CODEWITHSUNDEEP
clinuxpthreadscondition-variable
Earl Potters
Earl Potters

Reputation: 31

What happens when pthread_cond_broadcast is called and multiple threads are awoken only to compete for the same mutex?

What happens when you call pthread_cond_broadcast() and multiple threads wake up just to compete for the same mutex lock. One of the threads takes the mutex lock but what happens to the other threads? Do they go back to sleep? Or do they spin until the lock is available again?

Upvotes: 1

Views: 737

Answers (1)

John Bollinger
John Bollinger

Reputation: 180201

What happens when you call pthread_cond_broadcast() and multiple threads wake up just to compete for the same mutex lock. One of the threads takes the mutex lock but what happens to the other threads? Do they go back to sleep? Or do they spin until the lock is available again?

When you call pthread_cond_broadcast(), all threads then waiting on the specified condition variable stop doing so. All such threads will have passed (a pointer to) the same mutex to pthread_cond_wait(), else the behavior is undefined. Each thread that was unblocked will (re)acquire that mutex before returning successfully from pthread_cond_wait(). That may require some or even all of them to block, just as if they were all contending for the same mutex under any other circumstances. They do not spin, and they do not require any further interaction with the CV for them to resume, but each one will hold the mutex locked when it returns from pthread_cond_wait(), just as it did when it called that function.

Upvotes: 1

Related Questions