Question

According to the man page, pthread_cond_broadcast wakes up all the threads that are waiting on condition variable (condvar). And those waken threads will hold back the mutex lock and return from pthread_cond_wait.

But what I am confusing is: Isn’t it the mutex lock should held by only one thread in the same time?

Thanks in advance.

Answer 1

Condition variables work like this:

/* Lock a mutex. */
pthread_mutex_lock(&mtx);

/* Wait on condition variable. */
while (/* condition *.)
    pthread_cond_wait(&cond, &mtx);

/* When pthread_cond_wait returns mtx is atomically locked. */

/* ... */

/* Unlock the mutex. */
pthread_mutex_unlock(&mtx);

So the main point to understand is that many threads can wake up when a broadcast is sent, but only one will “win” the race and actually lock mtx and get out of the loop.