The Open Group Base Specifications Issue 7
IEEE Std 1003.1, 2013 Edition
Copyright © 2001-2013 The IEEE and The Open Group

#### NAME

> pthread_rwlock_unlock - unlock a read-write lock object

#### SYNOPSIS

> ``#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 this function is 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 shall not return an error code of [EINTR].

The following sections are informative.

#### EXAMPLES

> None.

#### APPLICATION USAGE

> None.

#### RATIONALE

> If an implementation detects that the value specified by the rwlock argument to pthread_rwlock_unlock() does not refer to an initialized read-write lock object, it is recommended that the function should fail and report an [EINVAL] error.

> If an implementation detects that the value specified by the rwlock argument to pthread_rwlock_unlock() refers to a read-write lock object for which the current thread does not hold a lock, it is recommended that the function should fail and report an [EPERM] error.

#### FUTURE DIRECTIONS

> None.

#### SEE ALSO

> pthread_rwlock_destroy, pthread_rwlock_rdlock, pthread_rwlock_timedrdlock, pthread_rwlock_timedwrlock, pthread_rwlock_trywrlock

> XBD Memory Synchronization, <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.

#### Issue 7

> SD5-XSH-ERN-183 is applied.

> The pthread_rwlock_unlock() function is moved from the Threads option to the Base.

> The [EINVAL] error for an uninitialized read-write lock object is removed; this condition results in undefined behavior.

> The [EPERM] error for a read-write lock object for which the current thread does not hold a lock is removed; this condition results in undefined behavior.

End of informative text.

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