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Technical Reference: Base Operating System and Extensions, Volume 1

pthread_rwlock_wrlock or pthread_rwlock_trywrlock Subroutines


Locks a read-write lock object for writing.


Threads Library (libpthreads.a)


#include <pthread.h>

int pthread_rwlock_wrlock (pthread_rwlock_t *rwlock );
int pthread_rwlock_trywrlock (pthread_rwlock_t *rwlock );


The pthread_rwlock_wrlock function applies a write lock to the read-write lock referenced by rwlock. The calling thread acquires the write lock if no other thread (reader or writer) holds the read-write lock rwlock. Otherwise, the thread blocks (that is, does not return from the pthread_rwlock_wrlock call) until it can acquire the lock. Results are undefined if the calling thread holds the read-write lock (whether a read or write lock) at the time the call is made.

Implementations are allowed to favour writers over readers to avoid writer starvation.

The function pthread_rwlock_trywrlock applies a write lock like the pthread_rwlock_wrlock function, with the exception that the function fails if any thread currently holds rwlock (for reading or writing).

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 writing, upon return from the signal handler the thread resumes waiting for the read-write lock for writing as if it was not interrupted.

Return Values

If successful, the pthread_rwlock_wrlock function returns zero. Otherwise, an error number is returned to indicate the error.

The function pthread_rwlock_trywrlock returns zero if the lock for writing 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_trywrlock function will fail if:

EBUSY The read-write lock could not be acquired for writing because it was already locked for reading or writing.

The pthread_rwlock_wrlock and pthread_rwlock_trywrlock 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 or reading.

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_unlock (pthread_rwlock_unlock Subroutine), pthread_rwlockattr_init (pthread_rwlockattr_init or pthread_rwlockattr_destroy Subroutines), pthread_rwlock_rdlock (pthread_rwlock_rdlock or pthread_rwlock_tryrdlock Subroutines) subroutines.

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