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<PART id="libc">
<TITLE>The ISO Standard C and Math Libraries</TITLE>
<CHAPTER id="c-and-math-library-overview">
<TITLE><!-- <xref> -->C and math library overview</TITLE>
<anchor id="iso-c-and-math-libraries">
<PARA><EMPHASIS>eCos</EMPHASIS> provides compatibility with the
	ISO 9899:1990 specification for the standard C library, which
	is essentially the same as the better-known ANSI C3.159-1989
	specification (C-89).</PARA>
<PARA>There are three aspects of this compatibility supplied by <EMPHASIS>eCos</EMPHASIS>.
First there is a <!-- <index></index> --><firstterm>C library</firstterm> which
implements the functions defined by the ISO standard, except for the
mathematical functions. This is provided by the eCos C library
packages. </PARA>
<PARA>Then <EMPHASIS>eCos</EMPHASIS> provides a <!-- <index></index> -->math
library, which implements the mathematical functions from the ISO
C library. This distinction between C and math libraries is frequently
drawn &mdash; most standard C library implementations provide
separate linkable files for the two, and the math library contains
all the functions from the <filename>math.h</filename> header
file.</PARA>
<PARA>There is a third element to the ISO C library, which is the
environment in which applications run when they use the standard
C library. This environment is set up by the C library startup procedure
(<xref linkend="c-library-startup">)
and it provides (among other things) a <FUNCTION>main()</FUNCTION> entry
point function, an <FUNCTION>exit()</FUNCTION> function that
does the cleanup required by the standard (including handlers registered
using the <FUNCTION>atexit()</FUNCTION> function), and an environment
that can be read with <FUNCTION>getenv()</FUNCTION>. </PARA>
<PARA>The description in this manual focuses on the <EMPHASIS>eCos</EMPHASIS>-specific
aspects of the C library (mostly related to <EMPHASIS>eCos</EMPHASIS>'s
configurability) as well as mentioning the omissions from the standard
in this release. We do not attempt to define the semantics of each
function, since that information can be found in the ISO, ANSI,
POSIX and IEEE standards, and the many good books that have been
written about the standard C library, that cover usage of these
functions in a more general and useful way.</PARA>
<SECT1 id="include-non-iso-functions">
<TITLE>Included non-ISO functions</TITLE>
<PARA>The following functions from the <!-- <index></index> -->POSIX specification
are included for convenience: </PARA>
<PARA>	<literal>extern char **</literal><EMPHASIS>environ</EMPHASIS> variable
(for setting up the environment for use with <FUNCTION>getenv()</FUNCTION>)</PARA>
<PARA>	<FUNCTION>_exit()</FUNCTION> </PARA>
<PARA>	<FUNCTION>strtok_r()</FUNCTION> </PARA>
<PARA>	<FUNCTION>rand_r()</FUNCTION> </PARA>
<PARA>	<FUNCTION>asctime_r()</FUNCTION> </PARA>
<PARA>	<FUNCTION>ctime_r()</FUNCTION> </PARA>
<PARA>	<FUNCTION>localtime_r()</FUNCTION> </PARA>
<PARA>	<FUNCTION>gmtime_r()</FUNCTION> </PARA>
<PARA><EMPHASIS>eCos</EMPHASIS> provides the following additional
implementation-specific functions within the standard C library
to adjust the date and time settings:</PARA>
<PROGRAMLISTING>void <FUNCTION>cyg_libc_time_setdst</FUNCTION>( 
  cyg_libc_time_dst state
);</PROGRAMLISTING>
<PARA>This function sets the state of Daylight Savings Time. The
values for state are:</PARA>
<PROGRAMLISTING>CYG_LIBC_TIME_DSTNA   unknown
CYG_LIBC_TIME_DSTOFF  off
CYG_LIBC_TIME_DSTON   on</PROGRAMLISTING>

<PROGRAMLISTING>void <FUNCTION>cyg_libc_time_setzoneoffsets</FUNCTION>( 
  time_t stdoffset, time_t dstoffset
);</PROGRAMLISTING>
<PARA>This function sets the offsets from UTC used when Daylight
Savings Time is enabled or disabled. The offsets are in time_t&rsquo;s,
which are seconds in the current inplementation.</PARA>
<PROGRAMLISTING><FUNCTION>Cyg_libc_time_dst cyg_libc_time_getzoneoffsets</FUNCTION>( 
  time_t *stdoffset, time_t *dstoffset
);</PROGRAMLISTING>
<PARA>This function retrieves the current setting for Daylight Savings
Time along with the offsets used for both STD and DST. The offsets
are both in time_t&rsquo;s, which are seconds in the
current implementation.</PARA>
<PROGRAMLISTING><FUNCTION>cyg_bool cyg_libc_time_settime</FUNCTION>( 
  time_t utctime
);</PROGRAMLISTING>
<PARA>This function sets the current time for the system The time
is specified as a <type>time_t</type> in UTC.
It returns non-zero on error.</PARA>
</SECT1>
<SECT1 id="math-library-compatibility-modes">
<TITLE>Math library compatibility modes</TITLE>
<PARA>This math library is capable of being operated in several
different compatibility modes. These options deal solely with how
errors are handled. </PARA>
<PARA>There are 4 compatibility modes: ANSI/POSIX 1003.1;
IEEE-754; X/Open Portability Guide issue 3 (XPG3); and
System V Interface Definition Edition 3. </PARA>
<PARA>In <!-- <index></index> -->IEEE mode, the <function>matherr()</function> function
(see below) is never called, no warning messages are printed on
the stderr output stream, and errno is never set. </PARA>
<PARA>In <!-- <index></index> -->ANSI/POSIX mode, errno is set correctly,
but <function>matherr()</function> is never called and no warning messages
are printed on the stderr output stream. </PARA>
<PARA>In <!-- <index></index> -->X/Open mode, errno is set correctly,
<function>matherr()</function> is called, but no warning messages are printed
on the stderr output stream. </PARA>
<PARA>In <!-- <index></index> -->SVID mode, functions which overflow return
a value HUGE (defined in <filename>math.h</filename>), which is the maximum
single precision floating point value (as opposed to
HUGE_VAL which is meant to stand for infinity). errno is
set correctly and <function>matherr()</function> is called. If
<function>matherr()</function> returns 0, warning messages are printed on
the stderr output stream for some errors. </PARA>
<PARA>The mode can be compiled-in as IEEE-only, or any one of the
above methods settable at run-time. </PARA>
<NOTE>
<PARA>This math library assumes that the hardware (or software floating
point emulation) supports IEEE-754 style arithmetic, 32-bit 2's
complement integer arithmetic, doubles are in 64-bit IEEE-754 format.</PARA>
</NOTE>
<SECT2>
<TITLE><!-- <index></index> -->	    matherr()</TITLE>
<PARA>As mentioned above, in X/Open or SVID modes, the user
	    can supply a function <FUNCTION>matherr()</FUNCTION> of
	    the form:</PARA>
<PROGRAMLISTING>int <FUNCTION>matherr</FUNCTION>( struct exception *e )
</PROGRAMLISTING>
<PARA>where struct exception is defined as:</PARA>
<PROGRAMLISTING>struct exception {
 int type;
 char *name;
 double arg1, arg2, retval;
}; </PROGRAMLISTING>
<PARA>type is the exception type and is one of:</PARA>
<VARIABLELIST>
<VARLISTENTRY>
<TERM>DOMAIN 	</TERM>
<LISTITEM>

<PARA>argument domain exception</PARA>
</LISTITEM>
</VARLISTENTRY>
<VARLISTENTRY>
<TERM>SING 	</TERM>
<LISTITEM>

<PARA>argument singularity</PARA>
</LISTITEM>
</VARLISTENTRY>
<VARLISTENTRY>
<TERM>OVERFLOW 	</TERM>
<LISTITEM>

<PARA>overflow range exception</PARA>
</LISTITEM>
</VARLISTENTRY>
<VARLISTENTRY>
<TERM>UNDERFLOW 	</TERM>
<LISTITEM>

<PARA>underflow range exception</PARA>
</LISTITEM>
</VARLISTENTRY>
<VARLISTENTRY>
<TERM>TLOSS 	</TERM>
<LISTITEM>

<PARA>total loss of significance</PARA>
</LISTITEM>
</VARLISTENTRY>
<VARLISTENTRY>
<TERM>PLOSS 	</TERM>
<LISTITEM>

<PARA>partial loss of significance</PARA>
</LISTITEM>
</VARLISTENTRY>
</VARIABLELIST>
<PARA><parameter>name</parameter> is a string containing the name of the
function</PARA>
<PARA><parameter>arg1</parameter> and <parameter>arg2</parameter> are the
arguments passed to the function</PARA>
<PARA><parameter>retval</parameter> is the default value that will be returned
by the function, and can be changed by <FUNCTION>matherr()</FUNCTION></PARA>
<NOTE>
<PARA>matherr must have &ldquo;C&rdquo; linkage, not &ldquo;C&plus;&plus;&rdquo; linkage.</PARA>
</NOTE>
<PARA>If matherr returns zero, or the user doesn't supply
their own matherr, then the following <EMPHASIS>usually</EMPHASIS> happens
in SVID mode:</PARA><!-- FIXME: must convert this table -->
	  <table>
<title>Behavior of math exception handling</title>
<tgroup cols="2">
<thead>
<row>
  <entry>Type</entry>
  <entry>Behavior</entry
</row>
</thead>
<tbody>
<row>
<entry>DOMAIN</entry><entry>0.0 returned,
errno=EDOM, and a message printed on stderr</entry></row>
<row>
<entry>SING</entry><entry>HUGE of appropriate
sign is returned, errno=EDOM, and a message is printed
on stderr</entry></row>
<row>
<entry>OVERFLOW</entry><entry>HUGE of
appropriate sign is returned, and errno=ERANGE</entry></row>
<row>
<entry>UNDERFLOW</entry><entry>0.0 is
returned and errno=ERANGE</entry></row>
<row>
<entry>TLOSS</entry><entry>0.0 is returned,
errno=ERANGE, and a message is printed on stderr</entry></row>
<row>
<entry>PLOSS</entry><entry>The current
implementation doesn't return this type</entry></row>
</tbody>
</tgroup>
</table>
<!-- -->
<PARA>X/Open mode is similar except that the message is
not printed on stderr and HUGE_VAL is used in place of
HUGE</PARA>
</SECT2>
<SECT2>
<TITLE>Thread-safety and re-entrancy</TITLE>
<PARA>With the appropriate configuration options set below, the
math library is fully thread-safe if:</PARA>
<ITEMIZEDLIST>
<LISTITEM>
<PARA>Depending on the compatibility mode, the
		setting of the errno variable from the C library is
		thread-safe</PARA>
</LISTITEM>
<LISTITEM>
<PARA>Depending on the compatibility mode, sending error messages
to the stderr output stream using the C library 
<FUNCTION>fputs()</FUNCTION>
 function is thread-safe </PARA>
</LISTITEM>
<LISTITEM>
<PARA>Depending on the compatibility mode, the user-supplied 
<FUNCTION>matherr()</FUNCTION>
 function and anything it depends on are thread-safe </PARA>
</LISTITEM>
</ITEMIZEDLIST>
<PARA>In addition, with the exception of the <FUNCTION>gamma*()</FUNCTION> and <FUNCTION>lgamma*()</FUNCTION> functions,
the math library is reentrant (and thus safe to use from interrupt handlers)
if the Math library is always in IEEE mode.</PARA>
</SECT2>
</SECT1>
<SECT1 id="libc-implementation-details">
<TITLE>Some implementation details</TITLE>
<PARA>Here are some details about the <!-- <index></index> -->implementation
which might be interesting, although they do not affect the ISO-defined
semantics of the library. </PARA>
<ITEMIZEDLIST>
<LISTITEM>
<PARA>It is possible to configure 
<EMPHASIS>eCos</EMPHASIS>
 to have the standard C library without the kernel. You might want
to do this to use less memory. But if you disable the kernel, you
will be unable to use memory allocation, thread-safety and certain
stdio functions such as input. Other C library functionality is
unaffected.</PARA>
</LISTITEM>
<LISTITEM>
<PARA>The opaque type returned by 
<FUNCTION>clock()</FUNCTION>
 is called clock_t, and is implemented as a 64 bit integer.
The value returned by 
<FUNCTION>clock()</FUNCTION>
 is only correct if the kernel is configured with real-time clock
support, as determined by the CYGVAR_KERNEL_COUNTERS_CLOCK
configuration option in 
<FILENAME>kernel.h</FILENAME>
.</PARA>
</LISTITEM>
<LISTITEM>
<PARA>The FILE type is not implemented as a structure, but rather
as a CYG_ADDRESS. </PARA>
</LISTITEM>
<LISTITEM>
<PARA>The GNU C compiler will place its own <EMPHASIS>built-in</EMPHASIS> implementations
instead of some C library functions. This can be turned off with
the <EMPHASIS>-fno-builtin</EMPHASIS> option. The functions affected
by this are 
<FUNCTION>abs()</FUNCTION>
, 
<FUNCTION>cos()</FUNCTION>
, 
<FUNCTION>fabs()</FUNCTION>
, 
<FUNCTION>labs()</FUNCTION>
, 
<FUNCTION>memcmp()</FUNCTION>
, 
<FUNCTION>memcpy()</FUNCTION>
, 
<FUNCTION>sin()</FUNCTION>
, 
<FUNCTION>sqrt()</FUNCTION>
, 
<FUNCTION>strcmp()</FUNCTION>
, 
<FUNCTION>strcpy()</FUNCTION>
, and 
<FUNCTION>strlen()</FUNCTION>
.</PARA>
</LISTITEM>
<LISTITEM>
<PARA>For faster execution speed you should avoid this option
and let the compiler use its built-ins. This can be turned off by
invoking 
<EMPHASIS>GCC</EMPHASIS>
 with the <EMPHASIS>-fno-builtin</EMPHASIS> option. </PARA>
</LISTITEM>
<LISTITEM>
<PARA><FUNCTION>memcpy()</FUNCTION>
 and 
<FUNCTION>memset()</FUNCTION>
 are located in the infrastructure package, not in the C library
package. This is because the compiler calls these functions, and
the kernel needs to resolve them even if the C library is not configured. </PARA>
</LISTITEM>
<LISTITEM>
<PARA>Error codes such as EDOM and ERANGE, as well as 
<FUNCTION>strerror()</FUNCTION>
, are implemented in the <EMPHASIS>error</EMPHASIS> package. The
error package is separate from the rest of the C and math libraries
so that the rest of 
<EMPHASIS>eCos</EMPHASIS>
 can use these error handling facilities even if the C library is
not configured. </PARA>
</LISTITEM>
<LISTITEM>
<PARA>When 
<FUNCTION>free()</FUNCTION>
 is invoked, heap memory will normally be coalesced. If the CYGSEM_KERNEL_MEMORY_COALESCE
configuration parameter is not set, memory will not be coalesced,
which might cause programs to fail. </PARA>
</LISTITEM>
<LISTITEM>
<PARA>Signals, as implemented by 
<filename>&lt;signal.h&gt;</filename>, are guaranteed to work
correctly if raised using the
<FUNCTION>raise()</FUNCTION>
 function from a normal working program context. Using signals from
within an ISR or DSR context is not expected to work. Also, it is
not guaranteed that if CYGSEM_LIBC_SIGNALS_HWEXCEPTIONS
is set, that handling a signal using 
<FUNCTION>signal()</FUNCTION>
 will necessarily catch that form of exception. For example, it
may be expected that a divide-by-zero error would be caught by handling 
<varname>SIGFPE</varname>. However it depends on the underlying HAL implementation to implement
the required hardware exception. And indeed the hardware itself
may not be capable of detecting these exceptions so it may not be
possible for the HAL implementer to do this in any case. Despite
this lack of guarantees in this respect, the signals implementation
is still ISO C compliant since ISO C does not offer any such guarantees
either. </PARA>
</LISTITEM>
<LISTITEM>
<PARA>The 
<FUNCTION>getenv()</FUNCTION>
 function is implemented (unless the CYGPKG_LIBC_ENVIRONMENT configuration
option is turned off), but there is no shell or 
<FUNCTION>putenv()</FUNCTION>
 function to set the environment dynamically. The environment is
set in a global variable environ, declared as:</PARA>
<PROGRAMLISTING>extern char **environ; // Standard environment definition</PROGRAMLISTING>
<PARA>The environment can be statically initialized at startup time
using the CYGDAT_LIBC_DEFAULT_ENVIRONMENT
option. If so, remember that the final entry of the array initializer
must be NULL. </PARA>
</LISTITEM>
</ITEMIZEDLIST>
<PARA>Here is a minimal <EMPHASIS>eCos</EMPHASIS> program which
demonstrates the use of environments (see also the test case in <filename>language/c/libc/current/tests/stdlib/getenv.c</filename>): </PARA>
<PROGRAMLISTING>#include &lt;stdio.h&gt;
#include &lt;stdlib.h&gt; // Main header for stdlib functions

extern char **environ; // Standard environment definition

int
main( int argc, char *argv[] )
{
 char *str;
 char *env[] = { "PATH=/usr/local/bin:/usr/bin",
 "HOME=/home/fred",
 "TEST=1234=5678",
 "home=hatstand",
 NULL };

 printf("Display the current PATH environment variable\n");

 environ = (char **)&amp;env;

 str = getenv("PATH");

 if (str==NULL) {
  printf("The current PATH is unset\n");
 } else {
  printf("The current PATH is \"%s\"\n", str);
 }
 return 0;
} </PROGRAMLISTING>
</SECT1>
<SECT1 id="libc-thread-safety">
<TITLE><!-- <index></index> -->Thread safety</TITLE>
<PARA>The ISO C library has configuration options that control thread
safety, i.e. working behavior if multiple threads call the same
function at the same time.</PARA>
<PARA>The following functionality has to be configured correctly,
or used carefully in a multi-threaded environment:</PARA>
<ITEMIZEDLIST>
<LISTITEM>
<PARA><function>mblen()</function></para>
</listitem>
<listitem>
<para><function>mbtowc()</function></para>
</listitem>
<listitem>
<para><function>wctomb()</function></para>
</listitem>
<listitem>
<para>
<FUNCTION>printf()</FUNCTION>
 (and all standard I/O functions except for 
<FUNCTION>sprintf()</FUNCTION>
 and 
<FUNCTION>sscanf()</FUNCTION></PARA>
</LISTITEM>
<LISTITEM>
<PARA><FUNCTION>strtok()</FUNCTION></PARA>
</LISTITEM>
<LISTITEM>
<PARA><FUNCTION>rand()</FUNCTION>
 and 
<FUNCTION>srand()</FUNCTION></PARA>
</LISTITEM>
<LISTITEM>
<PARA><FUNCTION>signal()</FUNCTION>
 and 
<FUNCTION>raise()</FUNCTION></PARA>
</LISTITEM>
<LISTITEM>
<PARA><FUNCTION>asctime()</FUNCTION>
, 
<FUNCTION>ctime()</FUNCTION>
, 
<FUNCTION>gmtime()</FUNCTION>
, and 
<FUNCTION>localtime()</FUNCTION></PARA>
</LISTITEM>
<LISTITEM>
<PARA>the 
<FUNCTION>errno</FUNCTION>
 variable</PARA>
</LISTITEM>
<LISTITEM>
<PARA>the 
<FUNCTION>environ</FUNCTION>
 variable</PARA>
</LISTITEM>
<LISTITEM>
<PARA>date and time settings</PARA>
</LISTITEM>
</ITEMIZEDLIST>
<PARA>In some cases, to make <EMPHASIS>eCos</EMPHASIS> development
easier, functions are provided (as specified by POSIX 1003.1) that define
re-entrant alternatives, i.e. <FUNCTION>rand_r()</FUNCTION>, <FUNCTION>strtok_r()</FUNCTION>, <FUNCTION>asctime_r()</FUNCTION>, <FUNCTION>ctime_r()</FUNCTION>, <FUNCTION>gmtime_r()</FUNCTION>,
and <FUNCTION>localtime_r()</FUNCTION>. In other cases,
configuration options are provided that control either locking of functions
or their shared data, such as with standard I/O streams,
or by using per-thread data, such as with the <FUNCTION>errno</FUNCTION> variable.</PARA>
<PARA>In some other cases, like the setting of date and time, no
re-entrant or thread-safe alternative or configuration is provided
as it is simply not a worthwhile addition (date and time should
rarely need to be set.)</PARA>
</SECT1>
<SECT1 id="c-library-startup">
<TITLE><!-- <index></index> --><!-- <xref> -->C library startup</TITLE>
<PARA>The C library includes a function declared as:</PARA>
<PROGRAMLISTING>void <FUNCTION>cyg_iso_c_start</FUNCTION>( void )</PROGRAMLISTING>
<PARA>This function is used to start an environment in which an
ISO C style program can run in the most compatible way.</PARA>
<PARA>What this function does is to create a thread which will invoke <FUNCTION>main()</FUNCTION> &mdash; normally
considered a program's entry point. In particular, it can
supply arguments to <FUNCTION>main()</FUNCTION> using the CYGDAT_LIBC_ARGUMENTS
configuration option, and when returning from <FUNCTION>main()</FUNCTION>,
or calling <FUNCTION>exit()</FUNCTION>, pending stdio file output
is flushed and any functions registered with <FUNCTION>atexit()</FUNCTION> are
invoked. This is all compliant with the ISO C standard in this respect. </PARA>
<PARA>This thread starts execution when the <EMPHASIS>eCos</EMPHASIS> scheduler
is started. If the <EMPHASIS>eCos</EMPHASIS> kernel package is not
available (and hence there is no scheduler), then <FUNCTION>cyg_iso_c_start()</FUNCTION> will
invoke the <FUNCTION>main()</FUNCTION> function directly, i.e.
it will not return until the <FUNCTION>main()</FUNCTION> function
returns. </PARA>
<PARA>The <FUNCTION>main()</FUNCTION> function should be defined
as the following, and if defined in a C&plus;&plus; file,
should have &ldquo;C&rdquo; linkage: </PARA>
<PROGRAMLISTING>extern int <FUNCTION>main</FUNCTION>( 
  int <EMPHASIS>argc,</EMPHASIS>
  char *<EMPHASIS>argv[] </EMPHASIS>)</PROGRAMLISTING>
<PARA>The thread that is started by <FUNCTION>cyg_iso_c_start()</FUNCTION> can
be manipulated directly, if you wish. For example you can suspend
it. The kernel C API needs a handle to do this, which is available
by including the following in your source code.</PARA>
<PROGRAMLISTING>extern cyg_handle_t cyg_libc_main_thread;</PROGRAMLISTING>
<PARA>Then for example, you can suspend the thread with the line:</PARA>
<PROGRAMLISTING>cyg_thread_suspend( cyg_libc_main_thread );</PROGRAMLISTING>
<PARA>If you call <FUNCTION>cyg_iso_c_start()</FUNCTION> and
do not provide your own <FUNCTION>main()</FUNCTION> function,
the system will provide a <FUNCTION>main()</FUNCTION> for you
which will simply return immediately.</PARA>
<PARA>In the default configuration, <FUNCTION>cyg_iso_c_start()</FUNCTION> is
invoked automatically by the <FUNCTION>cyg_package_start()</FUNCTION> function
in the infrastructure configuration. This means that in the simplest
case, your program can indeed consist of simply:</PARA>
<PROGRAMLISTING>int main( int argc, char *argv[] )
{
 printf("Hello eCos\n");
}</PROGRAMLISTING>
<PARA>If you override <FUNCTION>cyg_package_start()</FUNCTION> or <FUNCTION>cyg_start()</FUNCTION>,
or disable the infrastructure configuration option CYGSEM_START_ISO_C_COMPATIBILITY
then you must ensure that you call <FUNCTION>cyg_iso_c_start()</FUNCTION> yourself
if you want to be able to have your program start at the entry point
of <FUNCTION>main()</FUNCTION> automatically.</PARA>
</SECT1>
</CHAPTER>
</PART>
