Description

   This routine sends a cancelation request to the specified target
   thread. A cancelation request is a mechanism by which a calling
   thread requests the target thread to terminate as quickly as
   possible. Issuing a cancelation request does not guarantee that
   the target thread will receive or handle the request.

   When the cancelation request is acted on, all active cleanup
   handler routines for the target thread are called. When the last
   cleanup handler returns, the thread-specific data destructor
   routines are called for each thread-specific data key with a
   destructor and for which the target thread has a non-NULL value.
   Finally, the target thread is terminated.

   Note that cancelation of the target thread runs asynchronously
   with respect to the calling thread's returning from pthread_
   cancel(). The target thread's cancelability state and type
   determine when or if the cancelation takes place, as follows:

   1. The target thread can delay cancelation during critical
      operations by setting its cancelability state to PTHREAD_
      CANCEL_DISABLE.

   2. Because of communication delays, the calling thread can only
      rely on the fact that a cancelation request will eventually
      become pending in the target thread (provided that the target
      thread does not terminate beforehand).

   3. The calling thread has no guarantee that a pending cancelation
      request will be delivered because delivery is controlled by
      the target thread.

   When a cancelation request is delivered to a thread, termination
   processing is similar to that for pthread_exit(). For more
   information about thread termination, see the Thread Termination
   section of pthread_create().

   This routine is preferred in implementing an Ada abort statement
   and any other language- or software-defined construct for
   requesting thread cancelation.

   The results of this routine are unpredictable if the value
   specified in thread refers to a thread that does not currently
   exist.