#include <pthread.h> int pthread_rwlock_rdlock(pthread_rwlock_t *rwlock );
pthread_rwlock_rdlock(3THR) 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()) 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 favor writers over readers to avoid writer starvation. (For instance, the Solaris threads implementation favors writers over readers. See rw_rdlock(3THR).)
A thread can hold multiple concurrent read locks on rwlock (that is, successfully call pthread_rwlock_rdlock() n times) If so, the thread must perform matching unlocks (that is, it must call pthread_rwlock_unlock() n times).
Results are undefined if pthread_rwlock_rdlock() is called with an uninitialized read-write lock.
If a signal is delivered to a thread waiting for a read-write lock for reading, on return from the signal handler the thread resumes waiting for the read-write lock for reading as if it was not interrupted.
If successful, pthread_rwlock_rdlock() returns zero. Otherwise, an error number is returned to indicate the error.