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Technical Reference: Base Operating System and Extensions, Volume 1

pthread_rwlock_rdlock or pthread_rwlock_tryrdlock Subroutines


Locks a read-write lock object for reading.


Threads Library (libpthreads.a)


#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. 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 favour writers over readers to avoid writer starvation.

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 uninitialised 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 zero. Otherwise, an error number is returned to indicate the error.

The function pthread_rwlock_tryrdlock returns zero 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.

Error Codes

The pthread_rwlock_tryrdlock function will fail if:

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

The pthread_rwlock_rdlock and pthread_rwlock_tryrdlock functions will fail if:

EINVAL The value specified by rwlock does not refer to an initialised read-write lock object.
EDEADLK The current thread already owns the read-write lock for writing.
EAGAIN The read lock could not be acquired because the maximum number of read locks for rwlock has been exceeded.

Implementation Specifics

Similar functions are being developed by IEEE PASC. In keeping with its objective of ensuring that CAE Specifications are fully aligned with formal standards, The Open Group intends to add any new interfaces adopted by an official IEEE standard in this area.

Realtime applications may encounter priority inversion when using read-write locks. The problem occurs when a high priority thread 'locks' a read-write lock that is about to be 'unlocked' by a low priority thread, but the low priority thread is preempted by a medium priority thread. This scenario leads to priority inversion; a high priority thread is blocked by lower priority threads for an unlimited period of time. During system design, realtime programmers must take into account the possibility of this kind of priority inversion. They can deal with it in a number of ways, such as by having critical sections that are guarded by read-write locks execute at a high priority, so that a thread cannot be preempted while executing in its critical section.

Related Information

The pthread.h file.

The pthread_rwlock_init (pthread_rwlock_init, pthread_rwlock_destroy Subroutine), pthread_rwlock_wrlock (pthread_rwlock_wrlock or pthread_rwlock_trywrlock Subroutines), pthread_rwlockattr_init (pthread_rwlockattr_init or pthread_rwlockattr_destroy Subroutines), and pthread_rwlock_unlock (pthread_rwlock_unlock Subroutine) subroutines.

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