线程的条件变量问题

zhlyp

void *decrement_count(void *arg) {         pthread_mutex_lock(&count_lock);         while (count == 0)                 pthread_cond_wait(&count_nonzero, &count_lock);         count = count - 1;         printf("LINE:%d %d\n", __LINE__, count);         pthread_mutex_unlock(&count_lock);         pthread_exit(NULL); } void *increment_count(void *arg) {         pthread_mutex_lock(&count_lock);         if (count == 0)                 pthread_cond_signal(&count_nonzero);         count = count + 1;         printf("LINE:%d %d\n", __LINE__, count);         sleep(2);         pthread_mutex_unlock(&count_lock);         pthread_exit(NULL); }

不理解这句

while (count == 0)                 pthread_cond_wait(&count_nonzero, &count_lock);

为什么非要用while呢？在前面已经由pthread_mutex_lock(&count_lock);上了锁，所以在while (count == 0)和pthread_cond_wait(&count_nonzero, &count_lock);之间，是不会有别的进程产生count的变化了。所以不明白这个while存在的意义了，请大侠指教

[ 本帖最后由 zhlyp 于 2008-10-30 17:27 编辑 ]

timespace

有多种情况可以使pthread_cond_wait返回，线程signal或broadcast只是其中之一，wait返回后有可能count == 0，所以需要循环直到条件真正满足

另外互斥锁保护的代码尽量要少，否则线程的并发也没什么意义了，比如你代码中printf sleep这类东西就没必要放到互斥锁内。

zhlyp

了解了
原来pthread_cond_wait会解锁

action08

up

MyStar_api

if (count == 0)
                pthread_cond_signal(&count_nonzero);
        count = count + 1;
=============================
这里之间如果发生线程切换。。。。。。那么就要while了