虚假唤醒（spurious wakeup）

On a multi-processor, it may be impossible for an implementation of pthread_cond_signal() to avoid the unblocking of more than one thread blocked on a condition variable.

The effect is that more than one thread can return from its call to pthread_cond_wait() or pthread_cond_timedwait() as a result of one call to pthread_cond_signal(). This effect is called “spurious wakeup”. Note that the situation is self-correcting in that the number of threads that are so awakened is finite; for example, the next thread to call pthread_cond_wait() after the sequence of events above blocks.

While this problem could be resolved, the loss of efficiency for a  fringe condition that occurs only rarely is unacceptable, especially given that one has to check the predicate associated with a condition variable anyway. Correcting this problem would unnecessarily reduce the degree of concurrency in this basic building block for all higher-level synchronization operations.

在多核处理器下，pthread_cond_signal可能会激活多于一个线程（阻塞在条件变量上的线程）。结果是，当一个线程调用pthread_cond_signal()后，多个调用pthread_cond_wait()或pthread_cond_timedwait()的线程返回。这种效应成为”虚假唤醒”(spurious wakeup)。

虽然虚假唤醒在pthread_cond_wait函数中可以解决，为了发生概率很低的情况而降低边缘条件（fringe condition）效率是不值得的，纠正这个问题会降低对所有基于它的所有更高级的同步操作的并发度。所以pthread_cond_wait的实现上没有去解它。

static void *thread_func(void *arg)

{

    while (1) {

      pthread_mutex_lock(&mtx);          //这个mutex主要是用来保证pthread_cond_wait的并发性

      while (msg_list.empty())  {    //pthread_cond_wait里的线程可能会被意外唤醒（虚假唤醒），如果这个时候，则不是我们想要的情况。

//这个时候，应该让线程继续进入pthread_cond_wait

          pthread_cond_wait(&cond, &mtx);

      }

      msg = msg_list.pop();

      pthread_mutex_unlock(&mtx);            //临界区数据操作完毕，释放互斥锁

      // handle msg

    }

    return 0;

}