title: What does -D_XOPEN_SOURCE do/mean?
body: <p>I recently encountered some code that gcc would not compile without this arg. I checked the gcc man page, but did not find this specific option. I did find <code>XOPEN_SOURCE</code>, but there was little explanation of what it does.</p>
  
  <p>Can someone please elaborate? I know <code>-D_XOPEN_SOURCE</code> can be set to different values, such <code>400</code>, <code>600</code>, but what do those do?</p>