What happens if no threads are waiting and condition signal was sent?

问题

What happens if all threads are busy and main thread has sent thread cond signal ?

1 Main Thread and 3 pthreads in thread pool. 3 pthreads are in status of

    pthread_mutex_lock(&sync_mutex);
    pthread_cond_wait(&sync_cond, &sync_mutex);
    pthread_mutex_unlock(&sync_mutex);

Main thread has sent Signal to wake up the threads to process the work. In this situation, What if 3 threads are already busy and next signal has arrived?

回答1:

Nothing. The signal disappears.

回答2:

If your using one of the following functions:

pthread_cond_signal - restarts one of the threads that are waiting on the condition variable cond. pthread_cond_broadcast - wake up all threads blocked by the specified condition variable.

The manual states that

The pthread_cond_broadcast() and pthread_cond_signal() functions shall have no effect if there are no threads currently blocked on cond.

来源：https://stackoverflow.com/questions/9598034/what-happens-if-no-threads-are-waiting-and-condition-signal-was-sent