This is Info file libc.info, produced by Makeinfo version 1.67 from the
input file libc.texinfo.

   This file documents the GNU C library.

   This is Edition 0.07 DRAFT, last updated 4 Oct 1996, of `The GNU C
Library Reference Manual', for Version 2.00 Beta.

   Copyright (C) 1993, '94, '95, '96 Free Software Foundation, Inc.

   Permission is granted to make and distribute verbatim copies of this
manual provided the copyright notice and this permission notice are
preserved on all copies.

   Permission is granted to copy and distribute modified versions of
this manual under the conditions for verbatim copying, provided also
that the section entitled "GNU Library General Public License" is
included exactly as in the original, and provided that the entire
resulting derived work is distributed under the terms of a permission
notice identical to this one.

   Permission is granted to copy and distribute translations of this
manual into another language, under the above conditions for modified
versions, except that the text of the translation of the section
entitled "GNU Library General Public License" must be approved for
accuracy by the Foundation.


File: libc.info,  Node: Opening Streams,  Next: Closing Streams,  Prev: Standard Streams,  Up: I/O on Streams

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' (*note Stream Buffering::.) or a file
     positioning function such as `fseek' (*note 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 whatever 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 you are guaranteed will 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' you can are guaranteed that you will not
     clobber an existing file.  This is equivalent to the `O_EXCL'
     option to the `open' function (*note 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.  *Note
     Binary Streams::.

     Any other characters in OPENTYPE are simply ignored.  They may be
     meaningful in other systems.

     If the open fails, `fopen' returns a null pointer.

   You can have multiple streams (or file descriptors) pointing to the
same file open at the same time.  If you do only input, this works
straightforwardly, but you must be careful if any output streams are
included.  *Note Stream/Descriptor Precautions::.  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.  *Note File
Locks::.

 - 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;
     *note General Limits::..  In BSD and GNU, it is controlled by the
     `RLIMIT_NOFILE' resource limit; *note Limits on Resources::..

 - 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.


File: libc.info,  Node: Closing Streams,  Next: Simple Output,  Prev: Opening Streams,  Up: I/O on Streams

Closing Streams
===============

   When a stream is closed with `fclose', the connection between the
stream and the file is cancelled.  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 inputis 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 in only in special situation, e.g.,
     when an error occurred and the program must be aborted.  Normally
     each single stream should be closed separately so that problems
     with one stream can be identifier.  It is also problematic since
     the standard streams (*note 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 (*note Normal Termination::.), all open streams are
automatically closed properly.  If your program terminates in any other
manner, such as by calling the `abort' function (*note Aborting a
Program::.) or from a fatal signal (*note 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 *Note Stream Buffering::.


File: libc.info,  Node: Simple Output,  Next: Character Input,  Prev: Closing Streams,  Up: I/O on Streams

Simple Output by Characters or Lines
====================================

   This section describes functions for performing character- and
line-oriented output.

   These functions are declared in the header file `stdio.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: 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: int putchar (int C)
     The `putchar' function is equivalent to `putc' with `stdout' as
     the value of the STREAM argument.

 - 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 non-negative 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 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.");

 - 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 (*note Block Input/Output::.).


File: libc.info,  Node: Character Input,  Next: Line Input,  Prev: Simple Output,  Up: I/O on Streams

Character Input
===============

   This section describes functions for performing character-oriented
input.  These functions are declared in the header file `stdio.h'.

   These functions return an `int' value that is either a character of
input, or the special value `EOF' (usually -1).  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've 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: 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: int getchar (void)
     The `getchar' function is equivalent to `getc' with `stdin' as the
     value of the STREAM argument.

   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)'.

     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 (*note 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.


File: libc.info,  Node: Line Input,  Next: Unreading,  Prev: Character Input,  Up: I/O on Streams

Line-Oriented Input
===================

   Since many programs interpret input on the basis of lines, it's
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'.  *Note 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 *'
     which 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, `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 which
     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'.

 - 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'.


File: libc.info,  Node: Unreading,  Next: Block Input/Output,  Prev: Line Input,  Up: I/O on Streams

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.

* Menu:

* Unreading Idea::              An explanation of unreading with pictures.
* How Unread::                  How to call `ungetc' to do unreading.


File: libc.info,  Node: Unreading Idea,  Next: How Unread,  Up: Unreading

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'.


File: libc.info,  Node: How Unread,  Prev: Unreading Idea,  Up: Unreading

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 only supports one character of pushback--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' or `rewind'; *note File Positioning::.)
     is called, any pending pushed-back characters are discarded.

     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.

   Here is an example showing the use of `getc' and `ungetc' to skip
over whitespace characters.  When this function reaches a
non-whitespace 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);
     }


File: libc.info,  Node: Block Input/Output,  Next: Formatted Output,  Prev: Unreading,  Up: I/O on Streams

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 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.


File: libc.info,  Node: Formatted Output,  Next: Customizing Printf,  Prev: Block Input/Output,  Up: I/O on Streams

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.

* Menu:

* Formatted Output Basics::     Some examples to get you started.
* Output Conversion Syntax::    General syntax of conversion
                                 specifications.
* Table of Output Conversions:: Summary of output conversions and
                                 what they do.
* Integer Conversions::         Details about formatting of integers.
* Floating-Point Conversions::  Details about formatting of
                                 floating-point numbers.
* Other Output Conversions::    Details about formatting of strings,
                                 characters, pointers, and the like.
* Formatted Output Functions::  Descriptions of the actual functions.
* Dynamic Output::		Functions that allocate memory for the output.
* Variable Arguments Output::   `vprintf' and friends.
* Parsing a Template String::   What kinds of args does a given template
                                 call for?
* Example of Parsing::          Sample program using `parse_printf_format'.


File: libc.info,  Node: Formatted Output Basics,  Next: Output Conversion Syntax,  Up: Formatted Output

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.


File: libc.info,  Node: Output Conversion Syntax,  Next: Table of Output Conversions,  Prev: Formatted Output Basics,  Up: Formatted Output

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 (*note Extended Characters::.) are permitted in a
template string.

   The conversion specifications in a `printf' template string have the
general form:

     % FLAGS WIDTH [ . PRECISION ] 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 8 digits left-justified in a field
at least 10 characters wide.)

   In more detail, output conversion specifications consist of an
initial `%' character followed in sequence by:

   * 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.  *Note Declaring Attributes
of Functions: (gcc.info)Function Attributes, for more information.


File: libc.info,  Node: Table of Output Conversions,  Next: Integer Conversions,  Prev: Output Conversion Syntax,  Up: Formatted Output

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.  *Note Integer
     Conversions::, for details.  `%d' and `%i' are synonymous for
     output, but are different when used with `scanf' for input (*note
     Table of Input Conversions::.).

`%o'
     Print an integer as an unsigned octal number.  *Note Integer
     Conversions::, for details.

`%u'
     Print an integer as an unsigned decimal number.  *Note Integer
     Conversions::, for details.

`%x', `%X'
     Print an integer as an unsigned hexadecimal number.  `%x' uses
     lower-case letters and `%X' uses upper-case.  *Note Integer
     Conversions::, for details.

`%f'
     Print a floating-point number in normal (fixed-point) notation.
     *Note Floating-Point Conversions::, for details.

`%e', `%E'
     Print a floating-point number in exponential notation.  `%e' uses
     lower-case letters and `%E' uses upper-case.  *Note Floating-Point
     Conversions::, for details.

`%g', `%G'
     Print a floating-point number in either normal or exponential
     notation, whichever is more appropriate for its magnitude.  `%g'
     uses lower-case letters and `%G' uses upper-case.  *Note
     Floating-Point Conversions::, for details.

`%c'
     Print a single character.  *Note Other Output Conversions::.

`%s'
     Print a string.  *Note Other Output Conversions::.

`%p'
     Print the value of a pointer.  *Note Other Output Conversions::.

`%n'
     Get the number of characters printed so far.  *Note Other Output
     Conversions::.  Note that this conversion specification never
     produces any output.

`%m'
     Print the string corresponding to the value of `errno'.  (This is
     a GNU extension.) *Note Other Output Conversions::.

`%%'
     Print a literal `%' character.  *Note 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 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.


File: libc.info,  Node: Integer Conversions,  Next: Floating-Point Conversions,  Prev: Table of Output Conversions,  Up: Formatted Output

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 which can be parsed
     by the `strtoul' function (*note Parsing of Integers::.) and
     `scanf' with the `%i' conversion (*note Numeric Input
     Conversions::.).

`''
     Separate the digits into groups as specified by the locale
     specified for the `LC_NUMERIC' category; *note 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:

`h'
     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.

`l'
     Specifies that the argument is a `long int' or `unsigned long
     int', as appropriate.  Two `l' characters is like the `L'
     modifier, below.

`L'
`ll'
`q'
     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'.

`Z'
     Specifies that the argument is a `size_t'.  This is a GNU
     extension.

   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|

   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|  0x0|  0X0|0x00000000|
     |    1|    1|    1|    1|   01|  0x1|  0X1|0x00000001|
     |100000|303240|186a0|186A0|0303240|0x186a0|0X186A0|0x000186a0|


File: libc.info,  Node: Floating-Point Conversions,  Next: Other Output Conversions,  Prev: Integer Conversions,  Up: Formatted Output

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`.'DDD`e'[`+'|`-']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.  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 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.

`#'
     Specifies 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;
     *note 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 `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:

     "|%12.4f|%12.4e|%12.4g|\n"

   Here is the output:

     |      0.0000|  0.0000e+00|           0|
     |      1.0000|  1.0000e+00|           1|
     |     -1.0000| -1.0000e+00|          -1|
     |    100.0000|  1.0000e+02|         100|
     |   1000.0000|  1.0000e+03|        1000|
     |  10000.0000|  1.0000e+04|       1e+04|
     |  12345.0000|  1.2345e+04|   1.234e+04|
     | 100000.0000|  1.0000e+05|       1e+05|
     | 123456.0000|  1.2346e+05|   1.234e+05|

   Notice how the `%g' conversion drops trailing zeros.


File: libc.info,  Node: Other Output Conversions,  Next: Formatted Output Functions,  Prev: Floating-Point Conversions,  Up: Formatted Output

Other Output Conversions
------------------------

   This section describes miscellaneous conversions for `printf'.

   The `%c' conversion prints a single character.  The `int' argument
is first converted to an `unsigned char'.  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'.

   The `%s' conversion prints a string.  The corresponding argument
must be of type `char *' (or `const char *').  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 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'.  *Note 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, non-null 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 which 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.

