title: implicit declaration of function usleep
body: <pre><code>gcc (GCC) 4.6.3
  c89
  </code></pre>
  
  <p>I am trying to use <code>usleep</code>. However, I keep getting the following warning:</p>
  
  <blockquote>
    <p>implicit declaration of function usleep</p>
  </blockquote>
  
  <p>I have included the <code>unistd.h</code> header file.</p>
  
  <p>The man pages mentions something about this. But I am not sure I understand by it:</p>
  
  <pre><code>usleep():
     Since glibc 2.12:
         _BSD_SOURCE ||
             (_XOPEN_SOURCE &gt;= 500 ||
                 _XOPEN_SOURCE &amp;&amp; _XOPEN_SOURCE_EXTENDED) &amp;&amp;
             !(_POSIX_C_SOURCE &gt;= 200809L || _XOPEN_SOURCE &gt;= 700)
     Before glibc 2.12:
         _BSD_SOURCE || _XOPEN_SOURCE &gt;= 500 || _XOPEN_SOURCE &amp;&amp; _XOPEN_SOURCE_EXTENDED
  </code></pre>
  
  <p>But not sure what I a to do with the above?</p>