Linux下利用条件变量实现读写锁

首先介绍下pthread_cond_t。 在Linux下称之为状态变量，与之相关的有下面几个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);

这里就讲下2个api，pthread_cond_signal和pthread_cond_wait，一般的用法如下：

{

    pthread_mutex_lock(lock)

 …

    pthread_cond_wait(cond, lock);

 …

    pthread_cond_mutex_unlock(lock);

}

pthread_cond_wait会解锁lock，然后在cond上等待,这两步是atomic operation. 当pthread_cond_wait返回时，会同时对lock上锁. 这里我的理解是，如果获取不到lock锁，即使cond已经被激活，pthread_cond_wait依然不会返回。

{

    pthread_mutex_lock(lock);

 …

    pthread_cond_signal(cond);

 …

    pthread_mutex_unlock(lock);

}

pthread_cond_signal调用之前一定会先拿到lock锁。pthread_cond_signal不会去管lock锁，只是将cond激活，接下去释放lock锁。这时候pthread_cond_wait就可以得到lock从而返回了。

言归正传，下面是利用条件变量实现的一个读写锁的例子：

typedef struct pthread_rwlock

{

 int active_readers; /* -1 when writer lock locked, >0 when read lock locked */

 int pending_readers;

 int pending_writers;

 pthread_mutex_t mutex;

 pthread_cond_t ok_to_read;

 pthread_cond_t ok_to_write;

} pthread_rwlock_t ;

int pthread_rwlock_init(pthread_rwlock_t * lock, pthread_rwlockattr_t *attr)
{
 active_readers = 0;
 pending_readers = 0;
 pending_writers = 0;
 pthread_mutex_init(&lock->mutex, NULL);
 pthread_cond_init(&lock->ok_to_read, NULL);
 pthread_cond_init(&lock->ok_to_write, NULL);
 return 0;
}
int pthread_rwlock_destroy(pthread_rwlock_t * lock)
{
 pthread_mutex_destroy(&lock->mutex);
 pthread_cond_destroy(&lock->ok_to_read);
 pthread_cond_destroy(&lock->ok_to_write);
 return 0;
}
int pthread_rwlock_rdlock(pthread_rwlock_t * lock)
{
 pthread_mutex_lock(&lock->mutex);
 lock->pending_readers++;
 while(lock->active_readers < 0) /* the write lock locked */
  pthread_cond_wait(&lock->ok_to_read, &lock->mutex);
 lock->pending_readers–;
 lock->active_readers++;
 pthread_mutex_unlock(&lock->mutex);
 return 0;
}
int pthread_rwlock_wrlock(pthread_rwlock_t * lock)
{
 pthread_mutex_lock(&lock->mutex);
 lock->pending_writers++;
 while(lock->active_readers) /* the write lock or read lock locked */
  pthread_cond_wait(&lock->ok_to_write, &lock->mutex);
 lock->pending_writers–;
 lock->active_readers = -1;
 pthread_mutex_unlock(&lock->mutex);
 return 0;
}
int pthread_rwlock_unlock(pthread_rwlock_t * lock)
{
 pthread_mutex_lock(&lock->mutex);
 assert(lock->active_readers)
 if (lock->active_readers > 0) /* release the read lock */
 {
  lock->active_readers–;
  if (lock->active_readers == 0) /* no read lock locked */
  {
   pthread_cond_signal(&lock->ok_to_write);
  }
 } else if (lock->active_readers < 0) /* release the write lock */
 {
  lock->active_readers=0;
  /* it may be different, when write lock has higher priority than read lock */
  if (lock->pending_readers > 0) {
   pthread_cond_broadcast(&lock->ok_to_read);
  } else if (lock->pending_writers > 0) {
   pthread_cond_signal(&lock->ok_to_write);
  }
 }
 pthread_mutex_unlock(&lock->mutex);
 return 0;
}
还有pthread_rwlock_tryrdlock 和 pthread_rwlock_trywrlock，感兴趣的自己写吧，感觉这两个用的不多。