Condition Variable Broadcast Example

Since pthread_cond_broadcast() causes all threads blocked on the condition to contend again for the mutex lock, use it 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() it does not matter whether resources is updated first or if pthread_cond_broadcast() is called first inside the mutex lock.

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