Subj : Re: Help: Why is pthread_cond_broadcast not unblocking all threads
To   : comp.programming.threads
From : Loic Domaigne
Date : Sun Feb 13 2005 11:42 pm

Hi Harry,

> Hi: I am creating 3 threads and they all are waiting in on
> pthread_cond_wait(). A seperate thread is sending a signal
> pthread_cond_broadcast. However, only one thread comes out of the wait
> instead of all of the thread coming out.. Please help. Thanks a whole
> bunch

First note that, normally, a predicate should be associated to a 
condition variable. That is, you are normally waiting for the predicate 
to become true as follows:

pthread_mutex_lock (&mutex);
while ( !predicate) {
     pthread_cond_wait (&condition, &mutex)
}
// here the mutex is locked again


Whereas you should signal in a similar manner:

pthread_mutex_lock (&mutex);
predicate = true;
pthread_mutex_unlock (&mutex);
pthread_cond_broadcast (&condition);


Now assuming that you just want to test the "wake-up" mechanism and do 
not mind about the predicate, your code becomes:

void* WorkerThreads(void* )
{
      cout << "Starting to Wait[" << pthread_self() << "]" << endl;
      pthread_mutex_lock( &mutex );
      pthread_cond_wait( &condition, &mutex );
      pthread_mutex_unlock( &mutex );

      cout << "Out of Wait[" << pthread_self() << "]" << endl;
      return NULL;
}

void* Broadcast(void* )
{
      sleep(7);
      pthread_cond_broadcast( &condition );
      cout << "sent the broadcast signal" << endl;
      return NULL;
}

( rest unchanged )


Hope this Help,
Loic.

.