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.