如果在调用pthread_mutex_lock（）时线程未成功执行该怎么办？

Question

I'm doing a experiment, which, I think, is a guaranteed deadlock situation:

void *thread_1(void *vptr)
{
    pthread_mutex_lock(&a);
    sleep(1);
    pthread_mutex_lock(&b);
    pthread_mutex_unlock(&b);
    pthread_mutex_unlock(&a);
}

void *thread_2(void *vptr)
{
    pthread_mutex_lock(&b);
    sleep(1);
    pthread_mutex_lock(&a);
    pthread_mutex_unlock(&a);
    pthread_mutex_unlock(&b);
}

What happens if a thread cannot lock? Does it get into a queue, waiting for the mutex, or does it simply move on to the next instruction?

Answer 1

In your case both threads will be suspended forever waiting for the mutexes. If you start both threads at once.

Why don't you just create the mutexes and the threads and then run your example?

Answer 2

If a thread tries to lock a mutex that is already locked by another thread, it will be suspended. In this case, its execution will not continue until it has acquired the lock. (There is no "queue" though - just an unordered set of waiting threads. Any waiting thread could be the next to acquire the mutex).

If a thread tries to lock a mutex that is already locked by itself or not initialised properly, then pthread_mutex_lock() may return a non-zero error code, and execution will continue without the mutex being acquired.

Answer 3

It depends on the reason why it couldn't lock. Eg if the thread already holds the lock, an error might be returned. You should check the return value, anyhow, and check the manual page for the error codes that are return in errno . Mine (linux) states:

  The pthread_mutex_lock() function may fail if: EDEADLK The current thread already owns the mutex.