Hands-On System Programming with Linux

Is the mutex locked?

In certain situations, the developer might be tempted to ask: given a mutex, can I find out if it's in the locked or unlocked state? Perhaps the reasoning is: if locked, let's unlock it.

There is a way to test this: with the pthread_mutex_trylock(3) API. If it returns EBUSY, it implies that the mutex is currently locked (otherwise, it should return 0, implying it's unlocked). But wait! There is an inherent race condition here; just think about this:

if (pthread_mutex_trylock(&mylock) != EBUSY)) {    <-- time t1    // it's unlocked                               <-- time t2}// it's locked

By the time we reach time t2, there is no guarantee that another thread has not, by now, locked the mutex in question! So, this approach is incorrect. (The only realistic ...