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

Question

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?

If no one is waiting for a signal, the signal will have no effect. Or what do you mean? — Johan Kotlinski, Mar 07 '12 at 08:32

score 11 · Accepted Answer · answered Mar 07 '12 at 08:44

11

Nothing. The signal disappears.

answered Mar 07 '12 at 08:44

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Thank you, then to avoid this do you have any suggestion to solve this problem? – Jae Park Mar 07 '12 at 09:08
2

If you want to record the times/count of signals even if there is nothing waiting yet, use a semaphore. Semaphores have a count, condvars events etc. do not. – Martin James Mar 07 '12 at 09:14
Semaphores.. I'll check on it thanks. what about using a simple queue to store the events and dequeue from it when Thread gets available? – Jae Park Mar 07 '12 at 09:17
1

@Jae: a condition variable **signals a change in condition**. The receiving threads must wait for a condition variable until a condition is met, which when using queues means wait until the queue is not empty (see http://stackoverflow.com/a/5538447/104774). – stefaanv Mar 07 '12 at 10:09

score 4 · Answer 2 · answered Mar 07 '12 at 08:46

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.

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

2 Answers2

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