title: errno location on different systems
body: <p>I have confusions about what implementation of errno() should I use in my programs.</p>
  
  <p>As far as I know, the standard errno() is defined in errno.h. However, Visual Studio also has errno() in stdlib.h. Maybe that's incorrect, but for me stdlib.h's errno() is faster than errno.h's one. But errno() is also defined in stddef.h.</p>
  
  <p>Which one should I use? <code>#ifdef _WIN32 #include &lt;stdlib.h&gt; #else #include &lt;errno.h&gt; #endif</code>? </p>