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copied!<p>Curious...I can reproduce the problem on Mac OS X 10.9 Mavericks with GCC 4.8.2 and with Clang.</p> <p>POSIX says that some functions that fail will set <code>errno</code> (and <a href="http://pubs.opengroup.org/onlinepubs/9699919799/functions/fork.html" rel="nofollow"><code>fork()</code></a> is one of those functions), but does not say that functions that succeed will not set <code>errno</code>. For example, on Solaris, many standard I/O functions set <code>errno</code> if the output stream is not a terminal. However, resetting <code>errno = 0;</code> after the <code>printf()</code> doesn't alter the behaviour on Mac OS X.</p> <h3>POSIX 2008 (System Interfaces — General Information: 3. <a href="http://pubs.opengroup.org/onlinepubs/9699919799/functions/V2_chap02.html#tag_15_03" rel="nofollow">Error numbers</a>):</h3> <blockquote> <p>Some functions provide the error number in a variable accessed through the symbol <code>errno</code>, defined by including the <code><errno.h></code> header. The value of <code>errno</code> should only be examined when it is indicated to be valid by a function's return value. No function in this volume of POSIX.1-2008 shall set <code>errno</code> to zero. For each thread of a process, the value of <code>errno</code> shall not be affected by function calls or assignments to <code>errno</code> by other threads.</p> </blockquote> <p>If <code>fork()</code> failed, then <code>errno</code> would be set to indicate the failure. When it succeeds, it is not technically valid to inspect <code>errno</code>. And this is a demonstration of why.</p>

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