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

    This routine wakes one thread waiting on a condition variable.
    It can only be called from a software interrupt handler routine.
    Calling this routine implies that it might be possible for a
    single waiting thread to proceed. Call this routine when any
    thread waiting on the specified condition variable might find its
    predicate true.

    The scheduling policies of the waiting threads determine which
    thread is awakened. For policies SCHED_FIFO and SCHED_RR, a
    blocked thread is chosen in priority order, using first-in/first-
    out (FIFO) within priorities.

    You can call this routine when the associated mutex is either
    locked or unlocked. (Never try to lock a mutex from an interrupt
    handler.)

    This routine allows you to signal a thread from a software
    interrupt handler. Do not call this routine from noninterrupt
    code. If you want to signal a thread from the normal noninterrupt
    level, use pthread_cond_signal.

                                   NOTE

       If a waiting thread has a preemptive scheduling policy and
       a higher priority than the thread which was running when
       the interrupt occurred, then the waiting thread will preempt
       the interrupt routine and begin to run immediately. This
       is unlike pthread_cond_signal_int_np() which causes the
       condition variable to be signaled at a safe point after
       the interrupt has completed. pthread_cond_sig_preempt_int_
       np() avoids the possible latency which pthread_cond_signal_
       int_np() may introduce; however, a side effect of this is
       that during the call to pthread_cond_sig_preempt_int_np()
       other threads may run if a preemption occurs. Thus, once an
       interrupt routine calls pthread_cond_sig_preempt_int_np()
       it can no longer rely on any assumptions of exclusivity
       or atomicity which are typically provided by interrupt
       routines. Furthermore, once the call to pthread_cond_sig_
       preempt_int_np() is made, in addition to other threads
       running, subsequent interrupts may be delivered at any time
       as well (that is, they will not be blocked until the current
       interrupt completes). For this reason, it is recommended
       that pthread_cond_sig_preempt_int_np() be called as the last
       statement in the interrupt routine.