Description

   This routine requests delivery of a pending cancelation request
   to the calling thread. Thus, calling this routine creates a
   cancelation point within the calling thread.

   The cancelation request is delivered only if a request is pending
   for the calling thread and the calling thread's cancelability
   state is enabled. (A thread disables delivery of cancelation
   requests to itself by calling pthread_setcancelstate().)

   When called within very long loops, this routine ensures that
   a pending cancelation request is noticed by the calling thread
   within a reasonable amount of time.