title: What does it mean by "#define X X"?
body: <p>In Linux header file <code>epoll.h</code>, I found the following code:</p>
  
  <pre><code>enum EPOLL_EVENTS
  {
      EPOLLIN = 0x001,
  #define EPOLLIN EPOLLIN
  ...
  }
  </code></pre>
  
  <p>What does it mean by <code>#define EPOLLIN EPOLLIN</code>?</p>