The mutex passed to pthread_cond_wait protects the condition.The caller passes it locked to the function, which then atomically places them calling thread on the list of threads waiting for the condition and unlocks the mutex. This closes the window between the time that the condition is checked and the time that the thread goes to sleep waiting for the condition to change, so that the thread doesn't miss a change in the condition. When pthread_cond_wait returns, the mutex is again locked.
上面是APUE的原话，就是说pthread_cond_wait(pthread_cond_t *cond, pthread_mutex_t *mutex)函数传入的参数mutex用于保护条件，因为我们在调用pthread_cond_wait时，如果条件不成立我们就进入阻塞，但是进入阻 塞这个期间，如果条件变量改变了的话，那我们就漏掉了这个条件。因为这个线程还没有放到等待队列上，所以调用pthread_cond_wait前要先锁 互斥量，即调用pthread_mutex_lock(),pthread_cond_wait在把线程放进阻塞队列后，自动对mutex进行解锁，使得 其它线程可以获得加锁的权利。这样其它线程才能对临界资源进行访问并在适当的时候唤醒这个阻塞的进程。当pthread_cond_wait返回的时候又 自动给mutex加锁。
实际上边代码的加解锁过程如下：
/************pthread_cond_wait()的使用方法**********/
pthread_mutex_lock(&qlock); /*lock*/
pthread_cond_wait(&qready, &qlock); /*block-->unlock-->wait() return-->lock*/
pthread_mutex_unlock(&qlock); /*unlock*/
/*****************************************************/