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    <p>The Single UNIX Specification requires the existence of <a href="http://pubs.opengroup.org/onlinepubs/7908799/xsh/unistd.h.html"><code>unistd.h</code></a>, which can tell you the POSIX version (via the <code>_POSIX_VERSION</code> macro).</p> <p><em>But how can you include <code>unistd.h</code> if you don't know yet that you are in fact compiling on a UNIX?</em></p> <p>That is where <a href="http://gcc.gnu.org/onlinedocs/cpp/System_002dspecific-Predefined-Macros.html">this GCC document</a> comes handy. According to it, testing for the presence, or evaluation-to-true of <code>__unix__</code> should tell you that the system is a UNIX. So:</p> <pre><code>#ifdef __unix__ /* Yes it is a UNIX because __unix__ is defined. */ #include &lt;unistd.h&gt; /* You can find out the version with _POSIX_VERSION. .. .. */ #endif </code></pre> <p><code>__unix__</code> is not defined on Mac OS X, so to account for that, you could instead do:</p> <pre><code>#if defined (__unix__) || (defined (__APPLE__) &amp;&amp; defined (__MACH__)) </code></pre> <p>To get a list of system specific predefined macros on your system, you may execute:</p> <pre><code>cpp -dM /dev/null </code></pre> <p>For example, my GNU/Linux system also additionally defines <code>__linux__</code> and <code>__gnu_linux__</code> apart from <code>__unix__</code> and a bunch of other stuff.</p> <hr> <p>Another useful document that you must look at is <a href="http://sourceforge.net/apps/mediawiki/predef/index.php?title=Operating_Systems#UNIX_Environment">this Wiki</a>.</p> <p>It <a href="http://sourceforge.net/apps/mediawiki/predef/index.php?title=Standards#Unix_Standards">goes on to present a way of detecting the presence and version of POSIX</a> in a way similar to the one I described above.</p> <hr> <p><strong>EDIT:</strong> Since you really want to do all this because you want to decide which directory separator to use, look at <a href="http://msdn.microsoft.com/en-us/library/windows/desktop/aa365247%28v=vs.85%29.aspx">this URL</a>. It says:</p> <blockquote> <p><strong>Note</strong> File I/O functions in the Windows API convert "/" to "\" as part of converting the name to an NT-style name, except when using the "\?\" prefix as detailed in the following sections.</p> </blockquote> <p>I don't program on Windows, or know <s>much</s> anything about it, so I can't say I've banked on this.</p>
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    1. POHow can I determine if the operating system is POSIX in C?
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    1. POHow can I determine if the operating system is POSIX in C?
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    1. COWell you know that UNIX is (mostly) POSIX, but it doesn't mean non-UNIX is not POSIX though.
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    2. CO`unistd.h` defines the `_POSIX_*` macros. UNIX is POSIX, so there's a `unistd.h` for sure. My point was that you could have a POSIX compliant system that is not UNIX, where `__unix__` wouldn't be defined (speculations really). In short, POSIX is not UNIX. ;-)
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    3. CO@netcoder - I went about it in the reverse way. I wanted to be sure `unistd.h` is *there at all* before I can include it and see the POSIX version. Eventually, the only 'safe' way I could see was to look for `__unix__`. But you are right (POSIX != UNIX), I wouldn't say it's bullet proof.
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