title: Linux _SC_PHYS_PAGES not working on Mac OS X
body: <p>I am trying to compile a project on Mac OS that doesn't make any problems under Linux. I am using GCC on both operating systems. Besides other problems that I managed to fix, I get the following error when trying to compile on OSX:</p>
  
  <pre><code>error: ‘_SC_PHYS_PAGES’ was not declared in this scope
       long pages = sysconf(_SC_PHYS_PAGES);
                        ^
  </code></pre>
  
  <p><code>unistd.h</code> is included in the file where this error occurs.</p>
  
  <p>How can I fix this in a way that the code still compiles under Linux?</p>