pthread_rwlock_init(3pthread)

pthread_rwlock_init, pthread_rwlock_destroy -- initialize, destroy, a read-write lock

Synopsis

   cc [options] -Kthread file
   
   #include <pthread.h>
   

   int pthread_rwlock_init(pthread_rwlock_t *rwlock,
   	const pthread_rwlockattr_t  *attr);
   

   int pthread_rwlock_destroy(pthread_rwlock_t *rwlock);
   

   pthread_rwlock_t *rwlock=PTHREAD_RWLOCK_INITIALIZER

Description

pthread_rwlock_init initializes the read-write lock pointed to by rwlock with the attributes referenced by attr and in the unlocked state. Once initialized, the lock can be used any number of times without being re-initialized.

The rwlock parameter points to the reader-writer lock to be initialized or destroyed.

The attr parameter points to a read-write lock attributes object that will be used to initialize rwlock. If attr is NULL, the default read-write lock attributes are used; the effect is the same as passing the address of a default read-write attributes object.

If the pthread_rwlock_init function fails, rwlock is not initialized and its contents are undefined.

pthread_rwlock_destroy destroys the read-write object pointed to by rwlock and releases any resources used by the lock. This includes invalidating the lock and freeing any associated dynamically allocated resources. The effect of subsequent use of the lock is undefined until the lock is re-initialized by another call to pthread_rwlock_init. An implementation may cause pthread_rwlock_destroy to set the object referenced by rwlock to an invalid value. Results are undefined if pthread_rwlock_destroy is called when any thread holds rwlock. Attempting to destroy an uninitialized read-write lock results in undefined behavior. A destroyed read-write lock object can be re-initialized using pthread_rwlock_init; the results of otherwise referencing the read-write lock object after it has been destroyed are undefined.

Static reader-Writer initialization

In cases where default read-write lock attributes are appropriate, the macro PTHREAD_RWLOCK_INITIALIZER can be used to initialize read-write locks that are statically allocated. The effect is equivalent to dynamic initialization by a call to pthread_rwlock_init with attr specified as NULL, except that no error checks are performed.

Return values

pthread_rwlock_init and pthread_rwlock_destroy return zero on success. Otherwise, an error number is returned.

Diagnostics

The EBUSY and EINVAL error checks, if implemented, will act as if they were performed immediately at the beginning of processing for the function and caused an error return prior to modifying the state of the read-write lock specified by rwlock.

pthread_rwlock_init returns the following value and does not change the contents of rwlock if the corresponding condition is detected:

EINVAL: Invalid attr argument specified.
EINVAL: Invalid rwlock argument specified.

pthread_rwlock_destroy returns the following values if the corresponding conditions are detected:

EBUSY: rwlock is locked or another thread is waiting to acquire rwlock.
EINVAL: The value specified by rwlock is invalid.

Warnings

pthread_rwlock_init does not examine the rwlock argument before initializing it. If pthread_rwlock_init is called more than once for the same reader-writer lock, it will overwrite its state. It is the user's responsibility to ensure that pthread_rwlock_init is only called once for each reader-writer lock.

Most operations on read-write locks are not recursive--a thread can deadlock if it attempts to reacquire a read-write lock that it already has acquired.

Results are undefined if a read-write lock is used without first being initialized.

Standards Compliance

The Single UNIX Specification, Version 2; The Open Group.

References

Intro(3pthread), pthread(4), pthread_rwlockattr_init(3pthread), pthread_rwlock_rdlock(3pthread), pthread_rwlock_unlock(3pthread), pthread_rwlock_wrlock(3pthread)