title: Linux: Undefined reference to "sleep", even though I included unistd.h
body: <p>Trying to use <code>sleep()</code> to pause my program briefly.
  Except when I "make" I get an <code>undefined reference to sleep</code>, even though I <code>#include &lt;unistd.h&gt;</code> at the top.</p>
  
  <blockquote>
  <pre><code>In function `main':
  [...]/source/ball.c:232: undefined reference to `sleep' 
  [...]/source/ball.c:240: undefined reference to `sleep'
  collect2: error: ld returned 1 exit status
  </code></pre>
  </blockquote>
  
  <p>I'm using linux.
  Any suggestions?
  Thanks</p>
  
  <p>EDIT: This Is not a duplicate question because every other time this question has been asked, the user was running it on Windows. The solution. for me, is not simply to include the windows header.</p>
  
  <p>EDIT(2): The link to that other question does not help me. That's why I made a new post. I'm not on windows!!</p>