The pthread_rwlock_rdlock function acquires a read lock on lock provided that lock is not presently held for writing and no writer threads are presently blocked on the lock. If the read lock cannot be immediately acquired, the calling thread blocks until it can acquire the lock.
The pthread_rwlock_tryrdlock function performs the same action, but does not block if the lock cannot be immediately obtained (i.e., the lock is held for writing or there are waiting writers).
A thread may hold multiple concurrent read locks. If so, pthread_rwlock_unlock must be called once for each lock obtained.
The results of acquiring a read lock while the calling thread holds a write lock are undefined.
To prevent writer starvation, writers are favored over readers.
If successful, the pthread_rwlock_rdlock and pthread_rwlock_tryrdlock functions will return zero. Otherwise an error number will be returned to indicate the error.
The pthread_rwlock_tryrdlock function will fail if: