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The pthread_rwlock_rdlock() function applies 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. It is unspecified whether the calling thread acquires the lock when a writer does not hold the lock and there are writers waiting
for the lock. If a writer holds the lock, the calling thread will not acquire the read lock. If the read lock is not acquired, the calling thread blocks (that is, it does not return from the pthread_rwlock_rdlock() call) until it can acquire the lock. Results are undefined if the calling thread holds a write lock on rwlock at the time the call is made.
Implementations are allowed to favors writers over readers to avoid writer starvation. The current implementation favors writers over readers.
A thread may hold multiple concurrent read locks on rwlock (that is, successfully call the pthread_rwlock_rdlock() function n
times). If so, the thread must perform matching unlocks (that is, it must call the pthread_rwlock_unlock() function n times).
The function pthread_rwlock_tryrdlock() applies a read lock as in the pthread_rwlock_rdlock() function with the exception that the function fails if any thread
holds a write lock on rwlock or there are writers blocked on rwlock.
Results are undefined if any of these functions are called with an uninitialized read-write 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.
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