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<h3 class="section">31.3 Which Version of POSIX is Supported</h3>

<!-- unistd.h -->
<!-- POSIX.1 -->
<div class="defun">
&mdash; Macro: long int <b>_POSIX_VERSION</b><var><a name="index-g_t_005fPOSIX_005fVERSION-3522"></a></var><br>
<blockquote><p>This constant represents the version of the POSIX.1 standard to which
the implementation conforms.  For an implementation conforming to the
1995 POSIX.1 standard, the value is the integer <code>199506L</code>.

        <p><code>_POSIX_VERSION</code> is always defined (in <samp><span class="file">unistd.h</span></samp>) in any
POSIX system.

        <p><strong>Usage Note:</strong> Don't try to test whether the system supports POSIX
by including <samp><span class="file">unistd.h</span></samp> and then checking whether
<code>_POSIX_VERSION</code> is defined.  On a non-POSIX system, this will
probably fail because there is no <samp><span class="file">unistd.h</span></samp>.  We do not know of
<em>any</em> way you can reliably test at compilation time whether your
target system supports POSIX or whether <samp><span class="file">unistd.h</span></samp> exists.

        <p>The GNU C compiler predefines the symbol <code>__POSIX__</code> if the target
system is a POSIX system.  Provided you do not use any other compilers
on POSIX systems, testing <code>defined (__POSIX__)</code> will reliably
detect such systems. 
</p></blockquote></div>

<!-- unistd.h -->
<!-- POSIX.2 -->
<div class="defun">
&mdash; Macro: long int <b>_POSIX2_C_VERSION</b><var><a name="index-g_t_005fPOSIX2_005fC_005fVERSION-3523"></a></var><br>
<blockquote><p>This constant represents the version of the POSIX.2 standard which the
library and system kernel support.  We don't know what value this will
be for the first version of the POSIX.2 standard, because the value is
based on the year and month in which the standard is officially adopted.

        <p>The value of this symbol says nothing about the utilities installed on
the system.

        <p><strong>Usage Note:</strong> You can use this macro to tell whether a POSIX.1
system library supports POSIX.2 as well.  Any POSIX.1 system contains
<samp><span class="file">unistd.h</span></samp>, so include that file and then test <code>defined
(_POSIX2_C_VERSION)</code>. 
</p></blockquote></div>

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