POSIX 条件变量和互斥量 "competition"

Question

当一个线程等待一个条件变量时，关联的互斥锁被（原子地）释放（解锁）。当该条件变量（由不同的线程）发出信号时，一个（用于信号）或所有（用于广播）等待线程is/are被唤醒，自动重新获取（锁定）互斥量。

如果一个或多个其他线程正在等待获取（锁定）同一个互斥锁，但没有等待相同的条件，会发生什么情况？在其他线程获取（锁定）互斥锁之前，等待条件变量的线程是否保证被唤醒（并因此获取互斥锁），或者其他线程是否可以获取（锁定）互斥锁在等待条件变量的线程之前？

[注意：为清楚起见，下面的示例进行了简化。 Thread_B 并没有真正开始 Thread_C，但是 Thread_C 保证不会运行直到 Thread_B 获得了互斥量 - 它不会与 [= 竞争30=] 用于 Thread_A 等待条件变量后的互斥体]

Thread_A:

pthread_mutex_lock(&myMutex);
while (!someState) {
    pthread_cond_wait(&myCondVar,&myMutex);
}
// do something
pthread_mutex_unlock(&myMutex);

Thread_B:

pthread_mutex_lock(&myMutex);
// do other things
someState = true;
// start Thread_C here
pthread_cond_signal(&myCondVar);
pthread_mutex_unlock(&myMutex);

Thread_C:

pthread_mutex_lock(&myMutex);
// can I reach this point after Thread_B releases the mutex,
// but before Thread_A re-acquires it after being signaled?

// do things that may interfere with Thread_A...
pthread_mutex_unlock(&myMutex);

编辑：选择下面接受的答案是因为它清楚地表明，无论 reader 是否同意给出的解释，都存在足够的歧义，唯一安全的假设是受访者的假设。请注意，精通 C++ 标准语言的其他人可能会发现文本完全不含糊......我不属于该组。

Answer 1

与在 pthread_mutex_lock() 中尝试获取相同互斥量时已阻塞的任何其他线程相比，从 pthread_cond_[timed]wait() 唤醒时获取互斥量没有什么特别之处。

Per the POSIX 7 pthread_cond_signal() documentation（加粗我的）：

If more than one thread is blocked on a condition variable, the scheduling policy shall determine the order in which threads are unblocked. When each thread unblocked as a result of a pthread_cond_broadcast() or pthread_cond_signal() returns from its call to pthread_cond_wait() or pthread_cond_timedwait(), the thread shall own the mutex with which it called pthread_cond_wait() or pthread_cond_timedwait(). The thread(s) that are unblocked shall contend for the mutex according to the scheduling policy (if applicable), and as if each had called pthread_mutex_lock().

从 pthread_cond_[timed]wait() 唤醒后获取互斥量需要完全就好像线程调用了 pthread_mutex_lock().

简而言之，任何线程都可以获得互斥量。

POSIX 条件变量和互斥量 "competition"

POSIX condition variable & mutex "competition"

c++

multithreading

posix

mutex

condition-variable