VMS Help POSIX Threads, PTHREAD routines, pthread_setcancelstate, Description  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    This routine sets the calling thread's cancelability state and
    returns the calling thread's previous cancelability state in
    oldstate.

    When cancelability state is set to PTHREAD_CANCEL_DISABLE, a
    cancelation request cannot be delivered to the thread, even if a
    cancelable routine is called or asynchronous cancelability type
    is enabled.

    When a thread is created, its default cancelability state is
    PTHREAD_CANCEL_ENABLE.

    Possible Problems When Disabling Cancelability

    The most important use of thread cancelation is to ensure
    that indefinite wait operations are terminated. For example, a
    thread that waits on some network connection, which can possibly
    take days to respond (or might never respond), should be made
    cancelable.

    When a thread's cancelability is disabled, no routine in that
    thread is cancelable. As a result, the user is unable to
    cancel the operation performed by that thread. When disabling
    cancelability, be sure that no long waits can occur or that it is
    necessary for other reasons to defer cancelation requests around
    that particular region of code.