title: what does it mean if sa_sigaction is set to NULL when calling sigaction()?
body: <p>What does the following code do?</p>
  
  <pre><code>struct sigaction saStruct;
  saStruct.sa_flags = SA_NOCLDWAIT|SA_NODEFER|SA_SIGINFO;
  saStruct.sa_sigaction = NULL;
  sigemptyset(&amp;saStruct.sa_mask);
  sigaction(SIGCHLD, &amp;saStruct, NULL);
  </code></pre>
  
  <p>In particular, what does it mean that <code>sa_sigaction</code> is set to NULL? Is this a complicated way to express something much more simple? I couldn't find any reference to this usage in the <a href="http://man7.org/linux/man-pages/man2/sigaction.2.html" rel="nofollow">sigaction man page</a>.</p>
  
  <p>I'm primarily interested in answers for Linux (>= 2.6).</p>