pthread_cond_wait()的使用方法:－轉貼部落格

pthread_cond_wait()的使用方法:

pthread_mutex_lock(&qlock); //lock mutex
pthread_cond_wait(&qready, &qlock); //automatic unlock and wait for cond ; this is atomic operation
pthread_cond_wait return // check cond ok , then automatic lock mutex ; this is atomic operation
pthread_mutex_unlock(&qlock); //unlock for next

-----------------------------------

e.g

thread 1 :

pthread_mutex_lock(&mutex);1 ; 15(被 13卡住)
while (!空空如也)2
{
pthread_cond_wait(&cond, &mutex); 4(掛起，且unlock mutex 兩個動作是atomic) ; 9 (wait到8了，該function return 同時 lock mutex , 這兩動為atomic)
}
subsub();11 ; 17 (不做cond wait , 因為下面14已經++, something 為真)
pthread_mutex_unlock(&mutex); 12 ; 18

---------------------------
thread2:

pthread_mutex_lock(&mutex); 3(被上面1卡住) ; 5 (因為4解鎖，然後再lock 進到 6 ) ; 10 (被上面9卡住) ; 13 (12解鎖，且拿鎖才走到)
addadd(); 6 ; 14
pthread_mutex_unlock(&mutex); 7 ; 16
pthread_cond_signal(&cond) 8

********************************************************

wait()操作通常伴随着条件检测，如：

while(pass == 0)
    pthread_cond_wait(...);

signal*()函数通常伴随着条件改变，如：
```
pass = 1;
pthread_cond_signal(...)
```

由于此两处都涉及到变量pass，所以为了防止Race Condition，必须得加锁。所以代码会变成下面这样：

// 条件测试
pthread_mutex_lock(mtx);
while(pass == 0)
    pthread_cond_wait(...);
pthread_mutex_unlock(mtx);

// 条件发生修改，对应的signal代码
pthread_mutex_lock(mtx);
pass = 1;
pthread_mutex_unlock(mtx);
pthread_cond_signal(...);