Question

线程调用pthread_cond_timedwait后返回ETIMEDOUT，线程是否拥有互斥锁？

我最初会认为不是，但即使在pthread_mutex_unlock返回pthread_cond_timedwait之后，我们仍然必须致电ETIMEDOUT。

成功返回后，互斥锁应被锁定，并由调用线程拥有。

因此，在非成功返回（返回值！= 0）时，我认为互斥锁不属于。

但是，如果我们在pthread_mutex_unlock之后没有调用ETIMEDOUT，则互斥锁似乎处于断开状态（即我无法获得另一个线程来获取它，它只是停止）。

文档也提示这一点，因为无论pthread_cond_timedwait的返回值是什么，总是解锁互斥锁：

(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);

那么，线程是否始终在pthread_cond_timedwait之后获取互斥锁？它没有多大意义，因为为了再次获取互斥锁，调用必须阻止超过指定的时间。

Answer 1

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).

pthread_cond_timedwait超时后，线程是否拥有互斥锁？

1 个答案: