讀寫鎖的實現

首先介紹下pthread_cond_t。在linux下稱之為狀態變數，與之相關的有下面幾個api:

int pthread_cond_init (pthread_cond_t *cond,pthread_condattr_t *cond_attr);

int pthread_cond_signal (pthread_cond_t *cond);

int pthread_cond_broadcast (pthread_cond_t *cond);

int pthread_cond_wait (pthread_cond_t *cond, pthread_mutex_t *mutex);

int pthread_cond_timedwait (pthread_cond_t *cond, pthread_mutex_t *mutex, const struct timespec *abstime);

int pthread_cond_destroy (pthread_cond_t *cond);

這裡就講下2個api，pthread_cond_signal和pthread_cond_wait，一般的用法如下：

pthread_cond_wait會解鎖lock，然後在cond上等待,這兩步是atomic operation. 當pthread_cond_wait返回時，會同時對lock上鎖. 這裡我的理解是，如果獲取不到lock鎖，即使cond已經被啟用，pthread_cond_wait依然不會返回。

pthread_cond_signal呼叫之前一定會先拿到lock鎖。pthread_cond_signal不會去管lock鎖，只是將cond啟用，接下去釋放lock鎖。這時候pthread_cond_wait就可以得到lock從而返回了。

言歸正傳，下面是利用條件變數實現的乙個讀寫鎖的例子：

typedef struct pthread_rwlock

pthread_rwlock_t ;

int pthread_rwlock_init(pthread_rwlock_t * lock, pthread_rwlockattr_t *attr)

int pthread_rwlock_destroy(pthread_rwlock_t * lock)

int pthread_rwlock_rdlock(pthread_rwlock_t * lock)

int pthread_rwlock_wrlock(pthread_rwlock_t * lock)

int pthread_rwlock_unlock(pthread_rwlock_t * lock)

} else if (lock->active_readers < 0) /* release the write lock */

else if (lock->pending_writers > 0)

}pthread_mutex_unlock(&lock->mutex);

return 0;

}還有pthread_rwlock_tryrdlock 和 pthread_rwlock_trywrlock