title: Determine whether a thread is blocked
body: <p>Does anyone know of a way to determine whether a thread is currently blocking?  Basically, I want to check whether a certain thread is blocking (in this case on a AF_UNIX datagram socket receive call) and send it a signal to interrupt if it is.</p>
  
  <p>I'm working on Linux using Boost.Thread meaning underneath I'm using pthreads.  My system has NPTL.</p>
  
  <p>I think the answer is "no", but want to see if I'm missing something.</p>