Description

   This routine requests delivery of a pending cancelation request
   to the calling thread. Thus, this routine creates a cancelation
   point in 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
   tis_setcancelstate().)

   This routine, when called within very long loops, ensures that a
   pending cancelation request is noticed within a reasonable amount
   of time.