title: Eventfd role in Linux
body: <p>I read that eventfd is efficient as kernel will informs user process about event on file description.  I saw its example at  </p>
  
  <p> <a href="http://graphics.sci.ubu.ac.th/api/man-html/htmlman2/eventfd.2.html" rel="nofollow">http://graphics.sci.ubu.ac.th/api/man-html/htmlman2/eventfd.2.html</a>    </p>
  
  <ol>
  <li>I saw two argument :<br>
  a. one is unsigned int initval, which is kernel counter. how can i use this?<br/>
  b. int flags, what is its role?</li>
  <li>what is role and where can i use it?</li>
  </ol>
  
  <p>I saw example but its uses unable to understand.  </p>
  
  <p>Why is it required?</p>