Unlocking a Read-Write Lock

pthread_rwlock_unlock(3C) releases a lock held on the read-write lock object referenced by rwlock.

pthread_rwlock_unlock Syntax

#include <pthread.h>

int pthread_rwlock_unlock (pthread_rwlock_t  *rwlock);

Results are undefined if the read-write lock rwlock is not held by the calling thread.

If pthread_rwlock_unlock() is called to release a read lock from the read-write lock object, and other read locks are currently held on this lock object, the object remains in the read locked state. If pthread_rwlock_unlock() releases the calling thread's last read lock on this read-write lock object, the calling thread is no longer an owner of the object. If pthread_rwlock_unlock() releases the last read lock for this read-write lock object, the read-write lock object is put in the unlocked state with no owners.

If pthread_rwlock_unlock() is called to release a write lock for this read-write lock object, the lock object is put in the unlocked state with no owners.

If pthread_rwlock_unlock() unlocks the read-write lock object and multiple threads are waiting to acquire the lock object for writing, the scheduling policy determines which thread acquires the object for writing. If multiple threads are waiting to acquire the read-write lock object for reading, the scheduling policy determines the order the waiting threads acquire the object for reading. If multiple threads are blocked on rwlock for both read locks and write locks, whether the readers or the writer acquire the lock first is unspecified.

Results are undefined if pthread_rwlock_unlock() is called with an uninitialized read-write lock.

pthread_rwlock_unlock Return Values

If successful, pthread_rwlock_unlock() returns zero. Otherwise, an error number is returned to indicate the error.