Why Does Calling `pthread_cond_signal` Without a Mutex Lock Cause Unreliable Wakeups?

Calling pthread_cond_signal Without Locking Mutex: Unreliable Wakeups

Contrary to a common misconception, calling pthread_cond_signal or pthread_cond_broadcast without acquiring the mutex can lead to missed wakeups, compromising thread synchronization.

Why Mutex Locking is Essential

The pthread_cond_signal function notifies waiting threads that the associated condition has changed. However, if the mutex protecting the shared data related to the condition is not locked, a race condition can occur.

Consider a scenario where one thread (A) is waiting on the condition variable, and another thread (B) sets the condition to true and calls pthread_cond_signal. Without mutex locking, it's possible for thread B to signal while thread A is still checking the condition and has not yet entered the pthread_cond_wait call.

The Risk of Missed Wakeups

As a result of this race condition, thread A might miss the wakeup signal and remain waiting indefinitely. This situation is especially problematic when thread A is implementing a critical section and needs to be notified promptly of changes to shared data.

Correct Synchronization Pattern

To ensure reliable wakeups, the following synchronization pattern should be strictly followed:

pthread_mutex_lock(&mutex);
// Change shared data
pthread_cond_signal(&cond);
pthread_mutex_unlock(&mutex);

Thread B should only signal after acquiring the mutex and changing the shared data, ensuring that any pending wakeups will be received by thread A.

Conclusion

Locking a mutex around calls to pthread_cond_signal is crucial for safeguarding thread synchronization and preventing missed wakeups. Failure to do so can lead to unpredictable and potentially catastrophic race conditions.

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