Does a thread own a mutex after pthread_cond_timedwait times out?

Question

After a thread calls pthread_cond_timedwait, and it returns ETIMEDOUT, does the thread own the mutex?

I would initially think NO, but it appears that we must call pthread_mutex_unlock even after pthread_cond_timedwait returns ETIMEDOUT.

The documentation says:

Upon successful return, the mutex shall have been locked and shall be owned by the calling thread.

So, upon nonsuccessful return (return value != 0), the mutex is NOT owned, I would think.

But, if we do not call pthread_mutex_unlock after ETIMEDOUT, the mutex appears to be in a broken state (ie I cannot get another thread to acquire it, it just stalls).

The documentation also hints at this as well, as they always unlock the mutex regardless of the return value of pthread_cond_timedwait:

(void) pthread_mutex_lock(&t.mn);
                t.waiters++;
        clock_gettime(CLOCK_REALTIME, &ts);
        ts.tv_sec += 5;
        rc = 0;
        while (! mypredicate(&t) && rc == 0)
                rc = pthread_cond_timedwait(&t.cond, &t.mn, &ts);
        t.waiters--;
        if (rc == 0) setmystate(&t);
(void) pthread_mutex_unlock(&t.mn);

So, does the thread ALWAYS acquire the mutex after pthread_cond_timedwait? It does not really make sense because the call would have to block more than the specificed time in order to acquire the mutex again.

score 6 · Accepted Answer · answered Oct 20 '16 at 12:55

You are looking at an older issue of POSIX. Issue 7 has this clarified text:

When such timeouts occur, pthread_cond_timedwait() shall nonetheless release and re-acquire the mutex referenced by mutex, and may consume a condition signal directed concurrently at the condition variable.

If it didn't reacquire the mutex in this case you'd have to re-acquire it in the calling code anyway so you could re-test the condition after a timeout, because you might have consumed a condition signal. It's only if the condition you're waiting for hasn't occurred and a timeout occurred that you should treat it as the timeout case.

The timeout isn't guarding against the mutex being held for overly long, it's guarding against a condition signal not arriving in a timely manner (generally, the mutex should only be held for short, relatively deterministic periods whereas the condition being waited for might be influenced by external inputs).

Does a thread own a mutex after pthread_cond_timedwait times out?

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