pthread_cond_broadcast Syntax

int pthread_cond_broadcast(pthread_cond_t *cv);

#include <pthread.h>

pthread_cond_t cv;
int ret;

/* all condition variables are signaled */
ret = pthread_cond_broadcast(&cv); 

When no threads are blocked on the condition variable, pthread_cond_broadcast() has no effect.

Since pthread_cond_broadcast() causes all threads blocked on the condition to contend again for the mutex lock, use pthread_cond_broadcast() with care. For example, use pthread_cond_broadcast() to allow threads to contend for varying resource amounts when resources are freed, as shown in Example 4–10.

Example 4–10 Condition Variable Broadcast

pthread_mutex_t rsrc_lock;
pthread_cond_t rsrc_add;
unsigned int resources;

get_resources(int amount)
{
    pthread_mutex_lock(&rsrc_lock);
    while (resources < amount) {
        pthread_cond_wait(&rsrc_add, &rsrc_lock);
    }
    resources -= amount;
    pthread_mutex_unlock(&rsrc_lock);
}

add_resources(int amount)
{
    pthread_mutex_lock(&rsrc_lock);
    resources += amount;
    pthread_cond_broadcast(&rsrc_add);
    pthread_mutex_unlock(&rsrc_lock);
}

Note that in add_resources() whether resources are updated first, or if pthread_cond_broadcast() is called first inside the mutex lock does not matter.

Modify the associated condition under the protection of the same mutex that is used with the condition variable being signaled. Otherwise, the condition could be modified between its test and blocking in pthread_cond_wait(), which can cause an infinite wait.