title: What is "urgent data"?
body: <p>The man page of <a href="http://linux.die.net/man/2/epoll_ctl" rel="nofollow noreferrer">epoll_ctl()</a> says about <code>EPOLLPRI</code>:</p>
  
  <blockquote>
    <p>There is urgent data available for 
    read(2) operations.</p>
  </blockquote>
  
  <p>How exactly is "urgent data" defined and who decides which data has priority?</p>