title: Which preprocessor symbol to check to detect FreeBSD
body: <p>I want to compile some piece of c code if I'm on FreeBSD. What <code>#ifdef</code> do I use for this?</p>
  
  <p>I realize this is a basic question which the correct web search should answer, but I cannot find the correct search term(s). I've seen code checking <code>FREEBSD</code> and/or <code>__FreeBSD__</code> and/or <code>__FreeBSD</code> but nothing I found mentioned "this is what you need to do".</p>