title: How and when should I use _POSIX_C_SOURCE in C programs?
body: <p>I've gotten myself into having to maintain some C project which should also compile on older platforms. At the moment, for some platforms, the macro <code>_POSIX_C_SOURCE</code> is defined. I was wondering - if it's acceptable to have it defined, should I not just define it always, on all platforms? And perhaps with the highest relevant value?</p>
  
  <p>To generalize, I suppose I'm asking: When and under what conditions should <code>_POSIX_C_SOURCE</code> be used?</p>