问题描述
我米使用调用pthread_cond_wait（安培; cond_t，＆安培;互斥）; 在我的计划，我米奇怪，为什么这个功能需要作为第二个参数互斥变量
I m using pthread_cond_wait(&cond_t, &mutex); in my program and I m wondering why this function need as a second parameter a mutex variable.
请问调用pthread_cond_wait（）解锁在beggining互斥（执行的beggining 调用pthread_cond_wait（））和那么当它完成锁定（只是临走前调用pthread_cond_wait（））？
Does the pthread_cond_wait() unlock the mutex at the beggining (beggining of the execution pthread_cond_wait()) and then locked when it finish (just before leaving pthread_cond_wait())?
推荐答案
在第一个线程调用它释放互斥锁，并等待，直到条件 cond_t 的信号作为完整的和 互斥是可用的。
When the first thread calls pthread_cond_wait(&cond_t, &mutex); it releases the mutex and it waits till condition cond_t is signaled as complete and mutex is available.
所以，当是所谓的其他螺纹，它并不唤醒该等待尚未螺纹。 互斥必须先解锁，只有有一个机会，第一个线程将获得锁，这意味着的后的互斥应已被锁定，并由调用线程拥有。
So when pthread_cond_signal is called in the other thread, it doesn't "wake up" the thread that waits yet. mutex must be unlocked first, only then there is a chance that first thread will get a lock, which means that "upon successful return of pthread_cond_wait mutex shall have been locked and shall be owned by the calling thread."
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