title: Undefined reference to "sleep" but I did include <unistd.h>
body: <p>Well, this must be a silly one. Here below is a can-not-be-simpler code in C. It can not compile saying "undefined reference to sleep". But I think I include all system header I need...</p>
  
  <pre><code>#include &lt;stdio.h&gt;
  #include &lt;unistd.h&gt;
  int main()
  {
      printf("Test starts.\n");
      sleep(1);
      printf("Test ends.\n");
  
      return 1;
  }
  </code></pre>