条件变量 - 为什么在调用pthread_cond_wait（）之前调用pthread_cond_signal（）是一个逻辑错误？

pthread_mutex_lock(&mutex); 
while (!condition)
    pthread_cond_wait(&cond, &mutex); 
pthread_mutex_unlock(&mutex);

pthread_mutex_lock(&mutex); 
changeCondition(); 
pthread_mutex_unlock(&mutex); 
pthread_cond_signal(&cond)

// A shared mutex, global in this case.
boost::mutex myMutex;

// Condition variable
boost::condition_variable myCondition;

void threadProc()
{
    // Lock the mutex while the thread is running.
    boost::mutex::scoped_lock guard( myMutex );

    while( true )
    {
        // Do stuff, then...

        myCondition.wait( guard ); // Unlocks the mutex and waits for a notification.
    }
}

void func()
{
    // Function wants to trigger the other thread. Locks the mutex...
    boost::mutex::scoped_lock guard( myMutex );

    // Since the mutex is locked, we know that the other thread is
    // waiting on the condition variable...
    myCondition.notify_all();
}