Subj : Help: Why is pthread_cond_broadcast not unblocking all threads
To   : comp.programming.threads
From : harryr2003
Date : Sat Feb 12 2005 10:48 am

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

Harry.
The output from the following attached program is
$ pgm
Starting to Wait[4]
Starting to Wait[5]
Starting to Wait[6]
sent the broadcast signal
Out of Wait[4]

A simple program that spins off 3 detached thread that waits for a
signal. The other thread sends a signal.

#include <iostream>
#include <unistd.h>
#include    <pthread.h>
#include    <signal.h>
pthread_cond_t condition;
pthread_mutex_t mutex;
void DetachThreadAndRun(void * StartFunc_(void*), void * arg);
using namespace std;
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_mutex_unlock( &mutex );
     pthread_cond_broadcast( &condition );
     cout << "sent the broadcast signal" << endl;
     return NULL;
}

// PROGRAM..
int main(int argc, char **argv)
{
     pthread_mutex_init( &mutex, 0 );
     pthread_cond_init( &condition, 0 );

   for(int i=0; i < 3; i++)
     DetachThreadAndRun(WorkerThreads, (void *) NULL);

     DetachThreadAndRun(Broadcast, (void *) NULL);
     pause();

     pthread_mutex_destroy( &mutex );
     pthread_cond_destroy( &condition );

  return 0;
}

void DetachThreadAndRun(void * StartFunc_(void*), void * arg)
{
  pthread_t      tid; pthread_attr_t attr; int n;
  pthread_attr_init(&attr);
  pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_DETACHED);
  pthread_create(&tid, &attr, StartFunc_, (void *) arg);
  pthread_attr_destroy(&attr);
}

.