The pthread_rwlock_tryrdlock( ) function shall apply a read lock as in the pthread_rwlock_rdlock( ) function, with the exception that the function shall fail if the equivalent pthread_rwlock_rdlock( ) call would have blocked the calling thread. In no case shall the pthread_rwlock_tryrdlock( ) function ever block; it always either acquires the lock or fails and returns immediately. The pthread_rwlock_tryrdlock( ) function shall return zero if the lock for reading on the read-write lock object referenced by rwlock is acquired. Otherwise, an error number shall be returned to indicate the error. The pthread_rwlock_tryrdlock( ) function shall fail if: [EBUSY] The read-write lock could not be acquired for reading because a writer holds the lock or a writer with the appropriate priority was blocked on it. The function shall not return an error code of [EINTR].