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.