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

    This routine sets the calling thread's cancelability state to
    the value specified in the state argument and returns the calling
    thread's previous cancelability state in the location referenced
    by the oldstate argument.

    When a thread's 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 is enabled.

    When a thread is created, its default cancelability state is
    PTHREAD_CANCEL_ENABLE. When this routine is called prior to
    loading threads, the cancelability state propagates to the
    initial thread in the executing program.

    Possible Problems When Disabling Cancelability

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

    When a thread's cancelability state is disabled, no routine
    called within that thread is cancelable. As a result, the user
    is unable to cancel the operation. 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.