pthread_rwlock_rdlock( ) function shall apply a read lock to the read-write lock referenced by rwlock. The calling thread acquires the read lock if a writer does not hold the lock and there are no writers blocked on the lock. If the Thread Execution Scheduling option is supported, and the threads involved in the lock are executing with the scheduling policies SCHED_FIFO or SCHED_RR, the calling thread shall not acquire the lock if a writer holds the lock or if writers of higher or equal priority are blocked on the lock; otherwise, the calling thread shall acquire the lock. If the Threads Execution Scheduling option is supported, and the threads involved in the lock are executing with the SCHED_SPORADIC scheduling policy, the calling thread shall not acquire the lock if a writer holds the lock or if writers of higher or equal priority are blocked on the lock; otherwise, the calling thread shall acquire the lock. If a signal is delivered to a thread waiting for a read-write lock for reading, upon return from the signal handler the thread resumes waiting for the read-write lock for reading as if it was not interrupted. A thread may hold multiple concurrent read locks on 'rwlock' and the application shall ensure that the thread will perform matching unlocks for each read lock. If successful, the pthread_rwlock_rdlock( ) function shall return zero; otherwise, an error number shall be returned to indicate the error. These functions shall not return an error code of [EINTR].