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man pages section 3: Basic Library Functions

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Updated: Wednesday, July 27, 2022



pthread_rwlock_rdlock, pthread_rwlock_tryrdlock - lock or attempt to lock read-write lock object for reading


#include <pthread.h>

int pthread_rwlock_rdlock(pthread_rwlock_t *rwlock);
int pthread_rwlock_tryrdlock(pthread_rwlock_t *rwlock);


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.

The calling thread does not acquire the lock if a writer holds the lock or if writers of higher or equal priority are blocked on the lock; otherwise, the calling thread acquires the lock. If the read lock is not acquired, the calling thread blocks until it can acquire the lock.

A thread can 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 maximum number of concurrent read locks that a thread can hold on one read-write lock is currently set at 100,000, though this number could change in a future release. There is no imposed limit on the number of different threads that can apply a read lock to one read-write lock.

The pthread_rwlock_tryrdlock() function applies a read lock like the pthread_rwlock_rdlock() function, with the exception that the function fails if the equivalent pthread_rwlock_rdlock() call would have blocked the calling thread. In no case will the pthread_rwlock_tryrdlock() function ever block. It always either acquires the lock or fails and returns immediately.

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.

Return Values

If successful, the pthread_rwlock_rdlock() function returns 0. Otherwise, an error number is returned to indicate the error.

The pthread_rwlock_tryrdlock() function returns 0 if the lock for reading on the read-write lock object referenced by rwlock is acquired. Otherwise an error number is returned to indicate the error.


The pthread_rwlock_rdlock() and pthread_rwlock_tryrdlock() functions will fail if:


The read lock could not be acquired because the maximum number of read locks by the current thread for rwlock has been exceeded.

The pthread_rwlock_rdlock() function will fail if:


The current thread already owns the read-write lock for writing.

The pthread_rwlock_tryrdlock() function will fail if:


The read-write lock could not be acquired for reading because a writer holds the lock or a writer with the appropriate priority was blocked on it.


See attributes(7) for descriptions of the following attributes:

Interface Stability

See Also

pthread_rwlock_init(3C), pthread_rwlock_unlock(3C), pthread_rwlock_wrlock(3C), pthread_rwlockattr_init(3C), attributes(7), standards(7)