Should You Protect `pthread_cond_signal` with a Mutex?

pthread_cond_signal: When and Why to Protect with Mutex

When using condition variables for thread synchronization, the question arises whether it is necessary to acquire a mutex before calling pthread_cond_signal.

The Importance of Mutex Protection

The documentation mentions the need to lock the mutex before calling pthread_cond_signal to prevent potential issues with missed wakeups. Consider the following situation:

// 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 Process B signals the condition without locking the mutex, it's possible that Process A may miss the wakeup if its thread context switch occurs during the unprotected code block.

Ensuring Proper Wakeup

To ensure proper wakeup, it is crucial to modify Process B to acquire the mutex before changing the condition and signaling the wakeup:

// Process B (Correct)
pthread_mutex_lock(&mutex);
condition = TRUE;
pthread_cond_signal(&cond);
pthread_mutex_unlock(&mutex);

With the mutex protection in place, the wakeup will always be received by waiting threads like Process A, eliminating the potential for missed wakeups. While it is technically possible to signal the condition after releasing the mutex, this is not recommended for optimal scheduler performance and potential race conditions.

Therefore, it is essential to remember that within the code path that modifies the condition and signals the wakeup, acquiring and releasing the mutex is necessary to ensure proper synchronization and avoid wakeup failures.

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