Unblock a Specific Thread

pthread_cond_signal(3T)

Use pthread_cond_signal(3T) to unblock one thread that is blocked on the condition variable pointed to by cv. (For Solaris threads, see "cond_signal(3T)".)

Prototype:
int	pthread_cond_signal(pthread_cond_t *cv);

#include <pthread.h>

pthread_cond_t cv;
int ret;

/* one condition variable is signaled */
ret = pthread_cond_signal(&cv); 

Call pthread_cond_signal() under the protection of the same mutex 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.

The scheduling policy determines the order in which blocked threads are awakened. For SCHED_OTHER, threads are awakened in priority order.

When no threads are blocked on the condition variable, then calling pthread_cond_signal() has no effect.

Return Values

pthread_cond_signal() returns zero after completing successfully. Any other returned value indicates that an error occurred. When the following condition occurs, the function fails and returns the corresponding value.

EINVAL

cv points to an illegal address.

Example 4-8 Using pthread_cond_wait() and pthread_cond_signal()

pthread_mutex_t count_lock;
pthread_cond_t count_nonzero;
unsigned count;

decrement_count()
{
    pthread_mutex_lock(&count_lock);
    while (count == 0)
        pthread_cond_wait(&count_nonzero, &count_lock);
    count = count - 1;
    pthread_mutex_unlock(&count_lock);
}

increment_count()
{
    pthread_mutex_lock(&count_lock);
    if (count == 0)
        pthread_cond_signal(&count_nonzero);
    count = count + 1;
    pthread_mutex_unlock(&count_lock);
}