pthread_rwlock_wrlock or pthread_rwlock_trywrlock Subroutines

Purpose

Locks a read-write lock object for writing.

Library

Threads Library (libpthreads.a)

Syntax

#include <pthread.h>

int pthread_rwlock_wrlock (rwlock)
pthread_rwlock_t *rwlock;

int pthread_rwlock_trywrlock (rwlock)
pthread_rwlock_t *rwlock;

Description

The pthread_rwlock_wrlock subroutine 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 favor writers over readers to avoid writer starvation.

The pthread_rwlock_trywrlock subroutine applies a write lock like the pthread_rwlock_wrlock subroutine, 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 uninitialized 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.

Real-time 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 pre-empted 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. During system design, real-time 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 pre-empted while executing in its critical section.

Note: With a large number of readers and relatively few writers there is a possibility of writer starvation. If the threads are waiting for an exclusive write lock on the read-write lock, and there are threads that currently hold a shared read lock, the subsequent attempts to acquire a shared read lock request are granted, where as the attempts to acquire an exclusive write lock waits.

Parameters

Item	Description
rwlock	Specifies the read-write lock to be locked for writing.

Return Values

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

The pthread_rwlock_trywrlock subroutine 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 subroutine will fail if:

Item	Description
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 subroutines may fail if:

Item	Description
EINVAL	The value specified by rwlock does not refer to an initialized read-write lock object.
EDEADLK	The current thread already owns the read-write lock for writing or reading.