当存在一个读锁时，很多读操作均可使用这个锁。
When a read lock exists, many read operations may use this lock.
他们并不会阻止不同的事务在相同的间隙获取锁，因此间隙读锁和间隙写锁有相同的影响。
They do not prevent different transactions from taking gap locks on the same gap. Thus, a gap X-lock has the same effect as a gap S-lock.
如果超过了最大读锁数量（由实现定义），pthread_rwlock_rdlock 可能会失败。
pthread_rwlock_rdlock may fail if the maximum number of read locks (implementation defined) for the lock has been exceeded.
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