The Open Group Base Specifications Issue 6
IEEE Std 1003.1, 2004 Edition
Copyright © 2001-2004 The IEEE and The Open Group, All Rights reserved.

#### NAME

> pthread_rwlock_unlock - unlock a read-write lock object

#### SYNOPSIS

> ``[THR] #include <pthread.h>
> int pthread_rwlock_unlock(pthread_rwlock_t` *rwlock`);``
> `
> `

#### DESCRIPTION

> The pthread_rwlock_unlock() function shall release a lock held on the read-write lock object referenced by rwlock. Results are undefined if the read-write lock rwlock is not held by the calling thread.

> If this function is called to release a read lock from the read-write lock object and there are other read locks currently held on this read-write lock object, the read-write lock object remains in the read locked state. If this function releases the last read lock for this read-write lock object, the read-write lock object shall be put in the unlocked state with no owners.

> If this function is called to release a write lock for this read-write lock object, the read-write lock object shall be put in the unlocked state.

> If there are threads blocked on the lock when it becomes available, the scheduling policy shall determine which thread(s) shall acquire the lock. [TPS] If the Thread Execution Scheduling option is supported, when threads executing with the scheduling policies SCHED_FIFO, SCHED_RR, or SCHED_SPORADIC are waiting on the lock, they shall acquire the lock in priority order when the lock becomes available. For equal priority threads, write locks shall take precedence over read locks. If the Thread Execution Scheduling option is not supported, it is implementation-defined whether write locks take precedence over read locks.

> Results are undefined if any of these functions are called with an uninitialized read-write lock.

#### RETURN VALUE

> If successful, the pthread_rwlock_unlock() function shall return zero; otherwise, an error number shall be returned to indicate the error.

#### ERRORS

> The pthread_rwlock_unlock() function may fail if:

> [EINVAL]
>   The value specified by rwlock does not refer to an initialized read-write lock object.
> [EPERM]
>   The current thread does not hold a lock on the read-write lock.

> The pthread_rwlock_unlock() function shall not return an error code of [EINTR].

The following sections are informative.

#### EXAMPLES

> None.

#### APPLICATION USAGE

> None.

#### RATIONALE

> None.

#### FUTURE DIRECTIONS

> None.

#### SEE ALSO

> pthread_rwlock_destroy(), pthread_rwlock_rdlock(), pthread_rwlock_timedrdlock(), pthread_rwlock_timedwrlock(), pthread_rwlock_tryrdlock(), pthread_rwlock_trywrlock(), pthread_rwlock_wrlock(), the Base Definitions volume of IEEE Std 1003.1-2001, <pthread.h>

#### CHANGE HISTORY

> First released in Issue 5.

#### Issue 6

> The following changes are made for alignment with IEEE Std 1003.1j-2000:

> - The margin code in the SYNOPSIS is changed to THR to indicate that the functionality is now part of the Threads option (previously it was part of the Read-Write Locks option in IEEE Std 1003.1j-2000 and also part of the XSI extension).

> - The DESCRIPTION is updated as follows:

>   - The conditions under which writers have precedence over readers are specified.

>   - The concept of read-write lock owner is deleted.

> - The SEE ALSO section is updated.

End of informative text.

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