messages (see Argp Help Filtering).

     const char *argp_domain
          If nonzero, the strings used in the argp library are translated using the domain described by this string.  If zero, the current default domain is used.

Of the above group, options, parser, args_doc and the doc fields are usually all that are needed.  If an argp parser is defined as an initialized C variable, only the fields used need be specified in the initializer.  The rest will default to zero due to the way C structure initialization works.  This design is exploited in most argp structures; the most-used fields are grouped near the beginning and the unused fields are left unspecified.

Specifying Options in an Argp Parser
------------------------------------

The options field in a struct argp points to a vector of struct argp_option structures, each of which specifies an option that the argp parser supports.  Multiple entries may be used for a single option provided it has multiple names.  This should be terminated by an entry with zero in all fields.  Note that when using an initialized C array for options, writing { 0 } is enough to achieve this.

 -- Data Type: struct argp_option
     This structure specifies a single option that an argp parser understands, as well as how to parse and document that option.  It has the following fields:

     const char *name
          The long name for this option, corresponding to the long option --NAME; this field may be zero if this option only has a short name.  To specify multiple names for an option, additional entries may follow this one, with the OPTION_ALIAS flag set.  See Argp Option Flags.

     int key
          The integer key provided by the current option to the option parser.  If KEY has a value that is a printable ASCII character (i.e., isascii (KEY) is true), it also specifies a short option -CHAR, where CHAR is the ASCII character with the code KEY.

     const char *arg
          If non-zero, this is the name of an argument associated with this option, which must be provided (e.g., with the --NAME=VALUE or -CHAR VALUE syntaxes), unless the OPTION_ARG_OPTIONAL flag (see Argp Option Flags) is set, in which case it may be provided.

     int flags
          Flags associated with this option, some of which are referred to above. See Argp Option Flags.

     const char *doc
          A documentation string for this option, for printing in help messages.

          If both the name and key fields are zero, this string will be printed tabbed left from the normal option column, making it useful as a group header.  This will be the first thing printed in its group.  In this usage, it's conventional to end the string with a : character.

     int group
          Group identity for this option.

          In a long help message, options are sorted alphabetically within each group, and the groups presented in the order 0, 1, 2, ..., N, -M, ..., -2, -1.

          Every entry in an options array with this field 0 will inherit the group number of the previous entry, or zero if it's the first one.  If it's a group header with name and key fields both zero, the previous entry + 1 is the default.  Automagic options such as --help are put into group -1.

          Note that because of C structure initialization rules, this field often need not be specified, because 0 is the correct value.

Flags for Argp Options
......................

The following flags may be ORed together in the flags field of a struct argp_option.  These flags control various aspects of how that option is parsed or displayed in help messages:

OPTION_ARG_OPTIONAL
     The argument associated with this option is optional.

OPTION_HIDDEN
     This option isn't displayed in any help messages.

OPTION_ALIAS
     This option is an alias for the closest previous non-alias option. This means that it will be displayed in the same help entry and will inherit fields other than name and key from the option being aliased.

OPTION_DOC
     This option isn't actually an option and should be ignored by the actual option parser.  It is an arbitrary section of documentation that should be displayed in much the same manner as the options.  This is known as a documentation option.

     If this flag is set, then the option name field is displayed unmodified (e.g., no -- prefix is added) at the left margin where a short option would normally be displayed, and this documentation string is left in its usual place.  For purposes of sorting, any leading white space and punctuation is ignored, unless the first non-white-space character is -.  This entry is displayed after all options, after OPTION_DOC entries with a leading -, in the same group.

OPTION_NO_USAGE
     This option shouldn't be included in long usage messages, but should still be included in other help messages.  This is intended for options that are completely documented in an argp's args_doc field (see Argp Parsers).  Including this option in the generic usage list would be redundant, and should be avoided.

     For instance, if args_doc is FOO BAR\n-x BLAH, and the -x option's purpose is to distinguish these two cases, -x should probably be marked OPTION_NO_USAGE.

Argp Parser Functions
---------------------

The function pointed to by the parser field in a struct argp (see Argp Parsers) defines what actions take place in response to each option or argument parsed.  It is also used as a hook, allowing a parser to perform tasks at certain other points during parsing.

Argp parser functions have the following type signature:

     error_t PARSER (int KEY, char *ARG, struct argp_state *STATE)

where the arguments are as follows:

key
     For each option that is parsed, PARSER is called with a value of KEY from that option's key field in the option vector (see Argp Option Vectors).  PARSER is also called at other times with special reserved keys, such as ARGP_KEY_ARG for non-option arguments (see Argp Special Keys).

arg
     If KEY is an option, ARG is its given value.  This defaults to zero if no value is specified.  Only options that have a nonzero arg field can ever have a value.  These must always have a value unless the OPTION_ARG_OPTIONAL flag is specified.  If the input being parsed specifies a value for an option that doesn't allow one, an error results before PARSER ever gets called.

     If KEY is ARGP_KEY_ARG, ARG is a non-option argument.  Other special keys always have a zero ARG.

state
     STATE points to a struct argp_state, containing useful information about the current parsing state for use by PARSER (see Argp Parsing State).

When PARSER is called, it should perform whatever action is appropriate for KEY, and return 0 for success, ARGP_ERR_UNKNOWN if the value of KEY is not handled by this parser function, or a Unix error code if a real error occurred (see Error Codes).

 -- Macro: int ARGP_ERR_UNKNOWN
     Argp parser functions should return ARGP_ERR_UNKNOWN for any KEY value they do not recognize, or for non-option arguments (KEY == ARGP_KEY_ARG) that they are not equipped to handle.

A typical parser function uses a switch statement on KEY:

     error_t
     parse_opt (int key, char *arg, struct argp_state *state)
     {
       switch (key)
         {
         case OPTION_KEY:
           ACTION
           break;
         ...
         default:
           return ARGP_ERR_UNKNOWN;
         }
       return 0;
     }

Special Keys for Argp Parser Functions
......................................

In addition to key values corresponding to user options, the KEY argument to argp parser functions may have a number of other special values.  In the following example, ARG and STATE refer to parser function arguments (see Argp Parser Functions).

ARGP_KEY_ARG
     This is not an option at all, but rather a command-line argument, whose value is pointed to by ARG.

     When there are multiple parser functions in play due to argp parsers being combined, it's impossible to know which one will handle a specific argument.  Each is called until one returns 0 or an error other than ARGP_ERR_UNKNOWN; if an argument is not handled, argp_parse immediately returns success, without parsing any more arguments.

     Once a parser function returns success for this key, that fact is recorded, and the ARGP_KEY_NO_ARGS case won't be used.  However, if while processing the argument a parser function decrements the next field of its STATE argument, the option won't be considered processed; this is to allow you to actually modify the argument, perhaps into an option, and have it processed again.

ARGP_KEY_ARGS
     If a parser function returns ARGP_ERR_UNKNOWN for ARGP_KEY_ARG, it is immediately called again with the key ARGP_KEY_ARGS, which has a similar meaning, but is slightly more convenient for consuming all remaining arguments.  ARG is 0, and the tail of the argument vector may be found at STATE->argv + STATE->next.  If success is returned for this key, and STATE->next is unchanged, all remaining arguments are considered to have been consumed.  Otherwise, the amount by which STATE->next has been adjusted indicates how many were used.  Here is an example that uses both, for different args:

          ...
          case ARGP_KEY_ARG:
            if (STATE->arg_num == 0)
              /* First argument */
              first_arg = ARG;
            else
              /* Let the next case parse it.  */
              return ARGP_KEY_UNKNOWN;
            break;
          case ARGP_KEY_ARGS:
            remaining_args = STATE->argv + STATE->next;
            num_remaining_args = STATE->argc - STATE->next;
            break;

ARGP_KEY_END
     This indicates that there are no more command-line arguments.  Parser functions are called in a different order, children first.  This allows each parser to clean up its state for the parent.

ARGP_KEY_NO_ARGS
     Because it's common to do some special processing if there aren't any non-option args, parser functions are called with this key if they didn't successfully process any non-option arguments.  This is called just before ARGP_KEY_END, where more general validity checks on previously parsed arguments take place.

ARGP_KEY_INIT
     This is passed in before any parsing is done.  Afterwards, the values of each element of the child_input field of STATE, if any, are copied to each child's state to be the initial value of the input when their parsers are called.

ARGP_KEY_SUCCESS
     This is passed in when parsing has successfully been completed, even if arguments remain.

ARGP_KEY_ERROR
     This is passed in if an error has occurred and parsing is terminated. In this case, a call with a key of ARGP_KEY_SUCCESS is never made.

ARGP_KEY_FINI
     This is the final key ever seen by any parser, even after ARGP_KEY_SUCCESS and ARGP_KEY_ERROR.  Any resources allocated by ARGP_KEY_INIT may be freed here.  At times, certain resources allocated are to be returned to the caller after a successful parse.  In that case, those particular resources can be freed in the ARGP_KEY_ERROR case.

In all cases, ARGP_KEY_INIT is the first key seen by parser functions, and ARGP_KEY_FINI the last, unless an error was returned by the parser for ARGP_KEY_INIT.  Other keys can occur in one the following orders.  OPT refers to an arbitrary option key:

OPT... ARGP_KEY_NO_ARGS ARGP_KEY_END ARGP_KEY_SUCCESS
     The arguments being parsed did not contain any non-option arguments.

( OPT | ARGP_KEY_ARG )... ARGP_KEY_END ARGP_KEY_SUCCESS
     All non-option arguments were successfully handled by a parser function.  There may be multiple parser functions if multiple argp parsers were combined.

( OPT | ARGP_KEY_ARG )... ARGP_KEY_SUCCESS
     Some non-option argument went unrecognized.

     This occurs when every parser function returns ARGP_KEY_UNKNOWN for an argument, in which case parsing stops at that argument if ARG_INDEX is a null pointer.  Otherwise an error occurs.

In all cases, if a nonnull value for ARG_INDEX gets passed to argp_parse, the index of the first unparsed command-line argument is passed back in that value.

If an error occurs and is either detected by argp or because a parser function returned an error value, each parser is called with ARGP_KEY_ERROR.  No further calls are made, except the final call with ARGP_KEY_FINI.

Functions for Use in Argp Parsers
.................................

Argp provides a number of functions available to the user of argp (see Argp Parser Functions), mostly for producing error messages. These take as their first argument the STATE argument to the parser function (see Argp Parsing State).

 -- Function: void argp_usage (const struct argp_state *STATE)
     This outputs the standard usage message for the argp parser referred to by STATE to STATE->err_stream and terminates the program with exit (argp_err_exit_status) (see Argp Global Variables).

 -- Function: void argp_error (const struct argp_state *STATE, const char *FMT, ...)
     This prints the printf format string FMT and following args, preceded by the program name and :, and followed by a Try ... --help message, and terminates the program with an exit status of argp_err_exit_status (see Argp Global Variables).

 -- Function: void argp_failure (const struct argp_state *STATE, int STATUS, int ERRNUM, const char *FMT, ...)
     Similar to the standard GNU error-reporting function error, this prints the program name and :, the printf format string FMT, and the appropriate following args.  If it is nonzero, the standard Unix error text for ERRNUM is printed.  If STATUS is nonzero, it terminates the program with that value as its exit status.

     The difference between argp_failure and argp_error is that argp_error is for parsing errors, whereas argp_failure is for other problems that occur during parsing but don't reflect a syntactic problem with the input, such as illegal values for options, bad phase of the moon, etc.

 -- Function: void argp_state_help (const struct argp_state *STATE, FILE *STREAM, unsigned FLAGS)
     This outputs a help message to STREAM for the argp parser referred to by STATE.  The FLAGS argument determines what sort of help message is produced (see Argp Help Flags).

Error output is sent to STATE->err_stream, and the program name printed is STATE->name.

The output or program termination behavior of these functions may be suppressed if the ARGP_NO_EXIT or ARGP_NO_ERRS flags are passed to argp_parse (see Argp Flags).

This behavior is useful if an argp parser is exported for use by other programs (e.g., by a library), and may be used in a context where it is not desirable to terminate the program in response to parsing errors.  In argp parsers intended for such general use, and for the case where the program doesn't terminate, calls to any of these functions should be followed by code that returns the appropriate error code:

     if (BAD ARGUMENT SYNTAX)
       {
          argp_usage (STATE);
          return EINVAL;
       }

If a parser function will only be used when ARGP_NO_EXIT is not set, the return may be omitted.

Argp Parsing State
..................

The third argument to argp parser functions (see Argp Parser Functions) is a pointer to a struct argp_state, which contains information about the state of the option parsing.

 -- Data Type: struct argp_state
     This structure has the following fields, which may be modified as noted:

     const struct argp *const root_argp
          This is the top-level argp parser being parsed.  This is often not the same struct argp passed into argp_parse by the invoking program (see Argp).  It is an internal argp parser that contains options implemented by argp_parse itself, such as --help.

     int argc
     char **argv
          This is the argument vector being parsed.  This may be modified.

     int next
          This is the index in argv of the next argument to be parsed. This may be modified.

          One way to consume all remaining arguments in the input is to set STATE->next = STATE->argc, perhaps after recording the value of the next field to find the consumed arguments. The current option can be reparsed immediately by decrementing this field, then modifying STATE->argv[STATE->next] to reflect the option that should be reexamined.

     unsigned flags
          The flags supplied to argp_parse.  These may be modified, although some flags may only take effect when argp_parse is first invoked (see Argp Flags).

     unsigned arg_num
          While calling a parsing function with the KEY argument ARGP_KEY_ARG, this represents the number of the current arg, starting at 0.  It is incremented after each ARGP_KEY_ARG call returns.  At all other times, this is the number of ARGP_KEY_ARG arguments that have been processed.

     int quoted
          If nonzero, this is the index in argv of the first argument following a special -- argument.  This prevents anything that follows from being interpreted as an option.  It is only set after argument parsing has proceeded past this point.

     void *input
          This is an arbitrary pointer passed in from the caller of argp_parse, in the INPUT argument.

     void **child_inputs
          These are values that will be passed to child parsers.  This vector will be the same length as the number of children in the current parser.  Each child parser will be given the value of STATE->child_inputs[I] as its STATE->input field, where I is the index of the child in the this parser's children field (see Argp Children).

     void *hook
          This is for the parser function's use.  It is initialized to 0, but otherwise ignored by argp.

     char *name
          This is the name used when printing messages.  This is initialized to argv[0], or program_invocation_name if argv[0] is unavailable.

     FILE *err_stream
     FILE *out_stream
          These are the stdio streams used when argp prints.  Error messages are printed to err_stream and all other output, such as --help, is printed to out_stream.  These are initialized to stderr and stdout, respectively (see Standard Streams).

     void *pstate
          This is private, for use by the argp implementation.

Combining Multiple Argp Parsers
-------------------------------

The children field in a struct argp enables other argp parsers to be combined with the referencing one for the parsing of a single set of arguments.  This field should point to a vector of struct argp_child, which is terminated by an entry having a value of zero in the argp field.

Where conflicts between combined parsers arise, as when two specify an option with the same name, the parser conflicts are resolved in favor of the parent argp parser(s), or the earlier of the argp parsers in the list of children.

 -- Data Type: struct argp_child
     An entry in the list of subsidiary argp parsers pointed to by the children field in a struct argp.  The fields are as follows:

     const struct argp *argp
          This is the child argp parser, or zero to end of the list.

     int flags
          These are flags for this child.

     const char *header
          If nonzero, this is an optional header to be printed within help output before the child options.  As a side effect, a nonzero value forces the child options to be grouped together.  To achieve this effect without actually printing a header string, use a value of "".  As with header strings specified in an option entry, the conventional value of the last character is : (see Argp Option Vectors).

     int group
          This is where the child options are grouped relative to the other `consolidated' options in the parent argp parser.  The values are the same as the group field in struct argp_option (see Argp Option Vectors).  All child-groupings follow parent options at a particular group level.  If both this field and header are zero, then the child's options aren't grouped together; they are merged with parent options at the parent option group level.

Flags for argp_parse
--------------------

The default behavior of argp_parse is designed to be convenient for the most common case of parsing program command-line arguments. To modify these defaults, the following flags may be or'd together in the FLAGS argument to argp_parse:

ARGP_PARSE_ARGV0
     Do not ignore the first element of the ARGV argument to argp_parse.  Unless ARGP_NO_ERRS is set, the first element of the argument vector is skipped for option-parsing purposes, as it corresponds to the program name in a command line.

ARGP_NO_ERRS
     Do not print error messages for unknown options to stderr; unless this flag is set, ARGP_PARSE_ARGV0 is ignored, as argv[0] is used as the program name in the error messages.  This flag implies ARGP_NO_EXIT.  This is based on the assumption that silent exiting upon errors is bad behavior.

ARGP_NO_ARGS
     Do not parse any non-option args.  Normally, these are parsed by calling the parse functions with a key of ARGP_KEY_ARG, the actual argument being the value.  This flag needn't normally be set, as the default behavior is to stop parsing as soon as an argument fails to be parsed (see Argp Parser Functions).

ARGP_IN_ORDER
     Parse options and arguments in the same order they occur on the command line.  Normally, they are rearranged so that all options come first.

ARGP_NO_HELP
     Do not provide the standard long option --help, which ordinarily causes usage and option help information to be output to stdout and exit (0).

ARGP_NO_EXIT
     Do not exit on errors, although they may still result in error messages.

ARGP_LONG_ONLY
     Use the GNU getopt `long-only' rules for parsing arguments.  This allows long options to be recognized with only a single - (i.e. -help).  This results in a less useful interface, and its use is discouraged as it conflicts with the way most GNU programs work as well as the GNU coding standards.

ARGP_SILENT
     Turns off any message-printing/exiting options, specifically ARGP_NO_EXIT, ARGP_NO_ERRS, and ARGP_NO_HELP.

Customizing Argp Help Output
----------------------------

The help_filter field in a struct argp is a pointer to a function that filters the text of help messages before displaying them.  They have a function signature like:

     char *HELP-FILTER (int KEY, const char *TEXT, void *INPUT)

Where KEY is either a key from an option, in which case TEXT is that option's help text (see Argp Option Vectors). Alternately, one of the special keys with names beginning with ARGP_KEY_HELP_ might be used, describing which other help text TEXT will contain (see Argp Help Filter Keys).

The function should return either TEXT if it remains as-is, or a replacement string allocated using malloc.  This will either be freed by argp or be zero, which prints nothing.  The value of TEXT is supplied after any translation has been done, so if any of the replacement text needs translation, it will be done by the filter function.  INPUT is either the input supplied to argp_parse or it is zero, if argp_help was called directly by the user.

Special Keys for Argp Help Filter Functions
...........................................

The following special values may be passed to an argp help filter function as the first argument in addition to key values for user options.  They specify which help text the TEXT argument contains:

ARGP_KEY_HELP_PRE_DOC
     This is the help text preceding options.

ARGP_KEY_HELP_POST_DOC
     This is the help text following options.

ARGP_KEY_HELP_HEADER
     This is the option header string.

ARGP_KEY_HELP_EXTRA
     This is used after all other documentation; TEXT is zero for this key.

ARGP_KEY_HELP_DUP_ARGS_NOTE
     This is the explanatory note printed when duplicate option arguments have been suppressed.

ARGP_KEY_HELP_ARGS_DOC
     This is the argument doc string; formally the args_doc field from the argp parser (see Argp Parsers).

The argp_help Function
----------------------

Normally programs using argp need not be written with particular printing argument-usage-type help messages in mind, as the standard --help option is handled automatically by argp.  Typical error cases can be handled using argp_usage and argp_error (see Argp Helper Functions).  However, if it's desirable to print a help message in some context other than parsing the program options, argp offers the argp_help interface.

 -- Function: void argp_help (const struct argp *ARGP, FILE *STREAM, unsigned FLAGS, char *NAME)
     This outputs a help message for the argp parser ARGP to STREAM.  The type of messages printed will be determined by FLAGS.

     Any options such as --help that are implemented automatically by argp itself will not be present in the help output; for this reason it is best to use argp_state_help if calling from within an argp parser function (see Argp Helper Functions).

Flags for the argp_help Function
--------------------------------

When calling argp_help (see Argp Help) or argp_state_help (see Argp Helper Functions) the exact output is determined by the FLAGS argument.  This should consist of any of the following flags, or'd together:

ARGP_HELP_USAGE
     This is a Unix Usage: message that explicitly lists all options.

ARGP_HELP_SHORT_USAGE
     This is a Unix Usage: message that displays an appropriate placeholder to indicate where the options go; useful for showing the non-option argument syntax.

ARGP_HELP_SEE
     This is a Try ... for more help message; ... contains the program name and --help.

ARGP_HELP_LONG
     This is a verbose option help message that gives each option available along with its documentation string.

ARGP_HELP_PRE_DOC
     This is the part of the argp parser doc string preceding the verbose option help.

ARGP_HELP_POST_DOC
     This is the part of the argp parser doc string following the verbose option help.

ARGP_HELP_DOC
     (ARGP_HELP_PRE_DOC | ARGP_HELP_POST_DOC)

ARGP_HELP_BUG_ADDR
     This is a message that prints where to report bugs for this program, if the argp_program_bug_address variable contains this information.

ARGP_HELP_LONG_ONLY
     This will modify any output to reflect the ARGP_LONG_ONLY mode.

The following flags are only understood when used with argp_state_help.  They control whether the function returns after printing its output, or terminates the program:

ARGP_HELP_EXIT_ERR
     This will terminate the program with exit (argp_err_exit_status).

ARGP_HELP_EXIT_OK
     This will terminate the program with exit (0).

The following flags are combinations of the basic flags for printing standard messages:

ARGP_HELP_STD_ERR
     Assuming that an error message for a parsing error has printed, this prints a message on how to get help, and terminates the program with an error.

ARGP_HELP_STD_USAGE
     This prints a standard usage message and terminates the program with an error.  This is used when no other specific error messages are appropriate or available.

ARGP_HELP_STD_HELP
     This prints the standard response for a --help option, and terminates the program successfully.

Argp Examples
-------------

These example programs demonstrate the basic usage of argp.

A Minimal Program Using Argp
............................

This is perhaps the smallest program possible that uses argp.  It won't do much except give an error messages and exit when there are any arguments, and prints a rather pointless message for --help.

     /* Argp Example 1---a minimal program using argp */

     /* This is (probably) the smallest possible program that
        uses argp.  It won't do much except give an error
        message and exit when there are any arguments, and print
        a (rather pointless) message for --help. */

     #include <argp.h>

     int main (int argc, char **argv)
     {
       argp_parse (0, argc, argv, 0, 0, 0);
       exit (0);
     }

A Program Using Argp with Only Default Options
..............................................

This program doesn't use any options or arguments; it uses argp to be compliant with the GNU standard command-line format.

In addition to giving no arguments and implementing a --help option, this example has a --version option, which will put the given documentation string and bug address in the --help output, as per GNU standards.

The variable argp contains the argument parser specification.  Adding fields to this structure is the way most parameters are passed to argp_parse.  The first three fields are normally used, but they are not in this small program.  There are also two global variables that argp can use defined here, argp_program_version and argp_program_bug_address.  They are considered global variables because they will almost always be constant for a given program, even if they use different argument parsers for various tasks.

     /* Argp Example 2---a pretty minimal program using argp */

     /* This program doesn't use any options or arguments, but uses
        argp to be compliant with the GNU standard command-line
        format.

        In addition to making sure no arguments are given, and
        implementing a --help option, this example will have a
        --version option, and will put the given documentation string
        and bug address in the --help output, as per GNU standards.

        The variable ARGP contains the argument parser specification;
        adding fields to this structure is the way most parameters are
        passed to argp_parse (the first three fields are usually used,
        but not in this small program).  There are also two global
        variables that argp knows about defined here,
        ARGP_PROGRAM_VERSION and ARGP_PROGRAM_BUG_ADDRESS (they are
        global variables because they will almost always be constant
        for a given program, even if it uses different argument
        parsers for various tasks). */

     #include <argp.h>

     const char *argp_program_version =
       "argp-ex2 1.0";
     const char *argp_program_bug_address =
       "<bug-gnu-utils@gnu.org>";

     /* Program documentation */
     static char doc[] =
       "Argp Example 2---a pretty minimal program using argp";

     /* This is our argument parser.  The options, parser, and
        args_doc fields are zero because we have neither options nor
        arguments; doc and argp_program_bug_address will be
        used in the output for --help, and the --version
        option will print out argp_program_version. */
     static struct argp argp = { 0, 0, 0, doc };

     int main (int argc, char **argv)
     {
       argp_parse (&argp, argc, argv, 0, 0, 0);
       exit (0);
     }

A Program Using Argp with User Options
......................................

This program uses the same features as Example 2, adding user options and arguments.

We now use the first four fields in argp (see Argp Parsers) and specify parse_opt as the parser function (see Argp Parser Functions).

In this example, main uses a structure to communicate with the parse_opt function, a pointer to which it passes in the input argument to argp_parse (see Argp).  It is retrieved by parse_opt through the input field in its state argument (see Argp Parsing State).  Of course, it's also possible to use global variables instead, but using a structure like this is somewhat more flexible and clean.

     /* Argp Example 3---a program with options and arguments using argp */

     /* This program uses the same features as Example 2, and uses options and
        arguments.

        We now use the first four fields in ARGP, so here's a description of them:
          OPTIONS: A pointer to a vector of struct argp_option (see below)
          PARSER: A function to parse a single option, called by argp
          ARGS_DOC: A string describing how the non-option arguments should look
          DOC: A descriptive string about this program; if it contains a
                      vertical tab character (\v), the part after it will be
                      printed *following* the options.

        The function PARSER takes the following arguments:
          KEY: An integer specifying which option this is (taken
               from the KEY field in each struct argp_option), or
               a special key specifying something else; the only
               special keys we use here are ARGP_KEY_ARG, meaning
               a non-option argument, and ARGP_KEY_END, meaning
               that all arguments have been parsed
          ARG: For an option KEY, the string value of its
               argument, or NULL if it has none
          STATE: A pointer to a struct argp_state, containing
                 various useful information about the parsing state; used here
                 are the INPUT field, which reflects the INPUT argument to
                 argp_parse, and the ARG_NUM field, which is the number of the
                 current non-option argument being parsed
        It should return either 0, meaning success, ARGP_ERR_UNKNOWN, meaning the
        given KEY wasn't recognized, or an errno value indicating some other
        error.

        The OPTIONS field contains a pointer to a vector of struct argp_option's;
        that structure has the following fields (if you assign your option
        structures using array initialization like this example, unspecified
        fields will be defaulted to 0, and need not be specified):
          NAME: The name of this option's long option (may be zero)
          KEY: The KEY to pass to the PARSER function when parsing this option,
               *and* the name of this option's short option, if it is a
               printable ASCII character
          ARG: The name of this option's argument, if any
          FLAGS: Flags describing this option; some of them are:
                      OPTION_ARG_OPTIONAL: The argument to this option is optional
                      OPTION_ALIAS: This option is an alias for the
                                    previous option
                      OPTION_HIDDEN: Don't show this option in --help output
          DOC: A documentation string for this option, shown in --help output

        An options vector should be terminated by an option with all fields zero. */

     #include <argp.h>

     const char *argp_program_version =
       "argp-ex3 1.0";
     const char *argp_program_bug_address =
       "<bug-gnu-utils@gnu.org>";

     /* Program documentation */
     static char doc[] =
       "Argp Example 3---a program with options and arguments using argp";

     /* A description of the arguments we accept */
     static char args_doc[] = "ARG1 ARG2";

     /* The options we understand */
     static struct argp_option options[] = {
       {"verbose",  'v', 0,      0,  "Produce verbose output" },
       {"quiet",    'q', 0,      0,  "Don't produce any output" },
       {"silent",   's', 0,      OPTION_ALIAS },
       {"output",   'o', "FILE", 0,
        "Output to FILE instead of standard output" },
       { 0 }
     };

     /* Used by main to communicate with parse_opt */
     struct arguments
     {
       char *args[2];                /* ARG1 & ARG2 */
       int silent, verbose;
       char *output_file;
     };

     /* Parse a single option. */
     static error_t
     parse_opt (int key, char *arg, struct argp_state *state)
     {
       /* Get the INPUT argument from argp_parse, which we
          know is a pointer to our arguments structure. */
       struct arguments *arguments = state->input;

       switch (key)
         {
         case 'q': case 's':
           arguments->silent = 1;
           break;
         case 'v':
           arguments->verbose = 1;
           break;
         case 'o':
           arguments->output_file = arg;
           break;

         case ARGP_KEY_ARG:
           if (state->arg_num >= 2)
             /* Too many arguments */
             argp_usage (state);

           arguments->args[state->arg_num] = arg;

           break;

         case ARGP_KEY_END:
           if (state->arg_num < 2)
             /* Not enough arguments */
             argp_usage (state);
           break;

         default:
           return ARGP_ERR_UNKNOWN;
         }
       return 0;
     }

     /* Our argp parser */
     static struct argp argp = { options, parse_opt, args_doc, doc };

     int main (int argc, char **argv)
     {
       struct arguments arguments;

       /* Default values */
       arguments.silent = 0;
       arguments.verbose = 0;
       arguments.output_file = "-";

       /* Parse our arguments; every option seen by parse_opt will
          be reflected in arguments. */
       argp_parse (&argp, argc, argv, 0, 0, &arguments);

       printf ("ARG1 = %s\nARG2 = %s\nOUTPUT_FILE = %s\n"
               "VERBOSE = %s\nSILENT = %s\n",
               arguments.args[0], arguments.args[1],
               arguments.output_file,
               arguments.verbose ? "yes" : "no",
               arguments.silent ? "yes" : "no");

       exit (0);
     }

A Program Using Multiple Combined Argp Parsers
..............................................

This program uses the same features as Example 3, but it has more options and presents more structure in the --help output.  It also illustrates how you can ``steal'' the remainder of the input arguments past a certain point for programs that accept a list of items.  It also illustrates the KEY value ARGP_KEY_NO_ARGS, which is only given if no non-option arguments were supplied to the program (see Argp Special Keys).

For structuring help output, two features are used: headers and a two-part option string.  The headers are entries in the options vector (see Argp Option Vectors).  The first four fields are zero.  The two-part documentation string is in the variable doc, which allows documentation both before and after the options (see Argp Parsers).  The two parts of doc are separated by a vertical-tab character ('\v', or '\013'). By convention, the documentation before the options is a short string stating what the program does.  After any options, it is longer, describing the behavior in more detail.  All documentation strings are automatically filled for output, although newlines may be included to force a line break at a particular point.  In addition, documentation strings are passed to the gettext function, for possible translation into the current locale.

     /* Argp Example 4---a program with somewhat more complicated options */

     /* This program uses the same features as Example 3, but it has more
        options and somewhat more structure in the --help output.  It
        also shows how you can ``steal'' the remainder of the input
        arguments past a certain point, for programs that accept a
        list of items.  It also shows the special argp KEY value
        ARGP_KEY_NO_ARGS, which is only given if no non-option
        arguments were supplied to the program.

        For structuring the help output, two features are used:
        *headers*, which are entries in the options vector with the
        first four fields being zero, and a two-part documentation
        string (in the variable DOC), which allows documentation both
        before and after the options; the two parts of DOC are
        separated by a vertical-tab character ('\v', or '\013').  By
        convention, the documentation before the options is just a
        short string saying what the program does, and afterwards it
        is longer, describing the behavior in more detail.  All
        documentation strings are automatically filled for output,
        although newlines may be included to force a line break at a
        particular point.  All documentation strings are also passed to
        the `gettext' function, for possible translation into the
        current locale. */

     #include <stdlib.h>
     #include <error.h>
     #include <argp.h>

     const char *argp_program_version =
       "argp-ex4 1.0";
     const char *argp_program_bug_address =
       "<bug-gnu-utils@prep.ai.mit.edu>";

     /* Program documentation */
     static char doc[] =
       "Argp Example 4---a program with somewhat more complicated\
     options\
     \vThis part of the documentation comes *after* the options;\
      note that the text is automatically filled, but it's possible\
      to force a line-break, e.g.\n<-- here.";

     /* A description of the arguments we accept */
     static char args_doc[] = "ARG1 [STRING...]";

     /* Keys for options without short options */
     #define OPT_ABORT  1            /* --abort */

     /* The options we understand */
     static struct argp_option options[] = {
       {"verbose",  'v', 0,       0, "Produce verbose output" },
       {"quiet",    'q', 0,       0, "Don't produce any output" },
       {"silent",   's', 0,       OPTION_ALIAS },
       {"output",   'o', "FILE",  0,
        "Output to FILE instead of standard output" },

       {0,0,0,0, "The following options should be grouped together:" },
       {"repeat",   'r', "COUNT", OPTION_ARG_OPTIONAL,
        "Repeat the output COUNT (default 10) times"},
       {"abort",    OPT_ABORT, 0, 0, "Abort before showing any output"},

       { 0 }
     };

     /* Used by main to communicate with parse_opt. */
     struct arguments
     {
       char *arg1;                   /* ARG1 */
       char **strings;               /* [STRING...] */
       int silent, verbose, abort;   /* -s, -v, --abort */
       char *output_file;            /* FILE arg to --output */
       int repeat_count;             /* COUNT arg to --repeat */
     };

     /* Parse a single option. */
     static error_t
     parse_opt (int key, char *arg, struct argp_state *state)
     {
       /* Get the input argument from argp_parse, which we
          know is a pointer to our arguments structure. */
       struct arguments *arguments = state->input;

       switch (key)
         {
         case 'q': case 's':
           arguments->silent = 1;
           break;
         case 'v':
           arguments->verbose = 1;
           break;
         case 'o':
           arguments->output_file = arg;
           break;
         case 'r':
           arguments->repeat_count = arg ? atoi (arg) : 10;
           break;
         case OPT_ABORT:
           arguments->abort = 1;
           break;

         case ARGP_KEY_NO_ARGS:
           argp_usage (state);

         case ARGP_KEY_ARG:
           /* Here we know that state->arg_num == 0, since we
              force argument parsing to end before any more arguments can
              get here. */
           arguments->arg1 = arg;

           /* Now we consume all the rest of the arguments.
              state->next is the index in state->argv of the
              next argument to be parsed, which is the first STRING
              we're interested in, so we can just use
              &state->argv[state->next] as the value for
              arguments->strings.

              In addition, by setting state->next to the end
              of the arguments, we can force argp to stop parsing here and
              return. */
           arguments->strings = &state->argv[state->next];
           state->next = state->argc;

           break;

         default:
           return ARGP_ERR_UNKNOWN;
         }
       return 0;
     }

     /* Our argp parser */
     static struct argp argp = { options, parse_opt, args_doc, doc };

     int main (int argc, char **argv)
     {
       int i, j;
       struct arguments arguments;

       /* Default values */
       arguments.silent = 0;
       arguments.verbose = 0;
       arguments.output_file = "-";
       arguments.repeat_count = 1;
       arguments.abort = 0;

       /* Parse our arguments; every option seen by parse_opt will be
          reflected in arguments. */
       argp_parse (&argp, argc, argv, 0, 0, &arguments);

       if (arguments.abort)
         error (10, 0, "ABORTED");

       for (i = 0; i < arguments.repeat_count; i++)
         {
           printf ("ARG1 = %s\n", arguments.arg1);
           printf ("STRINGS = ");
           for (j = 0; arguments.strings[j]; j++)
             printf (j == 0 ? "%s" : ", %s", arguments.strings[j]);
           printf ("\n");
           printf ("OUTPUT_FILE = %s\nVERBOSE = %s\nSILENT = %s\n",
                   arguments.output_file,
                   arguments.verbose ? "yes" : "no",
                   arguments.silent ? "yes" : "no");
         }

       exit (0);
     }

Argp User Customization
-----------------------

The formatting of argp --help output may be controlled to some extent by a program's users, by setting the ARGP_HELP_FMT environment variable to a comma-separated list of tokens.  White space is ignored:

dup-args
no-dup-args
     These turn duplicate-argument-mode on or off.  In duplicate argument mode, if an option that accepts an argument has multiple names, the argument is shown for each name.  Otherwise, it is only shown for the first long option.  A note is subsequently printed so the user knows that it applies to other names as well.  The default is no-dup-args, which is less consistent, but prettier.

dup-args-note

no-dup-args-note
     These will enable or disable the note informing the user of suppressed option argument duplication.  The default is dup-args-note.

short-opt-col=N
     This prints the first short option in column N.  The default is 2.

long-opt-col=N
     This prints the first long option in column N.  The default is 6.

doc-opt-col=N
     This prints `documentation options' (see Argp Option Flags) in column N.  The default is 2.

opt-doc-col=N
     This prints the documentation for options starting in column N.  The default is 29.

header-col=N
     This will indent the group headers that document groups of options to column N.  The default is 1.

usage-indent=N
     This will indent continuation lines in Usage: messages to column N.  The default is 12.

rmargin=N
     This will word wrap help output at or before column N.  The default is 79.

Parsing of Suboptions
.....................

Having a single level of options is sometimes not enough.  There might be too many options that have to be available or a set of options that are closely related.

For these cases, some programs use suboptions.  One of the most prominent programs is certainly mount(8).  The -o option takes one argument, which itself is a comma-separated list of options. To ease the programming of code like this, the function getsubopt is available.

 -- Function: int getsubopt (char **OPTIONP, const char* const *TOKENS, char **VALUEP)

     The OPTIONP parameter must be a pointer to a variable containing the address of the string to process.  When the function returns, the reference is updated to point to the next suboption or to the terminating \0 character if there are no more suboptions available.

     The TOKENS parameter references an array of strings containing the known suboptions.  All strings must be \0 terminated and, to mark the end, a null pointer must be stored.  When getsubopt finds a possible legal suboption, it compares it with all strings available in the TOKENS array and returns the index in the string as the indicator.

     In case the suboption has an associated value introduced by an = character, a pointer to the value is returned in VALUEP.  The string is \0 terminated.  If no argument is available, VALUEP is set to the null pointer.  By doing this, the caller can check whether a necessary value is given or whether no unexpected value is present.

     In case the next suboption in the string is not mentioned in the TOKENS array, the starting address of the suboption including a possible value is returned in VALUEP, and the return value of the function is -1.

Parsing of Suboptions Example
-----------------------------

The code that might appear in the mount(8) program is a perfect example of the use of getsubopt:

     #include <stdio.h>
     #include <stdlib.h>
     #include <unistd.h>

     int do_all;
     const char *type;
     int read_size;
     int write_size;
     int read_only;

     enum
     {
       RO_OPTION = 0,
       RW_OPTION,
       READ_SIZE_OPTION,
       WRITE_SIZE_OPTION,
       THE_END
     };

     const char *mount_opts[] =
     {
       [RO_OPTION] = "ro",
       [RW_OPTION] = "rw",
       [READ_SIZE_OPTION] = "rsize",
       [WRITE_SIZE_OPTION] = "wsize",
       [THE_END] = NULL
     };

     int
     main (int argc, char *argv[])
     {
       char *subopts, *value;
       int opt;

       while ((opt = getopt (argc, argv, "at:o:")) != -1)
         switch (opt)
           {
           case 'a':
             do_all = 1;
             break;
           case 't':
             type = optarg;
             break;
           case 'o':
             subopts = optarg;
             while (*subopts != '\0')
               switch (getsubopt (&subopts, mount_opts, &value))
                 {
                 case RO_OPTION:
                   read_only = 1;
                   break;
                 case RW_OPTION:
                   read_only = 0;
                   break;
                 case READ_SIZE_OPTION:
                   if (value == NULL)
                     abort ();
                   read_size = atoi (value);
                   break;
                 case WRITE_SIZE_OPTION:
                   if (value == NULL)
                     abort ();
                   write_size = atoi (value);
                   break;
                 default:
                   /* Unknown suboption */
                   printf ("Unknown suboption `%s'\n", value);
                   break;
                 }
             break;
           default:
             abort ();
           }

       /* Do the real work. */

       return 0;
     }

Environment Variables
=====================

When a program is executed, it receives information about the context in which it was invoked in two ways.  The first mechanism uses the ARGV and ARGC arguments to its main function, and is discussed in Program Arguments.  The second mechanism uses environment variables and is discussed in this section.

The ARGV mechanism is typically used to pass command-line arguments specific to the particular program being invoked.  The environment, on the other hand, keeps track of information that is shared by many programs, changes infrequently and is less frequently used.

The environment variables discussed in this section are the same environment variables that you set using assignments and the export command in the shell.  Programs executed from the shell inherit all of the environment variables from the shell.

Standard environment variables are used for information about the user's home directory, terminal type, current locale and so on; you can define additional variables for other purposes.  The set of all environment variables that have values is collectively known as the environment.

Names of environment variables are case-sensitive and must not contain the character =.  System-defined environment variables are invariably uppercase.

The values of environment variables can be anything that can be represented as a string.  A value must not contain an embedded null character, since this is assumed to terminate the string.

Environment Access
------------------

The value of an environment variable can be accessed with the getenv function.  This is declared in the header file stdlib.h.  All of the following functions can be safely used in multithreaded programs.  It is ensured that concurrent modifications to the environment do not lead to errors.

 -- Function: char * getenv (const char *NAME)
     This function returns a string that is the value of the environment variable NAME.  You must not modify this string.  In some non-Unix systems not using the GNU library, it might be overwritten by subsequent calls to getenv (but not by any other library function).  If the environment variable NAME is not defined, the value is a null pointer.

 -- Function: int putenv (char *STRING)
     The putenv function adds or removes definitions from the environment. If the STRING is of the form NAME=VALUE, the definition is added to the environment.  Otherwise, the STRING is interpreted as the name of an environment variable, and any definition for this variable in the environment is removed.

     The difference to the setenv function is that the exact string given as the parameter STRING is put into the environment.  If the user should change the string after the putenv call, this will reflect automatically in the environment.  This also requires that STRING not be an automatic variable that could be deleted or could fall out of scope before the variable is removed from the environment.  The same applies to dynamically allocated variables that are freed later.

     This function is part of the extended Unix interface.  Since it was also available in old SVID libraries, you should define either _XOPEN_SOURCE or _SVID_SOURCE before including any header.

 -- Function: int setenv (const char *NAME, const char *VALUE, int REPLACE)
     The setenv function can be used to add a new definition to the environment.  The entry with the name NAME is replaced by the value NAME=VALUE.  This is also true if VALUE is the empty string.  To do this, a new string is created and the string's NAME and VALUE are copied.  A null pointer for the VALUE parameter is illegal.  If the environment already contains an entry with key NAME, then the REPLACE parameter controls the action.  If replace is zero, nothing happens. Otherwise, the old entry is replaced by the new one.

     You cannot remove an entry completely using this function.

     This function was originally part of the BSD library, but is now part of the Unix standard.

 -- Function: int unsetenv (const char *NAME)
     Using this function, you can remove an entry completely from the environment.  If the environment contains an entry with the key NAME, this whole entry is removed.  A call to this function is equivalent to a call to putenv when the VALUE part of the string is empty.

     The function returns -1 if NAME is a null pointer, points to an empty string, or points to a string containing an = character.  It returns 0 if the call succeeded.

     This function was originally part of the BSD library but is now part of the Unix standard.  The BSD version had no return value, though.

There is one more function to modify the whole environment.  This function is said to be used in the POSIX.9 (POSIX bindings for Fortran 77) and so one would expect it to have made it to POSIX.1.  But this never happened.  We still provide this function as a GNU extension to enable writing standard compliant Fortran environments.

 -- Function: int clearenv (void)
     The clearenv function removes all entries from the environment. Using putenv and setenv, new entries can be added again later.

     If the function is successful, it returns 0.  Otherwise, the return value is nonzero.

You can deal directly with the underlying representation of environment objects to add more variables to the environment (for example, to communicate with another program you are about to execute). (Ibid., ``Executing a File''.)

 -- Variable: char ** environ
     The environment is represented as an array of strings.  Each string is of the format NAME=VALUE.  The order in which strings appear in the environment is not significant, but the same NAME must not appear more than once.  The last element of the array is a null pointer.

     This variable is declared in the header file unistd.h.

     If you just want to get the value of an environment variable, use getenv.

Unix systems and the GNU system pass the initial value of environ as the third argument to main (see Program Arguments).

Standard Environment Variables
------------------------------

These environment variables have standard meanings.  This doesn't mean that they are always present in the environment; but if these variables are present, they have these meanings.  You shouldn't try to use these environment variable names for some other purpose.

HOME

     This is a string representing the user's home directory or initial default working directory.

     The user can set HOME to any value. If you need to make sure to obtain the proper home directory for a particular user, you should not use HOME; instead, look up the user's name in the user database. (Ibid., ``User Database''.)

     For most purposes, it is better to use HOME, precisely because this lets the user specify the value.

LOGNAME

     This is the name that the user used to log in.  Since the value in the environment can be tweaked arbitrarily, this is not a reliable way to identify the user who is running a program; a function like getlogin is better for that purpose. (Ibid., ``Identifying Who Logged In''.)

     For most purposes, it is better to use LOGNAME, precisely because this lets the user specify the value.

PATH

     A path is a sequence of directory names that is used for searching for a file.  The variable PATH holds a path used for searching for programs to be run.

     The execlp and execvp functions (Ibid., ``Executing a File''.) use this environment variable, as do many shells and other utilities that are implemented in terms of those functions.

     The syntax of a path is a sequence of directory names separated by colons.  An empty string instead of a directory name stands for the current directory. (Ibid., ``Working Directory''.)

     A typical value for this environment variable might be a string like:

          :/bin:/etc:/usr/bin:/usr/new/X11:/usr/new:/usr/local/bin

     This means that if the user tries to execute a program named foo, the system will look for files named foo, /bin/foo, /etc/foo and so on.  The first of these files that exists is the one that is executed.

TERM

     This specifies the kind of terminal that is receiving program output. Some programs can make use of this information to take advantage of special escape sequences or terminal modes supported by particular kinds of terminals.  Many programs that use the termcap library (See Richard M. Stallman, ``Finding a Terminal Description: tgetent'' in The Termcap Manual: The Termcap Library and Data Base, 2nd ed. (Boston, MA: GNU Press, December 1992), http:// www.gnu.org/ software/ termutils/ manual/ termcap-1.3/ termcap.html.)  use the TERM environment variable, for example.

TZ

     This specifies the time zone.  See TZ Variable, for information about the format of this string and how it is used.

LANG

     This specifies the default locale to use for attribute categories where neither LC_ALL nor the specific environment variable for that category is set.  See Locales, for more information about locales.

LC_ALL

     If this environment variable is set, it overrides the selection for all the locales done using the other LC_* environment variables.  The value of the other LC_* environment variables is simply ignored in this case.

LC_COLLATE

     This specifies what locale to use for string sorting.

LC_CTYPE

     This specifies what locale to use for character sets and character classification.

LC_MESSAGES

     This specifies what locale to use for printing messages and parsing responses.

LC_MONETARY

     This specifies what locale to use for formatting monetary values.

LC_NUMERIC

     This specifies what locale to use for formatting numbers.

LC_TIME

     This specifies what locale to use for formatting date and time values.

NLSPATH

     This specifies the directories in which the catopen function looks for message translation catalogs.

_POSIX_OPTION_ORDER

     If this environment variable is defined, it suppresses the usual reordering of command-line arguments by getopt and argp_parse (see Argument Syntax).

System Calls
============

A system call is a request for service that a program makes of the kernel.  The service is generally something that only the kernel has the privilege to do, such as doing I/O.  Programmers don't normally need to be concerned with system calls because there are functions in the GNU C Library to do virtually everything that system calls do. These functions work by making system calls themselves.  For example, there is a system call that changes the permissions of a file, but you don't need to know about it because you can just use the GNU C Library's chmod function.

System calls are sometimes called kernel calls.

However, there are times when you want to make a system call explicitly, and for that, the GNU C Library provides the syscall function. syscall is harder to use and less portable than functions like chmod, but easier and more portable than coding the system call in assembler instructions.

syscall is most useful when you are working with a system call that is special to your system or is newer than the GNU C Library you are using.  syscall is implemented in an entirely generic way; the function does not know anything about what a particular system call does or even if it is valid.

The description of syscall in this section assumes a certain protocol for system calls on the various platforms on which the GNU C Library runs.  That protocol is not defined by any strong authority, but we won't describe it here either, because anyone who is coding syscall probably won't accept anything less than kernel and C library source code as a specification of the interface between them anyway.

syscall is declared in unistd.h.

 -- Function: long int syscall (long int SYSNO, ...)

     syscall performs a generic system call.

     SYSNO is the system call number.  Each kind of system call is identified by a number.  Macros for all the possible system call numbers are defined in sys/syscall.h

     The remaining arguments are the arguments for the system call, in order, and their meanings depend on the kind of system call.  Each kind of system call has a definite number of arguments, from zero to five. If you code more arguments than the system call takes, the extra ones to the right are ignored.

     The return value is the return value from the system call, unless the system call failed.  In that case, syscall returns -1 and sets errno to an error code that the system call returned. System calls do not return -1 when they succeed.

     If you specify an invalid SYSNO, syscall returns -1 with errno = ENOSYS.

     Here is an example:

          #include <unistd.h>
          #include <sys/syscall.h>
          #include <errno.h>

          ...

          int rc;

          rc = syscall(SYS_chmod, "/etc/passwd", 0444);

          if (rc == -1)
             fprintf(stderr, "chmod failed, errno = %d\n", errno);

     This, if all of the compatibility stars are aligned, is equivalent to the following preferable code:

          #include <sys/types.h>
          #include <sys/stat.h>
          #include <errno.h>

          ...

          int rc;

          rc = chmod("/etc/passwd", 0444);
          if (rc == -1)
             fprintf(stderr, "chmod failed, errno = %d\n", errno);

Program Termination
===================

The usual way for a program to terminate is simply for its main function to return.  The exit status value returned from the main function is used to report information back to the process's parent process or shell.

A program can also terminate normally by calling the exit function.

In addition, programs can be terminated by signals. (Ibid., ``Signal Handling''.)  The abort function causes a signal that kills the program.

Normal Termination
------------------

A process terminates normally when its program signals it is done by calling exit.  Returning from main is equivalent to calling exit, and the value that main returns is used as the argument to exit.

 -- Function: void exit (int STATUS)
     The exit function tells the system that the program is done, which causes it to terminate the process.

     STATUS is the program's exit status, which becomes part of the process's termination status.  This function does not return.

Normal termination causes the following actions:

 1. Functions that were registered with the atexit or on_exit functions are called in the reverse order of their registration.  This mechanism allows your application to specify its own ``clean-up'' actions to be performed at program termination.  Typically, this is used to do things like saving program state information in a file, or unlocking locks in shared data bases.

 2. All open streams are closed, writing out any buffered output data (see Closing Streams).  In addition, temporary files opened with the tmpfile function are removed. (Ibid., ``Temporary Files''.)

 3. _exit is called, terminating the program (see Termination Internals).

Exit Status
-----------

When a program exits, it can return to the parent process a small amount of information about the cause of termination, using the exit status.  This is a value between 0 and 255 that the exiting process passes as an argument to exit.

Normally, you should use the exit status to report very broad information about success or failure.  You can't provide a lot of detail about the reasons for the failure, and most parent processes would not want much detail anyway.

There are conventions for what sorts of status values certain programs should return.  The most common convention is simply 0 for success and 1 for failure.  Programs that perform comparison use a different convention: they use status 1 to indicate a mismatch, and status 2 to indicate an inability to compare.  Your program should follow an existing convention if an existing convention makes sense for it.

A general convention reserves status values 128 and up for special purposes.  In particular, the value 128 is used to indicate failure to execute another program in a subprocess.  This convention is not universally obeyed, but it is a good idea to follow it in your programs.

Warning: Don't try to use the number of errors as the exit status.  This is actually not very useful; a parent process would generally not care how many errors occurred.  Worse than that, it does not work, because the status value is truncated to 8 bits.  Thus, if the program tried to report 256 errors, the parent would receive a report of 0 errors---that is, success.

For the same reason, it does not work to use the value of errno as the exit status---these can exceed 255.

Portability Note: Some non-POSIX systems use different conventions for exit status values.  For greater portability, you can use the macros EXIT_SUCCESS and EXIT_FAILURE for the conventional status value for success and failure, respectively.  They are declared in the file stdlib.h.

 -- Macro: int EXIT_SUCCESS
     This macro can be used with the exit function to indicate successful program completion.

     On POSIX systems, the value of this macro is 0.  On other systems, the value might be some other (possibly nonconstant) integer expression.

 -- Macro: int EXIT_FAILURE
     This macro can be used with the exit function to indicate unsuccessful program completion in a general sense.

     On POSIX systems, the value of this macro is 1.  On other systems, the value might be some other (possibly nonconstant) integer expression.  Other nonzero status values also indicate failures.  Certain programs use different nonzero status values to indicate particular kinds of ``nonsuccess''.  For example, diff uses status value 1 to mean that the files are different, and 2 or more to mean that there was difficulty in opening the files.

Don't confuse a program's exit status with a process's termination status.  There are lots of ways a process can terminate besides having its program finish.  In the event that the process termination is caused by program termination (i.e., exit), though, the program's exit status becomes part of the process's termination status.

Clean-Ups on Exit
-----------------

Your program can arrange to run its own clean-up functions if normal termination happens.  If you are writing a library for use in various application programs, then it is unreliable to insist that all applications call the library's clean-up functions explicitly before exiting.  It is much more robust to make the clean-up invisible to the application, by setting up a clean-up function in the library itself using atexit or on_exit.

 -- Function: int atexit (void (*FUNCTION) (void))
     The atexit function registers the function FUNCTION to be called at normal program termination.  The FUNCTION is called with no arguments.

     The return value from atexit is zero on success and nonzero if the function cannot be registered.

 -- Function: int on_exit (void (*FUNCTION)(int STATUS, void *ARG), void *ARG)
     This function is a somewhat more powerful variant of atexit. It accepts two arguments: a function FUNCTION and an arbitrary pointer ARG.  At normal program termination, the FUNCTION is called with two arguments: the STATUS value passed to exit, and the ARG.

     This function is included in the GNU C Library only for compatibility for SunOS, and may not be supported by other implementations.

Here's a trivial program that illustrates the use of exit and atexit:

     #include <stdio.h>
     #include <stdlib.h>

     void
     bye (void)
     {
       puts ("Goodbye, cruel world....");
     }

     int
     main (void)
     {
       atexit (bye);
       exit (EXIT_SUCCESS);
     }

When this program is executed, it just prints the message and exits.

Aborting a Program
------------------

You can abort your program using the abort function.  The prototype for this function is in stdlib.h.

 -- Function: void abort (void)
     The abort function causes abnormal program termination.  This does not execute clean-up functions registered with atexit or on_exit.

     This function actually terminates the process by raising a SIGABRT signal, and your program can include a handler to intercept this signal. (Ibid., ``Signal Handling''.)

Termination Internals
---------------------

The _exit function is the primitive used for process termination by exit.  It is declared in the header file unistd.h.

 -- Function: void _exit (int STATUS)
     The _exit function is the primitive for causing a process to terminate with status STATUS.  Calling this function does not execute clean-up functions registered with atexit or on_exit.

 -- Function: void _Exit (int STATUS)
     The _Exit function is the ISO C equivalent to _exit.  The ISO C committee members were not sure whether the definitions of _exit and _Exit were compatible, so they have not used the POSIX name.

     This function was introduced in ISO C99 and is declared in stdlib.h.

When a process terminates for any reason---either because the program terminates, or as a result of a signal---the following things happen:

  * All open file descriptors in the process are closed. (Ibid., ``Low-Level Input/Output''.)  Streams are not flushed automatically when the process terminates (see I/O on Streams).

  * A process exit status is saved to be reported back to the parent process via wait or waitpid. (Ibid., ``Process Completion''.)  If the program exited, this status includes the program exit status as its low-order 8 bits.

  * Any child processes of the process being terminated are assigned a new parent process.  On most systems, including GNU, this is the init process, with process ID 1.

  * A SIGCHLD signal is sent to the parent process.

  * If the process is a session leader that has a controlling terminal, then a SIGHUP signal is sent to each process in the foreground job, and the controlling terminal is disassociated from that session. (Ibid., ``Job Control''.)

  * If termination of a process causes a process group to become orphaned, and any member of that process group is stopped, then a SIGHUP signal and a SIGCONT signal are sent to each process in the group. (Ibid., ``Job Control''.)

Input/Output Overview
*********************

Most programs need to do either input (reading data) or output (writing data), or most frequently both, in order to do anything useful.  The GNU C Library provides such a large selection of input and output functions that the hardest part is often deciding which function is most appropriate!

This chapter introduces concepts and terminology relating to input and output.  Other chapters relating to the GNU I/O facilities are

  * I/O on Streams, covers the high-level functions that operate on streams, including formatted input and output.

  * Loosemore et al., ``Low-Level I/O'' (see chap. 1, n. 1), covers the basic I/O and control functions on file descriptors.

  * Loosemore et al., ``File-System Interface'', covers functions for operating on directories and for manipulating file attributes such as access modes and ownership.

  * Loosemore et al., ``Pipes and FIFOs'', includes information on the basic interprocess communication facilities.

  * Loosemore et al., ``Sockets'', covers a more complicated interprocess communication facility with support for networking.

  * Loosemore et al., ``Low-Level Terminal Interface'', covers functions for changing how input and output to terminals or other serial devices are processed.

Input/Output Concepts
=====================

Before you can read or write the contents of a file, you must establish a connection or communications channel to the file.  This process is called opening the file.  You can open a file for reading, writing, or both.

The connection to an open file is represented either as a stream or as a file descriptor.  You pass this as an argument to the functions that do the actual read or write operations, to tell them which file to operate on.  Certain functions expect streams, and others are designed to operate on file descriptors.

When you have finished reading to or writing from the file, you can terminate the connection by closing the file.  Once you have closed a stream or file descriptor, you cannot do any more input or output operations on it.

Streams and File Descriptors
----------------------------

When you want to do input or output to a file, you have a choice of two basic mechanisms for representing the connection between your program and the file: file descriptors and streams.  File descriptors are represented as objects of type int, while streams are represented as FILE * objects.

File descriptors provide a primitive, low-level interface to input and output operations.  Both file descriptors and streams can represent a connection to a device (such as a terminal), or a pipe or socket for communicating with another process, as well as a normal file.  But, if you want to do control operations that are specific to a particular kind of device, you must use a file descriptor; there are no facilities to use streams in this way.  You must also use file descriptors if your program needs to do input or output in special modes, such as nonblocking (or polled) input. (Loosemore et al., ``File Status Flags'' (see chap. 1, n. 1).)

Streams provide a higher-level interface, layered on top of the primitive file descriptor facilities.  The stream interface treats all kinds of files pretty much alike---the sole exception being the three styles of buffering that you can choose (see Stream Buffering).

The main advantage of using the stream interface is that the set of functions for performing actual input and output operations (as opposed to control operations) on streams is much richer and more powerful than the corresponding facilities for file descriptors.  The file descriptor interface provides only simple functions for transferring blocks of characters, but the stream interface also provides powerful formatted input and output functions (printf and scanf) as well as functions for character- and line-oriented input and output.

Since streams are implemented in terms of file descriptors, you can extract the file descriptor from a stream and perform low-level operations directly on the file descriptor.  You can also initially open a connection as a file descriptor and then make a stream associated with that file descriptor.

In general, you should stick with using streams rather than file descriptors, unless there is some specific operation you want to do that can only be done on a file descriptor.  If you are a beginning programmer and aren't sure what functions to use, we suggest that you concentrate on the formatted input functions (see Formatted Input) and formatted output functions (see Formatted Output).

If you are concerned about portability of your programs to systems other than GNU, you should also be aware that file descriptors are not as portable as streams.  You can expect any system running ISO C to support streams, but non-GNU systems may not support file descriptors at all, or may only implement a subset of the GNU functions that operate on file descriptors.  Most of the file descriptor functions in the GNU library are included in the POSIX.1 standard, however.

File Position
-------------

One of the attributes of an open file is its file position that keeps track of where in the file the next character is to be read or written.  In the GNU system, and all POSIX.1 systems, the file position is simply an integer representing the number of bytes from the beginning of the file.

The file position is normally set to the beginning of the file when it is opened, and each time a character is read or written, the file position is incremented.  In other words, access to the file is normally sequential.

Ordinary files permit read or write operations at any position within the file.  Some other kinds of files may also permit this.  Files which do permit this are sometimes referred to as random-access files.  You can change the file position using the fseek function on a stream (see File Positioning) or the lseek function on a file descriptor. (Ibid., ``Input and Output Primitives''.) If you try to change the file position on a file that doesn't support random access, you get the ESPIPE error.

Streams and descriptors that are opened for append access are treated specially for output; output to such files is always appended sequentially to the end of the file, regardless of the file position.  However, the file position is still used to control where in the file reading is done.

If you think about it, you'll realize that several programs can read a given file at the same time.  In order for each program to be able to read the file at its own pace, each program must have its own file pointer, which is not affected by anything the other programs do.

In fact, each opening of a file creates a separate file position. Thus, if you open a file twice even in the same program, you get two streams or descriptors with independent file positions.

By contrast, if you open a descriptor and then duplicate it to get another descriptor, these two descriptors share the same file position: changing the file position of one descriptor will affect the other.

File Names
==========

In order to open a connection to a file, or to perform other operations such as deleting a file, you need some way to refer to the file.  Nearly all files have names that are strings---even files that are actually devices such as tape drives or terminals.  These strings are called file names.  You specify the file name to say which file you want to open or operate on.

This section describes the conventions for file names and how the operating system works with them.

Directories
-----------

In order to understand the syntax of file names, you need to understand how the file system is organized into a hierarchy of directories.

A directory is a file that contains information to associate other files with names; these associations are called links or directory entries.  Sometimes, people speak of ``files in a directory'', but in reality, a directory only contains pointers to files, not the files themselves.

The name of a file contained in a directory entry is called a file-name component.  In general, a file name consists of a sequence of one or more such components, separated by the slash character (/).  A file name that is just one component names a file with respect to its directory.  A file name with multiple components names a directory, and then a file in that directory, and so on.

Some other documents, such as the POSIX standard, use the term pathname for what we call a file name, and either filename or pathname component for what this manual calls a file-name component.  We don't use this terminology because a path is something completely different (a list of directories to search), and we think that pathname used for something else will confuse users.  We always use file name and file-name component (or sometimes just component, where the context is obvious) in GNU documentation.  Some macros use the POSIX terminology in their names, such as PATH_MAX.  These macros are defined by the POSIX standard, so we cannot change their names.

You can find more detailed information about operations on directories in Loosemore et al., ``File-System Interface'' (see chap. 1, n. 1).

File-Name Resolution
--------------------

A file name consists of file-name components separated by slash (/) characters.  On the systems that the GNU C Library supports, multiple successive / characters are equivalent to a single / character.

The process of determining what file a file name refers to is called file-name resolution.  This is performed by examining the components that make up a file name in left-to-right order, and locating each successive component in the directory named by the previous component.  Of course, each of the files that are referenced as directories must actually exist, be directories instead of regular files, and have the appropriate permissions to be accessible by the process; otherwise the file-name resolution fails.

If a file name begins with a /, the first component in the file name is located in the root directory of the process (usually all processes on the system have the same root directory).  Such a file name is called an absolute file name.

Otherwise, the first component in the file name is located in the current working directory. (Ibid., ``Working Directory''.) This kind of file name is called a relative file name.

The file-name components . (dot) and .. (dot-dot) have special meanings.  Every directory has entries for these file-name components.  The file-name component . refers to the directory itself, while the file-name component .. refers to its parent directory (the directory that contains the link for the directory in question).  As a special case, .. in the root directory refers to the root directory itself, since it has no parent; thus /.. is the same as /.

Here are some examples of file names:

/a
     The file named a, in the root directory

/a/b
     The file named b, in the directory named a in the root directory

a
     The file named a, in the current working directory

/a/./b
     The same as /a/b

./a
     The file named a, in the current working directory

../a
     The file named a, in the parent directory of the current working directory

A file name that names a directory may optionally end in a /. You can specify a file name of / to refer to the root directory, but the empty string is not a meaningful file name.  If you want to refer to the current working directory, use a file name of . or ./.

Unlike some other operating systems, the GNU system doesn't have any built-in support for file types (or extensions) or file versions as part of its file-name syntax.  Many programs and utilities use conventions for file names---for example, files containing C source code usually have names suffixed with .c---but there is nothing in the file system itself that enforces this kind of convention.

File-Name Errors
----------------

Functions that accept file-name arguments usually detect these errno error conditions relating to the file-name syntax or trouble finding the named file.  These errors are referred to throughout this manual as the usual file-name errors.

EACCES
     The process does not have search permission for a directory component of the file name.

ENAMETOOLONG
     This error is used when either the total length of a file name is greater than PATH_MAX, or when an individual file-name component has a length greater than NAME_MAX. (Ibid., ``Limits on File-System Capacity''.)

     In the GNU system, there is no imposed limit on overall file-name length, but some file systems may place limits on the length of a component.

ENOENT
     This error is reported when a file referenced as a directory component in the file name doesn't exist, or when a component is a symbolic link whose target file does not exist. (Ibid., ``Symbolic Links''.)

ENOTDIR
     A file that is referenced as a directory component in the file name exists, but it isn't a directory.

ELOOP
     Too many symbolic links were resolved while trying to look up the file name.  The system has an arbitrary limit on the number of symbolic links that may be resolved in looking up a single file name, as a primitive way to detect loops. (Ibid., ``Symbolic Links''.)

Portability of File Names
-------------------------

The rules for the syntax of file names discussed in File Names, are the rules normally used by the GNU system and by other POSIX systems.  However, other operating systems may use other conventions.

There are two reasons why it can be important for you to be aware of file-name portability issues:

  * If your program makes assumptions about file-name syntax, or contains embedded literal file-name strings, it is more difficult to get it to run under other operating systems that use different syntax conventions.

  * Even if you are not concerned about running your program on machines that run other operating systems, it may still be possible to access files that use different naming conventions.  For example, you may be able to access file systems on another computer running a different operating system over a network, or read and write disks in formats used by other operating systems.

The ISO C standard says very little about file-name syntax, only that file names are strings.  In addition to varying restrictions on the length of file names and what characters can validly appear in a file name, different operating systems use different conventions and syntax for concepts such as structured directories and file types or extensions.  Some concepts, such as file versions, might be supported in some operating systems and not in others.

The POSIX.1 standard allows implementations to put additional restrictions on file-name syntax, concerning what characters are permitted in file names and on the length of file name and file-name component strings.  However, in the GNU system, you do not need to worry about these restrictions; any character except the null character is permitted in a file-name string, and there are no limits on the length of file-name strings.

Debugging Support
*****************

Applications are usually debugged using dedicated debugger programs. But sometimes this is not possible and, in any case, it is useful to provide the developer with as much information as possible at the time the problems are experienced.  For this reason, a few functions are provided that a program can use to help the developer more easily locate the problem.

Backtraces
==========

A backtrace is a list of the function calls that are currently active in a thread.  The usual way to inspect a backtrace of a program is to use an external debugger such as gdb.  However, sometimes it is useful to obtain a backtrace programmatically from within a program, e.g., for the purposes of logging or diagnostics.

The header file execinfo.h declares three functions that obtain and manipulate backtraces of the current thread.

 -- Function: int backtrace (void **BUFFER, int SIZE)
     The backtrace function obtains a backtrace for the current thread, as a list of pointers, and places the information into BUFFER.  The argument SIZE should be the number of void * elements that will fit into BUFFER.  The return value is the actual number of entries of BUFFER that are obtained, and is at most SIZE.

     The pointers placed in BUFFER are actually return addresses obtained by inspecting the stack, one return address per stack frame.

     Certain compiler optimizations may interfere with obtaining a valid backtrace.  Function in-lining causes the in-lined function to not have a stack frame; tail-call optimization replaces one stack frame with another; frame pointer elimination will stop backtrace from interpreting the stack contents correctly.

 -- Function: char ** backtrace_symbols (void *const *BUFFER, int SIZE)
     The backtrace_symbols function translates the information obtained from the backtrace function into an array of strings. The argument BUFFER should be a pointer to an array of addresses obtained via the backtrace function, and SIZE is the number of entries in that array (the return value of backtrace).

     The return value is a pointer to an array of strings, which has SIZE entries just like the array BUFFER.  Each string contains a printable representation of the corresponding element of BUFFER.  It includes the function name (if this can be determined), an offset into the function, and the actual return address (in hexadecimal).

     Currently, the function name and offset can only be obtained on systems that use the ELF binary format for programs and libraries.  On other systems, only the hexadecimal return address will be present. Also, you may need to pass additional flags to the linker to make the function names available to the program; for example, on systems using GNU ld, you must pass -rdynamic.

     The return value of backtrace_symbols is a pointer obtained via the malloc function, and it is the responsibility of the caller to free that pointer.  Only the return value need be freed, not the individual strings.

     The return value is NULL if sufficient memory for the strings cannot be obtained.

 -- Function: void backtrace_symbols_fd (void *const *BUFFER, int SIZE, int FD)
     The backtrace_symbols_fd function performs the same translation as the function backtrace_symbols function.  Instead of returning the strings to the caller, it writes the strings to the file descriptor FD, one per line.  It does not use the malloc function, and can therefore be used in situations where that function might fail.

The following program illustrates the use of these functions.  Note that the array to contain the return addresses returned by backtrace is allocated on the stack.  Therefore, code like this can be used in situations where the memory handling via malloc does not work anymore (in which case the backtrace_symbols has to be replaced by a backtrace_symbols_fd call as well).  The number of return addresses is normally not very large.  Even complicated programs rather seldom have a nesting level of more than fifty, and with two hundred possible entries, probably all programs should be covered.

     #include <execinfo.h>
     #include <stdio.h>
     #include <stdlib.h>

     /* Obtain a backtrace and print it to stdout. */
     void
     print_trace (void)
     {
       void *array[10];
       size_t size;
       char **strings;
       size_t i;

       size = backtrace (array, 10);
       strings = backtrace_symbols (array, size);

       printf ("Obtained %zd stack frames.\n", size);

       for (i = 0; i < size; i++)
          printf ("%s\n", strings[i]);

       free (strings);
     }

     /* A dummy function to make the backtrace more interesting */
     void
     dummy_function (void)
     {
       print_trace ();
     }

     int
     main (void)
     {
       dummy_function ();
       return 0;
     }

Input/Output on Streams
***********************

This chapter describes the functions for creating streams and performing input and output operations on them.  As discussed in I/O Overview, a stream is a fairly abstract, high-level concept representing a communications channel to a file, device, or process.

Streams
=======

For historical reasons, the type of the C data structure that represents a stream is called FILE rather than stream. Since most of the library functions deal with objects of type FILE *, sometimes the term file pointer is also used to mean stream.  This leads to unfortunate confusion over terminology in many books on C.  This manual, however, is careful to use the terms file and stream only in the technical sense.

The FILE type is declared in the header file stdio.h.

 -- Data Type: FILE
     This is the data type used to represent stream objects.  A FILE object holds all of the internal state information about the connection to the associated file, including such things as the file position indicator and buffering information.  Each stream also has error and end-of-file status indicators that can be tested with the ferror and feof functions (see EOF and Errors).

FILE objects are allocated and managed internally by the input/output library functions.  Don't try to create your own objects of type FILE; let the library do it.  Your programs should deal only with pointers to these objects (that is, FILE * values) rather than the objects themselves.

Standard Streams
================

When the main function of your program is invoked, it already has three predefined streams open and available for use.  These represent the ``standard'' input and output channels that have been established for the process.

These streams are declared in the header file stdio.h.

 -- Variable: FILE * stdin
     The standard input stream, which is the normal source of input for the program

 -- Variable: FILE * stdout
     The standard output stream, which is used for normal output from the program

 -- Variable: FILE * stderr
     The standard error stream, which is used for error messages and diagnostics issued by the program

In the GNU system, you can specify what files or processes correspond to these streams using the pipe and redirection facilities provided by the shell. (The primitives shells use to implement these facilities are described in Loosemore et al., ``File-System Interface'' (see chap. 1, n. 1).)  Most other operating systems provide similar mechanisms, but the details of how to use them can vary.

In the GNU C Library, stdin, stdout, and stderr are normal variables that you can set just like any others.  For example, to redirect the standard output to a file, you could do:

     fclose (stdout);
     stdout = fopen ("standard-output-file", "w");

However, in other systems, stdin, stdout and stderr are macros that you cannot assign to in the normal way. But you can use freopen to get the effect of closing one and reopening it (see Opening Streams).

The three streams stdin, stdout and stderr are not unoriented at program start (see Streams and I18N).

Opening Streams
===============

Opening a file with the fopen function creates a new stream and establishes a connection between the stream and a file.  This may involve creating a new file.

Everything described in this section is declared in the header file stdio.h.

 -- Function: FILE * fopen (const char *FILENAME, const char *OPENTYPE)
     The fopen function opens a stream for I/O to the file FILENAME, and returns a pointer to the stream.

     The OPENTYPE argument is a string that controls how the file is opened and specifies attributes of the resulting stream.  It must begin with one of the following sequences of characters:

     r
          Open an existing file for reading only.

     w
          Open the file for writing only.  If the file already exists, it is truncated to zero length.  Otherwise, a new file is created.

     a
          Open a file for append access; that is, writing at the end of file only. If the file already exists, its initial contents are unchanged and output to the stream is appended to the end of the file. Otherwise, a new, empty file is created.

     r+
          Open an existing file for both reading and writing.  The initial contents of the file are unchanged and the initial file position is at the beginning of the file.

     w+
          Open a file for both reading and writing.  If the file already exists, it is truncated to zero length.  Otherwise, a new file is created.

     a+
          Open or create file for both reading and appending.  If the file exists, its initial contents are unchanged.  Otherwise, a new file is created. The initial file position for reading is at the beginning of the file, but output is always appended to the end of the file.

     As you can see, + requests a stream that can do both input and output.  The ISO standard says that when using such a stream, you must call fflush (see Stream Buffering) or a file positioning function such as fseek (see File Positioning) when switching from reading to writing or vice versa.  Otherwise, internal buffers might not be emptied properly.  The GNU C Library does not have this limitation; you can do arbitrary reading and writing operations on a stream in any order.

     Additional characters may appear after these to specify flags for the call.  Always put the mode (r, w+, etc.) first; that is the only part guaranteed to be understood by all systems.

     The GNU C Library defines one additional character for use in OPENTYPE: the character x insists on creating a new file---if a file FILENAME already exists, fopen fails rather than opening it.  If you use x, it is guaranteed that you will not clobber an existing file.  This is equivalent to the O_EXCL option to the open function. (Ibid., ``Opening and Closing Files''.)

     The character b in OPENTYPE has a standard meaning; it requests a binary stream rather than a text stream.  But this makes no difference in POSIX systems (including the GNU system).  If both + and b are specified, they can appear in either order (see Binary Streams).

     If the OPENTYPE string contains the sequence ccs=STRING, then STRING is taken as the name of a coded character set and fopen will mark the stream as wide-oriented with appropriate conversion functions in place to convert from and to the character set STRING.  Any other stream is opened initially unoriented, and the orientation is decided with the first file operation.  If the first operation is a wide-character operation, the stream is not only marked as wide-oriented---the conversion functions to convert to the coded character set used for the current locale are also loaded.  This will not change anymore from this point on, even if the locale selected for the LC_CTYPE category is changed.

     Any other characters in OPENTYPE are simply ignored.  They may be meaningful in other systems.

     If the open fails, fopen returns a null pointer.

     When the sources are compiling with _FILE_OFFSET_BITS == 64 on a 32-bit machine, this function is in fact fopen64, since the LFS interface transparently replaces the old interface.

You can have multiple streams (or file descriptors) pointing to the same file open at the same time.  If you do only input, this will be straightforward, but you must be careful if any output streams are included. (Ibid., ``Dangers of Mixing Streams and Descriptors''.)  This is equally true whether the streams are in one program (not usual) or in several programs (which can easily happen).  It may be advantageous to use the file-locking facilities to avoid simultaneous access. (Ibid., ``File Locks''.)

 -- Function: FILE * fopen64 (const char *FILENAME, const char *OPENTYPE)
     This function is similar to fopen, but the stream it returns a pointer for is opened using open64.  Therefore, this stream can be used even on files larger then 2^31 bytes on 32-bit machines.

     The return type is still FILE *.  There is no special FILE type for the LFS interface.

     If the sources are compiled with _FILE_OFFSET_BITS == 64 on a 32-bit machine, this function is available under the name fopen and so transparently replaces the old interface.

 -- Macro: int FOPEN_MAX
     The value of this macro is an integer constant expression that represents the minimum number of streams that the implementation guarantees can be open simultaneously.  You might be able to open more than this many streams, but that is not guaranteed.  The value of this constant is at least eight, which includes the three standard streams stdin, stdout and stderr.  In POSIX.1 systems, this value is determined by the OPEN_MAX parameter. (Ibid., ``General Capacity-Limits''.)  In BSD and GNU, it is controlled by the RLIMIT_NOFILE resource limit. (Ibid., ``Limiting Resource Usage''.)

 -- Function: FILE * freopen (const char *FILENAME, const char *OPENTYPE, FILE *STREAM)
     This function is like a combination of fclose and fopen. It first closes the stream referred to by STREAM, ignoring any errors that are detected in the process.  Because errors are ignored, you should not use freopen on an output stream if you have actually done any output using the stream.  Then the file named by FILENAME is opened with mode OPENTYPE as for fopen, and associated with the same stream object STREAM.

     If the operation fails, a null pointer is returned; otherwise, freopen returns STREAM.

     freopen has traditionally been used to connect a standard stream such as stdin with a file of your own choice.  This is useful in programs in which use of a standard stream for certain purposes is hard-coded.  In the GNU C Library, you can simply close the standard streams and open new ones with fopen.  But other systems lack this ability, so using freopen is more portable.

     When the sources are compiling with _FILE_OFFSET_BITS == 64 on a 32-bit machine, this function is in fact freopen64, since the LFS interface transparently replaces the old interface.

 -- Function: FILE * freopen64 (const char *FILENAME, const char *OPENTYPE, FILE *STREAM)
     This function is similar to freopen.  The only difference is that on 32-bit machines, the stream returned is able to read beyond the 2^31-bytes limits imposed by the normal interface.  The stream pointed to by STREAM need not be opened using fopen64 or freopen64, since its mode is not important for this function.

     If the sources are compiled with _FILE_OFFSET_BITS == 64 on a 32-bit machine, this function is available under the name freopen and so transparently replaces the old interface.

In some situations, it is useful to know whether a given stream is available for reading or writing.  This information is normally not available and would have to be remembered separately.  Solaris introduced a few functions to get this information from the stream descriptor and these functions are also available in the GNU C Library.

 -- Function: int __freadable (FILE *STREAM)
     The __freadable function determines whether the stream STREAM was opened to allow reading.  In this case, the return value is nonzero.  For write-only streams, the function returns zero.

     This function is declared in stdio_ext.h.

 -- Function: int __fwritable (FILE *STREAM)
     The __fwritable function determines whether the stream STREAM was opened to allow writing.  In this case, the return value is nonzero.  For read-only streams, the function returns zero.

     This function is declared in stdio_ext.h.

For slightly different kinds of problems, there are two more functions.  They provide even finer-grained information.

 -- Function: int __freading (FILE *STREAM)
     The __freading function determines whether the stream STREAM was last read from or whether it is opened read-only.  In this case, the return value is nonzero. Otherwise, it is zero. Determining whether a stream opened for reading and writing was last used for writing allows you to draw conclusions about the content of the buffer, among other things.

     This function is declared in stdio_ext.h.

 -- Function: int __fwriting (FILE *STREAM)
     The __fwriting function determines whether the stream STREAM was last written to or whether it is opened write-only. In this case, the return value is nonzero. Otherwise, it is zero.

     This function is declared in stdio_ext.h.

Closing Streams
===============

When a stream is closed with fclose, the connection between the stream and the file is canceled.  After you have closed a stream, you cannot perform any additional operations on it.

 -- Function: int fclose (FILE *STREAM)
     This function causes STREAM to be closed and the connection to the corresponding file to be broken.  Any buffered output is written, and any buffered input is discarded.  The fclose function returns a value of 0 if the file was closed successfully, and EOF if an error was detected.

     It is important to check for errors when you call fclose to close an output stream, because real, everyday errors can be detected at this time.  For example, when fclose writes the remaining buffered output, it might get an error because the disk is full.  Even if you know the buffer is empty, errors can still occur when closing a file if you are using NFS.

     The function fclose is declared in stdio.h.

To close all streams currently available, the GNU C Library provides another function.

 -- Function: int fcloseall (void)
     This function causes all open streams of the process to be closed and the connection to corresponding files to be broken.  All buffered data is written, and any buffered input is discarded.  The fcloseall function returns a value of 0 if all the files were closed successfully, and EOF if an error was detected.

     This function should be used only in special situations, such as when an error occurred and the program must be aborted.  Normally, each single stream should be closed separately so that problems with individual streams can be identified.  It is also problematic since the standard streams (see Standard Streams) will also be closed.

     The function fcloseall is declared in stdio.h.

If the main function to your program returns, or if you call the exit function (see Normal Termination), all open streams are automatically closed properly.  If your program terminates in any other manner, such as by calling the abort function (see Aborting a Program), or from a fatal signal (Ibid., ``Signal Handling''.), open streams might not be closed properly. Buffered output might not be flushed, and files may be incomplete. For more information on buffering of streams, see Stream Buffering.

Streams and Threads
===================

Streams can be used in multithreaded applications in the same way they are used in single-threaded applications.  But the programmer must be aware of the possible complications.  It is also important to know about these if the program one writes never use threads, since the design and implementation of many stream functions is heavily influenced by the requirements added by multithreaded programming.

The POSIX standard requires that, by default, the stream operations are atomic---issuing two stream operations for the same stream in two threads at the same time will cause the operations to be executed as if they were issued sequentially.  The buffer operations performed while reading or writing are protected from other uses of the same stream.  To do this, each stream has an internal lock object that has to be (implicitly) acquired before any work can be done.

But there are situations where this is not enough, and there are also situations where this is not wanted.  The implicit locking is not enough if the program requires more than one stream function call to happen atomically.  One example would be if an output line a program wants to generate is created by several function calls.  The functions by themselves would ensure only atomicity of their own operation, but not atomicity over all the function calls.  For this, it is necessary to perform the stream locking in the application code.

 -- Function: void flockfile (FILE *STREAM)
     The flockfile function acquires the internal locking object associated with the stream STREAM.  This ensures that no other thread can explicitly through flockfile/ftrylockfile or implicitly through a call of a stream function lock the stream.  The thread will block until the lock is acquired.  An explicit call to funlockfile has to be used to release the lock.

 -- Function: int ftrylockfile (FILE *STREAM)
     The ftrylockfile function tries to acquire the internal locking object associated with the stream STREAM just like flockfile.  But unlike flockfile, this function does not block if the lock is not available.  ftrylockfile returns zero if the lock was successfully acquired.  Otherwise, the stream is locked by another thread.

 -- Function: void funlockfile (FILE *STREAM)
     The funlockfile function releases the internal locking object of the stream STREAM. The stream must have been locked before by a call to flockfile or a successful call of ftrylockfile.  The implicit locking performed by the stream operations do not count.  The funlockfile function does not return an error status, and the behavior of a call for a stream that is not locked by the current thread is undefined.

The following example shows how the functions above can be used to generate an output line atomically even in multithreaded applications (yes, the same job could be done with one fprintf call, but that is sometimes not possible):

     FILE *fp;
     {
        ...
        flockfile (fp);
        fputs ("This is test number ", fp);
        fprintf (fp, "%d\n", test);
        funlockfile (fp)
     }

Without the explicit locking, it would be possible for another thread to use the stream FP after the fputs call return and before fprintf was called, with the result that the number does not follow the word number.

From this description, it might already be clear that the locking objects in streams are no simple mutexes.  Since locking the same stream twice in the same thread is allowed, the locking objects must be equivalent to recursive mutexes.  These mutexes keep track of the owner and the number of times the lock is acquired.  The same number of funlockfile calls by the same threads is necessary to unlock the stream completely.  For instance:

     void
     foo (FILE *fp)
     {
       ftrylockfile (fp);
       fputs ("in foo\n", fp);
       /* This is very wrong!!!  */
       funlockfile (fp);
     }

It is important here that the funlockfile function is only called if the ftrylockfile function succeeded in locking the stream.  It is therefore always wrong to ignore the result of ftrylockfile.  And it makes no sense, since otherwise one would use flockfile.  The result of code like that above is that either funlockfile tries to free a stream that hasn't been locked by the current thread, or it frees the stream prematurely.  The code should look like this:

     void
     foo (FILE *fp)
     {
       if (ftrylockfile (fp) == 0)
         {
           fputs ("in foo\n", fp);
           funlockfile (fp);
         }
     }

Now that we covered why it is necessary to have locking, it is necessary to talk about situations when locking is unwanted and what can be done.  The locking operations (explicit or implicit) don't come for free.  Even if a lock is not taken, the cost is not zero.  The operations that have to be performed require memory operations that are safe in multiprocessor environments.  With the many local caches involved in such systems, this is quite costly.  So it is best to avoid the locking completely if it is not needed---because the code in question is never used in a context where two or more threads may use a stream at a time.  This can be determined most of the time for application code; for library code that can be used in many contexts, one should default to be conservative and use locking.

There are two basic mechanisms to avoid locking.  The first is to use the _unlocked variants of the stream operations.  The POSIX standard defines quite a few of those, and the GNU library adds a few more.  These variants of the functions behave just like the functions with the name without the suffix, except that they do not lock the stream.  Using these functions is very desirable, since they are potentially much faster.  This is not only because the locking operation itself is avoided.  More importantly, functions like putc and getc are very simple and traditionally (before the introduction of threads) were implemented as macros that are very fast if the buffer is not empty.  With the addition of locking requirements, these functions are no longer implemented as macros, since they would would expand to too much code.  But these macros are still available with the same functionality under the new names putc_unlocked and getc_unlocked.  This potentially huge difference in speed also suggests the use of the _unlocked functions even if locking is required.  The difference is that the locking then has to be performed in the program:

     void
     foo (FILE *fp, char *buf)
     {
       flockfile (fp);
       while (*buf != '/')
         putc_unlocked (*buf++, fp);
       funlockfile (fp);
     }

If in this example, the putc function would be used and the explicit locking would be missing, the putc function would have to acquire the lock in every call, potentially many times depending on when the loop terminates.  Writing it the way illustrated above allows the putc_unlocked macro to be used, which means no locking and direct manipulation of the buffer of the stream.

A second way to avoid locking is by using a nonstandard function that was introduced in Solaris and is available in the GNU C Library as well.

 -- Function: int __fsetlocking (FILE *STREAM, int TYPE)

     The __fsetlocking function can be used to select whether the stream operations will implicitly acquire the locking object of the stream STREAM.  By default, this is done, but it can be disabled and reinstated using this function.  There are three values defined for the TYPE parameter:

     FSETLOCKING_INTERNAL
          The stream stream will from now on use the default internal locking.  Every stream operation with the exception of the _unlocked variants will implicitly lock the stream.

     FSETLOCKING_BYCALLER
          After the __fsetlocking function returns, the user is responsible for locking the stream.  None of the stream operations will implicitly do this anymore until the state is set back to FSETLOCKING_INTERNAL.

     FSETLOCKING_QUERY
          __fsetlocking only queries the current locking state of the stream.  The return value will be FSETLOCKING_INTERNAL or FSETLOCKING_BYCALLER depending on the state.

     The return value of __fsetlocking is either FSETLOCKING_INTERNAL or FSETLOCKING_BYCALLER depending on the state of the stream before the call.

     This function and the values for the TYPE parameter are declared in stdio_ext.h.

This function is especially useful when program code has to be used that is written without knowledge of the _unlocked functions (or if the programmer was too lazy to use them).

Streams in Internationalized Applications
=========================================

ISO C90 introduced the new type wchar_t to allow handling larger character sets.  What was missing was the ability to output strings of wchar_t directly.  One had to convert them into multibyte strings using mbstowcs (there was no mbsrtowcs yet) and then use the normal stream functions.  While this is doable, it is very cumbersome, since performing the conversions is not trivial and greatly increases program complexity and size.

The Unix standard (XPG4.2 and later) includes two additional format specifiers for the printf and scanf families of functions.  Printing and reading of single wide characters was made possible using the %C specifier, and wide-character strings can be handled with %S.  These modifiers behave just like %c and %s, only they expect the corresponding argument to have the wide-character type and that the wide character and string are transformed into/from multibyte strings before being used.

This was a beginning, but it is still not good enough.  It is not always desirable to use printf and scanf.  The other, smaller and faster functions cannot handle wide characters.  Second, it is not possible to have a format string for printf and scanf consisting of wide characters.  The result is that format strings would have to be generated if they have to contain nonbasic characters.

In the Amendment 1 to ISO C90, a whole new set of functions was added to solve the problem.  Most of the stream functions got counterparts that take a wide character or wide-character string instead of a character or string respectively. The new functions operate on the same streams (like stdout). This is different from the model of the C++ run-time library where separate streams for wide and normal I/O are used.

Being able to use the same stream for wide and normal operations comes with a restriction: a stream can be used either for wide operations or for normal operations.  Once it is decided, there is no way back. Only a call to freopen or freopen64 can reset the orientation.  The orientation can be decided in three ways:

  * If any of the normal character functions is used (this includes the fread and fwrite functions), the stream is marked as not wide oriented.

  * If any of the wide-character functions is used, the stream is marked as wide oriented.

  * The fwide function can be used to set the orientation either way.

It is important to never mix the use of wide and not-wide operations on a stream.  There are no diagnostics issued.  The application behavior will simply be strange or the application will simply crash. The fwide function can help avoid this.

 -- Function: int fwide (FILE *STREAM, int MODE)

     The fwide function can be used to set and query the state of the orientation of the stream STREAM.  If the MODE parameter has a positive value, the streams get wide oriented, and for negative values they get narrow oriented.  It is not possible to overwrite previous orientations with fwide---if the stream STREAM was already oriented before the call, nothing is done.

     If MODE is zero, the current orientation state is queried, and nothing is changed.

     The fwide function returns a negative value, zero, or a positive value if the stream is narrow, not at all, or wide oriented, respectively.

     This function was introduced in Amendment 1 to ISO C90 and is declared in wchar.h.

It is generally a good idea to orient a stream as early as possible. This can prevent surprise especially for the standard streams stdin, stdout, and stderr.  If some library function in some situations uses one of these streams and this use orients the stream in a different way than the rest of the application expects, one might end up with hard-to-reproduce errors.  Remember that no errors are signalled if the streams are used incorrectly. Leaving a stream unoriented after creation is normally only necessary for library functions that create streams that can be used in different contexts.

When writing code that uses streams and that can be used in different contexts, it is important to query the orientation of the stream before using it (unless the rules of the library interface demand a specific orientation).  The following little, silly function illustrates this:

     void
     print_f (FILE *fp)
     {
       if (fwide (fp, 0) > 0)
         /* Positive return value means wide orientation.  */
         fputwc (L'f', fp);
       else
         fputc ('f', fp);
     }

In this case, the function print_f decides about the orientation of the stream if it was unoriented before (this will not happen if the advice above is followed).

The encoding used for the wchar_t values is unspecified, and the user must not make any assumptions about it.  For I/O of wchar_t values, this means that it is impossible to write these values directly to the stream.  This is not what follows from the ISO C locale model either.  What happens instead is that the bytes read from or written to the underlying media are first converted into the internal encoding chosen by the implementation for wchar_t.  The external encoding is determined by the LC_CTYPE category of the current locale, or by the ccs part of the mode specification given to fopen, fopen64, freopen or freopen64.  How and when the conversion happens is unspecified, and it happens invisibly to the user.

Since a stream is created in the unoriented state, it has at that point no conversion associated with it.  The conversion that will be used is determined by the LC_CTYPE category selected at the time the stream is oriented.  If the locales are changed at the run time, this might produce surprising results unless you pay attention. This is just another good reason to orient the stream explicitly as soon as possible, perhaps with a call to fwide.

Simple Output by Characters or Lines
====================================

This section describes functions for performing character- and line-oriented output.

These narrow stream functions are declared in the header file stdio.h and the wide stream functions in wchar.h.

 -- Function: int fputc (int C, FILE *STREAM)
     The fputc function converts the character C to type unsigned char, and writes it to the stream STREAM. EOF is returned if a write error occurs. Otherwise, the character C is returned.

 -- Function: wint_t fputwc (wchar_t WC, FILE *STREAM)
     The fputwc function writes the wide character WC to the stream STREAM.  WEOF is returned if a write error occurs. Otherwise, the character WC is returned.

 -- Function: int fputc_unlocked (int C, FILE *STREAM)
     The fputc_unlocked function is equivalent to the fputc function, except that it does not implicitly lock the stream.

 -- Function: wint_t fputwc_unlocked (wint_t WC, FILE *STREAM)
     The fputwc_unlocked function is equivalent to the fputwc function, except that it does not implicitly lock the stream.

     This function is a GNU extension.

 -- Function: int putc (int C, FILE *STREAM)
     This is just like fputc, except that most systems implement it as a macro, making it faster.  One consequence is that it may evaluate the STREAM argument more than once, which is an exception to the general rule for macros.  putc is usually the best function to use for writing a single character.

 -- Function: wint_t putwc (wchar_t WC, FILE *STREAM)
     This is just like fputwc, except that it can be implemented as a macro, making it faster.  One consequence is that it may evaluate the STREAM argument more than once, which is an exception to the general rule for macros.  putwc is usually the best function to use for writing a single wide character.

 -- Function: int putc_unlocked (int C, FILE *STREAM)
     The putc_unlocked function is equivalent to the putc function, except that it does not implicitly lock the stream.

 -- Function: wint_t putwc_unlocked (wchar_t WC, FILE *STREAM)
     The putwc_unlocked function is equivalent to the putwc function, except that it does not implicitly lock the stream.

     This function is a GNU extension.

 -- Function: int putchar (int C)
     The putchar function is equivalent to putc with stdout as the value of the STREAM argument.

 -- Function: wint_t putwchar (wchar_t WC)
     The putwchar function is equivalent to putwc with stdout as the value of the STREAM argument.

 -- Function: int putchar_unlocked (int C)
     The putchar_unlocked function is equivalent to the putchar function, except that it does not implicitly lock the stream.

 -- Function: wint_t putwchar_unlocked (wchar_t WC)
     The putwchar_unlocked function is equivalent to the putwchar function, except that it does not implicitly lock the stream.

     This function is a GNU extension.

 -- Function: int fputs (const char *S, FILE *STREAM)
     The function fputs writes the string S to the stream STREAM.  The terminating null character is not written. This function does not add a newline character, either. It outputs only the characters in the string.

     This function returns EOF if a write error occurs, and otherwise a nonnegative value.

     For example:

          fputs ("Are ", stdout);
          fputs ("you ", stdout);
          fputs ("hungry?\n", stdout);

     outputs the text, Are you hungry?, followed by a newline.

 -- Function: int fputws (const wchar_t *WS, FILE *STREAM)
     The function fputws writes the wide-character string WS to the stream STREAM.  The terminating null character is not written. This function does not add a newline character, either.  It outputs only the characters in the string.

     This function returns WEOF if a write error occurs, and otherwise a nonnegative value.

 -- Function: int fputs_unlocked (const char *S, FILE *STREAM)
     The fputs_unlocked function is equivalent to the fputs function, except that it does not implicitly lock the stream.

     This function is a GNU extension.

 -- Function: int fputws_unlocked (const wchar_t *WS, FILE *STREAM)
     The fputws_unlocked function is equivalent to the fputws function, except that it does not implicitly lock the stream.

     This function is a GNU extension.

 -- Function: int puts (const char *S)
     The puts function writes the string S to the stream stdout followed by a newline.  The terminating null character of the string is not written.  (Note that fputs does not write a newline as this function does.)

     puts is the most convenient function for printing simple messages.  For example:

          puts ("This is a message.");

     outputs the text, This is a message., followed by a newline.

 -- Function: int putw (int W, FILE *STREAM)
     This function writes the word W (that is, an int) to STREAM.  It is provided for compatibility with SVID, but we recommend you use fwrite instead (see Block Input/Output).

Character Input
===============

This section describes functions for performing character-oriented input.  These narrow stream functions are declared in the header file stdio.h, and the wide-character functions are declared in wchar.h.

These functions return an int or wint_t value (for narrow and wide stream functions respectively) that is either a character of input, or the special value EOF/WEOF (usually -1).  For the narrow stream functions, it is important to store the result of these functions in a variable of type int instead of char, even when you plan to use it only as a character.  Storing EOF in a char variable truncates its value to the size of a character, so that it is no longer distinguishable from the valid character (char) -1.  So always use an int for the result of getc and friends, and check for EOF after the call; once you have verified that the result is not EOF, you can be sure that it will fit in a char variable without loss of information.

 -- Function: int fgetc (FILE *STREAM)
     This function reads the next character as an unsigned char from the stream STREAM and returns its value, converted to an int.  If an end-of-file condition or read error occurs, EOF is returned instead.

 -- Function: wint_t fgetwc (FILE *STREAM)
     This function reads the next wide character from the stream STREAM and returns its value.  If an end-of-file condition or read error occurs, WEOF is returned instead.

 -- Function: int fgetc_unlocked (FILE *STREAM)
     The fgetc_unlocked function is equivalent to the fgetc function, except that it does not implicitly lock the stream.

 -- Function: wint_t fgetwc_unlocked (FILE *STREAM)
     The fgetwc_unlocked function is equivalent to the fgetwc function, except that it does not implicitly lock the stream.

     This function is a GNU extension.

 -- Function: int getc (FILE *STREAM)
     This is just like fgetc, except that it is permissible (and typical) for it to be implemented as a macro that evaluates the STREAM argument more than once.  getc is often highly optimized, so it is usually the best function to use to read a single character.

 -- Function: wint_t getwc (FILE *STREAM)
     This is just like fgetwc, except that it is permissible for it to be implemented as a macro that evaluates the STREAM argument more than once.  getwc can be highly optimized, so it is usually the best function to use to read a single wide character.

 -- Function: int getc_unlocked (FILE *STREAM)
     The getc_unlocked function is equivalent to the getc function, except that it does not implicitly lock the stream.

 -- Function: wint_t getwc_unlocked (FILE *STREAM)
     The getwc_unlocked function is equivalent to the getwc function, except that it does not implicitly lock the stream.

     This function is a GNU extension.

 -- Function: int getchar (void)
     The getchar function is equivalent to getc with stdin as the value of the STREAM argument.

 -- Function: wint_t getwchar (void)
     The getwchar function is equivalent to getwc with stdin as the value of the STREAM argument.

 -- Function: int getchar_unlocked (void)
     The getchar_unlocked function is equivalent to the getchar function, except that it does not implicitly lock the stream.

 -- Function: wint_t getwchar_unlocked (void)
     The getwchar_unlocked function is equivalent to the getwchar function, except that it does not implicitly lock the stream.

     This function is a GNU extension.

Here is an example of a function that does input using fgetc.  It would work just as well using getc instead, or using getchar () instead of fgetc (stdin).  The code would also work the same for the wide-character stream functions.

     int
     y_or_n_p (const char *question)
     {
       fputs (question, stdout);
       while (1)
         {
           int c, answer;
           /* Write a space to separate answer from question. */
           fputc (' ', stdout);
           /* Read the first character of the line.
              This should be the answer character, but might not be. */
           c = tolower (fgetc (stdin));
           answer = c;
           /* Discard rest of input line. */
           while (c != '\n' && c != EOF)
             c = fgetc (stdin);
           /* Obey the answer if it was valid. */
           if (answer == 'y')
             return 1;
           if (answer == 'n')
             return 0;
           /* Answer was invalid. Ask for valid answer. */
           fputs ("Please answer y or n:", stdout);
         }
     }

 -- Function: int getw (FILE *STREAM)
     This function reads a word (that is, an int) from STREAM. It's provided for compatibility with SVID.  We recommend you use fread instead (see Block Input/Output).  Unlike getc, any int value could be a valid result.  getw returns EOF when it encounters end-of-file or an error, but there is no way to distinguish this from an input word with value -1.

Line-Oriented Input
===================

Since many programs interpret input on the basis of lines, it is convenient to have functions to read a line of text from a stream.

Standard C has functions to do this, but they aren't very safe: null characters and even (for gets) long lines can confuse them.  So the GNU library provides the nonstandard getline function that makes it easy to read lines reliably.

Another GNU extension, getdelim, generalizes getline.  It reads a delimited record, defined as everything through the next occurrence of a specified delimiter character.

All these functions are declared in stdio.h.

 -- Function: ssize_t getline (char **LINEPTR, size_t *N, FILE *STREAM)
     This function reads an entire line from STREAM, storing the text (including the newline and a terminating null character) in a buffer and storing the buffer address in *LINEPTR.

     Before calling getline, you should place in *LINEPTR the address of a buffer *N bytes long, allocated with malloc.  If this buffer is long enough to hold the line, getline stores the line in this buffer. Otherwise, getline makes the buffer bigger using realloc, storing the new buffer address back in *LINEPTR and the increased size back in *N (see Unconstrained Allocation).

     If you set *LINEPTR to a null pointer, and *N to zero, before the call, then getline allocates the initial buffer for you by calling malloc.

     In either case, when getline returns, *LINEPTR is a char * that points to the text of the line.

     When getline is successful, it returns the number of characters read (including the newline, but not including the terminating null). This value enables you to distinguish null characters that are part of the line from the null character inserted as a terminator.

     This function is a GNU extension, but it is the recommended way to read lines from a stream.  The alternative standard functions are unreliable.

     If an error occurs or end of file is reached without any bytes read, getline returns -1.

 -- Function: ssize_t getdelim (char **LINEPTR, size_t *N, int DELIMITER, FILE *STREAM)
     This function is like getline, except that the character that tells it to stop reading is not necessarily newline.  The argument DELIMITER specifies the delimiter character; getdelim keeps reading until it sees that character (or end of file).

     The text is stored in LINEPTR, including the delimiter character and a terminating null.  Like getline, getdelim makes LINEPTR bigger if it isn't big enough.

     getline is in fact implemented in terms of getdelim, just like this:

          ssize_t
          getline (char **lineptr, size_t *n, FILE *stream)
          {
            return getdelim (lineptr, n, '\n', stream);
          }

 -- Function: char * fgets (char *S, int COUNT, FILE *STREAM)
     The fgets function reads characters from the stream STREAM up to and including a newline character and stores them in the string S, adding a null character to mark the end of the string.  You must supply COUNT characters worth of space in S, but the number of characters read is at most COUNT - 1.  The extra character space is used to hold the null character at the end of the string.

     If the system is already at end of file when you call fgets, then the contents of the array S are unchanged and a null pointer is returned.  A null pointer is also returned if a read error occurs. Otherwise, the return value is the pointer S.

     Warning:  If the input data has a null character, you can't tell. So don't use fgets unless you know the data cannot contain a null. Don't use it to read files edited by the user because, if the user inserts a null character, you should either handle it properly or print a clear error message.  We recommend using getline instead of fgets.

 -- Function: wchar_t * fgetws (wchar_t *WS, int COUNT, FILE *STREAM)
     The fgetws function reads wide characters from the stream STREAM up to and including a newline character and stores them in the string WS, adding a null wide character to mark the end of the string.  You must supply COUNT wide-characters worth of space in WS, but the number of characters read is at most COUNT - 1.  The extra character space is used to hold the null wide character at the end of the string.

     If the system is already at end of file when you call fgetws, then the contents of the array WS are unchanged and a null pointer is returned.  A null pointer is also returned if a read error occurs. Otherwise, the return value is the pointer WS.

     Warning: If the input data has a null wide character (which are null bytes in the input stream), you can't tell.  So don't use fgetws unless you know the data cannot contain a null.  Don't use it to read files edited by the user because, if the user inserts a null character, you should either handle it properly or print a clear error message.

 -- Function: char * fgets_unlocked (char *S, int COUNT, FILE *STREAM)
     The fgets_unlocked function is equivalent to the fgets function, except that it does not implicitly lock the stream.

     This function is a GNU extension.

 -- Function: wchar_t * fgetws_unlocked (wchar_t *WS, int COUNT, FILE *STREAM)
     The fgetws_unlocked function is equivalent to the fgetws function, except that it does not implicitly lock the stream.

     This function is a GNU extension.

 -- Deprecated function: char * gets (char *S)
     The function gets reads characters from the stream stdin up to the next newline character, and stores them in the string S.  The newline character is discarded (note that this differs from the behavior of fgets, which copies the newline character into the string).  If gets encounters a read error or end of file, it returns a null pointer. Otherwise, it returns S.

     Warning: The gets function is very dangerous because it provides no protection against overflowing the string S.  The GNU library includes it for compatibility only. You should always use fgets or getline instead. To remind you of this, the linker (if using GNU ld) will issue a warning whenever you use gets.

Unreading
=========

In parser programs, it is often useful to examine the next character in the input stream without removing it from the stream.  This is called peeking ahead at the input because your program gets a glimpse of the input it will read next.

Using stream I/O, you can peek ahead at input by first reading it and then unreading it (also called  pushing it back on the stream). Unreading a character makes it available to be input again from the stream, by  the next call to fgetc or other input function on that stream.

What Unreading Means
--------------------

Here is a pictorial explanation of unreading.  Suppose you have a stream reading a file that contains just six characters, the letters foobar.  Suppose you have read three characters so far.  The situation looks like this:

     f  o  o  b  a  r
              ^

so the next input character will be b.

If instead of reading b, you unread the letter o, you get a situation like this:

     f  o  o  b  a  r
              |
           o--
           ^

so that the next input characters will be o and b.

If you unread 9 instead of o, you get this situation:

     f  o  o  b  a  r
              |
           9--
           ^

so that the next input characters will be 9 and b.

Using ungetc to Do Unreading
----------------------------

The function to unread a character is called ungetc, because it reverses the action of getc.

 -- Function: int ungetc (int C, FILE *STREAM)
     The ungetc function pushes back the character C onto the input stream STREAM.  So the next input from STREAM will read C before anything else.

     If C is EOF, ungetc does nothing and just returns EOF.  This lets you call ungetc with the return value of getc without needing to check for an error from getc.

     The character that you push back doesn't have to be the same as the last character that was actually read from the stream.  In fact, it isn't necessary to actually read any characters from the stream before unreading them with ungetc!  But that is a strange way to write a program; usually ungetc is used only to unread a character that was just read from the same stream.  The GNU C Library supports this even on files opened in binary mode, but other systems might not.

     The GNU C Library only supports one character of push back---in other words, it does not work to call ungetc twice without doing input in between.  Other systems might let you push back multiple characters; then reading from the stream retrieves the characters in the reverse order that they were pushed.

     Pushing back characters doesn't alter the file; only the internal buffering for the stream is affected.  If a file-positioning function (such as fseek, fseeko or rewind) is called, any pending pushed-back characters are discarded (see File Positioning).

     Unreading a character on a stream that is at end of file clears the end-of-file indicator for the stream, because it makes the character of input available.  After you read that character, trying to read again will encounter end of file.

 -- Function: wint_t ungetwc (wint_t WC, FILE *STREAM)
     The ungetwc function behaves just like ungetc just that it pushes back a wide character.

Here is an example showing the use of getc and ungetc to skip over white-space characters.  When this function reaches a non-white-space character, it unreads that character to be seen again on the next read operation on the stream:

     #include <stdio.h>
     #include <ctype.h>

     void
     skip_whitespace (FILE *stream)
     {
       int c;
       do
         /* No need to check for EOF because it is not
            isspace, and ungetc ignores EOF.  */
         c = getc (stream);
       while (isspace (c));
       ungetc (c, stream);
     }

Block Input/Output
==================

This section describes how to do input and output operations on blocks of data.  You can use these functions to read and write binary data, as well as to read and write text in fixed-size blocks instead of by characters or lines.

Binary files are typically used to read and write blocks of data in the same format as is used to represent the data in a running program.  In other words, arbitrary blocks of memory---not just character or string objects---can be written to a binary file, and meaningfully read in again by the same program.

Storing data in binary form is often considerably more efficient than using the formatted I/O functions.  Also, for floating-point numbers, the binary form avoids possible loss of precision in the conversion process.  On the other hand, binary files can't be examined or modified easily using many standard file utilities (such as text editors), and are not portable between different implementations of the language, or different kinds of computers.

These functions are declared in stdio.h.

 -- Function: size_t fread (void *DATA, size_t SIZE, size_t COUNT, FILE *STREAM)
     This function reads up to COUNT objects of size SIZE into the array DATA, from the stream STREAM.  It returns the number of objects actually read, which might be less than COUNT if a read error occurs or the end of the file is reached.  This function returns a value of zero (and doesn't read anything) if either SIZE or COUNT is zero.

     If fread encounters end of file in the middle of an object, it returns the number of complete objects read, and discards the partial object.  Therefore, the stream remains at the actual end of the file.

 -- Function: size_t fread_unlocked (void *DATA, size_t SIZE, size_t COUNT, FILE *STREAM)
     The fread_unlocked function is equivalent to the fread function, except that it does not implicitly lock the stream.

     This function is a GNU extension.

 -- Function: size_t fwrite (const void *DATA, size_t SIZE, size_t COUNT, FILE *STREAM)
     This function writes up to COUNT objects of size SIZE from the array DATA, to the stream STREAM.  The return value is normally COUNT, if the call succeeds.  Any other value indicates some sort of error, such as running out of space.

 -- Function: size_t fwrite_unlocked (const void *DATA, size_t SIZE, size_t COUNT, FILE *STREAM)
     The fwrite_unlocked function is equivalent to the fwrite function, except that it does not implicitly lock the stream.

     This function is a GNU extension.

Formatted Output
================

The functions described in this section (printf and related functions) provide a convenient way to perform formatted output.  You call printf with a format string or template string that specifies how to format the values of the remaining arguments.

Unless your program is a filter that specifically performs line- or character-oriented processing, using printf or one of the other related functions described in this section is usually the easiest and most concise way to perform output.  These functions are especially useful for printing error messages, tables of data, and the like.

Formatted Output Basics
-----------------------

The printf function can be used to print any number of arguments. The template string argument you supply in a call provides information not only about the number of additional arguments, but also about their types and what style should be used for printing them.

Ordinary characters in the template string are simply written to the output stream as-is, while conversion specifications introduced by a % character in the template cause subsequent arguments to be formatted and written to the output stream.  For example:

     int pct = 37;
     char filename[] = "foo.txt";
     printf ("Processing of `%s' is %d%% finished.\nPlease be patient.\n",
             filename, pct);

produces output like:

     Processing of `foo.txt' is 37% finished.
     Please be patient.

This example shows the use of the %d conversion to specify that an int argument should be printed in decimal notation, the %s conversion to specify printing of a string argument, and the %% conversion to print a literal % character.

There are also conversions for printing an integer argument as an unsigned value in octal, decimal, or hexadecimal radix (%o, %u, or %x, respectively); or as a character value (%c).

Floating-point numbers can be printed in normal, fixed-point notation using the %f conversion or in exponential notation using the %e conversion.  The %g conversion uses either %e or %f format, depending on what is more appropriate for the magnitude of the particular number.

You can control formatting more precisely by writing modifiers between the % and the character that indicates which conversion to apply.  These slightly alter the ordinary behavior of the conversion. For example, most conversion specifications permit you to specify a minimum field width and a flag indicating whether you want the result left- or right-justified within the field.

The specific flags and modifiers that are permitted and their interpretation vary depending on the particular conversion.  They're all described in more detail in the following sections.  Don't worry if this all seems excessively complicated at first; you can almost always get reasonable free-format output without using any of the modifiers at all. The modifiers are mostly used to make the output look ``prettier'' in tables.

Output Conversion Syntax
------------------------

This section provides details about the precise syntax of conversion specifications that can appear in a printf template string.

Characters in the template string that are not part of a conversion specification are printed as-is to the output stream. Multibyte-character sequences (see Character-Set Handling) are permitted in a template string.

The conversion specifications in a printf template string have the general form:

     % [ PARAM-NO $] FLAGS WIDTH [ . PRECISION ] TYPE CONVERSION

or

     % [ PARAM-NO $] FLAGS WIDTH . * [ PARAM-NO $] TYPE CONVERSION

For example, in the conversion specifier %-10.8ld, the - is a flag, 10 specifies the field width, the precision is 8, the letter l is a type modifier, and d specifies the conversion style.  (This particular type specifier says to print a long int argument in decimal notation, with a minimum of eight digits left-justified in a field at least ten characters wide.)

In more detail, output conversion specifications consist of an initial % character followed in sequence by:

  * An optional specification of the parameter used for this format. Normally, the parameters to the printf function are assigned to the formats in the order of appearance in the format string.  But in some situations (such as message translation), this is not desirable, and this extension allows an explicit parameter to be specified.

     The PARAM-NO parts of the format must be integers in the range of 1 to the maximum number of arguments present to the function call.  Some implementations limit this number to a certainly upper bound.  The exact limit can be retrieved by the following constant.

      -- Macro: NL_ARGMAX
          The value of NL_ARGMAX is the maximum value allowed for the specification of a positional parameter in a printf call.  The actual value in effect at run time can be retrieved by using sysconf using the _SC_NL_ARGMAX parameter. (Ibid., ``Definition of sysconf''.)

          Some systems have quite a low limit, such as 9 for System V systems.  The GNU C Library has no real limit.

     If any of the formats has a specification for the parameter position, all of them in the format string shall have one.  Otherwise, the behavior is undefined.

  * Zero or more flag characters that modify the normal behavior of the conversion specification

  * An optional decimal integer specifying the minimum field width; if the normal conversion produces fewer characters than this, the field is padded with spaces to the specified width.  This is a minimum value; if the normal conversion produces more characters than this, the field is not truncated.  Normally, the output is right-justified within the field.

     You can also specify a field width of *.  This means that the next argument in the argument list (before the actual value to be printed) is used as the field width.  The value must be an int. If the value is negative, this means to set the - flag (see below), and to use the absolute value as the field width.

  * An optional precision to specify the number of digits to be written for the numeric conversions; if the precision is specified, it consists of a period (.) followed optionally by a decimal integer (which defaults to zero if omitted).

     You can also specify a precision of *.  This means that the next argument in the argument list (before the actual value to be printed) is used as the precision.  The value must be an int, and is ignored if it is negative.  If you specify * for both the field width and precision, the field width argument precedes the precision argument. Other C library versions may not recognize this syntax.

  * An optional type modifier character, which is used to specify the data type of the corresponding argument if it differs from the default type; for example, the integer conversions assume a type of int, but you can specify h, l, or L for other integer types.

  * A character that specifies the conversion to be applied

The exact options that are permitted and how they are interpreted vary between the different conversion specifiers.  See the descriptions of the individual conversions for information about the particular options that they use.

With the -Wformat option, the GNU C Compiler checks calls to printf and related functions.  It examines the format string and verifies that the correct number and types of arguments are supplied.  There is also a GNU C syntax to tell the compiler that a function you write uses a printf-style format string. (See Richard M. Stallman and the GCC Developer Community, ``Decaring Attributes of Functions'' in Using GCC: The GNU Compiler Collection Reference Manual (Boston, MA: GNU Press, October 2003), http:// gcc.gnu.org/ onlinedocs/ gcc-3.3.2/ gcc/.)

Table of Output Conversions
---------------------------

Here is a table summarizing what all the different conversions do:

%d, %i
     Print an integer as a signed decimal number (see Integer Conversions).  %d and %i are synonymous for output, but are different when used with scanf for input (see Table of Input Conversions).

%o
     Print an integer as an unsigned octal number (see Integer Conversions).

%u
     Print an integer as an unsigned decimal number (see Integer Conversions).

%x, %X
     Print an integer as an unsigned hexadecimal number.  %x uses lowercase letters and %X uses uppercase (see Integer Conversions).

%f
     Print a floating-point number in normal (fixed-point) notation (see Floating-Point Conversions).

%e, %E
     Print a floating-point number in exponential notation.  %e uses lowercase letters and %E uses uppercase (see Floating-Point Conversions).

%g, %G
     Print a floating-point number in either normal or exponential notation, whichever is more appropriate for its magnitude.  %g uses lowercase letters and %G uses uppercase (see Floating-Point Conversions).

%a, %A
     Print a floating-point number in a hexadecimal fractional notation which the exponent to base 2 represented in decimal digits.  %a uses lowercase letters and %A uses uppercase (see Floating-Point Conversions).

%c
     Print a single character (see Other Output Conversions).

%C
     This is an alias for %lc that is supported for compatibility with the Unix standard.

%s
     Print a string (see Other Output Conversions).

%S
     This is an alias for %ls that is supported for compatibility with the Unix standard.

%p
     Print the value of a pointer (see Other Output Conversions).

%n
     Get the number of characters printed so far (see Other Output Conversions).  This conversion specification never produces any output.

%m
     Print the string corresponding to the value of errno (see Other Output Conversions).  This is a GNU extension.

%%
     Print a literal % character (see Other Output Conversions).

If the syntax of a conversion specification is invalid, unpredictable things will happen, so don't do this.  If there aren't enough function arguments provided to supply values for all of the conversion specifications in the template string, or if the arguments are not of the correct types, the results are unpredictable.  If you supply more arguments than conversion specifications, the extra argument values are simply ignored; this is sometimes useful.

Integer Conversions
-------------------

This section describes the options for the %d, %i, %o, %u, %x and %X conversion specifications.  These conversions print integers in various formats.

The %d and %i conversion specifications both print an int argument as a signed decimal number; while %o, %u and %x print the argument as an unsigned octal, decimal, or hexadecimal number, respectively.  The %X conversion specification is just like %x, except that it uses the characters ABCDEF as digits instead of abcdef.

The following flags are meaningful:

-
     Left-justify the result in the field (instead of the normal right-justification).

+
     For the signed %d and %i conversions, print a plus sign if the value is positive.

     For the signed %d and %i conversions, if the result doesn't start with a plus or minus sign, prefix it with a space character instead.  Since the + flag ensures that the result includes a sign, this flag is ignored if you supply both of them.

#
     For the %o conversion, this forces the leading digit to be 0, as if by increasing the precision.  For %x or %X, this prefixes a leading 0x or 0X, respectively, to the result.  This doesn't do anything useful for the %d, %i, or %u conversions.  Using this flag produces output that can be parsed by the strtoul function (see Parsing of Integers) and scanf with the %i conversion (see Numeric Input Conversions).

'
     Separate the digits into groups as specified by the locale specified for the LC_NUMERIC category (see General Numeric).  This flag is a GNU extension.

0
     Pad the field with zeros instead of spaces.  The zeros are placed after any indication of sign or base.  This flag is ignored if the - flag is also specified, or if a precision is specified.

If a precision is supplied, it specifies the minimum number of digits to appear; leading zeros are produced if necessary.  If you don't specify a precision, the number is printed with as many digits as it needs.  If you convert a value of zero with an explicit precision of zero, then no characters at all are produced.

Without a type modifier, the corresponding argument is treated as an int (for the signed conversions %i and %d) or unsigned int (for the unsigned conversions %o, %u, %x and %X).  Recall that since printf and friends are variadic, any char and short arguments are automatically converted to int by the default argument promotions.  For arguments of other integer types, you can use these modifiers:

hh
     This modifier specifies that the argument is a signed char or unsigned char, as appropriate.  A char argument is converted to an int or unsigned int by the default argument promotions anyway, but the h modifier says to convert it back to a char again.

     This modifier was introduced in ISO C99.

h
     This modifier specifies that the argument is a short int or unsigned short int, as appropriate.  A short argument is converted to an int or unsigned int by the default argument promotions anyway, but the h modifier says to convert it back to a short again.

j
     This modifier specifies that the argument is an intmax_t or uintmax_t, as appropriate.

     It was introduced in ISO C99.

l
     This modifier specifies that the argument is a long int or unsigned long int, as appropriate.  Two l characters is like the L modifier, below.

     If used with %c or %s, the corresponding parameter is considered as a wide character or wide-character string respectively. This use of l was introduced in Amendment 1 to ISO C90.

L
ll
q
     This modifier specifies that the argument is a long long int.  (This type is an extension supported by the GNU C Compiler.  On systems that don't support extra-long integers, this is the same as long int.)

     The q modifier is another name for the same thing, which comes from 4.4 BSD; a long long int is sometimes called a quad int.

t
     This modifier specifies that the argument is a ptrdiff_t.

     It was introduced in ISO C99.

z
Z
     This modifier specifies that the argument is a size_t.

     z was introduced in ISO C99.  Z is a GNU extension pre-dating this addition and should not be used in new code.

Here is an example.  Using the template string:

     "|%5d|%-5d|%+5d|%+-5d|% 5d|%05d|%5.0d|%5.2d|%d|\n"

to print numbers using the different options for the %d conversion gives results like:

     |    0|0    |   +0|+0   |    0|00000|     |   00|0|
     |    1|1    |   +1|+1   |    1|00001|    1|   01|1|
     |   -1|-1   |   -1|-1   |   -1|-0001|   -1|  -01|-1|
     |100000|100000|+100000|+100000| 100000|100000|100000|100000|100000|

In particular, notice what happens in the last case where the number is too large to fit in the minimum field width specified.

Here are some more examples showing how unsigned integers print under various format options, using the template string:

     "|%5u|%5o|%5x|%5X|%#5o|%#5x|%#5X|%#10.8x|\n"

     |    0|    0|    0|    0|    0|    0|    0|  00000000|
     |    1|    1|    1|    1|   01|  0x1|  0X1|0x00000001|
     |100000|303240|186a0|186A0|0303240|0x186a0|0X186A0|0x000186a0|

Floating-Point Conversions
--------------------------

This section discusses the conversion specifications for floating-point numbers: the %f, %e, %E, %g and %G conversions.

The %f conversion prints its argument in fixed-point notation, producing output of the form [-]DDD.DDD, where the number of digits following the decimal point is controlled by the precision you specify.

The %e conversion prints its argument in exponential notation, producing output of the form [-]D.DDDe[+|-]DD. Again, the number of digits following the decimal point is controlled by the precision.  The exponent always contains at least two digits. The %E conversion is similar, but the exponent is marked with the letter E instead of e.

The %g and %G conversions print the argument in the style of %e or %E, respectively, if the exponent would be less than -4 or greater than or equal to the precision. Otherwise, they use the %f style.  A precision of 0, is taken as 1. is Trailing zeros are removed from the fractional portion of the result, and a decimal-point character appears only if it is followed by a digit.

The %a and %A conversions are meant for representing floating-point numbers exactly in textual form so that they can be exchanged as texts between different programs and/or machines.  The numbers are represented in the form [-]0xH.HHHp[+|-]DD. At the left of the decimal-point character exactly one digit is printed.  This character is only 0 if the number is de-normalized.  Otherwise, the value is unspecified; it is implementation dependent how many bits are used.  The number of hexadecimal digits on the right side of the decimal-point character is equal to the precision.  If the precision is zero, it is determined to be large enough to provide an exact representation of the number or it is large enough to distinguish two adjacent values if the FLT_RADIX is not a power of 2. (Ibid., ``Floating-Point Parameters''.)  For the %a conversion, lowercase characters are used to represent the hexadecimal number, and the prefix and exponent sign are printed as 0x and p respectively.  Otherwise, uppercase characters are used, and 0X and P are used for the representation of prefix and exponent string.  The exponent to the base of two is printed as a decimal number using at least one digit but at most as many digits as necessary to represent the value exactly.

If the value to be printed represents infinity or a NaN, the output is [-]inf or nan, respectively, if the conversion specifier is %a, %e, %f or %g. The output is [-]INF or NAN, respectively, if the conversion is %A, %E or %G.

The following flags can be used to modify the behavior:

-
     Left-justify the result in the field.  Normally the result is right-justified.

+
     Always include a plus or minus sign in the result.

     If the result doesn't start with a plus or minus sign, prefix it with a space instead.  Since the + flag ensures that the result includes a sign, this flag is ignored if you supply both of them.

#
     Specify that the result should always include a decimal point, even if no digits follow it.  For the %g and %G conversions, this also forces trailing zeros after the decimal point to be left in place where they would otherwise be removed.

'
     Separate the digits of the integer part of the result into groups as specified by the locale specified for the LC_NUMERIC category (see General Numeric).  This flag is a GNU extension.

0
     Pad the field with zeros instead of spaces; the zeros are placed after any sign.  This flag is ignored if the - flag is also specified.

The precision specifies how many digits follow the decimal-point character for the %f, %e and %E conversions.  For these conversions, the default precision is 6.  If the precision is explicitly 0, this suppresses the decimal-point character entirely.  For the %g and %G conversions, the precision specifies how many significant digits to print.  Significant digits are the first digit before the decimal point and all the digits after it.  If the precision is 0 or not specified for %g or %G, it is treated like a value of 1.  If the value being printed cannot be expressed accurately in the specified number of digits, the value is rounded to the nearest number that fits.

Without a type modifier, the floating-point conversions use an argument of type double.  (By the default argument promotions, any float arguments are automatically converted to double.) The following type modifier is supported:

L
     An uppercase L specifies that the argument is a long double.

Here are some examples showing how numbers print using the various floating-point conversions.  All of the numbers were printed using this template string:

     "|%13.4a|%13.4f|%13.4e|%13.4g|\n"

Here is the output:

     |  0x0.0000p+0|       0.0000|   0.0000e+00|            0|
     |  0x1.0000p-1|       0.5000|   5.0000e-01|          0.5|
     |  0x1.0000p+0|       1.0000|   1.0000e+00|            1|
     | -0x1.0000p+0|      -1.0000|  -1.0000e+00|           -1|
     |  0x1.9000p+6|     100.0000|   1.0000e+02|          100|
     |  0x1.f400p+9|    1000.0000|   1.0000e+03|         1000|
     | 0x1.3880p+13|   10000.0000|   1.0000e+04|        1e+04|
     | 0x1.81c8p+13|   12345.0000|   1.2345e+04|    1.234e+04|
     | 0x1.86a0p+16|  100000.0000|   1.0000e+05|        1e+05|
     | 0x1.e240p+16|  123456.0000|   1.2346e+05|    1.235e+05|

Notice how the %g conversion drops trailing zeros.

Other Output Conversions
------------------------

This section describes miscellaneous conversions for printf.

The %c conversion prints a single character.  In case there is no l modifier, the int argument is first converted to an unsigned char.  Then, if used in a wide stream function, the character is converted into the corresponding wide character.  The - flag can be used to specify left-justification in the field, but no other flags are defined, and no precision or type modifier can be given.  For example:

     printf ("%c%c%c%c%c", 'h', 'e', 'l', 'l', 'o');

prints hello.

If there is a l modifier present, the argument is expected to be of type wint_t.  If used in a multibyte function, the wide character is converted into a multibyte character before being added to the output.  In this case, more than 1 output byte can be produced.

The %s conversion prints a string.  If no l modifier is present, the corresponding argument must be of type char * (or const char *).  If used in a wide stream function the string is first converted in a wide-character string.  A precision can be specified to indicate the maximum number of characters to write. Otherwise, characters in the string up to but not including the terminating null character are written to the output stream.  The - flag can be used to specify left-justification in the field, but no other flags or type modifiers are defined for this conversion. For example:

     printf ("%3s%-6s", "no", "where");

prints  nowhere .

If there is a l modifier present, the argument is expected to be of type wchar_t (or const wchar_t *).

If you accidentally pass a null pointer as the argument for a %s conversion, the GNU library prints it as (null).  We think this is more useful than crashing.  But it's not good practice to pass a null argument intentionally.

The %m conversion prints the string corresponding to the error code in errno (see Error Messages).  Thus:

     fprintf (stderr, "can't open `%s': %m\n", filename);

is equivalent to:

     fprintf (stderr, "can't open `%s': %s\n", filename, strerror (errno));

The %m conversion is a GNU C Library extension.

The %p conversion prints a pointer value.  The corresponding argument must be of type void *.  In practice, you can use any type of pointer.

In the GNU system, nonnull pointers are printed as unsigned integers, as if a %#x conversion were used.  Null pointers print as (nil).  (Pointers might print differently in other systems.)

For example:

     printf ("%p", "testing");

prints 0x followed by a hexadecimal number---the address of the string constant "testing".  It does not print the word testing.

You can supply the - flag with the %p conversion to specify left-justification, but no other flags, precision, or type modifiers are defined.

The %n conversion is unlike any of the other output conversions.  It uses an argument that must be a pointer to an int, but instead of printing anything, it stores the number of characters printed so far by this call at that location.  The h and l type modifiers are permitted to specify that the argument is of type short int * or long int * instead of int *, but no flags, field width, or precision are permitted.

For example:

     int nchar;
     printf ("%d %s%n\n", 3, "bears", &nchar);

prints:

     3 bears

and sets nchar to 7, because 3 bears is seven characters.

The %% conversion prints a literal % character.  This conversion doesn't use an argument, and no flags, field width, precision, or type modifiers are permitted.

Formatted Output Functions
--------------------------

This section describes how to call printf and related functions. Prototypes for these functions are in the header file stdio.h. Because these functions take a variable number of arguments, you must declare prototypes for them before using them.  Of course, the easiest way to make sure you have all the right prototypes is to just include stdio.h.

 -- Function: int printf (const char *TEMPLATE, ...)
     The printf function prints the optional arguments under the control of the template string TEMPLATE to the stream stdout.  It returns the number of characters printed, or a negative value if there was an output error.

 -- Function: int wprintf (const wchar_t *TEMPLATE, ...)
     The wprintf function prints the optional arguments under the control of the wide template string TEMPLATE to the stream stdout.  It returns the number of wide characters printed, or a negative value if there was an output error.

 -- Function: int fprintf (FILE *STREAM, const char *TEMPLATE, ...)
     This function is just like printf, except that the output is written to the stream STREAM instead of stdout.

 -- Function: int fwprintf (FILE *STREAM, const wchar_t *TEMPLATE, ...)
     This function is just like wprintf, except that the output is written to the stream STREAM instead of stdout.

 -- Function: int sprintf (char *S, const char *TEMPLATE, ...)
     This is like printf, except that the output is stored in the character array S instead of written to a stream.  A null character is written to mark the end of the string.

     The sprintf function returns the number of characters stored in the array S, not including the terminating null character.

     The behavior of this function is undefined if copying takes place between objects that overlap---for example, if S is also given as an argument to be printed under control of the %s conversion (see Copying and Concatenation).

     Warning: The sprintf function can be dangerous because it can potentially output more characters than can fit in the allocation size of the string S.  Remember that the field width given in a conversion specification is only a minimum value.

     To avoid this problem, you can use snprintf or asprintf, described below.

 -- Function: int swprintf (wchar_t *S, size_t SIZE, const wchar_t *TEMPLATE, ...)
     This is like wprintf, except that the output is stored in the wide-character array WS instead of written to a stream.  A null wide character is written to mark the end of the string.  The SIZE argument specifies the maximum number of characters to produce.  The trailing null character is counted toward this limit, so you should allocate at least SIZE wide characters for the string WS.

     The return value is the number of characters generated for the given input, excluding the trailing null.  If not all output fits into the provided buffer, a negative value is returned.  You should try again with a bigger output string.  This is different from how snprintf handles this situation.

     The corresponding narrow stream function takes fewer parameters. swprintf in fact corresponds to the snprintf function. Since the sprintf function can be dangerous and should be avoided, the ISO C committee refused to make the same mistake again and decided to not define a function exactly corresponding to sprintf.

 -- Function: int snprintf (char *S, size_t SIZE, const char *TEMPLATE, ...)
     The snprintf function is similar to sprintf, except that the SIZE argument specifies the maximum number of characters to produce.  The trailing null character is counted toward this limit, so you should allocate at least SIZE characters for the string S.

     The return value is the number of characters that would be generated for the given input, excluding the trailing null.  If this value is greater or equal to SIZE, not all characters from the result have been stored in S.  You should try again with a bigger output string.  Here is an example of doing this:

          /* Construct a message describing the value of a variable
             whose name is NAME and whose value is VALUE. */
          char *
          make_message (char *name, char *value)
          {
            /* Guess we need no more than 100 chars of space. */
            int size = 100;
            char *buffer = (char *) xmalloc (size);
            int nchars;

            if (buffer == NULL)
              return NULL;

           /* Try to print in the allocated space. */
            nchars = snprintf (buffer, size, "value of %s is %s",
                               name, value);

            if (nchars >= size)
              {
                /* Reallocate buffer now that we know
                   how much space is needed. */
                buffer = (char *) xrealloc (buffer, nchars + 1);

                if (buffer != NULL)
                  /* Try again. */
                  snprintf (buffer, size, "value of %s is %s",
                            name, value);
              }
            /* The last call worked, return the string. */
            return buffer;
          }

     In practice, it is often easier just to use asprintf (see Dynamic Output).

     Attention: In versions of the GNU C Library prior to 2.1, the return value is the number of characters stored, not including the terminating null; unless there was not enough space in S to store the result, in which case -1 is returned. This was changed in order to comply with the ISO C99 standard.

Dynamically Allocating Formatted Output
---------------------------------------

The functions in this section do formatted output and place the results in dynamically allocated memory.

 -- Function: int asprintf (char **PTR, const char *TEMPLATE, ...)
     This function is similar to sprintf, except that it dynamically allocates a string (as with malloc; see Unconstrained Allocation) to hold the output, instead of putting the output in a buffer you allocate in advance.  The PTR argument should be the address of a char * object, and asprintf stores a pointer to the newly allocated string at that location.

     The return value is the number of characters allocated for the buffer, or less than zero if an error occurred. Usually this means that the buffer could not be allocated.

     Here is how to use asprintf to get the same result as the snprintf example, but more easily:

          /* Construct a message describing the value of a variable
             whose name is NAME and whose value is VALUE. */
          char *
          make_message (char *name, char *value)
          {
            char *result;
            if (asprintf (&result, "value of %s is %s", name, value) < 0)
              return NULL;
            return result;
          }

 -- Function: int obstack_printf (struct obstack *OBSTACK, const char *TEMPLATE, ...)
     This function is similar to asprintf, except that it uses the obstack OBSTACK to allocate the space (see Obstacks).

     The characters are written onto the end of the current object. To get at them, you must finish the object with obstack_finish (see Growing Objects).

Variable Arguments Output Functions
-----------------------------------

The functions vprintf and friends are provided so that you can define your own variadic printf-like functions that make use of the same internals as the built-in formatted output functions.

The most natural way to define such functions would be to use a language construct to say, ``Call printf and pass this template plus all of my arguments after the first five.''  But there is no way to do this in C, and it would be hard to provide a way, since at the C language level, there is no way to tell how many arguments your function received.

Since that method is impossible, we provide alternative functions, the vprintf series, which lets you pass a va_list to describe ``all of my arguments after the first five.''

When it is sufficient to define a macro rather than a real function, the GNU C Compiler provides a way to do this much more easily with macros. (See Richard M. Stallman and the GCC Developer Community, ``Macros with a Variable Number of Arguments'' in Using GCC: The GNU Compiler Collection Reference Manual (Boston, MA: GNU Press, October 2003), http:// gcc.gnu.org/ onlinedocs/ gcc-3.3.2/ gcc/.)

For example:

     #define myprintf(a, b, c, d, e, rest...) \
                 printf (mytemplate , ## rest)

See Macros with Variable Numbers of Arguments (Using GNU CC), for details.  But this is limited to macros, and does not apply to real functions at all.

Before calling vprintf or the other functions listed in this section, you must call va_start to initialize a pointer to the variable arguments. (Ibid., ``Variadic Functions''.) Then you can call va_arg to fetch the arguments that you want to handle yourself.  This advances the pointer past those arguments.

Once your va_list pointer is pointing at the argument of your choice, you are ready to call vprintf.  That argument and all subsequent arguments that were passed to your function are used by vprintf along with the template that you specified separately.

In some other systems, the va_list pointer may become invalid after the call to vprintf, so you must not use va_arg after you call vprintf.  Instead, you should call va_end to retire the pointer from service.  However, you can safely call va_start on another pointer variable and begin fetching the arguments again through that pointer.  Calling vprintf does not destroy the argument list of your function, merely the particular pointer that you passed to it.

GNU C does not have such restrictions.  You can safely continue to fetch arguments from a va_list pointer after passing it to vprintf, and va_end is a no-op.  Note, however, that subsequent va_arg calls will fetch the same arguments that vprintf previously used.

Prototypes for these functions are declared in stdio.h.

 -- Function: int vprintf (const char *TEMPLATE, va_list AP)
     This function is similar to printf except that, instead of taking a variable number of arguments directly, it takes an argument list pointer AP.

 -- Function: int vwprintf (const wchar_t *TEMPLATE, va_list AP)
     This function is similar to wprintf except that, instead of taking a variable number of arguments directly, it takes an argument list pointer AP.

 -- Function: int vfprintf (FILE *STREAM, const char *TEMPLATE, va_list AP)
     This is the equivalent of fprintf with the variable argument list specified directly as for vprintf.

 -- Function: int vfwprintf (FILE *STREAM, const wchar_t *TEMPLATE, va_list AP)
     This is the equivalent of fwprintf with the variable argument list specified directly as for vwprintf.

 -- Function: int vsprintf (char *S, const char *TEMPLATE, va_list AP)
     This is the equivalent of sprintf with the variable argument list specified directly as for vprintf.

 -- Function: int vswprintf (wchar_t *S, size_t SIZE, const wchar_t *TEMPLATE, va_list AP)
     This is the equivalent of swprintf with the variable argument list specified directly as for vwprintf.

 -- Function: int vsnprintf (char *S, size_t SIZE, const char *TEMPLATE, va_list AP)
     This is the equivalent of snprintf with the variable argument list specified directly as for vprintf.

 -- Function: int vasprintf (char **PTR, const char *TEMPLATE, va_list AP)
     The vasprintf function is the equivalent of asprintf with the variable argument list specified directly as for vprintf.

 -- Function: int obstack_vprintf (struct obstack *OBSTACK, const char *TEMPLATE, va_list AP)
     The obstack_vprintf function is the equivalent of obstack_printf with the variable argument list specified directly as for vprintf.

Here is an example showing how you might use vfprintf.  This is a function that prints error messages to the stream stderr, along with a prefix indicating the name of the program (see Error Messages, for a description of program_invocation_short_name).

     #include <stdio.h>
     #include <stdarg.h>

     void
     eprintf (const char *template, ...)
     {
       va_list ap;
       extern char *program_invocation_short_name;

       fprintf (stderr, "%s: ", program_invocation_short_name);
       va_start (ap, template);
       vfprintf (stderr, template, ap);
       va_end (ap);
     }

You could call eprintf like this:

     eprintf ("file `%s' does not exist\n", filename);

In GNU C, there is a special construct you can use to let the compiler know that a function uses a printf-style format string.  Then it can check the number and types of arguments in each call to the function, and warn you when they do not match the format string. For example, take this declaration of eprintf:

     void eprintf (const char *template, ...)
             __attribute__ ((format (printf, 1, 2)));

This tells the compiler that eprintf uses a format string like printf (as opposed to scanf; see Formatted Input); the format string appears as the first argument and the arguments to satisfy the format begin with the second. (See Richard M. Stallman and the GCC Developer Community, ``Declaring Attributes of Functions'' in Using GCC: The GNU Compiler Collection Reference Manual (Boston, MA: GNU Press, October 2003), http:// gcc.gnu.org/ onlinedocs/ gcc-3.3.2/ gcc/.)

Parsing a Template String
-------------------------

You can use the function parse_printf_format to obtain information about the number and types of arguments that are expected by a given template string.  This function permits interpreters that provide interfaces to printf to avoid passing along invalid arguments from the user's program, which could cause a crash.

All the symbols described in this section are declared in the header file printf.h.

 -- Function: size_t parse_printf_format (const char *TEMPLATE, size_t N, int *ARGTYPES)
     This function returns information about the number and types of arguments expected by the printf template string TEMPLATE. The information is stored in the array ARGTYPES; each element of this array describes one argument.  This information is encoded using the various PA_ macros, listed below.

     The argument N specifies the number of elements in the array ARGTYPES.  This is the maximum number of elements that parse_printf_format will try to write.

     parse_printf_format returns the total number of arguments required by TEMPLATE.  If this number is greater than N, then the information returned describes only the first N arguments.  If you want information about additional arguments, allocate a bigger array and call parse_printf_format again.

The argument types are encoded as a combination of a basic type and modifier flag bits.

 -- Macro: int PA_FLAG_MASK
     This macro is a bitmask for the type modifier flag bits.  You can write the expression (argtypes[i] & PA_FLAG_MASK) to extract just the flag bits for an argument, or (argtypes[i] & ~PA_FLAG_MASK) to extract just the basic type code.

Here are symbolic constants that represent the basic types; they stand for integer values:

PA_INT
     This specifies that the base type is int.

PA_CHAR
     This specifies that the base type is int, cast to char.

PA_STRING
     This specifies that the base type is char *, a null-terminated string.

PA_POINTER
     This specifies that the base type is void *, an arbitrary pointer.

PA_FLOAT
     This specifies that the base type is float.

PA_DOUBLE
     This specifies that the base type is double.

PA_LAST
     You can define additional base types for your own programs as offsets from PA_LAST.  For example, if you have data types foo and bar with their own specialized printf conversions, you could define encodings for these types as:

          #define PA_FOO  PA_LAST
          #define PA_BAR  (PA_LAST + 1)

Here are the flag bits that modify a basic type.  They are combined with the code for the basic type using inclusive-or.

PA_FLAG_PTR
     If this bit is set, it indicates that the encoded type is a pointer to the base type, rather than an immediate value. For example, PA_INT|PA_FLAG_PTR represents the type int *.

PA_FLAG_SHORT
     If this bit is set, it indicates that the base type is modified with short.  This corresponds to the h type modifier.

PA_FLAG_LONG
     If this bit is set, it indicates that the base type is modified with long.  This corresponds to the l type modifier.

PA_FLAG_LONG_LONG
     If this bit is set, it indicates that the base type is modified with long long.  This corresponds to the L type modifier.

PA_FLAG_LONG_DOUBLE
     This is a synonym for PA_FLAG_LONG_LONG, used by convention with a base type of PA_DOUBLE to indicate a type of long double.

For an example of using these facilities, see Example of Parsing.

Example of Parsing a Template String
------------------------------------

Here is an example of decoding argument types for a format string.  We assume this is part of an interpreter that contains arguments of type NUMBER, CHAR, STRING and STRUCTURE (and perhaps others that are not valid here).

     /* Test whether the NARGS specified objects
        in the vector ARGS are valid
        for the format string FORMAT;
        if so, return 1.
        If not, return 0 after printing an error message.  */

     int
     validate_args (char *format, int nargs, OBJECT *args)
     {
       int *argtypes;
       int nwanted;

       /* Get the information about the arguments.
          Each conversion specification must be at least two characters
          long, so there cannot be more specifications than half the
          length of the string.  */

       argtypes = (int *) alloca (strlen (format) / 2 * sizeof (int));
       nwanted = parse_printf_format (string, nelts, argtypes);

       /* Check the number of arguments.  */
       if (nwanted > nargs)
         {
           error ("too few arguments (at least %d required)", nwanted);
           return 0;
         }

       /* Check the C type wanted for each argument
          and see if the object given is suitable.  */
       for (i = 0; i < nwanted; i++)
         {
           int wanted;

           if (argtypes[i] & PA_FLAG_PTR)
             wanted = STRUCTURE;
           else
             switch (argtypes[i] & ~PA_FLAG_MASK)
               {
               case PA_INT:
               case PA_FLOAT:
               case PA_DOUBLE:
                 wanted = NUMBER;
                 break;
               case PA_CHAR:
                 wanted = CHAR;
                 break;
               case PA_STRING:
                 wanted = STRING;
                 break;
               case PA_POINTER:
                 wanted = STRUCTURE;
                 break;
               }
           if (TYPE (args[i]) != wanted)
             {
               error ("type mismatch for arg number %d", i);
               return 0;
             }
         }
       return 1;
     }

Customizing printf
==================

The GNU C Library lets you define your own custom conversion specifiers for printf template strings, to teach printf clever ways to print the important data structures of your program.

The way you do this is by registering the conversion with the function register_printf_function (see Registering New Conversions). One of the arguments you pass to this function is a pointer to a handler function that produces the actual output (see Defining the Output Handler, for information on how to write this function).

You can also install a function that just returns information about the number and type of arguments expected by the conversion specifier (see Parsing a Template String).

The facilities of this section are declared in the header file printf.h.

Portability Note: The ability to extend the syntax of printf template strings is a GNU extension.  ISO standard C has nothing similar.

Registering New Conversions
---------------------------

The function to register a new output conversion is register_printf_function, declared in printf.h.

 -- Function: int register_printf_function (int SPEC, printf_function HANDLER-FUNCTION, printf_arginfo_function ARGINFO-FUNCTION)
     This function defines the conversion specifier character SPEC. Thus, if SPEC is 'Y', it defines the conversion %Y. You can redefine the built-in conversions like %s, but flag characters like # and type modifiers like l can never be used as conversions; calling register_printf_function for those characters has no effect.  It is advisable not to use lowercase letters, since the ISO C standard warns that additional lowercase letters may be standardized in future editions of the standard.

     The HANDLER-FUNCTION is the function called by printf and friends when this conversion appears in a template string (see Defining the Output Handler, for information about how to define a function to pass as this argument).  If you specify a null pointer, any existing handler function for SPEC is removed.

     The ARGINFO-FUNCTION is the function called by parse_printf_format when this conversion appears in a template string (see Parsing a Template String).

     Attention: In the GNU C Library versions before 2.0, the ARGINFO-FUNCTION function did not need to be installed unless the user used the parse_printf_format function.  This has changed.  Now a call to any of the printf functions will call this function when this format specifier appears in the format string.

     The return value is 0 on success, and -1 on failure (which occurs if SPEC is out of range).

     You can redefine the standard output conversions, but this is probably not a good idea because of the potential for confusion.  Library routines written by other people could break if you do this.

Conversion Specifier Options
----------------------------

If you define a meaning for %A, what if the template contains %+23A or %-#A?  To implement a sensible meaning for these, the handler, when called, needs to be able to get the options specified in the template.

Both the HANDLER-FUNCTION and ARGINFO-FUNCTION accept an argument that points to a struct printf_info, which contains information about the options appearing in an instance of the conversion specifier.  This data type is declared in the header file printf.h.

 -- Type: struct printf_info
     This structure is used to pass information about the options appearing in an instance of a conversion specifier in a printf template string to the handler and arginfo functions for that specifier.  It contains the following members:

     int prec
          This is the precision specified.  The value is -1 if no precision was specified.  If the precision was given as *, the printf_info structure passed to the handler function contains the actual value retrieved from the argument list.  But the structure passed to the arginfo function contains a value of INT_MIN, since the actual value is not known.

     int width
          This is the minimum field width specified.  The value is 0 if no width was specified.  If the field width was given as *, the printf_info structure passed to the handler function contains the actual value retrieved from the argument list.  But the structure passed to the arginfo function contains a value of INT_MIN, since the actual value is not known.

     wchar_t spec
          This is the conversion specifier character specified.  It's stored in the structure so that you can register the same handler function for multiple characters, but still have a way to tell them apart when the handler function is called.

     unsigned int is_long_double
          This is a Boolean that is true if the L, ll, or q type modifier was specified.  For integer conversions, this indicates long long int, as opposed to long double for floating-point conversions.

     unsigned int is_char
          This is a Boolean that is true if the hh type modifier was specified.

     unsigned int is_short
          This is a Boolean that is true if the h type modifier was specified.

     unsigned int is_long
          This is a Boolean that is true if the l type modifier was specified.

     unsigned int alt
          This is a Boolean that is true if the # flag was specified.

     unsigned int space
          This is a Boolean that is true if the   flag was specified.

     unsigned int left
          This is a Boolean that is true if the - flag was specified.

     unsigned int showsign
          This is a Boolean that is true if the + flag was specified.

     unsigned int group
          This is a Boolean that is true if the ' flag was specified.

     unsigned int extra
          This flag has a special meaning depending on the context.  It could be used freely by the user-defined handlers, but when called from the printf function, this variable always contains the value 0.

     unsigned int wide
          This flag is set if the stream is wide oriented.

     wchar_t pad
          This is the character to use for padding the output to the minimum field width.  The value is '0' if the 0 flag was specified, and ' ' otherwise.

Defining the Output Handler
---------------------------

Now let's look at how to define the handler and arginfo functions that are passed as arguments to register_printf_function.

Compatibility Note: The interface changed in GNU libc version 2.0.  Previously, the third argument was of type va_list *.

You should define your handler functions with a prototype like:

     int FUNCTION (FILE *stream, const struct printf_info *info,
                         const void *const *args)

The STREAM argument passed to the handler function is the stream to which it should write output.

The INFO argument is a pointer to a structure that contains information about the various options that were included with the conversion in the template string.  You should not modify this structure inside your handler function (see Conversion Specifier Options, for a description of this data structure).

The ARGS is a vector of pointers to the arguments data. The number of arguments was determined by calling the argument information function provided by the user.

Your handler function should return a value just like printf does---it should return the number of characters it has written, or a negative value to indicate an error.

 -- Data Type: printf_function
     This is the data type that a handler function should have.

If you are going to use parse_printf_format in your application, you must also define a function to pass as the ARGINFO-FUNCTION argument for each new conversion you install with register_printf_function.

You have to define these functions with a prototype like:

     int FUNCTION (const struct printf_info *info,
                         size_t n, int *argtypes)

The return value from the function should be the number of arguments the conversion expects.  The function should also fill in no more than N elements of the ARGTYPES array with information about the types of each of these arguments.  This information is encoded using the various PA_ macros.  (This is the same calling convention that parse_printf_format itself uses.)

 -- Data Type: printf_arginfo_function
     This type is used to describe functions that return information about the number and type of arguments used by a conversion specifier.

printf Extension Example
------------------------

Here is an example showing how to define a printf handler function. This program defines a data structure called a Widget and defines the %W conversion to print information about Widget * arguments, including the pointer value and the name stored in the data structure.  The %W conversion supports the minimum field width and left-justification options, but ignores everything else.

     #include <stdio.h>
     #include <stdlib.h>
     #include <printf.h>

     typedef struct
     {
       char *name;
     }
     Widget;

     int
     print_widget (FILE *stream,
                   const struct printf_info *info,
                   const void *const *args)
     {
       const Widget *w;
       char *buffer;
       int len;

       /* Format the output into a string. */
       w = *((const Widget **) (args[0]));
       len = asprintf (&buffer, "<Widget %p: %s>", w, w->name);
       if (len == -1)
         return -1;

       /* Pad to the minimum field width and print to the stream. */
       len = fprintf (stream, "%*s",
                      (info->left ? -info->width : info->width),
                      buffer);

       /* Clean up and return. */
       free (buffer);
       return len;
     }

     int
     print_widget_arginfo (const struct printf_info *info, size_t n,
                           int *argtypes)
     {
       /* We always take exactly one argument and this is a pointer to the
          structure. */
       if (n > 0)
         argtypes[0] = PA_POINTER;
       return 1;
     }

     int
     main (void)
     {
       /* Make a widget to print. */
       Widget mywidget;
       mywidget.name = "mywidget";

       /* Register the print function for widgets. */
       register_printf_function ('W', print_widget, print_widget_arginfo);

       /* Now print the widget. */
       printf ("|%W|\n", &mywidget);
       printf ("|%35W|\n", &mywidget);
       printf ("|%-35W|\n", &mywidget);

       return 0;
     }

The output produced by this program looks like:

     |<Widget 0xffeffb7c: mywidget>|
     |      <Widget 0xffeffb7c: mywidget>|
     |<Widget 0xffeffb7c: mywidget>      |

Predefined printf Handlers
--------------------------

The GNU libc also contains a concrete and useful application of the printf handler extension.  There are two functions available that implement a special way to print floating-point numbers.

 -- Function: int printf_size (FILE *FP, const struct printf_info *INFO, const void *const *ARGS)
     Print a given floating-point number as for the format %f, except that there is a postfix character indicating the divisor for the number to make this less than 1000.  There are two possible divisors: powers of 1024 or powers of 1000.  Which one is used depends on the format character specified while registering this handler.  If the character is lowercase, 1024 is used.  For uppercase characters, 1000 is used.

     The postfix tag corresponds to bytes, kilobytes, megabytes, gigabytes, etc.  The full table is

     low   Multiplier    From    Upper   Multiplier
     ' '   1                     ' '     1
     k     2^10 (1024)   kilo    K       10^3 (1000)
     m     2^20          mega    M       10^6
     g     2^30          giga    G       10^9
     t     2^40          tera    T       10^12
     p     2^50          peta    P       10^15
     e     2^60          exa     E       10^18
     z     2^70          zetta   Z       10^21
     y     2^80          yotta   Y       10^24

     The default precision is 3, i.e., 1024 is printed with a lowercase format character as if it were %.3fk and will yield 1.000k.

Due to the requirements of register_printf_function, we must also provide the function that returns information about the arguments.

 -- Function: int printf_size_info (const struct printf_info *INFO, size_t N, int *ARGTYPES)
     This function will return in ARGTYPES the information about the used parameters in the way the vfprintf implementation expects it.  The format always takes one argument.

To use these functions, both functions must be registered with a call like:

     register_printf_function ('B', printf_size, printf_size_info);

Here we register the functions to print numbers as powers of 1000, since the format character 'B' is an uppercase character.  If we would additionally use 'b' in a line like:

     register_printf_function ('b', printf_size, printf_size_info);

we could also print using a power of 1024.  All that is different in these two lines is the format specifier.  The printf_size function knows about the difference between lowercase and uppercase format specifiers.

The use of 'B' and 'b' is no coincidence.  It is the preferred way to use this functionality, since it is available on some other systems that also use format specifiers.

Formatted Input
===============

The functions described in this section (scanf and related functions) provide facilities for formatted input analogous to the formatted output facilities.  These functions provide a mechanism for reading arbitrary values under the control of a format string or template string.

Formatted Input Basics
----------------------

Calls to scanf are superficially similar to calls to printf, in that arbitrary arguments are read under the control of a template string.  While the syntax of the conversion specifications in the template is very similar to that for printf, the interpretation of the template is oriented more toward free-format input and simple pattern matching, rather than fixed-field formatting.  For example, most scanf conversions skip over any amount of white space (including spaces, tabs and newlines) in the input file, and there is no concept of precision for the numeric input conversions as there is for the corresponding output conversions.  Ordinarily, non-white-space characters in the template are expected to match characters in the input stream exactly, but a matching failure is distinct from an input error on the stream.

Another area of difference between scanf and printf is that you must remember to supply pointers rather than immediate values as the optional arguments to scanf; the values that are read are stored in the objects that the pointers point to.  Even experienced programmers tend to forget this occasionally, so if your program is getting strange errors that seem to be related to scanf, you might want to double-check this.

When a matching failure occurs, scanf returns immediately, leaving the first nonmatching character as the next character to be read from the stream.  The normal return value from scanf is the number of values that were assigned, so you can use this to determine if a matching error happened before all the expected values were read.

The scanf function is typically used for things like reading in the contents of tables.  For example, here is a function that uses scanf to initialize an array of double:

     void
     readarray (double *array, int n)
     {
       int i;
       for (i=0; i<n; i++)
         if (scanf (" %lf", &(array[i])) != 1)
           invalid_input_error ();
     }

The formatted input functions are not used as frequently as the formatted output functions.  Partly, this is because it takes some care to use them properly.  Another reason is that it is difficult to recover from a matching error.

If you are trying to read input that doesn't match a single, fixed pattern, you may be better off using a tool such as Flex to generate a lexical scanner, (See G.T. Nicol, Flex: The Lexical Scanner Generator (Boston, MA: GNU Press, February 1993), http:// www.gnu.org/ software/ flex/ manual/.) or Bison to generate a parser, (See Charles Donnelly and Richard M. Stallman, The Bison Manual (Boston, MA: GNU Press, September 2003), http:// www.gnu.org/ software/ bison/ manual/.) rather than using scanf.

Input Conversion Syntax
-----------------------

A scanf template string is a string that contains ordinary multibyte characters interspersed with conversion specifications that start with %.

Any white-space character (as defined by the isspace function; see Classification of Characters) in the template causes any number of white-space characters in the input stream to be read and discarded. The white-space characters that are matched need not be exactly the same white-space characters that appear in the template string.  For example, write  ,  in the template to recognize a comma with optional white space before and after.

Other characters in the template string that are not part of conversion specifications must match characters in the input stream exactly; if this is not the case, a matching failure occurs.

The conversion specifications in a scanf template string have the general form:

     % FLAGS WIDTH TYPE CONVERSION

In more detail, an input conversion specification consists of an initial % character followed in sequence by:

  * An optional flag character *, which says to ignore the text read for this specification; when scanf finds a conversion specification that uses this flag, it reads input as directed by the rest of the conversion specification, but it discards this input, does not use a pointer argument, and does not increment the count of successful assignments.

  * An optional flag character a (valid with string conversions only), which requests allocation of a buffer long enough to store the string in (see Dynamic String Input).  This is a GNU extension.

  * An optional decimal integer that specifies the maximum field width; reading of characters from the input stream stops either when this maximum is reached or when a nonmatching character is found, whichever happens first.  Most conversions discard initial white-space characters (those that don't are explicitly documented), and these discarded characters don't count toward the maximum field width. String input conversions store a null character to mark the end of the input; the maximum field width does not include this terminator.

  * An optional type modifier character; for example, you can specify a type modifier of l with integer conversions such as %d to specify that the argument is a pointer to a long int rather than a pointer to an int.

  * A character that specifies the conversion to be applied

The exact options that are permitted and how they are interpreted vary between the different conversion specifiers.  See the descriptions of the individual conversions for information about the particular options that they allow.

With the -Wformat option, the GNU C Compiler checks calls to scanf and related functions.  It examines the format string and verifies that the correct number and types of arguments are supplied.  There is also a GNU C syntax to tell the compiler that a function you write uses a scanf-style format string. (See Richard M. Stallman and the GCC Developer Community, ``Declaring Attributes of Functions'' in Using GCC: The GNU Compiler Collection Reference Manual (Boston, MA: GNU Press, October 2003), http:// gcc.gnu.org/ onlinedocs/ gcc-3.3.2/ gcc/.)

Table of Input Conversions
--------------------------

Here is a table that summarizes the various conversion specifications:

%d
     Match an optionally signed integer written in decimal (see Numeric Input Conversions).

%i
     Match an optionally signed integer in any of the formats that the C language defines for specifying an integer constant (see Numeric Input Conversions).

%o
     Match an unsigned integer written in octal radix (see Numeric Input Conversions).

%u
     Match an unsigned integer written in decimal radix (see Numeric Input Conversions).

%x, %X
     Match an unsigned integer written in hexadecimal radix (see Numeric Input Conversions).

%e, %f, %g, %E, %G
     Match an optionally signed floating-point number (see Numeric Input Conversions).

%s

     Match a string containing only non-white-space characters (see String Input Conversions).  The presence of the l modifier determines whether the output is stored as a wide-character string or a multibyte string.  If %s is used in a wide-character function, the string is converted as with multiple calls to wcrtomb into a multibyte string.  This means that the buffer must provide room for MB_CUR_MAX bytes for each wide character read.  In case %ls is used in a multibyte function, the result is converted into wide characters, as with multiple calls of mbrtowc, before being stored in the user-provided buffer.

%S
     This is an alias for %ls that is supported for compatibility with the Unix standard.

%[
     Match a string of characters that belong to a specified set (see String Input Conversions).  The presence of the l modifier determines whether the output is stored as a wide-character string or a multibyte string.  If %[ is used in a wide-character function, the string is converted as with multiple calls to wcrtomb into a multibyte string.  This means that the buffer must provide room for MB_CUR_MAX bytes for each wide character read.  In case %l[ is used in a multibyte function, the result is converted into wide characters, as with multiple calls of mbrtowc, before being stored in the user-provided buffer.

%c
     Match a string of one or more characters; the number of characters read is controlled by the maximum field width given for the conversion (see String Input Conversions).

     If the %c is used in a wide stream function, the read value is converted from a wide character to the corresponding multibyte character before storing it.  This conversion can produce more than 1 byte of output, and therefore the provided buffer must be large enough for up to MB_CUR_MAX bytes for each character.  If %lc is used in a multibyte function, the input is treated as a multibyte sequence (and not bytes), and the result is converted as with calls to mbrtowc.

%C
     This is an alias for %lc that is supported for compatibility with the Unix standard.

%p
     Match a pointer value in the same implementation-defined format used by the %p output conversion for printf (see Other Input Conversions).

%n
     This conversion doesn't read any characters; it records the number of characters read so far by this call (see Other Input Conversions).

%%
     This matches a literal % character in the input stream.  No corresponding argument is used (see Other Input Conversions).

If the syntax of a conversion specification is invalid, the behavior is undefined.  If there aren't enough function arguments provided to supply addresses for all the conversion specifications in the template strings that perform assignments, or if the arguments are not of the correct types, the behavior is also undefined.  On the other hand, extra arguments are simply ignored.

Numeric Input Conversions
-------------------------

This section describes the scanf conversions for reading numeric values.

The %d conversion matches an optionally signed integer in decimal radix.  The syntax that is recognized is the same as that for the strtol function (see Parsing of Integers) with the value 10 for the BASE argument.

The %i conversion matches an optionally signed integer in any of the formats that the C language defines for specifying an integer constant.  The syntax that is recognized is the same as that for the strtol function (see Parsing of Integers) with the value 0 for the BASE argument.  You can print integers in this syntax with printf by using the # flag character with the %x, %o, or %d conversion (see Integer Conversions).

For example, any of the strings 10, 0xa or 012 could be read in as integers under the %i conversion.  Each of these specifies a number with decimal value 10.

The %o, %u and %x conversions match unsigned integers in octal, decimal and hexadecimal radices, respectively.  The syntax that is recognized is the same as that for the strtoul function (see Parsing of Integers) with the appropriate value (8, 10, or 16) for the BASE argument.

The %X conversion is identical to the %x conversion.  They both permit either uppercase or lowercase letters to be used as digits.

The default type of the corresponding argument for the %d and %i conversions is int *, and unsigned int * for the other integer conversions.  You can use the following type modifiers to specify other sizes of integer:

hh
     Specify that the argument is a signed char * or unsigned char *.

     This modifier was introduced in ISO C99.

h
     Specify that the argument is a short int * or unsigned short int *.

j
     Specify that the argument is a intmax_t * or uintmax_t *.

     This modifier was introduced in ISO C99.

l
     Specify that the argument is a long int * or unsigned long int *.  Two l characters is like the L modifier, below.

     If used with %c or %s, the corresponding parameter is considered as a pointer to a wide character or wide-character string respectively.  This use of l was introduced in Amendment 1 to ISO C90.

ll
L
q
     Specify that the argument is a long long int * or unsigned long long int * (the long long type is an extension supported by the GNU C Compiler.  For systems that don't provide extra-long integers, this is the same as long int).

     The q modifier is another name for the same thing, which comes from 4.4 BSD; a long long int is sometimes called a quad int.

t
     Specify that the argument is a ptrdiff_t *.

     This modifier was introduced in ISO C99.

z
     Specify that the argument is a size_t *.

     This modifier was introduced in ISO C99.

All of the %e, %f, %g, %E and %G input conversions are interchangeable.  They all match an optionally signed floating-point number, in the same syntax as for the strtod function (see Parsing of Floats).

For the floating-point input conversions, the default argument type is float *.  This is different from the corresponding output conversions, where the default type is double; remember that float arguments to printf are converted to double by the default argument promotions, but float * arguments are not promoted to double *.  You can specify other sizes of float using these type modifiers:

l
     Specify that the argument is of type double *.

L
     Specify that the argument is of type long double *.

For all of the above number-parsing formats, there is an additional optional flag '.  When this flag is given, the scanf function expects the number represented in the input string to be formatted according to the grouping rules of the currently selected locale (see General Numeric).

If the C or POSIX locale is selected, there is no difference.  But for a locale that specifies values for the appropriate fields in the locale, the input must have the correct form in the input.  Otherwise, the longest prefix with a correct form is processed.

String Input Conversions
------------------------

This section describes the scanf input conversions for reading string and character values: %s, %S, %[, %c and %C.

You have two options for how to receive the input from these conversions:

  * Provide a buffer to store it in.  This is the default.  You should provide an argument of type char * or wchar_t * (the latter if the l modifier is present).

     Warning: To make a robust program, you must make sure that the input (plus its terminating null) cannot possibly exceed the size of the buffer you provide.  In general, the only way to do this is to specify a maximum field width one less than the buffer size.  If you provide the buffer, always specify a maximum field width to prevent overflow.

  * Ask scanf to allocate a big enough buffer, by specifying the a flag character.  This is a GNU extension.  You should provide an argument of type char ** for the buffer address to be stored in (see Dynamic String Input).

The %c conversion is the simplest---it matches a fixed number of characters, always.  The maximum field width says how many characters to read; if you don't specify the maximum, the default is 1.  This conversion doesn't append a null character to the end of the text it reads.  It also does not skip over initial white-space characters.  It reads precisely the next N characters, and fails if it cannot get that many.  Since there is always a maximum field width with %c (whether specified, or 1 by default), you can always prevent overflow by making the buffer long enough.

If the format is %lc or %C, the function stores wide characters that are converted using the conversion determined at the time the stream was opened from the external byte stream.  The number of bytes read from the medium is limited by MB_CUR_LEN * N, but at most N wide characters get stored in the output string.

The %s conversion matches a string of non-white-space characters. It skips and discards initial white space, but stops when it encounters more white space after having read something.  It stores a null character at the end of the text that it reads.

For example, reading the input:

      hello, world

with the conversion %10c produces " hello, wo", but reading the same input with the conversion %10s produces "hello,".

Warning: If you do not specify a field width for %s, then the number of characters read is limited only by where the next white-space character appears.  This almost certainly means that invalid input can make your program crash---which is a bug.

The %ls and %S format are handled just like %s, except that the external byte-sequence is converted using the conversion associated with the stream to wide characters with their own encoding.  A width or precision specified with the format do not directly determine how many bytes are read from the stream since they measure wide characters.  But an upper limit can be computed by multiplying the value of the width or precision by MB_CUR_MAX.

To read in characters that belong to an arbitrary set of your choice, use the %[ conversion.  You specify the set between the [ character and a following ] character, using the same syntax used in regular expressions.  As special cases:

  * A literal ] character can be specified as the first character of the set.

  * An embedded - character (that is, one that is not the first or last character of the set) is used to specify a range of characters.

  * If a caret character ^ immediately follows the initial [, then the set of allowed input characters is the everything except the characters listed.

The %[ conversion does not skip over initial white-space characters.

Here are some examples of %[ conversions and what they mean:

%25[1234567890]
     Match a string of up to twenty-five digits.

%25[][]
     Match a string of up to twenty-five square brackets.

%25[^ \f\n\r\t\v]
     Match a string up to twenty-five characters long that doesn't contain any of the standard white-space characters.  This is slightly different from %s, because if the input begins with a white-space character, %[ reports a matching failure while %s simply discards the initial white space.

%25[a-z]
     Match up to twenty-five lowercase characters.

As for %c and %s, the %[ format is also modified to produce wide characters if the l modifier is present.  All that was said about %ls above is true for %l[.

The %s and %[ conversions are dangerous if you don't specify a maximum width or use the a flag, because input too long would overflow whatever buffer you have provided for it.  No matter how long your buffer is, a user could supply input that is longer.  A well-written program reports invalid input with a comprehensible error message, not with a crash.

Dynamically Allocating String Conversions
-----------------------------------------

A GNU extension to formatted input lets you safely read a string with no maximum size.  Using this feature, you don't supply a buffer; instead, scanf allocates a buffer big enough to hold the data and gives you its address.  To use this feature, write a as a flag character, as in %as or %a[0-9a-z].

The pointer argument you supply for where to store the input should have type char **.  The scanf function allocates a buffer and stores its address in the word that the argument points to.  You should free the buffer with free when you no longer need it.

Here is an example of using the a flag with the %[...] conversion specification to read a variable assignment of the form VARIABLE = VALUE.

     {
       char *variable, *value;

       if (2 > scanf ("%a[a-zA-Z0-9] = %a[^\n]\n",
                      &variable, &value))
         {
           invalid_input_error ();
           return 0;
         }

       ...
     }

Other Input Conversions
-----------------------

This section describes the miscellaneous input conversions.

The %p conversion is used to read a pointer value.  It recognizes the same syntax used by the %p output conversion for printf (see Other Output Conversions); that is, a hexadecimal number, just as the %x conversion accepts.  The corresponding argument should be of type void **; that is, the address of a place to store a pointer.

The resulting pointer value is not guaranteed to be valid if it was not originally written during the same program execution that reads it in.

The %n conversion produces the number of characters read so far by this call.  The corresponding argument should be of type int *.  This conversion works in the same way as the %n conversion for printf (see Other Output Conversions, for an example).

The %n conversion is the only mechanism for determining the success of literal matches or conversions with suppressed assignments. If the %n follows the locus of a matching failure, then no value is stored for it since scanf returns before processing the %n.  If you store -1 in that argument slot before calling scanf, the presence of -1 after scanf indicates an error occurred before the %n was reached.

Finally, the %% conversion matches a literal % character in the input stream, without using an argument.  This conversion does not permit any flags, field width, or type modifier to be specified.

Formatted Input Functions
-------------------------

Here are the descriptions of the functions for performing formatted input. Prototypes for these functions are in the header file stdio.h.

 -- Function: int scanf (const char *TEMPLATE, ...)
     The scanf function reads formatted input from the stream stdin under the control of the template string TEMPLATE. The optional arguments are pointers to the places that receive the resulting values.

     The return value is normally the number of successful assignments.  If an end-of-file condition is detected before any matches are performed, including matches against white-space and literal characters in the template, then EOF is returned.

 -- Function: int wscanf (const wchar_t *TEMPLATE, ...)
     The wscanf function reads formatted input from the stream stdin under the control of the template string TEMPLATE. The optional arguments are pointers to the places which receive the resulting values.

     The return value is normally the number of successful assignments.  If an end-of-file condition is detected before any matches are performed, including matches against white-space and literal characters in the template, then WEOF is returned.

 -- Function: int fscanf (FILE *STREAM, const char *TEMPLATE, ...)
     This function is just like scanf, except that the input is read from the stream STREAM instead of stdin.

 -- Function: int fwscanf (FILE *STREAM, const wchar_t *TEMPLATE, ...)
     This function is just like wscanf, except that the input is read from the stream STREAM instead of stdin.

 -- Function: int sscanf (const char *S, const char *TEMPLATE, ...)
     This is like scanf, except that the characters are taken from the null-terminated string S instead of from a stream.  Reaching the end of the string is treated as an end-of-file condition.

     The behavior of this function is undefined if copying takes place between objects that overlap---for example, if S is also given as an argument to receive a string read under control of the %s, %S, or %[ conversion.

 -- Function: int swscanf (const wchar_t *WS, const char *TEMPLATE, ...)
     This is like wscanf, except that the characters are taken from the null-terminated string WS instead of from a stream.  Reaching the end of the string is treated as an end-of-file condition.

     The behavior of this function is undefined if copying takes place between objects that overlap---for example, if WS is also given as an argument to receive a string read under control of the %s, %S, or %[ conversion.

Variable Arguments Input Functions
----------------------------------

The functions vscanf and friends are provided so that you can define your own variadic scanf-like functions that make use of the same internals as the built-in formatted output functions.  These functions are analogous to the vprintf series of output functions (see Variable Arguments Output, for important information on how to use them).

Portability Note: The functions listed in this section were introduced in ISO C99 and were previously available as GNU extensions.

 -- Function: int vscanf (const char *TEMPLATE, va_list AP)
     This function is similar to scanf, but instead of taking a variable number of arguments directly, it takes an argument list pointer AP of type va_list. (Ibid., ``Variadic Functions''.)

 -- Function: int vwscanf (const wchar_t *TEMPLATE, va_list AP)
     This function is similar to wscanf, but instead of taking a variable number of arguments directly, it takes an argument list pointer AP of type va_list. (Ibid., ``Variadic Functions''.)

 -- Function: int vfscanf (FILE *STREAM, const char *TEMPLATE, va_list AP)
     This is the equivalent of fscanf with the variable argument list specified directly as for vscanf.

 -- Function: int vfwscanf (FILE *STREAM, const wchar_t *TEMPLATE, va_list AP)
     This is the equivalent of fwscanf with the variable argument list specified directly as for vwscanf.

 -- Function: int vsscanf (const char *S, const char *TEMPLATE, va_list AP)
     This is the equivalent of sscanf with the variable argument list specified directly as for vscanf.

 -- Function: int vswscanf (const wchar_t *S, const wchar_t *TEMPLATE, va_list AP)
     This is the equivalent of swscanf with the variable argument list specified directly as for vwscanf.

In GNU C, there is a special construct you can use to let the compiler know that a function uses a scanf-style format string. Then it can check the number and types of arguments in each call to the function, and warn you when they do not match the format string. (See Richard M. Stallman and the GCC Developer Community, ``Declaring Attributes of Functions'' in Using GCC: The GNU Compiler Collection Reference Manual (Boston, MA: GNU Press, October 2003), http:// gcc.gnu.org/ onlinedocs/ gcc-3.3.2/ gcc/.)

End-of-File and Errors
======================

Many of the functions described in this chapter return the value of the macro EOF to indicate unsuccessful completion of the operation.  Since EOF is used to report both end-of-file and random errors, it's often better to use the feof function to check explicitly for end of file and ferror to check for errors.  These functions check indicators that are part of the internal state of the stream object, indicators set if the appropriate condition was detected by a previous I/O operation on that stream.

 -- Macro: int EOF
     This macro is an integer value that is returned by a number of narrow stream functions to indicate an end-of-file condition, or some other error situation.  With the GNU library, EOF is -1.  In other libraries, its value may be some other negative number.

     This symbol is declared in stdio.h.

 -- Macro: int WEOF
     This macro is an integer value that is returned by a number of wide stream functions to indicate an end-of-file condition, or some other error situation.  With the GNU library, WEOF is -1.  In other libraries, its value may be some other negative number.

     This symbol is declared in wchar.h.

 -- Function: int feof (FILE *STREAM)
     The feof function returns nonzero if and only if the end-of-file indicator for the stream STREAM is set.

     This symbol is declared in stdio.h.

 -- Function: int feof_unlocked (FILE *STREAM)
     The feof_unlocked function is equivalent to the feof function, except that it does not implicitly lock the stream.

     This function is a GNU extension.

     This symbol is declared in stdio.h.

 -- Function: int ferror (FILE *STREAM)
     The ferror function returns nonzero if and only if the error indicator for the stream STREAM is set, indicating that an error has occurred on a previous operation on the stream.

     This symbol is declared in stdio.h.

 -- Function: int ferror_unlocked (FILE *STREAM)
     The ferror_unlocked function is equivalent to the ferror function, except that it does not implicitly lock the stream.

     This function is a GNU extension.

     This symbol is declared in stdio.h.

In addition to setting the error indicator associated with the stream, the functions that operate on streams also set errno in the same way as the corresponding low-level functions that operate on file descriptors.  For example, all of the functions that perform output to a stream---such as fputc, printf and fflush---are implemented in terms of write, and all of the errno error conditions defined for write are meaningful for these functions. (For more information about the descriptor-level I/O functions, see Loosemore et al., ``Low-Level Input/Output''.)

Recovering from Errors
======================

You may explicitly clear the error and EOF flags with the clearerr function.

 -- Function: void clearerr (FILE *STREAM)
     This function clears the end-of-file and error indicators for the stream STREAM.

     The file-positioning functions (see File Positioning) also clear the end-of-file indicator for the stream.

 -- Function: void clearerr_unlocked (FILE *STREAM)
     The clearerr_unlocked function is equivalent to the clearerr function, except that it does not implicitly lock the stream.

     This function is a GNU extension.

It is not correct to just clear the error flag and retry a failed stream operation.  After a failed write, any number of characters since the last buffer flush may have been committed to the file, while some buffered data may have been discarded.  Merely retrying can thus cause lost or repeated data.

A failed read may leave the file pointer in an inappropriate position for a second try.  In both cases, you should seek to a known position before retrying.

Most errors that can happen are not recoverable---a second try will always fail again in the same way.  So usually it is best to give up and report the error to the user, rather than install complicated recovery logic.

One important exception is EINTR. (Ibid., ``Primitives Interrupted by Signals''.) Many stream I/O implementations will treat it as an ordinary error, which can be quite inconvenient.  You can avoid this hassle by installing all signals with the SA_RESTART flag.

For similar reasons, setting nonblocking I/O on a stream's file descriptor is not usually advisable.

Text and Binary Streams
=======================

The GNU system and other POSIX-compatible operating systems organize all files as uniform sequences of characters.  However, some other systems make a distinction between files containing text and files containing binary data, and the input and output facilities of ISO C provide for this distinction.  This section tells you how to write programs portable to such systems.

When you open a stream, you can specify either a text stream or a binary stream.  You indicate that you want a binary stream by specifying the b modifier in the OPENTYPE argument to fopen (see Opening Streams).  Without this option, fopen opens the file as a text stream.

Text and binary streams differ in several ways:

  * The data read from a text stream is divided into lines that are terminated by newline ('\n') characters, while a binary stream is simply a long series of characters.  A text stream might on some systems fail to handle lines more than 254 characters long (including the terminating newline character).

  * On some systems, text files can contain only printing characters, horizontal tab characters, and newlines, and so text streams may not support other characters.  However, binary streams can handle any character value.

  * Space characters that are written immediately preceding a newline character in a text stream may disappear when the file is read in again.

  * More generally, there need not be a one-to-one mapping between characters that are read from or written to a text stream, and the characters in the actual file.

Since a binary stream is always more capable and more predictable than a text stream, you might wonder what purpose text streams serve.  Why not simply always use binary streams?  The answer is that on these operating systems, text and binary streams use different file formats, and the only way to read or write ``an ordinary file of text'' that can work with other text-oriented programs is through a text stream.

In the GNU library, and on all POSIX systems, there is no difference between text streams and binary streams.  When you open a stream, you get the same kind of stream regardless of whether you ask for binary. This stream can handle any file content, and has none of the restrictions that text streams sometimes have.

File Positioning
================

The file position of a stream describes where in the file the stream is currently reading or writing.  I/O on the stream advances the file position through the file.  In the GNU system, the file position is represented as an integer, which counts the number of bytes from the beginning of the file (see File Position).

During I/O to an ordinary disk file, you can change the file position whenever you wish, so as to read or write any portion of the file.  Some other kinds of files may also permit this.  Files that support changing the file position are sometimes referred to as random-access files.

You can use the functions in this section to examine or modify the file position indicator associated with a stream.  The symbols listed below are declared in the header file stdio.h.

 -- Function: long int ftell (FILE *STREAM)
     This function returns the current file position of the stream STREAM.

     This function can fail if the stream doesn't support file positioning, or if the file position can't be represented in a long int, and possibly for other reasons as well.  If a failure occurs, a value of -1 is returned.

 -- Function: off_t ftello (FILE *STREAM)
     The ftello function is similar to ftell, except that it returns a value of type off_t.  Systems that support this type use it to describe all file positions, unlike the POSIX specification, which uses a long int.  The two are not necessarily the same size.  Therefore, using ftell can lead to problems if the implementation is written on top of a POSIX compliant low-level I/O implementation, and using ftello is preferable whenever it is available.

     If this function fails, it returns (off_t) -1.  This can happen due to missing support for file positioning or internal errors. Otherwise, the return value is the current file position.

     The function is an extension defined in the Unix Single Specification version 2.

     When the sources are compiled with _FILE_OFFSET_BITS == 64 on a 32-bit system, this function is in fact ftello64---the LFS interface transparently replaces the old interface.

 -- Function: off64_t ftello64 (FILE *STREAM)
     This function is similar to ftello except that the return value is of type off64_t.  This also requires that the stream STREAM was opened using either fopen64, freopen64 or tmpfile64, since otherwise the underlying file operations to position the file pointer beyond the 2^31 bytes limit might fail.

     If the sources are compiled with _FILE_OFFSET_BITS == 64 on a 32-bit machine, this function is available under the name ftello and so transparently replaces the old interface.

 -- Function: int fseek (FILE *STREAM, long int OFFSET, int WHENCE)
     The fseek function is used to change the file position of the stream STREAM.  The value of WHENCE must be one of the constants SEEK_SET, SEEK_CUR, or SEEK_END, to indicate whether the OFFSET is relative to the beginning of the file, the current file position, or the end of the file, respectively.

     This function returns a value of zero if the operation was successful, and a nonzero value to indicate failure.  A successful call also clears the end-of-file indicator of STREAM and discards any characters that were pushed back by the use of ungetc.

     fseek either flushes any buffered output before setting the file position, or else remembers it so it will be written later in its proper place in the file.

 -- Function: int fseeko (FILE *STREAM, off_t OFFSET, int WHENCE)
     This function is similar to fseek, but it corrects a problem with fseek in a system with POSIX types.  Using a value of type long int for the offset is not compatible with POSIX. fseeko uses the correct type off_t for the OFFSET parameter.

     For this reason, it is a good idea to prefer ftello whenever it is available, since its functionality is (if different at all) closer the underlying definition.

     The functionality and return value are the same as for fseek.

     The function is an extension defined in the Unix Single Specification version 2.

     When the sources are compiled with _FILE_OFFSET_BITS == 64 on a 32-bit system, this function is in fact fseeko64---the LFS interface transparently replaces the old interface.

 -- Function: int fseeko64 (FILE *STREAM, off64_t OFFSET, int WHENCE)
     This function is similar to fseeko except that the OFFSET parameter is of type off64_t.  This also requires that the stream STREAM was opened using either fopen64, freopen64 or tmpfile64, since otherwise the underlying file operations to position the file pointer beyond the 2^31 bytes limit might fail.

     If the sources are compiled with _FILE_OFFSET_BITS == 64 on a 32-bit machine, this function is available under the name fseeko and so transparently replaces the old interface.

Portability Note: In non-POSIX systems, ftell, ftello, fseek and fseeko might work reliably only on binary streams (see Binary Streams).

The following symbolic constants are defined for use as the WHENCE argument to fseek.  They are also used with the lseek function (Ibid., ``Input and Output Primitives''.) and to specify offsets for file locks. (Ibid., ``Control Operations on Files''.)

 -- Macro: int SEEK_SET
     This is an integer constant which, when used as the WHENCE argument to the fseek or fseeko function, specifies that the offset provided is relative to the beginning of the file.

 -- Macro: int SEEK_CUR
     This is an integer constant which, when used as the WHENCE argument to the fseek or fseeko function, specifies that the offset provided is relative to the current file position.

 -- Macro: int SEEK_END
     This is an integer constant which, when used as the WHENCE argument to the fseek or fseeko function, specifies that the offset provided is relative to the end of the file.

 -- Function: void rewind (FILE *STREAM)
     The rewind function positions the stream STREAM at the beginning of the file.  It is equivalent to calling fseek or fseeko on the STREAM with an OFFSET argument of 0L and a WHENCE argument of SEEK_SET, except that the return value is discarded and the error indicator for the stream is reset.

These three aliases for the SEEK_... constants exist for the sake of compatibility with older BSD systems.  They are defined in two different header files: fcntl.h and sys/file.h.

L_SET
     An alias for SEEK_SET

L_INCR
     An alias for SEEK_CUR

L_XTND
     An alias for SEEK_END

Portable File-Position Functions
================================

On the GNU system, the file position is truly a character count. You can specify any character count value as an argument to fseek or fseeko and get reliable results for any random-access file.  However, some ISO C systems do not represent file positions in this way.

On some systems where text streams truly differ from binary streams, it is impossible to represent the file position of a text stream as a count of characters from the beginning of the file.  For example, the file position on some systems must encode both a record offset within the file, and a character offset within the record.

As a consequence, if you want your programs to be portable to these systems, you must observe certain rules:

  * The value returned from ftell on a text stream has no predictable relationship to the number of characters you have read so far.  The only thing you can rely on is that you can use it subsequently as the OFFSET argument to fseek or fseeko to move back to the same file position.

  * In a call to fseek or fseeko on a text stream, either the OFFSET must be zero, or WHENCE must be SEEK_SET and the OFFSET must be the result of an earlier call to ftell on the same stream.

  * The value of the file-position indicator of a text stream is undefined while there are characters that have been pushed back with ungetc that haven't been read or discarded (see Unreading).

But even if you observe these rules, you may still have trouble with long files, because ftell and fseek use a long int value to represent the file position.  This type may not have room to encode all the file positions in a large file.  Using the ftello and fseeko functions might help here, since the off_t type is expected to be able to hold all file-position values but this still does not help to handle additional information that must be associated with a file position.

So if you do want to support systems with peculiar encodings for the file positions, it is better to use the functions fgetpos and fsetpos instead.  These functions represent the file position using the data type fpos_t, whose internal representation varies from system to system.

These symbols are declared in the header file stdio.h.

 -- Data Type: fpos_t
     This is the type of an object that can encode information about the file position of a stream, for use by the functions fgetpos and fsetpos.

     In the GNU system, fpos_t is an opaque data structure that contains internal data to represent file-offset and conversion-state information.  In other systems, it might have a different internal representation.

     When compiling with _FILE_OFFSET_BITS == 64 on a 32-bit machine, this type is in fact equivalent to fpos64_t, since the LFS interface transparently replaces the old interface.

 -- Data Type: fpos64_t
     This is the type of an object that can encode information about the file position of a stream, for use by the functions fgetpos64 and fsetpos64.

     In the GNU system, fpos64_t is an opaque data structure that contains internal data to represent file-offset and conversion-state information.  In other systems, it might have a different internal representation.

 -- Function: int fgetpos (FILE *STREAM, fpos_t *POSITION)
     This function stores the value of the file-position indicator for the stream STREAM in the fpos_t object pointed to by POSITION.  If successful, fgetpos returns zero. Otherwise, it returns a nonzero value and stores an implementation-defined positive value in errno.

     When the sources are compiled with _FILE_OFFSET_BITS == 64 on a 32-bit system, the function is in fact fgetpos64---the LFS interface transparently replaces the old interface.

 -- Function: int fgetpos64 (FILE *STREAM, fpos64_t *POSITION)
     This function is similar to fgetpos, but the file position is returned in a variable of type fpos64_t to which POSITION points.

     If the sources are compiled with _FILE_OFFSET_BITS == 64 on a 32-bit machine, this function is available under the name fgetpos, and so transparently replaces the old interface.

 -- Function: int fsetpos (FILE *STREAM, const fpos_t *POSITION)
     This function sets the file-position indicator for the stream STREAM to the position POSITION, which must have been set by a previous call to fgetpos on the same stream.  If successful, fsetpos clears the end-of-file indicator on the stream, discards any characters that were pushed back by the use of ungetc, and returns a value of zero.  Otherwise, fsetpos returns a nonzero value and stores an implementation-defined positive value in errno.

     When the sources are compiled with _FILE_OFFSET_BITS == 64 on a 32-bit system, the function is in fact fsetpos64---the LFS interface transparently replaces the old interface.

 -- Function: int fsetpos64 (FILE *STREAM, const fpos64_t *POSITION)
     This function is similar to fsetpos, but the file position used for positioning is provided in a variable of type fpos64_t to which POSITION points.

     If the sources are compiled with _FILE_OFFSET_BITS == 64 on a 32-bit machine, this function is available under the name fsetpos, and so transparently replaces the old interface.

Stream Buffering
================

Characters that are written to a stream are normally accumulated and transmitted asynchronously to the file in a block, instead of appearing as soon as they are output by the application program.  Similarly, streams often retrieve input from the host environment in blocks rather than on a character-by-character basis.  This is called buffering.

If you are writing programs that do interactive input and output using streams, you need to understand how buffering works when you design the user interface to your program.  Otherwise, you might find that output (such as progress or prompt messages) doesn't appear when you intended it to, or displays some other unexpected behavior.

This section deals only with controlling when characters are transmitted between the stream and the file or device, and not with how things like echoing, flow control and the like are handled on specific classes of devices. (For information on common control operations on terminal devices, see Loosemore et al., ``Low-Level Terminal Interface''.)

You can bypass the stream buffering facilities altogether by using the low-level input and output functions that operate on file descriptors instead. (Ibid., ``Low-Level Input/Output''.)

Buffering Concepts
------------------

There are three different kinds of buffering strategies:

  * Characters written to or read from an unbuffered stream are transmitted individually to or from the file as soon as possible.

  * Characters written to a line buffered stream are transmitted to the file in blocks when a newline character is encountered.

  * Characters written to or read from a fully buffered stream are transmitted to or from the file in blocks of arbitrary size.

Newly opened streams are normally fully buffered, with one exception: a stream connected to an interactive device such as a terminal is initially line buffered (see Controlling Buffering, for information on how to select a different kind of buffering).  Usually, the automatic selection gives you the most convenient kind of buffering for the file or device you open.

The use of line buffering for interactive devices implies that output messages ending in a newline will appear immediately---which is usually what you want.  Output that doesn't end in a newline might or might not show up immediately, so if you want them to appear immediately, you should flush buffered output explicitly with fflush, as described in Flushing Buffers.

Flushing Buffers
----------------

Flushing output on a buffered stream means transmitting all accumulated characters to the file.  There are many circumstances when buffered output on a stream is flushed automatically:

  * When you try to do output and the output buffer is full

  * When the stream is closed (see Closing Streams)

  * When the program terminates by calling exit (see Normal Termination)

  * When a newline is written, if the stream is line buffered

  * Whenever an input operation on any stream actually reads data from its file

If you want to flush the buffered output at another time, call fflush, which is declared in the header file stdio.h.

 -- Function: int fflush (FILE *STREAM)
     This function causes any buffered output on STREAM to be delivered to the file.  If STREAM is a null pointer, then fflush causes buffered output on all open output streams to be flushed.

     This function returns EOF if a write error occurs, or zero otherwise.

 -- Function: int fflush_unlocked (FILE *STREAM)
     The fflush_unlocked function is equivalent to the fflush function, except that it does not implicitly lock the stream.

The fflush function can be used to flush all streams currently opened. While this is useful in some situations, it often does more than necessary.  It could be used when terminal input is required and the programmer wants to be sure that all output is visible on the terminal. But this means that only line-buffered streams have to be flushed.  Solaris introduced a function especially for this.  It was always available in the GNU C Library in some form but never officially exported.

 -- Function: void _flushlbf (void)
     The _flushlbf function flushes all line buffered streams currently opened.

     This function is declared in the stdio_ext.h header.

Compatibility Note: Some operating systems have been known to be so thoroughly fixated on line-oriented input and output that flushing a line buffered stream causes a newline to be written! Fortunately, this ``feature'' seems to be becoming less common.  You do not need to worry about this in the GNU system.

In some situations, it might be useful to not flush the output pending for a stream but instead simply forget it.  If transmission is costly and the output is not needed anymore, this is valid reasoning.  In this situation, a nonstandard function introduced in Solaris and available in the GNU C Library can be used.

 -- Function: void __fpurge (FILE *STREAM)
     The __fpurge function causes the buffer of the stream STREAM to be emptied.  If the stream is currently in read mode, all input in the buffer is lost.  If the stream is in output mode, the buffered output is not written to the device (or whatever other underlying storage), and the buffer is cleared.

     This function is declared in stdio_ext.h.

Controlling Which Kind of Buffering
-----------------------------------

After opening a stream (but before any other operations have been performed on it), you can explicitly specify what kind of buffering you want it to have using the setvbuf function.

The facilities listed in this section are declared in the header file stdio.h.

 -- Function: int setvbuf (FILE *STREAM, char *BUF, int MODE, size_t SIZE)
     This function is used to specify that the stream STREAM should have the buffering mode MODE, which can be either _IOFBF (for full buffering), _IOLBF (for line buffering), or _IONBF (for unbuffered input/output).

     If you specify a null pointer as the BUF argument, then setvbuf allocates a buffer itself using malloc.  This buffer will be freed when you close the stream.

     Otherwise, BUF should be a character array that can hold at least SIZE characters.  You should not free the space for this array as long as the stream remains open and this array remains its buffer.  You should usually either allocate it statically, or malloc (see Unconstrained Allocation) the buffer.  Using an automatic array is not a good idea unless you close the file before exiting the block that declares the array.

     While the array remains a stream buffer, the stream I/O functions will use the buffer for their internal purposes.  You shouldn't try to access the values in the array directly while the stream is using it for buffering.

     The setvbuf function returns zero on success, or a nonzero value if the value of MODE is not valid, or if the request could not be honored.

 -- Macro: int _IOFBF
     The value of this macro is an integer constant expression that can be used as the MODE argument to the setvbuf function to specify that the stream should be fully buffered.

 -- Macro: int _IOLBF
     The value of this macro is an integer constant expression that can be used as the MODE argument to the setvbuf function to specify that the stream should be line buffered.

 -- Macro: int _IONBF
     The value of this macro is an integer constant expression that can be used as the MODE argument to the setvbuf function to specify that the stream should be unbuffered.

 -- Macro: int BUFSIZ
     The value of this macro is an integer constant expression that is good to use for the SIZE argument to setvbuf.  This value is guaranteed to be at least 256.

     The value of BUFSIZ is chosen on each system so as to make stream I/O efficient.  So it is a good idea to use BUFSIZ as the size for the buffer when you call setvbuf.

     Actually, you can get an even better value to use for the buffer size by means of the fstat system call---it is found in the st_blksize field of the file attributes. (Ibid., ``Attribute Meanings''.)

     Sometimes people also use BUFSIZ as the allocation size of buffers used for related purposes, such as strings used to receive a line of input with fgets (see Character Input).  There is no particular reason to use BUFSIZ for this instead of any other integer, except that it might lead to doing I/O in chunks of an efficient size.

 -- Function: void setbuf (FILE *STREAM, char *BUF)
     If BUF is a null pointer, the effect of this function is equivalent to calling setvbuf with a MODE argument of _IONBF.  Otherwise, it is equivalent to calling setvbuf with BUF, and a MODE of _IOFBF and a SIZE argument of BUFSIZ.

     The setbuf function is provided for compatibility with old code.  Use setvbuf in all new programs.

 -- Function: void setbuffer (FILE *STREAM, char *BUF, size_t SIZE)
     If BUF is a null pointer, this function makes STREAM unbuffered. Otherwise, it makes STREAM fully buffered using BUF as the buffer.  The SIZE argument specifies the length of BUF.

     This function is provided for compatibility with old BSD code. Use setvbuf instead.

 -- Function: void setlinebuf (FILE *STREAM)
     This function makes STREAM be line buffered, and allocates the buffer for you.

     This function is provided for compatibility with old BSD code.  Use setvbuf instead.

It is possible to query whether a given stream is line buffered or not using a nonstandard function introduced in Solaris and available in the GNU C Library.

 -- Function: int __flbf (FILE *STREAM)
     The __flbf function will return a nonzero value in case the stream STREAM is line buffered.  Otherwise, the return value is zero.

     This function is declared in the stdio_ext.h header.

Two more extensions allow you to determine the size of the buffer and how much of it is used.  These functions were also introduced in Solaris.

 -- Function: size_t __fbufsize (FILE *STREAM)
     The __fbufsize function returns the size of the buffer in the stream STREAM.  This value can be used to optimize the use of the stream.

     This function is declared in the stdio_ext.h header.

 -- Function: size_t __fpending (FILE *STREAM)
     The __fpending function returns the number of bytes currently in the output buffer.  For a wide-oriented stream, the measuring unit is wide characters.  This function should not be used on buffers in read mode or opened read-only.

     This function is declared in the stdio_ext.h header.

Other Kinds of Streams
======================

The GNU library provides ways for you to define additional kinds of streams that do not necessarily correspond to an open file.

One such type of stream takes input from or writes output to a string. These kinds of streams are used internally to implement the sprintf and sscanf functions.  You can also create such a stream explicitly, using the functions described in String Streams.

More generally, you can define streams that do input/output to arbitrary objects using functions supplied by your program.  This protocol is discussed in Custom Streams.

Portability Note: The facilities described in this section are specific to GNU.  Other systems or C implementations might or might not provide equivalent functionality.

String Streams
--------------

The fmemopen and open_memstream functions allow you to do I/O to a string or memory buffer.  These facilities are declared in stdio.h.

 -- Function: FILE * fmemopen (void *BUF, size_t SIZE, const char *OPENTYPE)
     This function opens a stream that allows the access specified by the OPENTYPE argument, that reads from or writes to the buffer specified by the argument BUF.  This array must be at least SIZE bytes long.

     If you specify a null pointer as the BUF argument, fmemopen dynamically allocates an array SIZE bytes long (as with malloc; see Unconstrained Allocation).  This is really only useful if you are going to write things to the buffer and then read them back in again, because you have no way of actually getting a pointer to the buffer (for this, try open_memstream, below).  The buffer is freed when the stream is closed.

     The argument OPENTYPE is the same as in fopen (see Opening Streams).  If the OPENTYPE specifies append mode, then the initial file position is set to the first null character in the buffer.  Otherwise, the initial file position is at the beginning of the buffer.

     When a stream open for writing is flushed or closed, a null character (zero byte) is written at the end of the buffer if it fits.  You should add an extra byte to the SIZE argument to account for this. Attempts to write more than SIZE bytes to the buffer result in an error.

     For a stream open for reading, null characters (0 bytes) in the buffer do not count as end of file.  Read operations indicate end of file only when the file position advances past SIZE bytes.  So, if you want to read characters from a null-terminated string, you should supply the length of the string as the SIZE argument.

Here is an example of using fmemopen to create a stream for reading from a string:

     #include <stdio.h>

     static char buffer[] = "foobar";

     int
     main (void)
     {
       int ch;
       FILE *stream;

       stream = fmemopen (buffer, strlen (buffer), "r");
       while ((ch = fgetc (stream)) != EOF)
         printf ("Got %c\n", ch);
       fclose (stream);

       return 0;
     }

This program produces the following output:

     Got f
     Got o
     Got o
     Got b
     Got a
     Got r

 -- Function: FILE * open_memstream (char **PTR, size_t *SIZELOC)
     This function opens a stream for writing to a buffer.  The buffer is allocated dynamically (as with malloc; see Unconstrained Allocation) and grown as necessary.

     When the stream is closed with fclose or flushed with fflush, the locations PTR and SIZELOC are updated to contain the pointer to the buffer and its size.  The values thus stored remain valid only as long as no further output on the stream takes place.  If you do more output, you must flush the stream again to store new values before you use them again.

     A null character is written at the end of the buffer.  This null character is not included in the size value stored at SIZELOC.

     You can move the stream's file position with fseek or fseeko (see File Positioning).  Moving the file position past the end of the data already written fills the intervening space with zeros.

Here is an example of using open_memstream:

     #include <stdio.h>

     int
     main (void)
     {
       char *bp;
       size_t size;
       FILE *stream;

       stream = open_memstream (&bp, &size);
       fprintf (stream, "hello");
       fflush (stream);
       printf ("buf = `%s', size = %d\n", bp, size);
       fprintf (stream, ", world");
       fclose (stream);
       printf ("buf = `%s', size = %d\n", bp, size);

       return 0;
     }

This program produces the following output:

     buf = `hello', size = 5
     buf = `hello, world', size = 12

Obstack Streams
---------------

You can open an output stream that puts it data in an obstack (see Obstacks).

 -- Function: FILE * open_obstack_stream (struct obstack *OBSTACK)
     This function opens a stream for writing data into the obstack OBSTACK. This starts an object in the obstack and makes it grow as data is written (see Growing Objects).

     Calling fflush on this stream updates the current size of the object to match the amount of data that has been written.  After a call to fflush, you can examine the object temporarily.

     You can move the file position of an obstack stream with fseek or fseeko (see File Positioning).  Moving the file position past the end of the data written fills the intervening space with zeros.

     To make the object permanent, update the obstack with fflush, and then use obstack_finish to finalize the object and get its address. The following write to the stream starts a new object in the obstack, and later writes add to that object until you do another fflush and obstack_finish.

     But how do you find out how long the object is?  You can get the length in bytes by calling obstack_object_size (see Status of an Obstack), or you can null-terminate the object like this:

          obstack_1grow (OBSTACK, 0);

     Whichever one you do (you can do both if you wish), you must do it before calling obstack_finish.

Here is a sample function that uses open_obstack_stream:

     char *
     make_message_string (const char *a, int b)
     {
       FILE *stream = open_obstack_stream (&message_obstack);
       output_task (stream);
       fprintf (stream, ": ");
       fprintf (stream, a, b);
       fprintf (stream, "\n");
       fclose (stream);
       obstack_1grow (&message_obstack, 0);
       return obstack_finish (&message_obstack);
     }

Programming Your Own Custom Streams
-----------------------------------

This section describes how you can make a stream that gets input from an arbitrary data source or writes output to an arbitrary data sink programmed by you.  We call these custom streams.  The functions and types described here are all GNU extensions.

Custom Streams and Cookies
..........................

Inside every custom stream is a special object called the cookie.  This is an object supplied by you that records where to fetch or store the data read or written.  It is up to you to define a data type to use for the cookie.  The stream functions in the library never refer directly to its contents, and they don't even know what the type is; they record its address with type void *.

To implement a custom stream, you must specify how to fetch or store the data in the specified place.  You do this by defining hook functions to read, write, change file position, and close the stream.  All four of these functions will be passed the stream's cookie so they can tell where to fetch or store the data.  The library functions don't know what's inside the cookie, but your functions will know.

When you create a custom stream, you must specify the cookie pointer, and also the four hook functions stored in a structure of type cookie_io_functions_t.

These facilities are declared in stdio.h.

 -- Data Type: cookie_io_functions_t
     This is a structure type that holds the functions that define the communications protocol between the stream and its cookie.  It has the following members:

     cookie_read_function_t *read
          This is the function that reads data from the cookie.  If the value is a null pointer instead of a function, then read operations on this stream always return EOF.

     cookie_write_function_t *write
          This is the function that writes data to the cookie.  If the value is a null pointer instead of a function, then data written to the stream is discarded.

     cookie_seek_function_t *seek
          This is the function that performs the equivalent of file positioning on the cookie.  If the value is a null pointer instead of a function, calls to fseek or fseeko on this stream can only seek to locations within the buffer; any attempt to seek outside the buffer will return an ESPIPE error.

     cookie_close_function_t *close
          This function performs any appropriate clean-up on the cookie when closing the stream.  If the value is a null pointer instead of a function, nothing special is done to close the cookie when the stream is closed.

 -- Function: FILE * fopencookie (void *COOKIE, const char *OPENTYPE, cookie_io_functions_t IO-FUNCTIONS)
     This function actually creates the stream for communicating with the COOKIE using the functions in the IO-FUNCTIONS argument. The OPENTYPE argument is interpreted as for fopen (see Opening Streams)---but note that the truncate on open option is ignored. The new stream is fully buffered.

     The fopencookie function returns the newly created stream, or a null pointer in case of an error.

Custom Stream Hook Functions
............................

Here are more details on how you should define the four hook functions that a custom stream needs.

You should define the function to read data from the cookie as:

     ssize_t READER (void *COOKIE, char *BUFFER, size_t SIZE)

This is very similar to the read function. (Ibid., ``I/O Primitives''.)  Your function should transfer up to SIZE bytes into the BUFFER, and return the number of bytes read, or zero to indicate end of file.  You can return a value of -1 to indicate an error.

You should define the function to write data to the cookie as:

     ssize_t WRITER (void *COOKIE, const char *BUFFER, size_t SIZE)

This is very similar to the write function. (Ibid., ``I/O Primitives''.)  Your function should transfer up to SIZE bytes from the buffer, and return the number of bytes written.  You can return a value of -1 to indicate an error.

You should define the function to perform seek operations on the cookie as:

     int SEEKER (void *COOKIE, fpos_t *POSITION, int WHENCE)

For this function, the POSITION and WHENCE arguments are interpreted as for fgetpos (see Portable Positioning).  In the GNU library, fpos_t is equivalent to off_t or long int, and simply represents the number of bytes from the beginning of the file.

After doing the seek operation, your function should store the resulting file position relative to the beginning of the file in POSITION. Your function should return a value of 0 on success and -1 to indicate an error.

You should define the function to do clean-up operations on the cookie appropriate for closing the stream as:

     int CLEANER (void *COOKIE)

Your function should return -1 to indicate an error, and 0 otherwise.

 -- Data Type: cookie_read_function
     This is the data type that the read function for a custom stream should have.  If you declare the function as shown above, this is the type it will have.

 -- Data Type: cookie_write_function
     This is the data type of the write function for a custom stream.

 -- Data Type: cookie_seek_function
     This is the data type of the seek function for a custom stream.

 -- Data Type: cookie_close_function
     This is the data type of the close function for a custom stream.

Formatted Messages
==================

On systems that are based on System V, messages of programs (especially the system tools) are printed in a strict form using the fmtmsg function.  The uniformity sometimes helps the user to interpret messages, and the strictness tests of the fmtmsg function ensure that the programmer follows some minimal requirements.

Printing Formatted Messages
---------------------------

Messages can be printed to standard error and/or to the console.  To select the destination, the programmer can use the following two values, bit-wise OR combined if wanted, for the CLASSIFICATION parameter of fmtmsg:

MM_PRINT
     Display the message in standard error.

MM_CONSOLE
     Display the message on the system console.

The erroneous piece of the system can be signalled by exactly one of the following values that also is bit-wise ORed with the CLASSIFICATION parameter to fmtmsg:

MM_HARD
     The source of the condition is some hardware.

MM_SOFT
     The source of the condition is some software.

MM_FIRM
     The source of the condition is some firmware.

A third component of the CLASSIFICATION parameter to fmtmsg can describe the part of the system that detects the problem.  This is done by using exactly one of the following values:

MM_APPL
     The erroneous condition is detected by the application.

MM_UTIL
     The erroneous condition is detected by a utility.

MM_OPSYS
     The erroneous condition is detected by the operating system.

A last component of CLASSIFICATION can signal the results of this message.  Exactly one of the following values can be used:

MM_RECOVER
     It is a recoverable error.

MM_NRECOV
     It is a nonrecoverable error.

 -- Function: int fmtmsg (long int CLASSIFICATION, const char *LABEL, int SEVERITY, const char *TEXT, const char *ACTION, const char *TAG)
     Display a message described by its parameters on the device(s) specified in the CLASSIFICATION parameter.  The LABEL parameter identifies the source of the message.  The string should consist of two colon-separated parts where the first part has not more than ten and the second part not more than fourteen characters.  The TEXT parameter describes the condition of the error, the ACTION parameter describes possible steps to recover from the error, and the TAG parameter is a reference to the online documentation where more information can be found.  It should contain the LABEL value and a unique identification number.

     Each of the parameters can be a special value that means this value is to be omitted.  The symbolic names for these values are

     MM_NULLLBL
          Ignore LABEL parameter.

     MM_NULLSEV
          Ignore SEVERITY parameter.

     MM_NULLMC
          Ignore CLASSIFICATION parameter.  This implies that nothing is actually printed.

     MM_NULLTXT
          Ignore TEXT parameter.

     MM_NULLACT
          Ignore ACTION parameter.

     MM_NULLTAG
          Ignore TAG parameter.

     There is another way certain fields can be omitted from the output to standard error.  This is described below in the description of environment variables influencing the behavior.

     The SEVERITY parameter can have one of the values in the following table:

     MM_NOSEV
          Nothing is printed; this value is the same as MM_NULLSEV.

     MM_HALT
          This value is printed as HALT.

     MM_ERROR
          This value is printed as ERROR.

     MM_WARNING
          This value is printed as WARNING.

     MM_INFO
          This value is printed as INFO.

     The numeric values of these five macros are between 0 and 4.  Using the environment variable SEV_LEVEL, or using the addseverity function, one can add more severity levels with their corresponding string to print.  This is described below (see Adding Severity Classes).

     If no parameter is ignored, the output looks like this:

          LABEL: SEVERITY-STRING: TEXT
          TO FIX: ACTION TAG

     The colons, newline characters and the TO FIX string are inserted if necessary, i.e., if the corresponding parameter is not ignored.

     This function is specified in the X/Open Portability Guide. (X/Open Company, X/Open Portability Guide, Issue 4, Version 2 (Reading, UK: X/Open Company, Ltd., 1994).)  It is also available on all systems derived from System V.

     The function returns the value MM_OK if no error occurred.  If only the printing to standard error failed, it returns MM_NOMSG.  If printing to the console fails, it returns MM_NOCON.  If nothing is printed, MM_NOTOK is returned. Among situations where all outputs fail, this last value is also returned if a parameter value is incorrect.

There are two environment variables that influence the behavior of fmtmsg.  The first is MSGVERB.  It is used to control the output actually happening on standard error (not the console output).  Each of the five fields can explicitly be enabled. To do this, the user has to put the MSGVERB variable with a format like the following in the environment before calling the fmtmsg function the first time:

     MSGVERB=KEYWORD[:KEYWORD[:...]]

Valid KEYWORDs are label, severity, text, action and tag.  If the environment variable is not given or is an empty string, an unsupported keyword is given, or the value is somehow else invalid, no part of the message is masked out.

The second environment variable that influences the behavior of fmtmsg is SEV_LEVEL.  This variable and the change in the behavior of fmtmsg are not specified in the X/Open Portability Guide. (X/Open Company, X/Open Portability Guide, Issue 4, Version 2 (Reading, UK: X/Open Company, Ltd., 1994).) It is available in System V systems, though.  It can be used to introduce new severity levels.  By default, only the five severity levels described above are available.  Any other numeric value would make fmtmsg print nothing.

If the user puts SEV_LEVEL with a format like:

     SEV_LEVEL=[DESCRIPTION[:DESCRIPTION[:...]]]

in the environment of the process before the first call to fmtmsg, where DESCRIPTION has a value of the form:

     SEVERITY-KEYWORD,LEVEL,PRINTSTRING

The SEVERITY-KEYWORD part is not used by fmtmsg, but it has to be present.  The LEVEL part is a string representation of a number.  The numeric value must be a number greater than 4.  This value must be used in the SEVERITY parameter of fmtmsg to select this class.  It is not possible to overwrite any of the predefined classes.  The PRINTSTRING is the string printed when a message of this class is processed by fmtmsg (see above; fmtsmg does not print the numeric value but instead the string representation).

Adding Severity Classes
-----------------------

There is another way to introduce severity classes besides using the environment variable SEV_LEVEL.  This simplifies the task of introducing new classes in a running program.  One could use the setenv or putenv function to set the environment variable, but this is toilsome.

 -- Function: int addseverity (int SEVERITY, const char *STRING)
     This function allows the introduction of new severity classes that can be addressed by the SEVERITY parameter of the fmtmsg function.  The SEVERITY parameter of addseverity must match the value for the parameter with the same name of fmtmsg, and STRING is the string printed in the actual messages instead of the numeric value.

     If STRING is NULL, the severity class with the numeric value according to SEVERITY is removed.

     It is not possible to overwrite or remove one of the default severity classes.  All calls to addseverity with SEVERITY set to one of the values for the default classes will fail.

     The return value is MM_OK if the task was successfully performed.  If the return value is MM_NOTOK, something went wrong.  This could mean that no more memory is available, or a class is not available when it has to be removed.

     This function is not specified in the X/Open Portability Guide (X/Open Company, X/Open Portability Guide, Issue 4, Version 2 (Reading, UK: X/Open Company, Ltd., 1994).), although the fmtsmg function is.  It is available on System V systems.

How to Use fmtmsg and addseverity
---------------------------------

Here is a simple example program to illustrate the use of both of the functions described in this section:

     #include <fmtmsg.h>

     int
     main (void)
     {
       addseverity (5, "NOTE2");
       fmtmsg (MM_PRINT, "only1field", MM_INFO, "text2", "action2", "tag2");
       fmtmsg (MM_PRINT, "UX:cat", 5, "invalid syntax", "refer to manual",
               "UX:cat:001");
       fmtmsg (MM_PRINT, "label:foo", 6, "text", "action", "tag");
       return 0;
     }

The second call to fmtmsg illustrates a use of this function as it usually occurs on System V systems, which heavily use this function.  It seems worthwhile to give a short explanation here of how this system works on System V.  The value of the LABEL field (UX:cat) says that the error occurred in the Unix program cat.  The explanation of the error follows and the value for the ACTION parameter is refer to manual.  One could be more specific here, if necessary.  The TAG field contains, as proposed above, the value of the string given for the LABEL parameter, and additionally a unique ID (001 in this case).  For a GNU environment, this string could contain a reference to the corresponding node in the Info page for the program.

Running this program without specifying the MSGVERB and SEV_LEVEL function produces the following output:

     UX:cat: NOTE2: invalid syntax
     TO FIX: refer to manual UX:cat:001

We see the different fields of the message and how the extra glue (the colons and the TO FIX string) is printed.  But only one of the three calls to fmtmsg produced output.  The first call does not print anything because the LABEL parameter is not in the correct form.  The string must contain two fields, separated by a colon (see Printing Formatted Messages).  The third fmtmsg call produced no output, since the class with the numeric value 6 is not defined.  Although a class with numeric value 5 is also not defined by default, the call to addseverity introduces it, and the second call to fmtmsg produces the above output.

When we change the environment of the program to contain SEV_LEVEL=XXX,6,NOTE, we get a different result:

     UX:cat: NOTE2: invalid syntax
     TO FIX: refer to manual UX:cat:001
     label:foo: NOTE: text
     TO FIX: action tag

Now the third call to fmtmsg produced some output, and we see how the string NOTE from the environment variable appears in the message.

Now we can reduce the output by specifying which fields we are interested in.  If we additionally set the environment variable MSGVERB to the value severity:label:action we get the following output:

     UX:cat: NOTE2
     TO FIX: refer to manual
     label:foo: NOTE
     TO FIX: action

The output produced by the TEXT and the TAG parameters to fmtmsg vanished.  Please also note that now there is no colon after the NOTE and NOTE2 strings in the output.  This is not necessary, since there is no more output on this line because the text is missing.

Summary of Library Facilities
*****************************

This appendix is a complete list of the facilities declared within the header files supplied with the GNU C Library.  Each entry also lists the standard or other source from which each facility is derived, and tells you where in the manual you can find more information about how to use it.

long int a64l (const char *STRING)

     stdlib.h (XPG):  Encode Binary Data.

void abort (void)

     stdlib.h (ISO):  Aborting a Program.

int abs (int NUMBER)

     stdlib.h (ISO):  Absolute Value.

double acos (double X)

     math.h (ISO):  Inverse Trig Functions.

float acosf (float X)

     math.h (ISO):  Inverse Trig Functions.

double acosh (double X)

     math.h (ISO):  Hyperbolic Functions.

float acoshf (float X)

     math.h (ISO):  Hyperbolic Functions.

long double acoshl (long double X)

     math.h (ISO):  Hyperbolic Functions.

long double acosl (long double X)

     math.h (ISO):  Inverse Trig Functions.

int adjtime (const struct timeval *DELTA, struct timeval *OLDDELTA)

     sys/time.h (BSD):  High-Resolution Calendar.

int adjtimex (struct timex *TIMEX)

     sys/timex.h (GNU):  High-Resolution Calendar.

unsigned int alarm (unsigned int SECONDS)

     unistd.h (POSIX.1):  Setting an Alarm.

void * alloca (size_t SIZE);

     stdlib.h (GNU, BSD):  Variable Size Automatic.

error_t argp_err_exit_status

     argp.h (GNU):  Argp Global Variables.

void argp_error (const struct argp_state *STATE, const char *FMT, ...)

     argp.h (GNU):  Argp Helper Functions.

int ARGP_ERR_UNKNOWN

     argp.h (GNU):  Argp Parser Functions.

void argp_failure (const struct argp_state *STATE, int STATUS, int ERRNUM, const char *FMT, ...)

     argp.h (GNU):  Argp Helper Functions.

void argp_help (const struct argp *ARGP, FILE *STREAM, unsigned FLAGS, char *NAME)

     argp.h (GNU):  Argp Help.

ARGP_IN_ORDER

     argp.h (GNU):  Argp Flags.

ARGP_KEY_ARG

     argp.h (GNU):  Argp Special Keys.

ARGP_KEY_ARGS

     argp.h (GNU):  Argp Special Keys.

ARGP_KEY_END

     argp.h (GNU):  Argp Special Keys.

ARGP_KEY_ERROR

     argp.h (GNU):  Argp Special Keys.

ARGP_KEY_FINI

     argp.h (GNU):  Argp Special Keys.

ARGP_KEY_HELP_ARGS_DOC

     argp.h (GNU):  Argp Help Filter Keys.

ARGP_KEY_HELP_DUP_ARGS_NOTE

     argp.h (GNU):  Argp Help Filter Keys.

ARGP_KEY_HELP_EXTRA

     argp.h (GNU):  Argp Help Filter Keys.

ARGP_KEY_HELP_HEADER

     argp.h (GNU):  Argp Help Filter Keys.

ARGP_KEY_HELP_POST_DOC

     argp.h (GNU):  Argp Help Filter Keys.

ARGP_KEY_HELP_PRE_DOC

     argp.h (GNU):  Argp Help Filter Keys.

ARGP_KEY_INIT

     argp.h (GNU):  Argp Special Keys.

ARGP_KEY_NO_ARGS

     argp.h (GNU):  Argp Special Keys.

ARGP_KEY_SUCCESS

     argp.h (GNU):  Argp Special Keys.

ARGP_LONG_ONLY

     argp.h (GNU):  Argp Flags.

ARGP_NO_ARGS

     argp.h (GNU):  Argp Flags.

ARGP_NO_ERRS

     argp.h (GNU):  Argp Flags.

ARGP_NO_EXIT

     argp.h (GNU):  Argp Flags.

ARGP_NO_HELP

     argp.h (GNU):  Argp Flags.

error_t argp_parse (const struct argp *ARGP, int ARGC, char **ARGV, unsigned FLAGS, int *ARG_INDEX, void *INPUT)

     argp.h (GNU):  Suboptions.

ARGP_PARSE_ARGV0

     argp.h (GNU):  Argp Flags.

const char * argp_program_bug_address

     argp.h (GNU):  Argp Global Variables.

const char * argp_program_version

     argp.h (GNU):  Argp Global Variables.

argp_program_version_hook

     argp.h (GNU):  Argp Global Variables.

ARGP_SILENT

     argp.h (GNU):  Argp Flags.

void argp_state_help (const struct argp_state *STATE, FILE *STREAM, unsigned FLAGS)

     argp.h (GNU):  Argp Helper Functions.

void argp_usage (const struct argp_state *STATE)

     argp.h (GNU):  Argp Helper Functions.

error_t argz_add (char **ARGZ, size_t *ARGZ_LEN, const char *STR)

     argz.h (GNU):  Argz Functions.

error_t argz_add_sep (char **ARGZ, size_t *ARGZ_LEN, const char *STR, int DELIM)

     argz.h (GNU):  Argz Functions.

error_t argz_append (char **ARGZ, size_t *ARGZ_LEN, const char *BUF, size_t BUF_LEN)

     argz.h (GNU):  Argz Functions.

size_t argz_count (const char *ARGZ, size_t ARG_LEN)

     argz.h (GNU):  Argz Functions.

error_t argz_create (char *const ARGV[], char **ARGZ, size_t *ARGZ_LEN)

     argz.h (GNU):  Argz Functions.

error_t argz_create_sep (const char *STRING, int SEP, char **ARGZ, size_t *ARGZ_LEN)

     argz.h (GNU):  Argz Functions.

error_t argz_delete (char **ARGZ, size_t *ARGZ_LEN, char *ENTRY)

     argz.h (GNU):  Argz Functions.

void argz_extract (char *ARGZ, size_t ARGZ_LEN, char **ARGV)

     argz.h (GNU):  Argz Functions.

error_t argz_insert (char **ARGZ, size_t *ARGZ_LEN, char *BEFORE, const char *ENTRY)

     argz.h (GNU):  Argz Functions.

char * argz_next (char *ARGZ, size_t ARGZ_LEN, const char *ENTRY)

     argz.h (GNU):  Argz Functions.

error_t argz_replace (char **ARGZ, size_t *ARGZ_LEN, const char *STR, const char *WITH, unsigned *REPLACE_COUNT)

     argz.h (GNU):  Argz Functions.

void argz_stringify (char *ARGZ, size_t LEN, int SEP)

     argz.h (GNU):  Argz Functions.

char * asctime (const struct tm *BROKENTIME)

     time.h (ISO):  Formatting Calendar Time.

char * asctime_r (const struct tm *BROKENTIME, char *BUFFER)

     time.h (POSIX.1c):  Formatting Calendar Time.

double asin (double X)

     math.h (ISO):  Inverse Trig Functions.

float asinf (float X)

     math.h (ISO):  Inverse Trig Functions.

double asinh (double X)

     math.h (ISO):  Hyperbolic Functions.

float asinhf (float X)

     math.h (ISO):  Hyperbolic Functions.

long double asinhl (long double X)

     math.h (ISO):  Hyperbolic Functions.

long double asinl (long double X)

     math.h (ISO):  Inverse Trig Functions.

int asprintf (char **PTR, const char *TEMPLATE, ...)

     stdio.h (GNU):  Dynamic Output.

double atan (double X)

     math.h (ISO):  Inverse Trig Functions.

double atan2 (double Y, double X)

     math.h (ISO):  Inverse Trig Functions.

float atan2f (float Y, float X)

     math.h (ISO):  Inverse Trig Functions.

long double atan2l (long double Y, long double X)

     math.h (ISO):  Inverse Trig Functions.

float atanf (float X)

     math.h (ISO):  Inverse Trig Functions.

double atanh (double X)

     math.h (ISO):  Hyperbolic Functions.

float atanhf (float X)

     math.h (ISO):  Hyperbolic Functions.

long double atanhl (long double X)

     math.h (ISO):  Hyperbolic Functions.

long double atanl (long double X)

     math.h (ISO):  Inverse Trig Functions.

int atexit (void (*FUNCTION) (void))

     stdlib.h (ISO):  Clean-Ups on Exit.

double atof (const char *STRING)

     stdlib.h (ISO):  Parsing of Floats.

int atoi (const char *STRING)

     stdlib.h (ISO):  Parsing of Integers.

long int atol (const char *STRING)

     stdlib.h (ISO):  Parsing of Integers.

long long int atoll (const char *STRING)

     stdlib.h (ISO):  Parsing of Integers.

int backtrace (void **BUFFER, int SIZE)

     execinfo.h (GNU):  Backtraces.

char ** backtrace_symbols (void *const *BUFFER, int SIZE)

     execinfo.h (GNU):  Backtraces.

void backtrace_symbols_fd (void *const *BUFFER, int SIZE, int FD)

     execinfo.h (GNU):  Backtraces.

char * basename (char *PATH)

     libgen.h (XPG):  Finding Tokens in a String.

char * basename (const char *FILENAME)

     string.h (GNU):  Finding Tokens in a String.

int bcmp (const void *A1, const void *A2, size_t SIZE)

     string.h (BSD):  String/Array Comparison.

void bcopy (const void *FROM, void *TO, size_t SIZE)

     string.h (BSD):  Copying and Concatenation.

char * bindtextdomain (const char *DOMAINNAME, const char *DIRNAME)

     libintl.h (GNU):  Locating gettext catalog.

char * bind_textdomain_codeset (const char *DOMAINNAME, const char *CODESET)

     libintl.h (GNU):  Charset conversion in gettext.

int brk (void *ADDR)

     unistd.h (BSD):  Resizing the Data Segment.

_BSD_SOURCE

     (GNU):  Feature-Test Macros.

void * bsearch (const void *KEY, const void *ARRAY, size_t COUNT, size_t SIZE, comparison_fn_t COMPARE)

     stdlib.h (ISO):  Array Search Function.

wint_t btowc (int C)

     wchar.h (ISO):  Converting a Character.

int BUFSIZ

     stdio.h (ISO):  Controlling Buffering.

void bzero (void *BLOCK, size_t SIZE)

     string.h (BSD):  Copying and Concatenation.

double cabs (complex double Z)

     complex.h (ISO):  Absolute Value.

float cabsf (complex float Z)

     complex.h (ISO):  Absolute Value.

long double cabsl (complex long double Z)

     complex.h (ISO):  Absolute Value.

complex double cacos (complex double Z)

     complex.h (ISO):  Inverse Trig Functions.

complex float cacosf (complex float Z)

     complex.h (ISO):  Inverse Trig Functions.

complex double cacosh (complex double Z)

     complex.h (ISO):  Hyperbolic Functions.

complex float cacoshf (complex float Z)

     complex.h (ISO):  Hyperbolic Functions.

complex long double cacoshl (complex long double Z)

     complex.h (ISO):  Hyperbolic Functions.

complex long double cacosl (complex long double Z)

     complex.h (ISO):  Inverse Trig Functions.

void * calloc (size_t COUNT, size_t ELTSIZE)

     malloc.h, stdlib.h (ISO):  Allocating Cleared Space.

double carg (complex double Z)

     complex.h (ISO):  Operations on Complex.

float cargf (complex float Z)

     complex.h (ISO):  Operations on Complex.

long double cargl (complex long double Z)

     complex.h (ISO):  Operations on Complex.

complex double casin (complex double Z)

     complex.h (ISO):  Inverse Trig Functions.

complex float casinf (complex float Z)

     complex.h (ISO):  Inverse Trig Functions.

complex double casinh (complex double Z)

     complex.h (ISO):  Hyperbolic Functions.

complex float casinhf (complex float Z)

     complex.h (ISO):  Hyperbolic Functions.

complex long double casinhl (complex long double Z)

     complex.h (ISO):  Hyperbolic Functions.

complex long double casinl (complex long double Z)

     complex.h (ISO):  Inverse Trig Functions.

complex double catan (complex double Z)

     complex.h (ISO):  Inverse Trig Functions.

complex float catanf (complex float Z)

     complex.h (ISO):  Inverse Trig Functions.

complex double catanh (complex double Z)

     complex.h (ISO):  Hyperbolic Functions.

complex float catanhf (complex float Z)

     complex.h (ISO):  Hyperbolic Functions.

complex long double catanhl (complex long double Z)

     complex.h (ISO):  Hyperbolic Functions.

complex long double catanl (complex long double Z)

     complex.h (ISO):  Inverse Trig Functions.

nl_catd catopen (const char *CAT_NAME, int FLAG)

     nl_types.h (X/Open):  The catgets Functions.

double cbrt (double X)

     math.h (BSD):  Exponents and Logarithms.

float cbrtf (float X)

     math.h (BSD):  Exponents and Logarithms.

long double cbrtl (long double X)

     math.h (BSD):  Exponents and Logarithms.

complex double ccos (complex double Z)

     complex.h (ISO):  Trig Functions.

complex float ccosf (complex float Z)

     complex.h (ISO):  Trig Functions.

complex double ccosh (complex double Z)

     complex.h (ISO):  Hyperbolic Functions.

complex float ccoshf (complex float Z)

     complex.h (ISO):  Hyperbolic Functions.

complex long double ccoshl (complex long double Z)

     complex.h (ISO):  Hyperbolic Functions.

complex long double ccosl (complex long double Z)

     complex.h (ISO):  Trig Functions.

double ceil (double X)

     math.h (ISO):  Rounding Functions.

float ceilf (float X)

     math.h (ISO):  Rounding Functions.

long double ceill (long double X)

     math.h (ISO):  Rounding Functions.

complex double cexp (complex double Z)

     complex.h (ISO):  Exponents and Logarithms.

complex float cexpf (complex float Z)

     complex.h (ISO):  Exponents and Logarithms.

complex long double cexpl (complex long double Z)

     complex.h (ISO):  Exponents and Logarithms.

void cfree (void *PTR)

     stdlib.h (Sun):  Freeing after Malloc.

double cimag (complex double Z)

     complex.h (ISO):  Operations on Complex.

float cimagf (complex float Z)

     complex.h (ISO):  Operations on Complex.

long double cimagl (complex long double Z)

     complex.h (ISO):  Operations on Complex.

int clearenv (void)

     stdlib.h (GNU):  Environment Access.

void clearerr (FILE *STREAM)

     stdio.h (ISO):  Error Recovery.

void clearerr_unlocked (FILE *STREAM)

     stdio.h (GNU):  Error Recovery.

int CLK_TCK

     time.h (POSIX.1):  CPU Time.

clock_t clock (void)

     time.h (ISO):  CPU Time.

int CLOCKS_PER_SEC

     time.h (ISO):  CPU Time.

clock_t

     time.h (ISO):  CPU Time.

complex double clog (complex double Z)

     complex.h (ISO):  Exponents and Logarithms.

complex double clog10 (complex double Z)

     complex.h (GNU):  Exponents and Logarithms.

complex float clog10f (complex float Z)

     complex.h (GNU):  Exponents and Logarithms.

complex long double clog10l (complex long double Z)

     complex.h (GNU):  Exponents and Logarithms.

complex float clogf (complex float Z)

     complex.h (ISO):  Exponents and Logarithms.

complex long double clogl (complex long double Z)

     complex.h (ISO):  Exponents and Logarithms.

complex double conj (complex double Z)

     complex.h (ISO):  Operations on Complex.

complex float conjf (complex float Z)

     complex.h (ISO):  Operations on Complex.

complex long double conjl (complex long double Z)

     complex.h (ISO):  Operations on Complex.

cookie_close_function

     stdio.h (GNU):  Hook Functions.

cookie_io_functions_t

     stdio.h (GNU):  Streams and Cookies.

cookie_read_function

     stdio.h (GNU):  Hook Functions.

cookie_seek_function

     stdio.h (GNU):  Hook Functions.

cookie_write_function

     stdio.h (GNU):  Hook Functions.

double copysign (double X, double Y)

     math.h (ISO):  FP Bit Twiddling.

float copysignf (float X, float Y)

     math.h (ISO):  FP Bit Twiddling.

long double copysignl (long double X, long double Y)

     math.h (ISO):  FP Bit Twiddling.

double cos (double X)

     math.h (ISO):  Trig Functions.

float cosf (float X)

     math.h (ISO):  Trig Functions.

double cosh (double X)

     math.h (ISO):  Hyperbolic Functions.

float coshf (float X)

     math.h (ISO):  Hyperbolic Functions.

long double coshl (long double X)

     math.h (ISO):  Hyperbolic Functions.

long double cosl (long double X)

     math.h (ISO):  Trig Functions.

complex double cpow (complex double BASE, complex double POWER)

     complex.h (ISO):  Exponents and Logarithms.

complex float cpowf (complex float BASE, complex float POWER)

     complex.h (ISO):  Exponents and Logarithms.

complex long double cpowl (complex long double BASE, complex long double POWER)

     complex.h (ISO):  Exponents and Logarithms.

complex double cproj (complex double Z)

     complex.h (ISO):  Operations on Complex.

complex float cprojf (complex float Z)

     complex.h (ISO):  Operations on Complex.

complex long double cprojl (complex long double Z)

     complex.h (ISO):  Operations on Complex.

double creal (complex double Z)

     complex.h (ISO):  Operations on Complex.

float crealf (complex float Z)

     complex.h (ISO):  Operations on Complex.

long double creall (complex long double Z)

     complex.h (ISO):  Operations on Complex.

complex double csin (complex double Z)

     complex.h (ISO):  Trig Functions.

complex float csinf (complex float Z)

     complex.h (ISO):  Trig Functions.

complex double csinh (complex double Z)

     complex.h (ISO):  Hyperbolic Functions.

complex float csinhf (complex float Z)

     complex.h (ISO):  Hyperbolic Functions.

complex long double csinhl (complex long double Z)

     complex.h (ISO):  Hyperbolic Functions.

complex long double csinl (complex long double Z)

     complex.h (ISO):  Trig Functions.

complex double csqrt (complex double Z)

     complex.h (ISO):  Exponents and Logarithms.

complex float csqrtf (complex float Z)

     complex.h (ISO):  Exponents and Logarithms.

complex long double csqrtl (complex long double Z)

     complex.h (ISO):  Exponents and Logarithms.

complex double ctan (complex double Z)

     complex.h (ISO):  Trig Functions.

complex float ctanf (complex float Z)

     complex.h (ISO):  Trig Functions.

complex double ctanh (complex double Z)

     complex.h (ISO):  Hyperbolic Functions.

complex float ctanhf (complex float Z)

     complex.h (ISO):  Hyperbolic Functions.

complex long double ctanhl (complex long double Z)

     complex.h (ISO):  Hyperbolic Functions.

complex long double ctanl (complex long double Z)

     complex.h (ISO):  Trig Functions.

char * ctime (const time_t *TIME)

     time.h (ISO):  Formatting Calendar Time.

char * ctime_r (const time_t *TIME, char *BUFFER)

     time.h (POSIX.1c):  Formatting Calendar Time.

int daylight

     time.h (SVID):  Time Zone Functions.

char * dcgettext (const char *DOMAINNAME, const char *MSGID, int CATEGORY)

     libintl.h (GNU):  Translation with gettext.

char * dcngettext (const char *DOMAIN, const char *MSGID1, const char *MSGID2, unsigned long int N, int CATEGORY)

     libintl.h (GNU):  Advanced gettext functions.

char * dgettext (const char *DOMAINNAME, const char *MSGID)

     libintl.h (GNU):  Translation with gettext.

double difftime (time_t TIME1, time_t TIME0)

     time.h (ISO):  Elapsed Time.

char * dirname (char *PATH)

     libgen.h (XPG):  Finding Tokens in a String.

div_t div (int NUMERATOR, int DENOMINATOR)

     stdlib.h (ISO):  Integer Division.

div_t

     stdlib.h (ISO):  Integer Division.

char * dngettext (const char *DOMAIN, const char *MSGID1, const char *MSGID2, unsigned long int N)

     libintl.h (GNU):  Advanced gettext functions.

double drand48 (void)

     stdlib.h (SVID):  SVID Random.

int drand48_r (struct drand48_data *BUFFER, double *RESULT)

     stdlib.h (GNU):  SVID Random.

double drem (double NUMERATOR, double DENOMINATOR)

     math.h (BSD):  Remainder Functions.

float dremf (float NUMERATOR, float DENOMINATOR)

     math.h (BSD):  Remainder Functions.

long double dreml (long double NUMERATOR, long double DENOMINATOR)

     math.h (BSD):  Remainder Functions.

int E2BIG

     errno.h (POSIX.1: Argument list too long):  Error Codes.

int EACCES

     errno.h (POSIX.1: Permission denied):  Error Codes.

int EADDRINUSE

     errno.h (BSD: Address already in use):  Error Codes.

int EADDRNOTAVAIL

     errno.h (BSD: Cannot assign requested address):  Error Codes.

int EADV

     errno.h (Undocumented: Advertise error):  Error Codes.

int EAFNOSUPPORT

     errno.h (BSD: Address family not supported by protocol):  Error Codes.

int EAGAIN

     errno.h (POSIX.1: Resource temporarily unavailable):  Error Codes.

int EALREADY

     errno.h (BSD: Operation already in progress):  Error Codes.

int EAUTH

     errno.h (BSD: Authentication error):  Error Codes.

int EBACKGROUND

     errno.h (GNU: Inappropriate operation for background process):  Error Codes.

int EBADE

     errno.h (Undocumented: Invalid exchange):  Error Codes.

int EBADF

     errno.h (POSIX.1: Bad file descriptor):  Error Codes.

int EBADFD

     errno.h (Undocumented: File descriptor in bad state): Error Codes.

int EBADMSG

     errno.h (XOPEN: Bad message):  Error Codes.

int EBADR

     errno.h (Undocumented: Invalid request descriptor):  Error Codes.

int EBADRPC

     errno.h (BSD: RPC struct is bad):  Error Codes.

int EBADRQC

     errno.h (Undocumented: Invalid request code):  Error Codes.

int EBADSLT

     errno.h (Undocumented: Invalid slot):  Error Codes.

int EBFONT

     errno.h (Undocumented: Bad font file format):  Error Codes.

int EBUSY

     errno.h (POSIX.1: Device or resource busy):  Error Codes.

int ECANCELED

     errno.h (POSIX.1: Operation canceled):  Error Codes.

int ECHILD

     errno.h (POSIX.1: No child processes):  Error Codes.

int ECHRNG

     errno.h (: Channel number out of range):  Error Codes.

int ECOMM

     errno.h (Undocumented: Communication error on send):  Error Codes.

int ECONNABORTED

     errno.h (BSD: Software caused connection abort):  Error Codes.

int ECONNREFUSED

     errno.h (BSD: Connection refused):  Error Codes.

int ECONNRESET

     errno.h (BSD: Connection reset by peer):  Error Codes.

char * ecvt (double VALUE, int NDIGIT, int *DECPT, int *NEG)

     stdlib.h (SVID, Unix98):  System V Number Conversion.

char * ecvt_r (double VALUE, int NDIGIT, int *DECPT, int *NEG, char *BUF, size_t LEN)

     stdlib.h (GNU):  System V Number Conversion.

int ED

     errno.h (GNU: Undocumented):  Error Codes.

int EDEADLK

     errno.h (POSIX.1: Resource deadlock avoided):  Error Codes.

int EDEADLOCK

     errno.h (Undocumented: File-locking deadlock error):  Error Codes.

int EDESTADDRREQ

     errno.h (BSD: Destination address required):  Error Codes.

int EDIED

     errno.h (GNU: Translator died):  Error Codes.

int EDOM

     errno.h (ISO: Numerical argument out of domain):  Error Codes.

int EDOTDOT

     errno.h (Undocumented: RFS specific error):  Error Codes.

int EDQUOT

     errno.h (BSD: Disk quota exceeded):  Error Codes.

int EEXIST

     errno.h (POSIX.1: File exists):  Error Codes.

int EFAULT

     errno.h (POSIX.1: Bad address):  Error Codes.

int EFBIG

     errno.h (POSIX.1: File too large):  Error Codes.

int EFTYPE

     errno.h (BSD: Inappropriate file type or format):  Error Codes.

int EGRATUITOUS

     errno.h (GNU: Gratuitous error):  Error Codes.

int EGREGIOUS

     errno.h (GNU: You really blew it this time):  Error Codes.

int EHOSTDOWN

     errno.h (BSD: Host is down):  Error Codes.

int EHOSTUNREACH

     errno.h (BSD: No route to host):  Error Codes.

int EIDRM

     errno.h (XOPEN: Identifier removed):  Error Codes.

int EIEIO

     errno.h (GNU: Computer bought the farm):  Error Codes.

int EILSEQ

     errno.h (ISO: Invalid or incomplete multibyte or wide character):  Error Codes.

int EINPROGRESS

     errno.h (BSD: Operation now in progress):  Error Codes.

int EINTR

     errno.h (POSIX.1: Interrupted system call):  Error Codes.

int EINVAL

     errno.h (POSIX.1: Invalid argument):  Error Codes.

int EIO

     errno.h (POSIX.1: Input/output error):  Error Codes.

int EISCONN

     errno.h (BSD: Transport endpoint is already connected):  Error Codes.

int EISDIR

     errno.h (POSIX.1: Is a directory):  Error Codes.

int EISNAM

     errno.h (Undocumented: Is a named type file):  Error Codes.

int EL2HLT

     errno.h (Obsolete: Level 2 halted):  Error Codes.

int EL2NSYNC

     errno.h (Obsolete: Level 2 not synchronized):  Error Codes.

int EL3HLT

     errno.h (Obsolete: Level 3 halted):  Error Codes.

int EL3RST

     errno.h (Obsolete: Level 3 reset):  Error Codes.

int ELIBACC

     errno.h (Undocumented: Can not access a needed shared library):  Error Codes.

int ELIBBAD

     errno.h (Undocumented: Accessing a corrupted shared library):  Error Codes.

int ELIBEXEC

     errno.h (Undocumented: Cannot exec a shared library directly):  Error Codes.

int ELIBMAX

     errno.h (Undocumented: Attempting to link in too many shared libraries):  Error Codes.

int ELIBSCN

     errno.h (Undocumented: .lib section in a.out corrupted):  Error Codes.

int ELNRNG

     errno.h (Undocumented: Link number out of range):  Error Codes.

int ELOOP

     errno.h (BSD: Too many levels of symbolic links):  Error Codes.

int EMEDIUMTYPE

     errno.h (Undocumented: Wrong medium type):  Error Codes.

int EMFILE

     errno.h (POSIX.1: Too many open files):  Error Codes.

int EMLINK

     errno.h (POSIX.1: Too many links):  Error Codes.

int EMSGSIZE

     errno.h (BSD: Message too long):  Error Codes.

int EMULTIHOP

     errno.h (XOPEN: Multihop attempted):  Error Codes.

int ENAMETOOLONG

     errno.h (POSIX.1: File name too long):  Error Codes.

int ENAVAIL

     errno.h (Undocumented: No XENIX semaphores available):  Error Codes.

int ENEEDAUTH

     errno.h (BSD: Need authenticator):  Error Codes.

int ENETDOWN

     errno.h (BSD: Network is down):  Error Codes.

int ENETRESET

     errno.h (BSD: Network dropped connection on reset):  Error Codes.

int ENETUNREACH

     errno.h (BSD: Network is unreachable):  Error Codes.

int ENFILE

     errno.h (POSIX.1: Too many open files in system):  Error Codes.

int ENOANO

     errno.h (Undocumented: No anode):  Error Codes.

int ENOBUFS

     errno.h (BSD: No buffer space available):  Error Codes.

int ENOCSI

     errno.h (Undocumented: No CSI structure available):  Error Codes.

int ENODATA

     errno.h (XOPEN: No data available):  Error Codes.

int ENODEV

     errno.h (POSIX.1: No such device):  Error Codes.

int ENOENT

     errno.h (POSIX.1: No such file or directory):  Error Codes.

int ENOEXEC

     errno.h (POSIX.1: Exec format error):  Error Codes.

int ENOLCK

     errno.h (POSIX.1: No locks available):  Error Codes.

int ENOLINK

     errno.h (XOPEN: Link has been severed):  Error Codes.

int ENOMEDIUM

     errno.h (Undocumented: No medium found):  Error Codes.

int ENOMEM

     errno.h (POSIX.1: Cannot allocate memory):  Error Codes.

int ENOMSG

     errno.h (XOPEN: No message of desired type):  Error Codes.

int ENONET

     errno.h (Undocumented: Machine is not on the network):  Error Codes.

int ENOPKG

     errno.h (Undocumented: Package not installed):  Error Codes.

int ENOPROTOOPT

     errno.h (BSD: Protocol not available):  Error Codes.

int ENOSPC

     errno.h (POSIX.1: No space left on device):  Error Codes.

int ENOSR

     errno.h (XOPEN: Out of streams resources):  Error Codes.

int ENOSTR

     errno.h (XOPEN: Device not a stream):  Error Codes.

int ENOSYS

     errno.h (POSIX.1: Function not implemented):  Error Codes.

int ENOTBLK

     errno.h (BSD: Block device required):  Error Codes.

int ENOTCONN

     errno.h (BSD: Transport endpoint is not connected):  Error Codes.

int ENOTDIR

     errno.h (POSIX.1: Not a directory):  Error Codes.

int ENOTEMPTY

     errno.h (POSIX.1: Directory not empty):  Error Codes.

int ENOTNAM

     errno.h (Undocumented: Not a XENIX named type file):  Error Codes.

int ENOTSOCK

     errno.h (BSD: Socket operation on nonsocket):  Error Codes.

int ENOTSUP

     errno.h (POSIX.1: Not supported):  Error Codes.

int ENOTTY

     errno.h (POSIX.1: Inappropriate ioctl for device):  Error Codes.

int ENOTUNIQ

     errno.h (Undocumented: Name not unique on network):  Error Codes.

char ** environ

     unistd.h (POSIX.1):  Environment Access.

error_t envz_add (char **ENVZ, size_t *ENVZ_LEN, const char *NAME, const char *VALUE)

     envz.h (GNU):  Envz Functions.

char * envz_entry (const char *ENVZ, size_t ENVZ_LEN, const char *NAME)

     envz.h (GNU):  Envz Functions.

char * envz_get (const char *ENVZ, size_t ENVZ_LEN, const char *NAME)

     envz.h (GNU):  Envz Functions.

error_t envz_merge (char **ENVZ, size_t *ENVZ_LEN, const char *ENVZ2, size_t ENVZ2_LEN, int OVERRIDE)

     envz.h (GNU):  Envz Functions.

void envz_strip (char **ENVZ, size_t *ENVZ_LEN)

     envz.h (GNU):  Envz Functions.

int ENXIO

     errno.h (POSIX.1: No such device or address):  Error Codes.

int EOF

     stdio.h (ISO):  EOF and Errors.

int EOPNOTSUPP

     errno.h (BSD: Operation not supported):  Error Codes.

int EOVERFLOW

     errno.h (XOPEN: Value too large for defined data type):  Error Codes.

int EPERM

     errno.h (POSIX.1: Operation not permitted):  Error Codes.

int EPFNOSUPPORT

     errno.h (BSD: Protocol family not supported):  Error Codes.

int EPIPE

     errno.h (POSIX.1: Broken pipe):  Error Codes.

int EPROCLIM

     errno.h (BSD: Too many processes):  Error Codes.

int EPROCUNAVAIL

     errno.h (BSD: RPC bad procedure for program):  Error Codes.

int EPROGMISMATCH

     errno.h (BSD: RPC program version wrong):  Error Codes.

int EPROGUNAVAIL

     errno.h (BSD: RPC program not available):  Error Codes.

int EPROTO

     errno.h (XOPEN: Protocol error):  Error Codes.

int EPROTONOSUPPORT

     errno.h (BSD: Protocol not supported):  Error Codes.

int EPROTOTYPE

     errno.h (BSD: Protocol wrong type for socket):  Error Codes.

double erand48 (unsigned short int XSUBI[3])

     stdlib.h (SVID):  SVID Random.

int erand48_r (unsigned short int XSUBI[3], struct drand48_data *BUFFER, double *RESULT)

     stdlib.h (GNU):  SVID Random.

int ERANGE

     errno.h (ISO: Numerical result out of range):  Error Codes.

int EREMCHG

     errno.h (Undocumented: Remote address changed):  Error Codes.

int EREMOTE

     errno.h (BSD: Object is remote):  Error Codes.

int EREMOTEIO

     errno.h (Undocumented: Remote I/O error):  Error Codes.

int ERESTART

     errno.h (Undocumented: Interrupted system call should be restarted):  Error Codes.

double erf (double X)

     math.h (SVID):  Special Functions.

double erfc (double X)

     math.h (SVID):  Special Functions.

float erfcf (float X)

     math.h (SVID):  Special Functions.

long double erfcl (long double X)

     math.h (SVID):  Special Functions.

float erff (float X)

     math.h (SVID):  Special Functions.

long double erfl (long double X)

     math.h (SVID):  Special Functions.

int EROFS

     errno.h (POSIX.1: Read-only file system):  Error Codes.

int ERPCMISMATCH

     errno.h (BSD: RPC version wrong):  Error Codes.

void err (int STATUS, const char *FORMAT, ...)

     err.h (BSD):  Error Messages.

volatile int errno

     errno.h (ISO):  Checking for Errors.

void error (int STATUS, int ERRNUM, const char *FORMAT, ...)

     error.h (GNU):  Error Messages.

void error_at_line (int STATUS, int ERRNUM, const char *FNAME, unsigned int LINENO, const char *FORMAT, ...)

     error.h (GNU):  Error Messages.

unsigned int error_message_count

     error.h (GNU):  Error Messages.

int error_one_per_line

     error.h (GNU):  Error Messages.

void (* error_print_progname ) (void)

     error.h (GNU):  Error Messages.

void errx (int STATUS, const char *FORMAT, ...)

     err.h (BSD):  Error Messages.

int ESHUTDOWN

     errno.h (BSD: Cannot send after transport endpoint shutdown):  Error Codes.

int ESOCKTNOSUPPORT

     errno.h (BSD: Socket type not supported):  Error Codes.

int ESPIPE

     errno.h (POSIX.1: Illegal seek):  Error Codes.

int ESRCH

     errno.h (POSIX.1: No such process):  Error Codes.

int ESRMNT

     errno.h (Undocumented: Srmount error):  Error Codes.

int ESTALE

     errno.h (BSD: Stale NFS file handle):  Error Codes.

int ESTRPIPE

     errno.h (Undocumented: Streams pipe error):  Error Codes.

int ETIME

     errno.h (XOPEN: Timer expired):  Error Codes.

int ETIMEDOUT

     errno.h (BSD: Connection timed out):  Error Codes.

int ETOOMANYREFS

     errno.h (BSD: Too many references: cannot splice):  Error Codes.

int ETXTBSY

     errno.h (BSD: Text file busy):  Error Codes.

int EUCLEAN

     errno.h (Undocumented: Structure needs cleaning):  Error Codes.

int EUNATCH

     errno.h (Undocumented: Protocol driver not attached):  Error Codes.

int EUSERS

     errno.h (BSD: Too many users):  Error Codes.

int EWOULDBLOCK

     errno.h (BSD: Operation would block):  Error Codes.

int EXDEV

     errno.h (POSIX.1: Invalid cross-device link):  Error Codes.

int EXFULL

     errno.h (Undocumented: Exchange full):  Error Codes.

void exit (int STATUS)

     stdlib.h (ISO):  Normal Termination.

void _Exit (int STATUS)

     stdlib.h (ISO):  Termination Internals.

void _exit (int STATUS)

     unistd.h (POSIX.1):  Termination Internals.

int EXIT_FAILURE

     stdlib.h (ISO):  Exit Status.

int EXIT_SUCCESS

     stdlib.h (ISO):  Exit Status.

double exp (double X)

     math.h (ISO):  Exponents and Logarithms.

double exp10 (double X)

     math.h (GNU):  Exponents and Logarithms.

float exp10f (float X)

     math.h (GNU):  Exponents and Logarithms.

long double exp10l (long double X)

     math.h (GNU):  Exponents and Logarithms.

double exp2 (double X)

     math.h (ISO):  Exponents and Logarithms.

float exp2f (float X)

     math.h (ISO):  Exponents and Logarithms.

long double exp2l (long double X)

     math.h (ISO):  Exponents and Logarithms.

float expf (float X)

     math.h (ISO):  Exponents and Logarithms.

long double expl (long double X)

     math.h (ISO):  Exponents and Logarithms.

double expm1 (double X)

     math.h (ISO):  Exponents and Logarithms.

float expm1f (float X)

     math.h (ISO):  Exponents and Logarithms.

long double expm1l (long double X)

     math.h (ISO):  Exponents and Logarithms.

double fabs (double NUMBER)

     math.h (ISO):  Absolute Value.

float fabsf (float NUMBER)

     math.h (ISO):  Absolute Value.

long double fabsl (long double NUMBER)

     math.h (ISO):  Absolute Value.

size_t __fbufsize (FILE *STREAM)

     stdio_ext.h (GNU):  Controlling Buffering.

int fclose (FILE *STREAM)

     stdio.h (ISO):  Closing Streams.

int fcloseall (void)

     stdio.h (GNU):  Closing Streams.

char * fcvt (double VALUE, int NDIGIT, int *DECPT, int *NEG)

     stdlib.h (SVID, Unix98):  System V Number Conversion.

char * fcvt_r (double VALUE, int NDIGIT, int *DECPT, int *NEG, char *BUF, size_t LEN)

     stdlib.h (SVID, Unix98):  System V Number Conversion.

double fdim (double X, double Y)

     math.h (ISO):  Misc FP Arithmetic.

float fdimf (float X, float Y)

     math.h (ISO):  Misc FP Arithmetic.

long double fdiml (long double X, long double Y)

     math.h (ISO):  Misc FP Arithmetic.

int feclearexcept (int EXCEPTS)

     fenv.h (ISO):  Status bit operations.

int fedisableexcept (int EXCEPTS)

     fenv.h (GNU):  Control Functions.

FE_DIVBYZERO

     fenv.h (ISO):  Status bit operations.

FE_DOWNWARD

     fenv.h (ISO):  Rounding.

int feenableexcept (int EXCEPTS)

     fenv.h (GNU):  Control Functions.

int fegetenv (fenv_t *ENVP)

     fenv.h (ISO):  Control Functions.

int fegetexcept (int EXCEPTS)

     fenv.h (GNU):  Control Functions.

int fegetexceptflag (fexcept_t *FLAGP, int EXCEPTS)

     fenv.h (ISO):  Status bit operations.

int fegetround (void)

     fenv.h (ISO):  Rounding.

int feholdexcept (fenv_t *ENVP)

     fenv.h (ISO):  Control Functions.

FE_INEXACT

     fenv.h (ISO):  Status bit operations.

FE_INVALID

     fenv.h (ISO):  Status bit operations.

int feof (FILE *STREAM)

     stdio.h (ISO):  EOF and Errors.

int feof_unlocked (FILE *STREAM)

     stdio.h (GNU):  EOF and Errors.

FE_OVERFLOW

     fenv.h (ISO):  Status bit operations.

int feraiseexcept (int EXCEPTS)

     fenv.h (ISO):  Status bit operations.

int ferror (FILE *STREAM)

     stdio.h (ISO):  EOF and Errors.

int ferror_unlocked (FILE *STREAM)

     stdio.h (GNU):  EOF and Errors.

int fesetenv (const fenv_t *ENVP)

     fenv.h (ISO):  Control Functions.

int fesetexceptflag (const fexcept_t *FLAGP, int

     fenv.h (ISO):  Status bit operations.

int fesetround (int ROUND)

     fenv.h (ISO):  Rounding.

int fetestexcept (int EXCEPTS)

     fenv.h (ISO):  Status bit operations.

FE_TONEAREST

     fenv.h (ISO):  Rounding.

FE_TOWARDZERO

     fenv.h (ISO):  Rounding.

FE_UNDERFLOW

     fenv.h (ISO):  Status bit operations.

int feupdateenv (const fenv_t *ENVP)

     fenv.h (ISO):  Control Functions.

FE_UPWARD

     fenv.h (ISO):  Rounding.

int fflush (FILE *STREAM)

     stdio.h (ISO):  Flushing Buffers.

int fflush_unlocked (FILE *STREAM)

     stdio.h (POSIX):  Flushing Buffers.

int fgetc (FILE *STREAM)

     stdio.h (ISO):  Character Input.

int fgetc_unlocked (FILE *STREAM)

     stdio.h (POSIX):  Character Input.

int fgetpos (FILE *STREAM, fpos_t *POSITION)

     stdio.h (ISO):  Portable Positioning.

int fgetpos64 (FILE *STREAM, fpos64_t *POSITION)

     stdio.h (Unix98):  Portable Positioning.

char * fgets (char *S, int COUNT, FILE *STREAM)

     stdio.h (ISO):  Line Input.

char * fgets_unlocked (char *S, int COUNT, FILE *STREAM)

     stdio.h (GNU):  Line Input.

wint_t fgetwc (FILE *STREAM)

     wchar.h (ISO):  Character Input.

wint_t fgetwc_unlocked (FILE *STREAM)

     wchar.h (GNU):  Character Input.

wchar_t * fgetws (wchar_t *WS, int COUNT, FILE *STREAM)

     wchar.h (ISO):  Line Input.

wchar_t * fgetws_unlocked (wchar_t *WS, int COUNT, FILE *STREAM)

     wchar.h (GNU):  Line Input.

FILE

     stdio.h (ISO):  Streams.

int finite (double X)

     math.h (BSD):  Floating-Point Classes.

int finitef (float X)

     math.h (BSD):  Floating-Point Classes.

int finitel (long double X)

     math.h (BSD):  Floating-Point Classes.

int __flbf (FILE *STREAM)

     stdio_ext.h (GNU):  Controlling Buffering.

void flockfile (FILE *STREAM)

     stdio.h (POSIX):  Streams and Threads.

double floor (double X)

     math.h (ISO):  Rounding Functions.

float floorf (float X)

     math.h (ISO):  Rounding Functions.

long double floorl (long double X)

     math.h (ISO):  Rounding Functions.

void _flushlbf (void)

     stdio_ext.h (GNU):  Flushing Buffers.

double fma (double X, double Y, double Z)

     math.h (ISO):  Misc FP Arithmetic.

float fmaf (float X, float Y, float Z)

     math.h (ISO):  Misc FP Arithmetic.

long double fmal (long double X, long double Y, long double Z)

     math.h (ISO):  Misc FP Arithmetic.

double fmax (double X, double Y)

     math.h (ISO):  Misc FP Arithmetic.

float fmaxf (float X, float Y)

     math.h (ISO):  Misc FP Arithmetic.

long double fmaxl (long double X, long double Y)

     math.h (ISO):  Misc FP Arithmetic.

FILE * fmemopen (void *BUF, size_t SIZE, const char *OPENTYPE)

     stdio.h (GNU):  String Streams.

double fmin (double X, double Y)

     math.h (ISO):  Misc FP Arithmetic.

float fminf (float X, float Y)

     math.h (ISO):  Misc FP Arithmetic.

long double fminl (long double X, long double Y)

     math.h (ISO):  Misc FP Arithmetic.

double fmod (double NUMERATOR, double DENOMINATOR)

     math.h (ISO):  Remainder Functions.

float fmodf (float NUMERATOR, float DENOMINATOR)

     math.h (ISO):  Remainder Functions.

long double fmodl (long double NUMERATOR, long double DENOMINATOR)

     math.h (ISO):  Remainder Functions.

int fmtmsg (long int CLASSIFICATION, const char *LABEL, int SEVERITY, const char *TEXT, const char *ACTION, const char *TAG)

     fmtmsg.h (XPG):  Printing Formatted Messages.

int fnmatch (const char *PATTERN, const char *STRING, int FLAGS)

     fnmatch.h (POSIX.2):  Wildcard Matching.

FNM_CASEFOLD

     fnmatch.h (GNU):  Wildcard Matching.

FNM_EXTMATCH

     fnmatch.h (GNU):  Wildcard Matching.

FNM_FILE_NAME

     fnmatch.h (GNU):  Wildcard Matching.

FNM_LEADING_DIR

     fnmatch.h (GNU):  Wildcard Matching.

FNM_NOESCAPE

     fnmatch.h (POSIX.2):  Wildcard Matching.

FNM_PATHNAME

     fnmatch.h (POSIX.2):  Wildcard Matching.

FNM_PERIOD

     fnmatch.h (POSIX.2):  Wildcard Matching.

FILE * fopen (const char *FILENAME, const char *OPENTYPE)

     stdio.h (ISO):  Opening Streams.

FILE * fopen64 (const char *FILENAME, const char *OPENTYPE)

     stdio.h (Unix98):  Opening Streams.

FILE * fopencookie (void *COOKIE, const char *OPENTYPE, cookie_io_functions_t IO-FUNCTIONS)

     stdio.h (GNU):  Streams and Cookies.

int FOPEN_MAX

     stdio.h (ISO):  Opening Streams.

int fpclassify (float-type X)

     math.h (ISO):  Floating-Point Classes.

size_t __fpending (FILE *STREAM) The __fpending

     stdio_ext.h (GNU):  Controlling Buffering.

int FP_ILOGB0

     math.h (ISO):  Exponents and Logarithms.

int FP_ILOGBNAN

     math.h (ISO):  Exponents and Logarithms.

fpos64_t

     stdio.h (Unix98):  Portable Positioning.

fpos_t

     stdio.h (ISO):  Portable Positioning.

int fprintf (FILE *STREAM, const char *TEMPLATE, ...)

     stdio.h (ISO):  Formatted Output Functions.

void __fpurge (FILE *STREAM)

     stdio_ext.h (GNU):  Flushing Buffers.

int fputc (int C, FILE *STREAM)

     stdio.h (ISO):  Simple Output.

int fputc_unlocked (int C, FILE *STREAM)

     stdio.h (POSIX):  Simple Output.

int fputs (const char *S, FILE *STREAM)

     stdio.h (ISO):  Simple Output.

int fputs_unlocked (const char *S, FILE *STREAM)

     stdio.h (GNU):  Simple Output.

wint_t fputwc (wchar_t WC, FILE *STREAM)

     wchar.h (ISO):  Simple Output.

wint_t fputwc_unlocked (wint_t WC, FILE *STREAM)

     wchar.h (POSIX):  Simple Output.

int fputws (const wchar_t *WS, FILE *STREAM)

     wchar.h (ISO):  Simple Output.

int fputws_unlocked (const wchar_t *WS, FILE *STREAM)

     wchar.h (GNU):  Simple Output.

size_t fread (void *DATA, size_t SIZE, size_t COUNT, FILE *STREAM)

     stdio.h (ISO):  Block Input/Output.

int __freadable (FILE *STREAM)

     stdio_ext.h (GNU):  Opening Streams.

int __freading (FILE *STREAM)

     stdio_ext.h (GNU):  Opening Streams.

size_t fread_unlocked (void *DATA, size_t SIZE, size_t COUNT, FILE *STREAM)

     stdio.h (GNU):  Block Input/Output.

void free (void *PTR)

     malloc.h, stdlib.h (ISO):  Freeing after Malloc.

__free_hook

     malloc.h (GNU):  Hooks for Malloc.

FILE * freopen (const char *FILENAME, const char *OPENTYPE, FILE *STREAM)

     stdio.h (ISO):  Opening Streams.

FILE * freopen64 (const char *FILENAME, const char *OPENTYPE, FILE *STREAM)

     stdio.h (Unix98):  Opening Streams.

double frexp (double VALUE, int *EXPONENT)

     math.h (ISO):  Normalization Functions.

float frexpf (float VALUE, int *EXPONENT)

     math.h (ISO):  Normalization Functions.

long double frexpl (long double VALUE, int *EXPONENT)

     math.h (ISO):  Normalization Functions.

int fscanf (FILE *STREAM, const char *TEMPLATE, ...)

     stdio.h (ISO):  Formatted Input Functions.

int fseek (FILE *STREAM, long int OFFSET, int WHENCE)

     stdio.h (ISO):  File Positioning.

int fseeko (FILE *STREAM, off_t OFFSET, int WHENCE)

     stdio.h (Unix98):  File Positioning.

int fseeko64 (FILE *STREAM, off64_t OFFSET, int WHENCE)

     stdio.h (Unix98):  File Positioning.

int __fsetlocking (FILE *STREAM, int TYPE)

     stdio_ext.h (GNU):  Streams and Threads.

int fsetpos (FILE *STREAM, const fpos_t *POSITION)

     stdio.h (ISO):  Portable Positioning.

int fsetpos64 (FILE *STREAM, const fpos64_t *POSITION)

     stdio.h (Unix98):  Portable Positioning.

long int ftell (FILE *STREAM)

     stdio.h (ISO):  File Positioning.

off_t ftello (FILE *STREAM)

     stdio.h (Unix98):  File Positioning.

off64_t ftello64 (FILE *STREAM)

     stdio.h (Unix98):  File Positioning.

int ftrylockfile (FILE *STREAM)

     stdio.h (POSIX):  Streams and Threads.

void funlockfile (FILE *STREAM)

     stdio.h (POSIX):  Streams and Threads.

int fwide (FILE *STREAM, int MODE)

     wchar.h (ISO):  Streams and I18N.

int fwprintf (FILE *STREAM, const wchar_t *TEMPLATE, ...)

     wchar.h (ISO):  Formatted Output Functions.

int __fwritable (FILE *STREAM)

     stdio_ext.h (GNU):  Opening Streams.

size_t fwrite (const void *DATA, size_t SIZE, size_t COUNT, FILE *STREAM)

     stdio.h (ISO):  Block Input/Output.

size_t fwrite_unlocked (const void *DATA, size_t SIZE, size_t COUNT, FILE *STREAM)

     stdio.h (GNU):  Block Input/Output.

int __fwriting (FILE *STREAM)

     stdio_ext.h (GNU):  Opening Streams.

int fwscanf (FILE *STREAM, const wchar_t *TEMPLATE, ...)

     wchar.h (ISO):  Formatted Input Functions.

double gamma (double X)

     math.h (SVID):  Special Functions.

float gammaf (float X)

     math.h (SVID):  Special Functions.

long double gammal (long double X)

     math.h (SVID):  Special Functions.

void (*__gconv_end_fct) (struct gconv_step *)

     gconv.h (GNU):  glibc iconv Implementation.

int (*__gconv_fct) (struct __gconv_step *, struct __gconv_step_data *, const char **, const char *, size_t *, int)

     gconv.h (GNU):  glibc iconv Implementation.

int (*__gconv_init_fct) (struct __gconv_step *)

     gconv.h (GNU):  glibc iconv Implementation.

char * gcvt (double VALUE, int NDIGIT, char *BUF)

     stdlib.h (SVID, Unix98):  System V Number Conversion.

int getc (FILE *STREAM)

     stdio.h (ISO):  Character Input.

int getchar (void)

     stdio.h (ISO):  Character Input.

int getchar_unlocked (void)

     stdio.h (POSIX):  Character Input.

int getc_unlocked (FILE *STREAM)

     stdio.h (POSIX):  Character Input.

struct tm * getdate (const char *STRING)

     time.h (Unix98):  General Time String Parsing.

getdate_err

     time.h (Unix98):  General Time String Parsing.

int getdate_r (const char *STRING, struct tm *TP)

     time.h (GNU):  General Time String Parsing.

ssize_t getdelim (char **LINEPTR, size_t *N, int DELIMITER, FILE *STREAM)

     stdio.h (GNU):  Line Input.

char * getenv (const char *NAME)

     stdlib.h (ISO):  Environment Access.

int getitimer (int WHICH, struct itimerval *OLD)

     sys/time.h (BSD):  Setting an Alarm.

ssize_t getline (char **LINEPTR, size_t *N, FILE *STREAM)

     stdio.h (GNU):  Line Input.

int getopt (int ARGC, char **ARGV, const char *OPTIONS)

     unistd.h (POSIX.2):  Using Getopt.

int getopt_long (int ARGC, char *const *ARGV, const char *SHORTOPTS, const struct option *LONGOPTS, int *INDEXPTR)

     getopt.h (GNU):  Getopt Long Options.

int getopt_long_only (int ARGC, char *const *ARGV, const char *SHORTOPTS, const struct option *LONGOPTS, int *INDEXPTR)

     getopt.h (GNU):  Getopt Long Options.

char * gets (char *S)

     stdio.h (ISO):  Line Input.

int getsubopt (char **OPTIONP, const char* const *TOKENS, char **VALUEP)

     stdlib.h (stdlib.h):  Suboptions Example.

char * gettext (const char *MSGID)

     libintl.h (GNU):  Translation with gettext.

int gettimeofday (struct timeval *TP, struct timezone *TZP)

     sys/time.h (BSD):  High-Resolution Calendar.

int getw (FILE *STREAM)

     stdio.h (SVID):  Character Input.

wint_t getwc (FILE *STREAM)

     wchar.h (ISO):  Character Input.

wint_t getwchar (void)

     wchar.h (ISO):  Character Input.

wint_t getwchar_unlocked (void)

     wchar.h (GNU):  Character Input.

wint_t getwc_unlocked (FILE *STREAM)

     wchar.h (GNU):  Character Input.

int glob (const char *PATTERN, int FLAGS, int (*ERRFUNC) (const char *FILENAME, int ERROR-CODE), glob_t *VECTOR-PTR)

     glob.h (POSIX.2):  Calling Glob.

int glob64 (const char *PATTERN, int FLAGS, int (*ERRFUNC) (const char *FILENAME, int ERROR-CODE), glob64_t *VECTOR-PTR)

     glob.h (GNU):  Calling Glob.

glob64_t

     glob.h (GNU):  Calling Glob.

GLOB_ABORTED

     glob.h (POSIX.2):  Calling Glob.

GLOB_ALTDIRFUNC

     glob.h (GNU):  More Flags for Globbing.

GLOB_APPEND

     glob.h (POSIX.2):  Flags for Globbing.

GLOB_BRACE

     glob.h (GNU):  More Flags for Globbing.

GLOB_DOOFFS

     glob.h (POSIX.2):  Flags for Globbing.

GLOB_ERR

     glob.h (POSIX.2):  Flags for Globbing.

void globfree (glob_t *PGLOB)

     glob.h (POSIX.2):  More Flags for Globbing.

void globfree64 (glob64_t *PGLOB)

     glob.h (GNU):  More Flags for Globbing.

GLOB_MAGCHAR

     glob.h (GNU):  More Flags for Globbing.

GLOB_MARK

     glob.h (POSIX.2):  Flags for Globbing.

GLOB_NOCHECK

     glob.h (POSIX.2):  Flags for Globbing.

GLOB_NOESCAPE

     glob.h (POSIX.2):  Flags for Globbing.

GLOB_NOMAGIC

     glob.h (GNU):  More Flags for Globbing.

GLOB_NOMATCH

     glob.h (POSIX.2):  Calling Glob.

GLOB_NOSORT

     glob.h (POSIX.2):  Flags for Globbing.

GLOB_NOSPACE

     glob.h (POSIX.2):  Calling Glob.

GLOB_ONLYDIR

     glob.h (GNU):  More Flags for Globbing.

GLOB_PERIOD

     glob.h (GNU):  More Flags for Globbing.

glob_t

     glob.h (POSIX.2):  Calling Glob.

GLOB_TILDE

     glob.h (GNU):  More Flags for Globbing.

GLOB_TILDE_CHECK

     glob.h (GNU):  More Flags for Globbing.

struct tm * gmtime (const time_t *TIME)

     time.h (ISO):  Broken-down Time.

struct tm * gmtime_r (const time_t *TIME, struct tm *RESULTP)

     time.h (POSIX.1c):  Broken-down Time.

_GNU_SOURCE

     (GNU):  Feature-Test Macros.

int hcreate (size_t NEL)

     search.h (SVID):  Hash Search Function.

int hcreate_r (size_t NEL, struct hsearch_data *HTAB)

     search.h (GNU):  Hash Search Function.

void hdestroy (void)

     search.h (SVID):  Hash Search Function.

void hdestroy_r (struct hsearch_data *HTAB)

     search.h (GNU):  Hash Search Function.

ENTRY * hsearch (ENTRY ITEM, ACTION ACTION)

     search.h (SVID):  Hash Search Function.

int hsearch_r (ENTRY ITEM, ACTION ACTION, ENTRY **RETVAL, struct hsearch_data *HTAB)

     search.h (GNU):  Hash Search Function.

double HUGE_VAL

     math.h (ISO):  Math Error Reporting.

float HUGE_VALF

     math.h (ISO):  Math Error Reporting.

long double HUGE_VALL

     math.h (ISO):  Math Error Reporting.

double hypot (double X, double Y)

     math.h (ISO):  Exponents and Logarithms.

float hypotf (float X, float Y)

     math.h (ISO):  Exponents and Logarithms.

long double hypotl (long double X, long double Y)

     math.h (ISO):  Exponents and Logarithms.

size_t iconv (iconv_t CD, char **INBUF, size_t *INBYTESLEFT, char **OUTBUF, size_t *OUTBYTESLEFT)

     iconv.h (XPG2):  Generic Conversion Interface.

int iconv_close (iconv_t CD)

     iconv.h (XPG2):  Generic Conversion Interface.

iconv_t iconv_open (const char *TOCODE, const char *FROMCODE)

     iconv.h (XPG2):  Generic Conversion Interface.

iconv_t

     iconv.h (XPG2):  Generic Conversion Interface.

int ilogb (double X)

     math.h (ISO):  Exponents and Logarithms.

int ilogbf (float X)

     math.h (ISO):  Exponents and Logarithms.

int ilogbl (long double X)

     math.h (ISO):  Exponents and Logarithms.

intmax_t imaxabs (intmax_t NUMBER)

     inttypes.h (ISO):  Absolute Value.

imaxdiv_t imaxdiv (intmax_t NUMERATOR, intmax_t DENOMINATOR)

     inttypes.h (ISO):  Integer Division.

imaxdiv_t

     inttypes.h (ISO):  Integer Division.

char * index (const char *STRING, int C)

     string.h (BSD):  Search Functions.

float INFINITY

     math.h (ISO):  Infinity and NaN.

void * initstate (unsigned int SEED, void *STATE, size_t SIZE)

     stdlib.h (BSD):  BSD Random.

int initstate_r (unsigned int SEED, char *restrict STATEBUF, size_t STATELEN, struct random_data *restrict BUF)

     stdlib.h (GNU):  BSD Random.

int _IOFBF

     stdio.h (ISO):  Controlling Buffering.

int _IOLBF

     stdio.h (ISO):  Controlling Buffering.

int _IONBF

     stdio.h (ISO):  Controlling Buffering.

int isalnum (int C)

     ctype.h (ISO):  Classification of Characters.

int isalpha (int C)

     ctype.h (ISO):  Classification of Characters.

int isascii (int C)

     ctype.h (SVID, BSD):  Classification of Characters.

int isblank (int C)

     ctype.h (GNU):  Classification of Characters.

int iscntrl (int C)

     ctype.h (ISO):  Classification of Characters.

int isdigit (int C)

     ctype.h (ISO):  Classification of Characters.

int isfinite (float-type X)

     math.h (ISO):  Floating-Point Classes.

int isgraph (int C)

     ctype.h (ISO):  Classification of Characters.

int isgreater (real-floating X, real-floating Y)

     math.h (ISO):  FP Comparison Functions.

int isgreaterequal (real-floating X, real-floating Y)

     math.h (ISO):  FP Comparison Functions.

int isinf (double X)

     math.h (BSD):  Floating-Point Classes.

int isinff (float X)

     math.h (BSD):  Floating-Point Classes.

int isinfl (long double X)

     math.h (BSD):  Floating-Point Classes.

int isless (real-floating X, real-floating Y)

     math.h (ISO):  FP Comparison Functions.

int islessequal (real-floating X, real-floating Y)

     math.h (ISO):  FP Comparison Functions.

int islessgreater (real-floating X, real-floating Y)

     math.h (ISO):  FP Comparison Functions.

int islower (int C)

     ctype.h (ISO):  Classification of Characters.

int isnan (double X)

     math.h (BSD):  Floating-Point Classes.

int isnan (float-type X)

     math.h (ISO):  Floating-Point Classes.

int isnanf (float X)

     math.h (BSD):  Floating-Point Classes.

int isnanl (long double X)

     math.h (BSD):  Floating-Point Classes.

int isnormal (float-type X)

     math.h (ISO):  Floating-Point Classes.

_ISOC99_SOURCE

     (GNU):  Feature-Test Macros.

int isprint (int C)

     ctype.h (ISO):  Classification of Characters.

int ispunct (int C)

     ctype.h (ISO):  Classification of Characters.

int isspace (int C)

     ctype.h (ISO):  Classification of Characters.

int isunordered (real-floating X, real-floating Y)

     math.h (ISO):  FP Comparison Functions.

int isupper (int C)

     ctype.h (ISO):  Classification of Characters.

int iswalnum (wint_t WC)

     wctype.h (ISO):  Classification of Wide Characters.

int iswalpha (wint_t WC)

     wctype.h (ISO):  Classification of Wide Characters.

int iswblank (wint_t WC)

     wctype.h (GNU):  Classification of Wide Characters.

int iswcntrl (wint_t WC)

     wctype.h (ISO):  Classification of Wide Characters.

int iswctype (wint_t WC, wctype_t DESC)

     wctype.h (ISO):  Classification of Wide Characters.

int iswdigit (wint_t WC)

     wctype.h (ISO):  Classification of Wide Characters.

int iswgraph (wint_t WC)

     wctype.h (ISO):  Classification of Wide Characters.

int iswlower (wint_t WC)

     ctype.h (ISO):  Classification of Wide Characters.

int iswprint (wint_t WC)

     wctype.h (ISO):  Classification of Wide Characters.

int iswpunct (wint_t WC)

     wctype.h (ISO):  Classification of Wide Characters.

int iswspace (wint_t WC)

     wctype.h (ISO):  Classification of Wide Characters.

int iswupper (wint_t WC)

     wctype.h (ISO):  Classification of Wide Characters.

int iswxdigit (wint_t WC)

     wctype.h (ISO):  Classification of Wide Characters.

int isxdigit (int C)

     ctype.h (ISO):  Classification of Characters.

ITIMER_PROF

     sys/time.h (BSD):  Setting an Alarm.

ITIMER_REAL

     sys/time.h (BSD):  Setting an Alarm.

ITIMER_VIRTUAL

     sys/time.h (BSD):  Setting an Alarm.

double j0 (double X)

     math.h (SVID):  Special Functions.

float j0f (float X)

     math.h (SVID):  Special Functions.

long double j0l (long double X)

     math.h (SVID):  Special Functions.

double j1 (double X)

     math.h (SVID):  Special Functions.

float j1f (float X)

     math.h (SVID):  Special Functions.

long double j1l (long double X)

     math.h (SVID):  Special Functions.

double jn (int n, double X)

     math.h (SVID):  Special Functions.

float jnf (int n, float X)

     math.h (SVID):  Special Functions.

long double jnl (int n, long double X)

     math.h (SVID):  Special Functions.

long int jrand48 (unsigned short int XSUBI[3])

     stdlib.h (SVID):  SVID Random.

int jrand48_r (unsigned short int XSUBI[3], struct drand48_data *BUFFER, long int *RESULT)

     stdlib.h (GNU):  SVID Random.

char * l64a (long int N)

     stdlib.h (XPG):  Encode Binary Data.

long int labs (long int NUMBER)

     stdlib.h (ISO):  Absolute Value.

LANG

     locale.h (ISO):  Locale Categories.

LC_ALL

     locale.h (ISO):  Locale Categories.

LC_COLLATE

     locale.h (ISO):  Locale Categories.

LC_CTYPE

     locale.h (ISO):  Locale Categories.

LC_MESSAGES

     locale.h (XOPEN):  Locale Categories.

LC_MONETARY

     locale.h (ISO):  Locale Categories.

LC_NUMERIC

     locale.h (ISO):  Locale Categories.

void lcong48 (unsigned short int PARAM[7])

     stdlib.h (SVID):  SVID Random.

int lcong48_r (unsigned short int PARAM[7], struct drand48_data *BUFFER)

     stdlib.h (GNU):  SVID Random.

LC_TIME

     locale.h (ISO):  Locale Categories.

double ldexp (double VALUE, int EXPONENT)

     math.h (ISO):  Normalization Functions.

float ldexpf (float VALUE, int EXPONENT)

     math.h (ISO):  Normalization Functions.

long double ldexpl (long double VALUE, int EXPONENT)

     math.h (ISO):  Normalization Functions.

ldiv_t ldiv (long int NUMERATOR, long int DENOMINATOR)

     stdlib.h (ISO):  Integer Division.

ldiv_t

     stdlib.h (ISO):  Integer Division.

void * lfind (const void *KEY, void *BASE, size_t *NMEMB, size_t SIZE, comparison_fn_t COMPAR)

     search.h (SVID):  Array Search Function.

double lgamma (double X)

     math.h (SVID):  Special Functions.

float lgammaf (float X)

     math.h (SVID):  Special Functions.

float lgammaf_r (float X, int *SIGNP)

     math.h (XPG):  Special Functions.

long double lgammal (long double X)

     math.h (SVID):  Special Functions.

long double lgammal_r (long double X, int *SIGNP)

     math.h (XPG):  Special Functions.

double lgamma_r (double X, int *SIGNP)

     math.h (XPG):  Special Functions.

L_INCR

     sys/file.h (BSD):  File Positioning.

long long int llabs (long long int NUMBER)

     stdlib.h (ISO):  Absolute Value.

lldiv_t lldiv (long long int NUMERATOR, long long int DENOMINATOR)

     stdlib.h (ISO):  Integer Division.

lldiv_t

     stdlib.h (ISO):  Integer Division.

long long int llrint (double X)

     math.h (ISO):  Rounding Functions.

long long int llrintf (float X)

     math.h (ISO):  Rounding Functions.

long long int llrintl (long double X)

     math.h (ISO):  Rounding Functions.

long long int llround (double X)

     math.h (ISO):  Rounding Functions.

long long int llroundf (float X)

     math.h (ISO):  Rounding Functions.

long long int llroundl (long double X)

     math.h (ISO):  Rounding Functions.

struct lconv * localeconv (void)

     locale.h (ISO):  The Lame Way to Locale Data.

struct tm * localtime (const time_t *TIME)

     time.h (ISO):  Broken-down Time.

struct tm * localtime_r (const time_t *TIME, struct tm *RESULTP)

     time.h (POSIX.1c):  Broken-down Time.

double log (double X)

     math.h (ISO):  Exponents and Logarithms.

double log10 (double X)

     math.h (ISO):  Exponents and Logarithms.

float log10f (float X)

     math.h (ISO):  Exponents and Logarithms.

long double log10l (long double X)

     math.h (ISO):  Exponents and Logarithms.

double log1p (double X)

     math.h (ISO):  Exponents and Logarithms.

float log1pf (float X)

     math.h (ISO):  Exponents and Logarithms.

long double log1pl (long double X)

     math.h (ISO):  Exponents and Logarithms.

double log2 (double X)

     math.h (ISO):  Exponents and Logarithms.

float log2f (float X)

     math.h (ISO):  Exponents and Logarithms.

long double log2l (long double X)

     math.h (ISO):  Exponents and Logarithms.

double logb (double X)

     math.h (ISO):  Exponents and Logarithms.

float logbf (float X)

     math.h (ISO):  Exponents and Logarithms.

long double logbl (long double X)

     math.h (ISO):  Exponents and Logarithms.

float logf (float X)

     math.h (ISO):  Exponents and Logarithms.

long double logl (long double X)

     math.h (ISO):  Exponents and Logarithms.

long int lrand48 (void)

     stdlib.h (SVID):  SVID Random.

int lrand48_r (struct drand48_data *BUFFER, double *RESULT)

     stdlib.h (GNU):  SVID Random.

long int lrint (double X)

     math.h (ISO):  Rounding Functions.

long int lrintf (float X)

     math.h (ISO):  Rounding Functions.

long int lrintl (long double X)

     math.h (ISO):  Rounding Functions.

long int lround (double X)

     math.h (ISO):  Rounding Functions.

long int lroundf (float X)

     math.h (ISO):  Rounding Functions.

long int lroundl (long double X)

     math.h (ISO):  Rounding Functions.

void * lsearch (const void *KEY, void *BASE, size_t *NMEMB, size_t SIZE, comparison_fn_t COMPAR)

     search.h (SVID):  Array Search Function.

L_SET

     sys/file.h (BSD):  File Positioning.

L_XTND

     sys/file.h (BSD):  File Positioning.

struct mallinfo mallinfo (void)

     malloc.h (SVID):  Statistics of Malloc.

void * malloc (size_t SIZE)

     malloc.h, stdlib.h (ISO):  Basic Allocation.

__malloc_hook

     malloc.h (GNU):  Hooks for Malloc.

__malloc_initialize_hook

     malloc.h (GNU):  Hooks for Malloc.

int MB_CUR_MAX

     stdlib.h (ISO):  Selecting the Conversion.

int mblen (const char *STRING, size_t SIZE)

     stdlib.h (ISO):  Nonreentrant Character Conversion.

int MB_LEN_MAX

     limits.h (ISO):  Selecting the Conversion.

size_t mbrlen (const char *restrict S, size_t N, mbstate_t *PS)

     wchar.h (ISO):  Converting a Character.

size_t mbrtowc (wchar_t *restrict PWC, const char *restrict S, size_t N, mbstate_t *restrict PS)

     wchar.h (ISO):  Converting a Character.

int mbsinit (const mbstate_t *PS)

     wchar.h (ISO):  Keeping the state.

size_t mbsnrtowcs (wchar_t *restrict DST, const char **restrict SRC, size_t NMC, size_t LEN, mbstate_t *restrict PS)

     wchar.h (GNU):  Converting Strings.

size_t mbsrtowcs (wchar_t *restrict DST, const char **restrict SRC, size_t LEN, mbstate_t *restrict PS)

     wchar.h (ISO):  Converting Strings.

mbstate_t

     wchar.h (ISO):  Keeping the state.

size_t mbstowcs (wchar_t *WSTRING, const char *STRING, size_t SIZE)

     stdlib.h (ISO):  Nonreentrant String Conversion.

int mbtowc (wchar_t *restrict RESULT, const char *restrict STRING, size_t SIZE)

     stdlib.h (ISO):  Nonreentrant Character Conversion.

int mcheck (void (*ABORTFN) (enum mcheck_status STATUS))

     mcheck.h (GNU):  Heap Consistency Checking.

void * memalign (size_t BOUNDARY, size_t SIZE)

     malloc.h (BSD):  Aligned Memory Blocks.

__memalign_hook

     malloc.h (GNU):  Hooks for Malloc.

void * memccpy (void *restrict TO, const void *restrict FROM, int C, size_t SIZE)

     string.h (SVID):  Copying and Concatenation.

void * memchr (const void *BLOCK, int C, size_t SIZE)

     string.h (ISO):  Search Functions.

int memcmp (const void *A1, const void *A2, size_t SIZE)

     string.h (ISO):  String/Array Comparison.

void * memcpy (void *restrict TO, const void *restrict FROM, size_t SIZE)

     string.h (ISO):  Copying and Concatenation.

void * memfrob (void *MEM, size_t LENGTH)

     string.h (GNU):  Trivial Encryption.

void * memmem (const void *HAYSTACK, size_t HAYSTACK-LEN,
const void *NEEDLE, size_t NEEDLE-LEN)

     string.h (GNU):  Search Functions.

void * memmove (void *TO, const void *FROM, size_t SIZE)

     string.h (ISO):  Copying and Concatenation.

void * mempcpy (void *restrict TO, const void *restrict FROM, size_t SIZE)

     string.h (GNU):  Copying and Concatenation.

void * memrchr (const void *BLOCK, int C, size_t SIZE)

     string.h (GNU):  Search Functions.

void * memset (void *BLOCK, int C, size_t SIZE)

     string.h (ISO):  Copying and Concatenation.

time_t mktime (struct tm *BROKENTIME)

     time.h (ISO):  Broken-down Time.

int mlock (const void *ADDR, size_t LEN)

     sys/mman.h (POSIX.1b):  Page Lock Functions.

int mlockall (int FLAGS)

     sys/mman.h (POSIX.1b):  Page Lock Functions.

double modf (double VALUE, double *INTEGER-PART)

     math.h (ISO):  Rounding Functions.

float modff (float VALUE, float *INTEGER-PART)

     math.h (ISO):  Rounding Functions.

long double modfl (long double VALUE, long double *INTEGER-PART)

     math.h (ISO):  Rounding Functions.

long int mrand48 (void)

     stdlib.h (SVID):  SVID Random.

int mrand48_r (struct drand48_data *BUFFER, double *RESULT)

     stdlib.h (GNU):  SVID Random.

void mtrace (void)

     mcheck.h (GNU):  Tracing malloc.

int munlock (const void *ADDR, size_t LEN)

     sys/mman.h (POSIX.1b):  Page Lock Functions.

int munlockall (void)

     sys/mman.h (POSIX.1b):  Page Lock Functions.

void muntrace (void)

     mcheck.h (GNU):  Tracing malloc.

double nan (const char *TAGP)

     math.h (ISO):  FP Bit Twiddling.

float NAN

     math.h (GNU):  Infinity and NaN.

float nanf (const char *TAGP)

     math.h (ISO):  FP Bit Twiddling.

long double nanl (const char *TAGP)

     math.h (ISO):  FP Bit Twiddling.

int nanosleep (const struct timespec *REQUESTED_TIME, struct timespec *REMAINING)

     time.h (POSIX.1):  Sleeping.

double nearbyint (double X)

     math.h (ISO):  Rounding Functions.

float nearbyintf (float X)

     math.h (ISO):  Rounding Functions.

long double nearbyintl (long double X)

     math.h (ISO):  Rounding Functions.

double nextafter (double X, double Y)

     math.h (ISO):  FP Bit Twiddling.

float nextafterf (float X, float Y)

     math.h (ISO):  FP Bit Twiddling.

long double nextafterl (long double X, long double Y)

     math.h (ISO):  FP Bit Twiddling.

double nexttoward (double X, long double Y)

     math.h (ISO):  FP Bit Twiddling.

float nexttowardf (float X, long double Y)

     math.h (ISO):  FP Bit Twiddling.

long double nexttowardl (long double X, long double Y)

     math.h (ISO):  FP Bit Twiddling.

char * ngettext (const char *MSGID1, const char *MSGID2, unsigned long int N)

     libintl.h (GNU):  Advanced gettext functions.

char * nl_langinfo (nl_item ITEM)

     langinfo.h (XOPEN):  The Elegant and Fast Way.

long int nrand48 (unsigned short int XSUBI[3])

     stdlib.h (SVID):  SVID Random.

int nrand48_r (unsigned short int XSUBI[3], struct drand48_data *BUFFER, long int *RESULT)

     stdlib.h (GNU):  SVID Random.

int ntp_adjtime (struct timex *TPTR)

     sys/timex.h (GNU):  High-Accuracy Clock.

int ntp_gettime (struct ntptimeval *TPTR)

     sys/timex.h (GNU):  High-Accuracy Clock.

void obstack_1grow (struct obstack *OBSTACK-PTR, char C)

     obstack.h (GNU):  Growing Objects.

void obstack_1grow_fast (struct obstack *OBSTACK-PTR, char C)

     obstack.h (GNU):  Extra-Fast Growing.

int obstack_alignment_mask (struct obstack *OBSTACK-PTR)

     obstack.h (GNU):  Obstacks Data Alignment.

void * obstack_alloc (struct obstack *OBSTACK-PTR, int SIZE)

     obstack.h (GNU):  Allocation in an Obstack.

obstack_alloc_failed_handler

     obstack.h (GNU):  Preparing for Obstacks.

void * obstack_base (struct obstack *OBSTACK-PTR)

     obstack.h (GNU):  Status of an Obstack.

void obstack_blank (struct obstack *OBSTACK-PTR, int SIZE)

     obstack.h (GNU):  Growing Objects.

void obstack_blank_fast (struct obstack *OBSTACK-PTR, int SIZE)

     obstack.h (GNU):  Extra-Fast Growing.

int obstack_chunk_size (struct obstack *OBSTACK-PTR)

     obstack.h (GNU):  Obstack Chunks.

void * obstack_copy (struct obstack *OBSTACK-PTR, void *ADDRESS, int SIZE)

     obstack.h (GNU):  Allocation in an Obstack.

void * obstack_copy0 (struct obstack *OBSTACK-PTR, void *ADDRESS, int SIZE)

     obstack.h (GNU):  Allocation in an Obstack.

void * obstack_finish (struct obstack *OBSTACK-PTR)

     obstack.h (GNU):  Growing Objects.

void obstack_free (struct obstack *OBSTACK-PTR, void *OBJECT)

     obstack.h (GNU):  Freeing Obstack Objects.

void obstack_grow (struct obstack *OBSTACK-PTR, void *DATA, int SIZE)

     obstack.h (GNU):  Growing Objects.

void obstack_grow0 (struct obstack *OBSTACK-PTR, void *DATA, int SIZE)

     obstack.h (GNU):  Growing Objects.

int obstack_init (struct obstack *OBSTACK-PTR)

     obstack.h (GNU):  Preparing for Obstacks.

void obstack_int_grow (struct obstack *OBSTACK-PTR, int DATA)

     obstack.h (GNU):  Growing Objects.

void obstack_int_grow_fast (struct obstack *OBSTACK-PTR, int DATA)

     obstack.h (GNU):  Extra-Fast Growing.

void * obstack_next_free (struct obstack *OBSTACK-PTR)

     obstack.h (GNU):  Status of an Obstack.

int obstack_object_size (struct obstack *OBSTACK-PTR)

     obstack.h (GNU):  Growing Objects.

int obstack_object_size (struct obstack *OBSTACK-PTR)

     obstack.h (GNU):  Status of an Obstack.

int obstack_printf (struct obstack *OBSTACK, const char *TEMPLATE, ...)

     stdio.h (GNU):  Dynamic Output.

void obstack_ptr_grow (struct obstack *OBSTACK-PTR, void *DATA)

     obstack.h (GNU):  Growing Objects.

void obstack_ptr_grow_fast (struct obstack *OBSTACK-PTR, void *DATA)

     obstack.h (GNU):  Extra-Fast Growing.

int obstack_room (struct obstack *OBSTACK-PTR)

     obstack.h (GNU):  Extra-Fast Growing.

int obstack_vprintf (struct obstack *OBSTACK, const char *TEMPLATE, va_list AP)

     stdio.h (GNU):  Variable Arguments Output.

int on_exit (void (*FUNCTION)(int STATUS, void *ARG), void *ARG)

     stdlib.h (SunOS):  Clean-Ups on Exit.

FILE * open_memstream (char **PTR, size_t *SIZELOC)

     stdio.h (GNU):  String Streams.

FILE * open_obstack_stream (struct obstack *OBSTACK)

     stdio.h (GNU):  Obstack Streams.

char * optarg

     unistd.h (POSIX.2):  Using Getopt.

int opterr

     unistd.h (POSIX.2):  Using Getopt.

int optind

     unistd.h (POSIX.2):  Using Getopt.

OPTION_ALIAS

     argp.h (GNU):  Argp Option Flags.

OPTION_ARG_OPTIONAL

     argp.h (GNU):  Argp Option Flags.

OPTION_DOC

     argp.h (GNU):  Argp Option Flags.

OPTION_HIDDEN

     argp.h (GNU):  Argp Option Flags.

OPTION_NO_USAGE

     argp.h (GNU):  Argp Option Flags.

int optopt

     unistd.h (POSIX.2):  Using Getopt.

PA_CHAR

     printf.h (GNU):  Parsing a Template String.

PA_DOUBLE

     printf.h (GNU):  Parsing a Template String.

PA_FLAG_LONG

     printf.h (GNU):  Parsing a Template String.

PA_FLAG_LONG_DOUBLE

     printf.h (GNU):  Parsing a Template String.

PA_FLAG_LONG_LONG

     printf.h (GNU):  Parsing a Template String.

int PA_FLAG_MASK

     printf.h (GNU):  Parsing a Template String.

PA_FLAG_PTR

     printf.h (GNU):  Parsing a Template String.

PA_FLAG_SHORT

     printf.h (GNU):  Parsing a Template String.

PA_FLOAT

     printf.h (GNU):  Parsing a Template String.

PA_INT

     printf.h (GNU):  Parsing a Template String.

PA_LAST

     printf.h (GNU):  Parsing a Template String.

PA_POINTER

     printf.h (GNU):  Parsing a Template String.

size_t parse_printf_format (const char *TEMPLATE, size_t N, int *ARGTYPES)

     printf.h (GNU):  Parsing a Template String.

PA_STRING

     printf.h (GNU):  Parsing a Template String.

void perror (const char *MESSAGE)

     stdio.h (ISO):  Error Messages.

_POSIX_C_SOURCE

     (POSIX.2):  Feature-Test Macros.

int posix_memalign (void **MEMPTR, size_t ALIGNMENT, size_t SIZE)

     stdlib.h (POSIX):  Aligned Memory Blocks.

_POSIX_SOURCE

     (POSIX.1):  Feature-Test Macros.

double pow (double BASE, double POWER)

     math.h (ISO):  Exponents and Logarithms.

double pow10 (double X)

     math.h (GNU):  Exponents and Logarithms.

float pow10f (float X)

     math.h (GNU):  Exponents and Logarithms.

long double pow10l (long double X)

     math.h (GNU):  Exponents and Logarithms.

float powf (float BASE, float POWER)

     math.h (ISO):  Exponents and Logarithms.

long double powl (long double BASE, long double POWER)

     math.h (ISO):  Exponents and Logarithms.

int printf (const char *TEMPLATE, ...)

     stdio.h (ISO):  Formatted Output Functions.

printf_arginfo_function

     printf.h (GNU):  Defining the Output Handler.

printf_function

     printf.h (GNU):  Defining the Output Handler.

int printf_size (FILE *FP, const struct printf_info *INFO, const void *const *ARGS)

     printf.h (GNU):  Predefined Printf Handlers.

int printf_size_info (const struct printf_info *INFO, size_t N, int *ARGTYPES)

     printf.h (GNU):  Predefined Printf Handlers.

char * program_invocation_name

     errno.h (GNU):  Error Messages.

char * program_invocation_short_name

     errno.h (GNU):  Error Messages.

int putc (int C, FILE *STREAM)

     stdio.h (ISO):  Simple Output.

int putchar (int C)

     stdio.h (ISO):  Simple Output.

int putchar_unlocked (int C)

     stdio.h (POSIX):  Simple Output.

int putc_unlocked (int C, FILE *STREAM)

     stdio.h (POSIX):  Simple Output.

int putenv (char *STRING)

     stdlib.h (SVID):  Environment Access.

int puts (const char *S)

     stdio.h (ISO):  Simple Output.

int putw (int W, FILE *STREAM)

     stdio.h (SVID):  Simple Output.

wint_t putwc (wchar_t WC, FILE *STREAM)

     wchar.h (ISO):  Simple Output.

wint_t putwchar (wchar_t WC)

     wchar.h (ISO):  Simple Output.

wint_t putwchar_unlocked (wchar_t WC)

     wchar.h (GNU):  Simple Output.

wint_t putwc_unlocked (wchar_t WC, FILE *STREAM)

     wchar.h (GNU):  Simple Output.

char * qecvt (long double VALUE, int NDIGIT, int *DECPT, int *NEG)

     stdlib.h (GNU):  System V Number Conversion.

char * qecvt_r (long double VALUE, int NDIGIT, int *DECPT, int *NEG, char *BUF, size_t LEN)

     stdlib.h (GNU):  System V Number Conversion.

char * qfcvt (long double VALUE, int NDIGIT, int *DECPT, int *NEG)

     stdlib.h (GNU):  System V Number Conversion.

char * qfcvt_r (long double VALUE, int NDIGIT, int *DECPT, int *NEG, char *BUF, size_t LEN)

     stdlib.h (GNU):  System V Number Conversion.

char * qgcvt (long double VALUE, int NDIGIT, char *BUF)

     stdlib.h (GNU):  System V Number Conversion.

void qsort (void *ARRAY, size_t COUNT, size_t SIZE, comparison_fn_t COMPARE)

     stdlib.h (ISO):  Array Sort Function.

int rand (void)

     stdlib.h (ISO):  ISO Random.

int RAND_MAX

     stdlib.h (ISO):  ISO Random.

long int random (void)

     stdlib.h (BSD):  BSD Random.

int random_r (struct random_data *restrict BUF, int32_t *restrict RESULT)

     stdlib.h (GNU):  BSD Random.

int rand_r (unsigned int *SEED)

     stdlib.h (POSIX.1):  ISO Random.

void * rawmemchr (const void *BLOCK, int C)

     string.h (GNU):  Search Functions.

void * realloc (void *PTR, size_t NEWSIZE)

     malloc.h, stdlib.h (ISO):  Changing Block Size.

__realloc_hook

     malloc.h (GNU):  Hooks for Malloc.

_REENTRANT

     (GNU):  Feature-Test Macros.

REG_BADBR

     regex.h (POSIX.2):  POSIX Regexp Compilation.

REG_BADPAT

     regex.h (POSIX.2):  POSIX Regexp Compilation.

REG_BADRPT

     regex.h (POSIX.2):  POSIX Regexp Compilation.

int regcomp (regex_t *COMPILED, const char *PATTERN, int CFLAGS)

     regex.h (POSIX.2):  POSIX Regexp Compilation.

REG_EBRACE

     regex.h (POSIX.2):  POSIX Regexp Compilation.

REG_EBRACK

     regex.h (POSIX.2):  POSIX Regexp Compilation.

REG_ECOLLATE

     regex.h (POSIX.2):  POSIX Regexp Compilation.

REG_ECTYPE

     regex.h (POSIX.2):  POSIX Regexp Compilation.

REG_EESCAPE

     regex.h (POSIX.2):  POSIX Regexp Compilation.

REG_EPAREN

     regex.h (POSIX.2):  POSIX Regexp Compilation.

REG_ERANGE

     regex.h (POSIX.2):  POSIX Regexp Compilation.

size_t regerror (int ERRCODE, regex_t *COMPILED, char *BUFFER, size_t LENGTH)

     regex.h (POSIX.2):  Regexp Clean-Up.

REG_ESPACE

     regex.h (POSIX.2):  Matching POSIX Regexps.

REG_ESPACE

     regex.h (POSIX.2):  POSIX Regexp Compilation.

REG_ESUBREG

     regex.h (POSIX.2):  POSIX Regexp Compilation.

int regexec (regex_t *COMPILED, char *STRING, size_t NMATCH, regmatch_t MATCHPTR [], int EFLAGS)

     regex.h (POSIX.2):  Matching POSIX Regexps.

regex_t

     regex.h (POSIX.2):  POSIX Regexp Compilation.

REG_EXTENDED

     regex.h (POSIX.2):  Flags for POSIX Regexps.

void regfree (regex_t *COMPILED)

     regex.h (POSIX.2):  Regexp Clean-Up.

REG_ICASE

     regex.h (POSIX.2):  Flags for POSIX Regexps.

int register_printf_function (int SPEC, printf_function HANDLER-FUNCTION, printf_arginfo_function ARGINFO-FUNCTION)

     printf.h (GNU):  Registering New Conversions.

regmatch_t

     regex.h (POSIX.2):  Regexp Subexpressions.

REG_NEWLINE

     regex.h (POSIX.2):  Flags for POSIX Regexps.

REG_NOMATCH

     regex.h (POSIX.2):  Matching POSIX Regexps.

REG_NOSUB

     regex.h (POSIX.2):  Flags for POSIX Regexps.

REG_NOTBOL

     regex.h (POSIX.2):  Matching POSIX Regexps.

REG_NOTEOL

     regex.h (POSIX.2):  Matching POSIX Regexps.

regoff_t

     regex.h (POSIX.2):  Regexp Subexpressions.

double remainder (double NUMERATOR, double DENOMINATOR)

     math.h (BSD):  Remainder Functions.

float remainderf (float NUMERATOR, float DENOMINATOR)

     math.h (BSD):  Remainder Functions.

long double remainderl (long double NUMERATOR, long double DENOMINATOR)

     math.h (BSD):  Remainder Functions.

void rewind (FILE *STREAM)

     stdio.h (ISO):  File Positioning.

char * rindex (const char *STRING, int C)

     string.h (BSD):  Search Functions.

double rint (double X)

     math.h (ISO):  Rounding Functions.

float rintf (float X)

     math.h (ISO):  Rounding Functions.

long double rintl (long double X)

     math.h (ISO):  Rounding Functions.

double round (double X)

     math.h (ISO):  Rounding Functions.

float roundf (float X)

     math.h (ISO):  Rounding Functions.

long double roundl (long double X)

     math.h (ISO):  Rounding Functions.

int rpmatch (const char *RESPONSE)

     stdlib.h (stdlib.h):  Yes-or-No Questions.

int sbrk (ptrdiff_t DELTA)

     unistd.h (BSD):  Resizing the Data Segment.

double scalb (double VALUE, int EXPONENT)

     math.h (BSD):  Normalization Functions.

float scalbf (float VALUE, int EXPONENT)

     math.h (BSD):  Normalization Functions.

long double scalbl (long double VALUE, int EXPONENT)

     math.h (BSD):  Normalization Functions.

long long int scalbln (double X, long int n)

     math.h (BSD):  Normalization Functions.

long long int scalblnf (float X, long int n)

     math.h (BSD):  Normalization Functions.

long long int scalblnl (long double X, long int n)

     math.h (BSD):  Normalization Functions.

long long int scalbn (double X, int n)

     math.h (BSD):  Normalization Functions.

long long int scalbnf (float X, int n)

     math.h (BSD):  Normalization Functions.

long long int scalbnl (long double X, int n)

     math.h (BSD):  Normalization Functions.

int scanf (const char *TEMPLATE, ...)

     stdio.h (ISO):  Formatted Input Functions.

unsigned short int * seed48 (unsigned short int SEED16V[3])

     stdlib.h (SVID):  SVID Random.

int seed48_r (unsigned short int SEED16V[3], struct drand48_data *BUFFER)

     stdlib.h (GNU):  SVID Random.

int SEEK_CUR

     stdio.h (ISO):  File Positioning.

int SEEK_END

     stdio.h (ISO):  File Positioning.

int SEEK_SET

     stdio.h (ISO):  File Positioning.

void setbuf (FILE *STREAM, char *BUF)

     stdio.h (ISO):  Controlling Buffering.

void setbuffer (FILE *STREAM, char *BUF, size_t SIZE)

     stdio.h (BSD):  Controlling Buffering.

int setenv (const char *NAME, const char *VALUE, int REPLACE)

     stdlib.h (BSD):  Environment Access.

int setitimer (int WHICH, struct itimerval *NEW, struct itimerval *OLD)

     sys/time.h (BSD):  Setting an Alarm.

void setlinebuf (FILE *STREAM)

     stdio.h (BSD):  Controlling Buffering.

char * setlocale (int CATEGORY, const char *LOCALE)

     locale.h (ISO):  Setting the Locale.

void * setstate (void *STATE)

     stdlib.h (BSD):  BSD Random.

int setstate_r (char *restrict STATEBUF, struct random_data *restrict BUF)

     stdlib.h (GNU):  BSD Random.

int settimeofday (const struct timeval *TP, const struct timezone *TZP)

     sys/time.h (BSD):  High-Resolution Calendar.

int setvbuf (FILE *STREAM, char *BUF, int MODE, size_t SIZE)

     stdio.h (ISO):  Controlling Buffering.

int signbit (float-type X)

     math.h (ISO):  FP Bit Twiddling.

long long int significand (double X)

     math.h (BSD):  Normalization Functions.

long long int significandf (float X)

     math.h (BSD):  Normalization Functions.

long long int significandl (long double X)

     math.h (BSD):  Normalization Functions.

double sin (double X)

     math.h (ISO):  Trig Functions.

void sincos (double X, double *SINX, double *COSX)

     math.h (GNU):  Trig Functions.

void sincosf (float X, float *SINX, float *COSX)

     math.h (GNU):  Trig Functions.

void sincosl (long double X, long double *SINX, long double *COSX)

     math.h (GNU):  Trig Functions.

float sinf (float X)

     math.h (ISO):  Trig Functions.

double sinh (double X)

     math.h (ISO):  Hyperbolic Functions.

float sinhf (float X)

     math.h (ISO):  Hyperbolic Functions.

long double sinhl (long double X)

     math.h (ISO):  Hyperbolic Functions.

long double sinl (long double X)

     math.h (ISO):  Trig Functions.

unsigned int sleep (unsigned int SECONDS)

     unistd.h (POSIX.1):  Sleeping.

int snprintf (char *S, size_t SIZE, const char *TEMPLATE, ...)

     stdio.h (GNU):  Formatted Output Functions.

int sprintf (char *S, const char *TEMPLATE, ...)

     stdio.h (ISO):  Formatted Output Functions.

double sqrt (double X)

     math.h (ISO):  Exponents and Logarithms.

float sqrtf (float X)

     math.h (ISO):  Exponents and Logarithms.

long double sqrtl (long double X)

     math.h (ISO):  Exponents and Logarithms.

void srand (unsigned int SEED)

     stdlib.h (ISO):  ISO Random.

void srand48 (long int SEEDVAL)

     stdlib.h (SVID):  SVID Random.

int srand48_r (long int SEEDVAL, struct drand48_data *BUFFER)

     stdlib.h (GNU):  SVID Random.

void srandom (unsigned int SEED)

     stdlib.h (BSD):  BSD Random.

int srandom_r (unsigned int SEED, struct random_data *BUF)

     stdlib.h (GNU):  BSD Random.

int sscanf (const char *S, const char *TEMPLATE, ...)

     stdio.h (ISO):  Formatted Input Functions.

FILE * stderr

     stdio.h (ISO):  Standard Streams.

FILE * stdin

     stdio.h (ISO):  Standard Streams.

FILE * stdout

     stdio.h (ISO):  Standard Streams.

int stime (time_t *NEWTIME)

     time.h (SVID, XPG):  Simple Calendar Time.

char * stpcpy (char *restrict TO, const char *restrict FROM)

     string.h (Unknown origin):  Copying and Concatenation.

char * stpncpy (char *restrict TO, const char *restrict FROM, size_t SIZE)

     string.h (GNU):  Copying and Concatenation.

int strcasecmp (const char *S1, const char *S2)

     string.h (BSD):  String/Array Comparison.

char * strcasestr (const char *HAYSTACK, const char *NEEDLE)

     string.h (GNU):  Search Functions.

char * strcat (char *restrict TO, const char *restrict FROM)

     string.h (ISO):  Copying and Concatenation.

char * strchr (const char *STRING, int C)

     string.h (ISO):  Search Functions.

char * strchrnul (const char *STRING, int C)

     string.h (GNU):  Search Functions.

int strcmp (const char *S1, const char *S2)

     string.h (ISO):  String/Array Comparison.

int strcoll (const char *S1, const char *S2)

     string.h (ISO):  Collation Functions.

char * strcpy (char *restrict TO, const char *restrict FROM)

     string.h (ISO):  Copying and Concatenation.

size_t strcspn (const char *STRING, const char *STOPSET)

     string.h (ISO):  Search Functions.

char * strdup (const char *S)

     string.h (SVID):  Copying and Concatenation.

char * strdupa (const char *S)

     string.h (GNU):  Copying and Concatenation.

char * strerror (int ERRNUM)

     string.h (ISO):  Error Messages.

char * strerror_r (int ERRNUM, char *BUF, size_t N)

     string.h (GNU):  Error Messages.

char * strfry (char *STRING)

     string.h (GNU):  strfry.

size_t strftime (char *S, size_t SIZE, const char *TEMPLATE, const struct tm *BROKENTIME)

     time.h (ISO):  Formatting Calendar Time.

size_t strlen (const char *S)

     string.h (ISO):  String Length.

int strncasecmp (const char *S1, const char *S2, size_t N)

     string.h (BSD):  String/Array Comparison.

char * strncat (char *restrict TO, const char *restrict FROM, size_t SIZE)

     string.h (ISO):  Copying and Concatenation.

int strncmp (const char *S1, const char *S2, size_t SIZE)

     string.h (ISO):  String/Array Comparison.

char * strncpy (char *restrict TO, const char *restrict FROM, size_t SIZE)

     string.h (ISO):  Copying and Concatenation.

char * strndup (const char *S, size_t SIZE)

     string.h (GNU):  Copying and Concatenation.

char * strndupa (const char *S, size_t SIZE)

     string.h (GNU):  Copying and Concatenation.

size_t strnlen (const char *S, size_t MAXLEN)

     string.h (GNU):  String Length.

char * strpbrk (const char *STRING, const char *STOPSET)

     string.h (ISO):  Search Functions.

char * strptime (const char *S, const char *FMT, struct tm *TP)

     time.h (XPG4):  Low-Level Time String Parsing.

char * strrchr (const char *STRING, int C)

     string.h (ISO):  Search Functions.

char * strsep (char **STRING_PTR, const char *DELIMITER)

     string.h (BSD):  Finding Tokens in a String.

size_t strspn (const char *STRING, const char *SKIPSET)

     string.h (ISO):  Search Functions.

char * strstr (const char *HAYSTACK, const char *NEEDLE)

     string.h (ISO):  Search Functions.

double strtod (const char *restrict STRING, char **restrict TAILPTR)

     stdlib.h (ISO):  Parsing of Floats.

float strtof (const char *STRING, char **TAILPTR)

     stdlib.h (ISO):  Parsing of Floats.

intmax_t strtoimax (const char *restrict STRING, char **restrict TAILPTR, int BASE)

     inttypes.h (ISO):  Parsing of Integers.

char * strtok (char *restrict NEWSTRING, const char *restrict DELIMITERS)

     string.h (ISO):  Finding Tokens in a String.

char * strtok_r (char *NEWSTRING, const char *DELIMITERS, char **SAVE_PTR)

     string.h (POSIX):  Finding Tokens in a String.

long int strtol (const char *restrict STRING, char **restrict TAILPTR, int BASE)

     stdlib.h (ISO):  Parsing of Integers.

long double strtold (const char *STRING, char **TAILPTR)

     stdlib.h (ISO):  Parsing of Floats.

long long int strtoll (const char *restrict STRING, char **restrict TAILPTR, int BASE)

     stdlib.h (ISO):  Parsing of Integers.

long long int strtoq (const char *restrict STRING, char **restrict TAILPTR, int BASE)

     stdlib.h (BSD):  Parsing of Integers.

unsigned long int strtoul (const char *retrict STRING, char **restrict TAILPTR, int BASE)

     stdlib.h (ISO):  Parsing of Integers.

unsigned long long int strtoull (const char *restrict STRING, char **restrict TAILPTR, int BASE)

     stdlib.h (ISO):  Parsing of Integers.

uintmax_t strtoumax (const char *restrict STRING, char **restrict TAILPTR, int BASE)

     inttypes.h (ISO):  Parsing of Integers.

unsigned long long int strtouq (const char *restrict STRING, char **restrict TAILPTR, int BASE)

     stdlib.h (BSD):  Parsing of Integers.

struct argp

     argp.h (GNU):  Argp Parsers.

struct argp_child

     argp.h (GNU):  Argp Children.

struct argp_option

     argp.h (GNU):  Argp Option Vectors.

struct argp_state

     argp.h (GNU):  Argp Parsing State.

struct __gconv_step

     gconv.h (GNU):  glibc iconv Implementation.

struct __gconv_step_data

     gconv.h (GNU):  glibc iconv Implementation.

struct itimerval

     sys/time.h (BSD):  Setting an Alarm.

struct lconv

     locale.h (ISO):  The Lame Way to Locale Data.

struct mallinfo

     malloc.h (GNU):  Statistics of Malloc.

struct obstack

     obstack.h (GNU):  Creating Obstacks.

struct option

     getopt.h (GNU):  Getopt Long Options.

struct printf_info

     printf.h (GNU):  Conversion Specifier Options.

struct random_data

     stdlib.h (GNU):  BSD Random.

struct timespec

     sys/time.h (POSIX.1):  Elapsed Time.

struct timeval

     sys/time.h (BSD):  Elapsed Time.

struct timezone

     sys/time.h (BSD):  High-Resolution Calendar.

struct tm

     time.h (ISO):  Broken-down Time.

struct tms

     sys/times.h (POSIX.1):  Processor Time.

int strverscmp (const char *S1, const char *S2)

     string.h (GNU):  String/Array Comparison.

size_t strxfrm (char *restrict TO, const char *restrict FROM, size_t SIZE)

     string.h (ISO):  Collation Functions.

_SVID_SOURCE

     (GNU):  Feature-Test Macros.

int swprintf (wchar_t *S, size_t SIZE, const wchar_t *TEMPLATE, ...)

     wchar.h (GNU):  Formatted Output Functions.

int swscanf (const wchar_t *WS, const char *TEMPLATE, ...)

     wchar.h (ISO):  Formatted Input Functions.

long int syscall (long int SYSNO, ...)

     unistd.h (Undocumented):  System Calls.

double tan (double X)

     math.h (ISO):  Trig Functions.

float tanf (float X)

     math.h (ISO):  Trig Functions.

double tanh (double X)

     math.h (ISO):  Hyperbolic Functions.

float tanhf (float X)

     math.h (ISO):  Hyperbolic Functions.

long double tanhl (long double X)

     math.h (ISO):  Hyperbolic Functions.

long double tanl (long double X)

     math.h (ISO):  Trig Functions.

void * tdelete (const void *KEY, void **ROOTP, comparison_fn_t COMPAR)

     search.h (SVID):  Tree Search Function.

void tdestroy (void *VROOT, __free_fn_t FREEFCT)

     search.h (GNU):  Tree Search Function.

char * textdomain (const char *DOMAINNAME)

     libintl.h (GNU):  Locating gettext catalog.

void * tfind (const void *KEY, void *const *ROOTP, comparison_fn_t COMPAR)

     search.h (SVID):  Tree Search Function.

double tgamma (double X)

     math.h (XPG, ISO):  Special Functions.

float tgammaf (float X)

     math.h (XPG, ISO):  Special Functions.

long double tgammal (long double X)

     math.h (XPG, ISO):  Special Functions.

time_t time (time_t *RESULT)

     time.h (ISO):  Simple Calendar Time.

time_t timegm (struct tm *BROKENTIME)

     time.h (Undocumented):  Broken-down Time.

time_t timelocal (struct tm *BROKENTIME)

     time.h (Undocumented):  Broken-down Time.

clock_t times (struct tms *BUFFER)

     sys/times.h (POSIX.1):  Processor Time.

time_t

     time.h (ISO):  Simple Calendar Time.

long int timezone

     time.h (SVID):  Time Zone Functions.

int toascii (int C)

     ctype.h (SVID, BSD):  Case Conversion.

int tolower (int C)

     ctype.h (ISO):  Case Conversion.

int _tolower (int C)

     ctype.h (SVID):  Case Conversion.

int toupper (int C)

     ctype.h (ISO):  Case Conversion.

int _toupper (int C)

     ctype.h (SVID):  Case Conversion.

wint_t towctrans (wint_t WC, wctrans_t DESC)

     wctype.h (ISO):  Wide-Character Case Conversion.

wint_t towlower (wint_t WC)

     wctype.h (ISO):  Wide-Character Case Conversion.

wint_t towupper (wint_t WC)

     wctype.h (ISO):  Wide-Character Case Conversion.

double trunc (double X)

     math.h (ISO):  Rounding Functions.

float truncf (float X)

     math.h (ISO):  Rounding Functions.

long double truncl (long double X)

     math.h (ISO):  Rounding Functions.

void * tsearch (const void *KEY, void **ROOTP, comparison_fn_t COMPAR)

     search.h (SVID):  Tree Search Function.

void twalk (const void *ROOT, __action_fn_t ACTION)

     search.h (SVID):  Tree Search Function.

char * tzname [2]

     time.h (POSIX.1):  Time Zone Functions.

void tzset (void)

     time.h (POSIX.1):  Time Zone Functions.

int ungetc (int C, FILE *STREAM)

     stdio.h (ISO):  How Unread.

wint_t ungetwc (wint_t WC, FILE *STREAM)

     wchar.h (ISO):  How Unread.

int unsetenv (const char *NAME)

     stdlib.h (BSD):  Environment Access.

void * valloc (size_t SIZE)

     malloc.h, stdlib.h (BSD):  Aligned Memory Blocks.

int vasprintf (char **PTR, const char *TEMPLATE, va_list AP)

     stdio.h (GNU):  Variable Arguments Output.

void verr (int STATUS, const char *FORMAT, va_list)

     err.h (BSD):  Error Messages.

void verrx (int STATUS, const char *FORMAT, va_list)

     err.h (BSD):  Error Messages.

int vfprintf (FILE *STREAM, const char *TEMPLATE, va_list AP)

     stdio.h (ISO):  Variable Arguments Output.

int vfscanf (FILE *STREAM, const char *TEMPLATE, va_list AP)

     stdio.h (ISO):  Variable Arguments Input.

int vfwprintf (FILE *STREAM, const wchar_t *TEMPLATE, va_list AP)

     wchar.h (ISO):  Variable Arguments Output.

int vfwscanf (FILE *STREAM, const wchar_t *TEMPLATE, va_list AP)

     wchar.h (ISO):  Variable Arguments Input.

int vprintf (const char *TEMPLATE, va_list AP)

     stdio.h (ISO):  Variable Arguments Output.

int vscanf (const char *TEMPLATE, va_list AP)

     stdio.h (ISO):  Variable Arguments Input.

int vsnprintf (char *S, size_t SIZE, const char *TEMPLATE, va_list AP)

     stdio.h (GNU):  Variable Arguments Output.

int vsprintf (char *S, const char *TEMPLATE, va_list AP)

     stdio.h (ISO):  Variable Arguments Output.

int vsscanf (const char *S, const char *TEMPLATE, va_list AP)

     stdio.h (ISO):  Variable Arguments Input.

int vswprintf (wchar_t *S, size_t SIZE, const wchar_t *TEMPLATE, va_list AP)

     wchar.h (GNU):  Variable Arguments Output.

int vswscanf (const wchar_t *S, const wchar_t *TEMPLATE, va_list AP)

     wchar.h (ISO):  Variable Arguments Input.

void vwarn (const char *FORMAT, va_list)

     err.h (BSD):  Error Messages.

void vwarnx (const char *FORMAT, va_list)

     err.h (BSD):  Error Messages.

int vwprintf (const wchar_t *TEMPLATE, va_list AP)

     wchar.h (ISO):  Variable Arguments Output.

int vwscanf (const wchar_t *TEMPLATE, va_list AP)

     wchar.h (ISO):  Variable Arguments Input.

void warn (const char *FORMAT, ...)

     err.h (BSD):  Error Messages.

void warnx (const char *FORMAT, ...)

     err.h (BSD):  Error Messages.

wint_t WCHAR_MAX

     wchar.h (ISO):  Extended Char Intro.

wint_t WCHAR_MIN

     wchar.h (ISO):  Extended Char Intro.

wchar_t

     stddef.h (ISO):  Extended Char Intro.

wchar_t * wcpcpy (wchar_t *restrict WTO, const wchar_t *restrict WFROM)

     wchar.h (GNU):  Copying and Concatenation.

wchar_t * wcpncpy (wchar_t *restrict WTO, const wchar_t *restrict WFROM, size_t SIZE)

     wchar.h (GNU):  Copying and Concatenation.

size_t wcrtomb (char *restrict S, wchar_t WC, mbstate_t *restrict PS)

     wchar.h (ISO):  Converting a Character.

int wcscasecmp (const wchar_t *WS1, const wchar_T *WS2)

     wchar.h (GNU):  String/Array Comparison.

wchar_t * wcscat (wchar_t *restrict WTO, const wchar_t *restrict WFROM)

     wchar.h (ISO):  Copying and Concatenation.

wchar_t * wcschr (const wchar_t *WSTRING, int WC)

     wchar.h (ISO):  Search Functions.

wchar_t * wcschrnul (const wchar_t *WSTRING, wchar_t WC)

     wchar.h (GNU):  Search Functions.

int wcscmp (const wchar_t *WS1, const wchar_t *WS2)

     wchar.h (ISO):  String/Array Comparison.

int wcscoll (const wchar_t *WS1, const wchar_t *WS2)

     wchar.h (ISO):  Collation Functions.

wchar_t * wcscpy (wchar_t *restrict WTO, const wchar_t *restrict WFROM)

     wchar.h (ISO):  Copying and Concatenation.

size_t wcscspn (const wchar_t *WSTRING, const wchar_t *STOPSET)

     wchar.h (ISO):  Search Functions.

wchar_t * wcsdup (const wchar_t *WS)

     wchar.h (GNU):  Copying and Concatenation.

size_t wcsftime (wchar_t *S, size_t SIZE, const wchar_t *TEMPLATE, const struct tm *BROKENTIME)

     time.h (ISO/Amend1):  Formatting Calendar Time.

size_t wcslen (const wchar_t *WS)

     wchar.h (ISO):  String Length.

int wcsncasecmp (const wchar_t *WS1, const wchar_t *S2, size_t N)

     wchar.h (GNU):  String/Array Comparison.

wchar_t * wcsncat (wchar_t *restrict WTO, const wchar_t *restrict WFROM, size_t SIZE)

     wchar.h (ISO):  Copying and Concatenation.

int wcsncmp (const wchar_t *WS1, const wchar_t *WS2, size_t SIZE)

     wchar.h (ISO):  String/Array Comparison.

wchar_t * wcsncpy (wchar_t *restrict WTO, const wchar_t *restrict WFROM, size_t SIZE)

     wchar.h (ISO):  Copying and Concatenation.

size_t wcsnlen (const wchar_t *WS, size_t MAXLEN)

     wchar.h (GNU):  String Length.

size_t wcsnrtombs (char *restrict DST, const wchar_t **restrict SRC, size_t NWC, size_t LEN, mbstate_t *restrict PS)

     wchar.h (GNU):  Converting Strings.

wchar_t * wcspbrk (const wchar_t *WSTRING, const wchar_t *STOPSET)

     wchar.h (ISO):  Search Functions.

wchar_t * wcsrchr (const wchar_t *WSTRING, wchar_t C)

     wchar.h (ISO):  Search Functions.

size_t wcsrtombs (char *restrict DST, const wchar_t **restrict SRC, size_t LEN, mbstate_t *restrict PS)

     wchar.h (ISO):  Converting Strings.

size_t wcsspn (const wchar_t *WSTRING, const wchar_t *SKIPSET)

     wchar.h (ISO):  Search Functions.

wchar_t * wcsstr (const wchar_t *HAYSTACK, const wchar_t *NEEDLE)

     wchar.h (ISO):  Search Functions.

double wcstod (const wchar_t *restrict STRING, wchar_t **restrict TAILPTR)

     wchar.h (ISO):  Parsing of Floats.

float wcstof (const wchar_t *STRING, wchar_t **TAILPTR)

     stdlib.h (ISO):  Parsing of Floats.

intmax_t wcstoimax (const wchar_t *restrict STRING, wchar_t **restrict TAILPTR, int BASE)

     wchar.h (ISO):  Parsing of Integers.

wchar_t * wcstok (wchar_t *NEWSTRING, const char *DELIMITERS)

     wchar.h (ISO):  Finding Tokens in a String.

long int wcstol (const wchar_t *restrict STRING, wchar_t **restrict TAILPTR, int BASE)

     wchar.h (ISO):  Parsing of Integers.

long double wcstold (const wchar_t *STRING, wchar_t **TAILPTR)

     stdlib.h (ISO):  Parsing of Floats.

long long int wcstoll (const wchar_t *restrict STRING, wchar_t **restrict TAILPTR, int BASE)

     wchar.h (ISO):  Parsing of Integers.

size_t wcstombs (char *STRING, const wchar_t *WSTRING, size_t SIZE)

     stdlib.h (ISO):  Nonreentrant String Conversion.

long long int wcstoq (const wchar_t *restrict STRING, wchar_t **restrict TAILPTR, int BASE)

     wchar.h (GNU):  Parsing of Integers.

unsigned long int wcstoul (const wchar_t *restrict STRING, wchar_t **restrict TAILPTR, int BASE)

     wchar.h (ISO):  Parsing of Integers.

unsigned long long int wcstoull (const wchar_t *restrict STRING, wchar_t **restrict TAILPTR, int BASE)

     wchar.h (ISO):  Parsing of Integers.

uintmax_t wcstoumax (const wchar_t *restrict STRING, wchar_t **restrict TAILPTR, int BASE)

     wchar.h (ISO):  Parsing of Integers.

unsigned long long int wcstouq (const wchar_t *restrict STRING, wchar_t **restrict TAILPTR, int BASE)

     wchar.h (GNU):  Parsing of Integers.

wchar_t * wcswcs (const wchar_t *HAYSTACK, const wchar_t *NEEDLE)

     wchar.h (XPG):  Search Functions.

size_t wcsxfrm (wchar_t *restrict WTO, const wchar_t *WFROM, size_t SIZE)

     wchar.h (ISO):  Collation Functions.

int wctob (wint_t C)

     wchar.h (ISO):  Converting a Character.

int wctomb (char *STRING, wchar_t WCHAR)

     stdlib.h (ISO):  Nonreentrant Character Conversion.

wctrans_t wctrans (const char *PROPERTY)

     wctype.h (ISO):  Wide-Character Case Conversion.

wctrans_t

     wctype.h (ISO):  Wide-Character Case Conversion.

wctype_t wctype (const char *PROPERTY)

     wctype.h (ISO):  Classification of Wide Characters.

wctype_t

     wctype.h (ISO):  Classification of Wide Characters.

int WEOF

     wchar.h (ISO):  EOF and Errors.

wint_t WEOF

     wchar.h (ISO):  Extended Char Intro.

wint_t

     wchar.h (ISO):  Extended Char Intro.

wchar_t * wmemchr (const wchar_t *BLOCK, wchar_t WC, size_t SIZE)

     wchar.h (ISO):  Search Functions.

int wmemcmp (const wchar_t *A1, const wchar_t *A2, size_t SIZE)

     wcjar.h (ISO):  String/Array Comparison.

wchar_t * wmemcpy (wchar_t *restrict WTO, const wchar_t *restruct WFROM, size_t SIZE)

     wchar.h (ISO):  Copying and Concatenation.

wchar_t * wmemmove (wchar *WTO, const wchar_t *WFROM, size_t SIZE)

     wchar.h (ISO):  Copying and Concatenation.

wchar_t * wmempcpy (wchar_t *restrict WTO, const wchar_t *restrict WFROM, size_t SIZE)

     wchar.h (GNU):  Copying and Concatenation.

wchar_t * wmemset (wchar_t *BLOCK, wchar_t WC, size_t SIZE)

     wchar.h (ISO):  Copying and Concatenation.

int wordexp (const char *WORDS, wordexp_t *WORD-VECTOR-PTR, int FLAGS)

     wordexp.h (POSIX.2):  Calling Wordexp.

wordexp_t

     wordexp.h (POSIX.2):  Calling Wordexp.

void wordfree (wordexp_t *WORD-VECTOR-PTR)

     wordexp.h (POSIX.2):  Calling Wordexp.

int wprintf (const wchar_t *TEMPLATE, ...)

     wchar.h (ISO):  Formatted Output Functions.

WRDE_APPEND

     wordexp.h (POSIX.2):  Flags for Wordexp.

WRDE_BADCHAR

     wordexp.h (POSIX.2):  Calling Wordexp.

WRDE_BADVAL

     wordexp.h (POSIX.2):  Calling Wordexp.

WRDE_CMDSUB

     wordexp.h (POSIX.2):  Calling Wordexp.

WRDE_DOOFFS

     wordexp.h (POSIX.2):  Flags for Wordexp.

WRDE_NOCMD

     wordexp.h (POSIX.2):  Flags for Wordexp.

WRDE_NOSPACE

     wordexp.h (POSIX.2):  Calling Wordexp.

WRDE_REUSE

     wordexp.h (POSIX.2):  Flags for Wordexp.

WRDE_SHOWERR

     wordexp.h (POSIX.2):  Flags for Wordexp.

WRDE_SYNTAX

     wordexp.h (POSIX.2):  Calling Wordexp.

WRDE_UNDEF

     wordexp.h (POSIX.2):  Flags for Wordexp.

int wscanf (const wchar_t *TEMPLATE, ...)

     wchar.h (ISO):  Formatted Input Functions.

_XOPEN_SOURCE

     (X/Open):  Feature-Test Macros.

_XOPEN_SOURCE_EXTENDED

     (X/Open):  Feature-Test Macros.

double y0 (double X)

     math.h (SVID):  Special Functions.

float y0f (float X)

     math.h (SVID):  Special Functions.

long double y0l (long double X)

     math.h (SVID):  Special Functions.

double y1 (double X)

     math.h (SVID):  Special Functions.

float y1f (float X)

     math.h (SVID):  Special Functions.

long double y1l (long double X)

     math.h (SVID):  Special Functions.

double yn (int n, double X)

     math.h (SVID):  Special Functions.

float ynf (int n, float X)

     math.h (SVID):  Special Functions.

long double ynl (int n, long double X)

     math.h (SVID):  Special Functions.

Contributors to the GNU C Library
*********************************

The GNU C Library was written originally by Roland McGrath, and is currently maintained by Ulrich Drepper.  Some parts of the library were contributed or worked on by other people.

  * The getopt function and related code were written by Richard Stallman, David J. MacKenzie and Roland McGrath.

  * The merge sort function qsort was written by Michael J. Haertel.

  * The quick sort function used as a fallback by qsort was written by Douglas C. Schmidt.

  * The memory allocation functions malloc, realloc and free and related code were written by Michael J. Haertel, Wolfram Gloger and Doug Lea.

  * Fast implementations of many of the string functions (memcpy, strlen, etc.) were written by Torbjörn Granlund.

  * The tar.h header file was written by David J. MacKenzie.

  * The port to the MIPS DECStation running Ultrix 4 (mips-dec-ultrix4) was contributed by Brendan Kehoe and Ian Lance Taylor.

  * The DES encryption function crypt and related functions were contributed by Michael Glad.

  * The ftw and nftw functions were contributed by Ulrich Drepper.

  * The start-up code to support SunOS shared libraries was contributed by Tom Quinn.

  * The mktime function was contributed by Paul Eggert.

  * The port to the Sequent Symmetry running Dynix version 3 (i386-sequent-bsd) was contributed by Jason Merrill.

  * The time zone support code is derived from the public-domain time zone package by Arthur David Olson and his many contributors.

  * The port to the DEC Alpha running OSF/1 (alpha-dec-osf1) was contributed by Brendan Kehoe, using some code written by Roland McGrath.

  * The port to SGI machines running Irix 4 (mips-sgi-irix4) was contributed by Tom Quinn.

  * The port of the Mach and Hurd code to the MIPS architecture (mips-ANYTHING-gnu) was contributed by Kazumoto Kojima.

  * The floating-point printing function used by printf and friends, and the floating-point reading function used by scanf, strtod and friends were written by Ulrich Drepper.  The multiprecision integer functions used in those functions are taken from GNU MP, which was contributed by Torbjörn Granlund.

  * The internationalization support in the library, and the support programs locale and localedef, were written by Ulrich Drepper.  Ulrich Drepper adapted the support code for message catalogs (libintl.h, etc.) from the GNU gettext package, which he also wrote.  He also contributed the catgets support and the entire suite of multibyte- and wide-character support functions (wctype.h, wchar.h, etc.).

  * The implementations of the nsswitch.conf mechanism and the files and DNS backends for it were designed and written by Ulrich Drepper and Roland McGrath, based on a backend interface defined by Peter Eriksson.

  * The port to Linux i386/ELF (i386-ANYTHING-linux) was contributed by Ulrich Drepper, based in large part on work done in Hongjiu Lu's Linux version of the GNU C Library.

  * The port to Linux/m68k (m68k-ANYTHING-linux) was contributed by Andreas Schwab.

  * The ports to Linux/ARM (arm-ANYTHING-linuxaout) and ARM standalone (arm-ANYTHING-none), as well as parts of the IPv6 support code, were contributed by Philip Blundell.

  * Richard Henderson contributed the ELF dynamic linking code and other support for the Alpha processor.

  * David Mosberger-Tang contributed the port to Linux/Alpha (alpha-ANYTHING-linux).

  * The port to Linux on PowerPC (powerpc-ANYTHING-linux) was contributed by Geoffrey Keating.

  * Miles Bader wrote the argp argument-parsing package, and the argz/envz interfaces.

  * Stephen R. van den Berg contributed a highly-optimized strstr function.

  * Ulrich Drepper contributed the hsearch and drand48 families of functions; reentrant ..._r versions of the random family; System V shared memory and IPC support code; and several highly-optimized string functions for iX86 processors.

  * The math functions are taken from fdlibm-5.1 by Sun Microsystems, as modified by J.T. Conklin, Ian Lance Taylor, Ulrich Drepper, Andreas Schwab and Roland McGrath.

  * The libio library used to implement stdio functions on some platforms was written by Per Bothner and modified by Ulrich Drepper.

  * Eric Youngdale and Ulrich Drepper implemented versioning of objects on the symbol level.

  * Thorsten Kukuk provided an implementation for NIS (YP) and NIS+, securelevel 0, 1 and 2.

  * Andreas Jaeger provided a test suite for the math library.

  * Mark Kettenis implemented the utmpx interface and an utmp daemon.

  * Ulrich Drepper added character conversion functions (iconv).

  * Thorsten Kukuk provided an implementation for a caching daemon for NSS (nscd).

  * Tim Waugh provided an implementation of the POSIX.2 wordexp function family.

  * Mark Kettenis provided a Hesiod NSS module.

  * The Internet-related code (most of the inet subdirectory) and several other miscellaneous functions and header files have been included from 4.4 BSD with little or no modification.  The copying permission notice for this code can be found in the file LICENSES in the source distribution.

  * The random-number generation functions random, srandom, setstate and initstate, which are also the basis for the rand and srand functions, were written by Earl T. Cohen for the University of California at Berkeley and are copyrighted by the Regents of the University of California.  They have undergone minor changes to fit into the GNU C Library and to fit the ISO C standard, but the functional code is Berkeley's.

  * The DNS resolver code is taken directly from BIND 4.9.5, which includes copyrighted code from UC Berkeley and from Digital Equipment Corporation.  See the file LICENSES for the text of the DEC license.

  * The code to support Sun RPC is taken verbatim from Sun's RPCSRC-4.0 distribution; see the file LICENSES for the text of the license.

  * Some of the support code for Mach is taken from Mach 3.0 by CMU; the file if_ppp.h is also copyright by CMU, but under a different license; see the file LICENSES for the text of the licenses.

  * Many of the IA64 math functions are taken from a collection of ``Highly Optimized Mathematical Functions for Itanium'' that Intel makes available under a free license; see the file LICENSES for details.

  * The getaddrinfo and getnameinfo functions and supporting code were written by Craig Metz; see the file LICENSES for details on their licensing.

  * Many of the IEEE 64-bit double precision math functions (in the sysdeps/ieee754/dbl-64 subdirectory) come from the IBM Accurate Mathematical Library, contributed by IBM.

Free Software Needs Free Documentation
**************************************

The biggest deficiency in the free software community today is not in the software---it is the lack of good free documentation that we can include with the free software.  Many of our most important programs do not come with free reference manuals and free introductory texts.  Documentation is an essential part of any software package; when an important free software package does not come with a free manual and a free tutorial, that is a major gap.  We have many such gaps today.

Consider Perl, for instance.  The tutorial manuals that people normally use are nonfree.  How did this come about?  Because the authors of those manuals published them with restrictive terms---no copying, no modification, source files not available---which exclude them from the free software world.

That wasn't the first time this sort of thing happened, and it was far from the last.  Many times we have heard a GNU user eagerly describe a manual that he is writing, his intended contribution to the community, only to learn that he had ruined everything by signing a publication contract to make it nonfree.

Free documentation, like free software, is a matter of freedom, not price.  The problem with the nonfree manual is not that publishers charge a price for printed copies---that in itself is fine.  (The Free Software Foundation sells printed copies of manuals, too.)  The problem is the restrictions on the use of the manual.  Free manuals are available in source code form, and give you permission to copy and modify.  Nonfree manuals do not allow this.

The criteria of freedom for a free manual are roughly the same as for free software.  Redistribution (including the normal kinds of commercial redistribution) must be permitted, so that the manual can accompany every copy of the program, both on-line and on paper.

Permission for modification of the technical content is crucial too. When people modify the software, adding or changing features, if they are conscientious they will change the manual too---so they can provide accurate and clear documentation for the modified program.  A manual that leaves you no choice but to write a new manual to document a changed version of the program is not really available to our community.

Some kinds of limits on the way modification is handled are acceptable.  For example, requirements to preserve the original author's copyright notice, the distribution terms, or the list of authors, are ok.  It is also no problem to require modified versions to include notice that they were modified.  Even entire sections that may not be deleted or changed are acceptable, as long as they deal with nontechnical topics (like this one).  These kinds of restrictions are acceptable because they don't obstruct the community's normal use of the manual.

However, it must be possible to modify all the technical content of the manual, and then distribute the result in all the usual media, through all the usual channels.  Otherwise, the restrictions obstruct the use of the manual, it is not free, and we need another manual to replace it.

Please spread the word about this issue.  Our community continues to lose manuals to proprietary publishing.  If we spread the word that free software needs free reference manuals and free tutorials, perhaps the next person who wants to contribute by writing documentation will realize, before it is too late, that only free manuals contribute to the free software community.

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The Free Software Foundation maintains a list of free documentation published by other publishers, at http:// www.fsf.org/ doc/ other-free-books.html.

GNU Lesser General Public License
*********************************

Version 2.1, February 1999

     Copyright © 1991, 1999 Free Software Foundation, Inc.
     51 Franklin St -- Fifth Floor, Boston, MA 02110-1301, USA

     Everyone is permitted to copy and distribute verbatim copies
     of this license document, but changing it is not allowed.

     [This is the first released version of the Lesser GPL.  It also counts
     as the successor of the GNU Library Public License, version 2, hence the
     version number 2.1.]

Preamble
--------

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Although the Lesser General Public License is Less protective of the users' freedom, it does ensure that the user of a program that is linked with the Library has the freedom and the wherewithal to run that program using a modified version of the Library.

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GNU LESSER GENERAL PUBLIC LICENSE
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END OF TERMS AND CONDITIONS

How to Apply These Terms to Your New Libraries
----------------------------------------------

If you develop a new library, and you want it to be of the greatest possible use to the public, we recommend making it free software that everyone can redistribute and change.  You can do so by permitting redistribution under these terms (or, alternatively, under the terms of the ordinary General Public License).

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Also add information on how to contact you by electronic and paper mail.

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That's all there is to it!

GNU Free Documentation License
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Version 1.1, March 2000

     Copyright © 2000 Free Software Foundation, Inc.
     51 Franklin St, Fifth Floor, Boston, MA  02110-1301, USA

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      M. Delete any section entitled ``Endorsements''.  Such a section may not be included in the Modified Version.

      N. Do not retitle any existing section as ``Endorsements'' or to conflict in title with any Invariant Section.

     If the Modified Version includes new front-matter sections or appendices that qualify as Secondary Sections and contain no material copied from the Document, you may at your option designate some or all of these sections as invariant.  To do this, add their titles to the list of Invariant Sections in the Modified Version's license notice. These titles must be distinct from any other section titles.

     You may add a section entitled ``Endorsements'', provided it contains nothing but endorsements of your Modified Version by various parties---for example, statements of peer review or that the text has been approved by an organization as the authoritative definition of a standard.

     You may add a passage of up to five words as a Front-Cover Text, and a passage of up to 25 words as a Back-Cover Text, to the end of the list of Cover Texts in the Modified Version.  Only one passage of Front-Cover Text and one of Back-Cover Text may be added by (or through arrangements made by) any one entity.  If the Document already includes a cover text for the same cover, previously added by you or by arrangement made by the same entity you are acting on behalf of, you may not add another; but you may replace the old one, on explicit permission from the previous publisher that added the old one.

     The author(s) and publisher(s) of the Document do not by this License give permission to use their names for publicity for or to assert or imply endorsement of any Modified Version.

 5. COMBINING DOCUMENTS

     You may combine the Document with other documents released under this License, under the terms defined in section 4 above for modified versions, provided that you include in the combination all of the Invariant Sections of all of the original documents, unmodified, and list them all as Invariant Sections of your combined work in its license notice.

     The combined work need only contain one copy of this License, and multiple identical Invariant Sections may be replaced with a single copy.  If there are multiple Invariant Sections with the same name but different contents, make the title of each such section unique by adding at the end of it, in parentheses, the name of the original author or publisher of that section if known, or else a unique number. Make the same adjustment to the section titles in the list of Invariant Sections in the license notice of the combined work.

     In the combination, you must combine any sections entitled ``History'' in the various original documents, forming one section entitled ``History''; likewise combine any sections entitled ``Acknowledgments'', and any sections entitled ``Dedications''.  You must delete all sections entitled ``Endorsements.''

 6. COLLECTIONS OF DOCUMENTS

     You may make a collection consisting of the Document and other documents released under this License, and replace the individual copies of this License in the various documents with a single copy that is included in the collection, provided that you follow the rules of this License for verbatim copying of each of the documents in all other respects.

     You may extract a single document from such a collection, and distribute it individually under this License, provided you insert a copy of this License into the extracted document, and follow this License in all other respects regarding verbatim copying of that document.

 7. AGGREGATION WITH INDEPENDENT WORKS

     A compilation of the Document or its derivatives with other separate and independent documents or works, in or on a volume of a storage or distribution medium, does not as a whole count as a Modified Version of the Document, provided no compilation copyright is claimed for the compilation.  Such a compilation is called an ``aggregate'', and this License does not apply to the other self-contained works thus compiled with the Document, on account of their being thus compiled, if they are not themselves derivative works of the Document.

     If the Cover Text requirement of section 3 is applicable to these copies of the Document, then if the Document is less than one quarter of the entire aggregate, the Document's Cover Texts may be placed on covers that surround only the Document within the aggregate. Otherwise they must appear on covers around the whole aggregate.

 8. TRANSLATION

     Translation is considered a kind of modification, so you may distribute translations of the Document under the terms of section 4. Replacing Invariant Sections with translations requires special permission from their copyright holders, but you may include translations of some or all Invariant Sections in addition to the original versions of these Invariant Sections.  You may include a translation of this License provided that you also include the original English version of this License.  In case of a disagreement between the translation and the original English version of this License, the original English version will prevail.

 9. TERMINATION

     You may not copy, modify, sublicense, or distribute the Document except as expressly provided for under this License.  Any other attempt to copy, modify, sublicense or distribute the Document is void, and will automatically terminate your rights under this License.  However, parties who have received copies, or rights, from you under this License will not have their licenses terminated so long as such parties remain in full compliance.

 10. FUTURE REVISIONS OF THIS LICENSE

     The Free Software Foundation may publish new, revised versions of the GNU Free Documentation License from time to time.  Such new versions will be similar in spirit to the present version, but may differ in detail to address new problems or concerns.  See http:// www.gnu.org/ copyleft/.

     Each version of the License is given a distinguishing version number. If the Document specifies that a particular numbered version of this License ``or any later version'' applies to it, you have the option of following the terms and conditions either of that specified version or of any later version that has been published (not as a draft) by the Free Software Foundation.  If the Document does not specify a version number of this License, you may choose any version ever published (not as a draft) by the Free Software Foundation.

ADDENDUM: How to Use This License for Your Documents
----------------------------------------------------

To use this License in a document you have written, include a copy of the License in the document and put the following copyright and license notices just after the title page:

       Copyright (C)  YEAR  YOUR NAME.
       Permission is granted to copy, distribute and/or modify this document
       under the terms of the GNU Free Documentation License, Version 1.1
       or any later version published by the Free Software Foundation;
       with the Invariant Sections being LIST THEIR TITLES, with the
       Front-Cover Texts being LIST, and with the Back-Cover Texts being LIST.
       A copy of the license is included in the section entitled ``GNU
       Free Documentation License''.

If you have no Invariant Sections, write ``with no Invariant Sections'' instead of saying which ones are invariant.  If you have no Front-Cover Texts, write ``no Front-Cover Texts'' instead of ``Front-Cover Texts being LIST''; likewise for Back-Cover Texts.

If your document contains nontrivial examples of program code, we recommend releasing these examples in parallel under your choice of free software license, such as the GNU General Public License, to permit their use in free software.