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Technical Reference: Base Operating System and Extensions, Volume 1

pthread_rwlock_init, pthread_rwlock_destroy Subroutine


Initialises or destroys a read-write lock object.


Threads Library (libthreads.a)


#include <pthread.h>

int pthread_rwlock_init (pthread_rwlock_t *rwlock, const pthread_rwlock attr_t *attr);
int pthread_rwlock_destroy (pthread_rwlock_t *rwlock);
pthread_rwlock_t rwlock=PTHREAD_RWLOCK_INITIALIZER;


The pthread_rwlock_init function initialises the read-write lock referenced by rwlock with the attributes referenced by attr. 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 lock attributes object. Once initialised, the lock can be used any number of times without being re-initialised. Upon successful initialisation, the state of the read-write lock becomes initialised and unlocked. Results are undefined if pthread_rwlock_init is called specifying an already initialised read-write lock. Results are undefined if a read-write lock is used without first being initialised.

If the pthread_rwlock_init function fails, rwlock is not initialised and the contents of rwlock are undefined.

The pthread_rwlock_destroy function destroys the read-write lock object referenced by rwlock and releases any resources used by the lock. The effect of subsequent use of the lock is undefined until the lock is re-initialised 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 uninitialised read-write lock results in undefined behaviour. A destroyed read-write lock object can be re-initialised using pthread_rwlock_init; the results of otherwise referencing the read-write lock object after it has been destroyed are undefined.

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

Return Values

If successful, the pthread_rwlock_init and pthread_rwlock_destroy functions return zero. Otherwise, an error number is returned to indicate the error. 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.

Error Codes

The pthread_rwlock_init function will fail if:

ENOMEM Insufficient memory exists to initialise the read-write lock.
EINVAL The value specified by attr is invalid.

The pthread_rwlock_destroy function will fail if:

EBUSY The implementation has detected an attempt to destroy the object referenced by rwlock while it is locked.
EINVAL The value specified by attr is invalid.

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.

Related Information

The pthread.h file.

The pthread_rwlock_rdlock (pthread_rwlock_rdlock or pthread_rwlock_tryrdlock Subroutines), 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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