The Open Group Base Specifications Issue 7
IEEE Std 1003.1-2008, 2016 Edition
Copyright © 2001-2016 The IEEE and The Open Group

#### NAME

> pthread_rwlock_destroy, pthread_rwlock_init - destroy and initialize a read-write lock object

#### SYNOPSIS

> ``#include <pthread.h>
> int pthread_rwlock_destroy(pthread_rwlock_t *`rwlock`);
> int pthread_rwlock_init(pthread_rwlock_t *restrict` rwlock`,
> const pthread_rwlockattr_t *restrict` attr`);
> pthread_rwlock_t` rwlock `= PTHREAD_RWLOCK_INITIALIZER;
> ``

#### DESCRIPTION

> The pthread_rwlock_destroy() function shall destroy the read-write lock object referenced by rwlock and release any resources used by the lock. The effect of subsequent use of the lock is undefined until the lock is reinitialized 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.

> The pthread_rwlock_init() function shall allocate any resources required to use the read-write lock referenced by rwlock and initializes the lock to an unlocked state with attributes referenced by attr. If attr is NULL, the default read-write lock attributes shall be used; the effect is the same as passing the address of a default read-write lock attributes object. Once initialized, the lock can be used any number of times without being reinitialized. Results are undefined if pthread_rwlock_init() is called specifying an already initialized read-write lock. Results are undefined if a read-write lock is used without first being initialized.

> If the pthread_rwlock_init() function fails, rwlock shall not be initialized and the contents of rwlock are undefined.

> See Synchronization Object Copies and Alternative Mappings for further requirements.

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

> The behavior is undefined if the value specified by the attr argument to pthread_rwlock_init() does not refer to an initialized read-write lock attributes object.

#### RETURN VALUE

> If successful, the pthread_rwlock_destroy() and pthread_rwlock_init() functions shall return zero; otherwise, an error number shall be returned to indicate the error.

#### ERRORS

> The pthread_rwlock_init() function shall fail if:

> [EAGAIN]
>   The system lacked the necessary resources (other than memory) to initialize another read-write lock.
> [ENOMEM]
>   Insufficient memory exists to initialize the read-write lock.
> [EPERM]
>   The caller does not have the privilege to perform the operation.

> These functions shall not return an error code of [EINTR].

The following sections are informative.

#### EXAMPLES

> None.

#### APPLICATION USAGE

> Applications using these and related read-write lock functions may be subject to priority inversion, as discussed in XBD Priority Inversion.

#### RATIONALE

> If an implementation detects that the value specified by the rwlock argument to pthread_rwlock_destroy() 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 attr argument to pthread_rwlock_init() does not refer to an initialized read-write lock attributes 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_destroy() or pthread_rwlock_init() refers to a locked read-write lock object, or detects that the value specified by the rwlock argument to pthread_rwlock_init() refers to an already initialized read-write lock object, it is recommended that the function should fail and report an [EBUSY] error.

#### FUTURE DIRECTIONS

> None.

#### SEE ALSO

> pthread_rwlock_rdlock, pthread_rwlock_timedrdlock, pthread_rwlock_timedwrlock, pthread_rwlock_trywrlock, pthread_rwlock_unlock

> XBD Priority Inversion, <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 initializer macro is also deleted from the SYNOPSIS.

> - The DESCRIPTION is updated as follows:

>   - It explicitly notes allocation of resources upon initialization of a read-write lock object.

>   - A paragraph is added specifying that copies of read-write lock objects may not be used.

> - An [EINVAL] error is added to the ERRORS section for pthread_rwlock_init(), indicating that the rwlock value is invalid.

> - The SEE ALSO section is updated.

> The restrict keyword is added to the pthread_rwlock_init() prototype for alignment with the ISO/IEC 9899:1999 standard.

> IEEE Std 1003.1-2001/Cor 1-2002, item XSH/TC1/D6/45 is applied, adding APPLICATION USAGE relating to priority inversion.

#### Issue 7

> Austin Group Interpretation 1003.1-2001 #048 is applied, adding the PTHREAD_RWLOCK_INITIALIZER macro.

> The pthread_rwlock_destroy() and pthread_rwlock_init() functions are moved from the Threads option to the Base.

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

> The [EBUSY] error for a locked read-write lock object or an already initialized read-write lock object is removed; this condition results in undefined behavior.

> POSIX.1-2008, Technical Corrigendum 1, XSH/TC1-2008/0465 [70] is applied.

> POSIX.1-2008, Technical Corrigendum 2, XSH/TC2-2008/0288 [972] and XSH/TC2-2008/0289 [758] are applied.

End of informative text.

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