Is Calling `pthread_cond_signal` Without a Mutex Safe?

Does Calling pthread_cond_signal Without Mutex Compromise Signaling?

Some literature suggests that calling pthread_cond_signal requires locking the mutex beforehand and unlocking it afterward. However, can you call pthread_cond_signal or pthread_cond_broadcast without mutex locking?

Answer:

No, it is unsafe to call pthread_cond_signal or pthread_cond_broadcast without locking the mutex first. This is illustrated through a simple example involving two processes (A and B).

Process A:

pthread_mutex_lock(&mutex);
while (condition == FALSE)
    pthread_cond_wait(&cond, &mutex);
pthread_mutex_unlock(&mutex);

Process B (Incorrect):

condition = TRUE;
pthread_cond_signal(&cond);

If condition starts as FALSE and Process B attempts to signal without mutex locking, it is possible for Process A to miss the wake-up signal due to an interleaving of instructions.

Process B (Correct):

pthread_mutex_lock(&mutex);
condition = TRUE;
pthread_cond_signal(&cond);
pthread_mutex_unlock(&mutex);

Locking the mutex in Process B prevents this issue, ensuring Process A receives the wake-up signal.

Note: While it is technically possible to move pthread_cond_signal() after pthread_mutex_unlock(), it is not recommended as it reduces thread scheduling efficiency.

The above is the detailed content of Is Calling `pthread_cond_signal` Without a Mutex Safe?. For more information, please follow other related articles on the PHP Chinese website!