Why Must a Mutex Be Locked Before Calling `pthread_cond_signal()`?

Threading: The Significance of Mutex Locking for Condition Signaling

Asynchronous processes in multithreaded programming often require coordination and synchronization to ensure proper execution flow. Condition variables and mutexes play crucial roles in this context.

However, a common question arises regarding the necessity of locking a mutex before calling pthread_cond_signal. Despite varying opinions, it is crucial to recognize that locking the mutex is essential.

Why Mutex Locking Matters

Without locking the mutex, the potential for missed wakeups exists. Consider the following scenario:

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);

In this case, Process A may miss the wakeup signal from Process B if it is currently waiting on the condition variable. The mutex locking in Process A ensures that it has exclusive access to shared resources, preventing it from being woken up prematurely.

Process B (correct):
pthread_mutex_lock(&mutex);
condition = TRUE;
pthread_cond_signal(&cond);
pthread_mutex_unlock(&mutex);

By locking the mutex in Process B, the wakeup signal is guaranteed to reach Process A appropriately. Although it is technically possible to move pthread_cond_signal() after pthread_mutex_unlock(), this can lead to suboptimal scheduling and unnecessary locking.

Therefore, it is crucial to adhere to the practice of locking the mutex before calling pthread_cond_signal. This ensures reliable and predictable threading operations.

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