title: Difference between poll_disconnected, poll_in, poll_out and cancellations
body: <p>I am debugging <a href="https://twistedmatrix.com/trac/" rel="nofollow noreferrer">twisted</a> python application </p>
  
  <pre><code>    from twisted.internet import reactor
      threadpool = reactor.getThreadPool()
  </code></pre>
  
  <p>And seeing that:</p>
  
  <pre><code>_POLL_DISCONNECTED = 24 
  _POLL_IN = 1
  _POLL_OUT = 22
  _cancellations = 100
  </code></pre>
  
  <p>What does this constants or parameters mean? Does they come from POSIX? What does cancellations mean?</p>