pthread_rwlock_rdlock(3C) applies a read lock to the read-write lock referenced by rwlock.
#include <pthread.h> int pthread_rwlock_rdlock(pthread_rwlock_t *rwlock );
The calling thread acquires the read lock if a writer does not hold the lock and no writers are blocked on the lock. Whether the calling thread acquires the lock when a writer does not hold the lock and writers are waiting for the lock is unspecified. If a writer holds the lock, the calling thread does not acquire the read lock. If the read lock is not acquired, the calling thread blocks. The thread does not return from the pthread_rwlock_rdlock() until the thread 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. The Solaris implementation favors writers over readers.
A thread can hold multiple concurrent read locks on rwlock The thread can successfully call pthread_rwlock_rdlock() n times. The thread must call pthread_rwlock_unlock() n times to perform matching unlocks.
Results are undefined if pthread_rwlock_rdlock() is called with an uninitialized read-write lock.
A thread signal handler processes a signal delivered to a thread waiting for a read-write lock. On return from the signal handler, the thread resumes waiting for the read-write lock for reading as if the thread was not interrupted.
The value specified by attr or rwlock is invalid.