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_trywrlock, pthread_rwlock_wrlock - lock a read-write lock object for writing

#### SYNOPSIS

> ``#include <pthread.h>
> int pthread_rwlock_trywrlock(pthread_rwlock_t *`rwlock`);
> int pthread_rwlock_wrlock(pthread_rwlock_t *`rwlock`);
> ``

#### DESCRIPTION

> The pthread_rwlock_trywrlock() function shall apply a write lock like the pthread_rwlock_wrlock() function, with the exception that the function shall fail if any thread currently holds rwlock (for reading or writing).

> The pthread_rwlock_wrlock() function shall apply a write lock to the read-write lock referenced by rwlock. The calling thread acquires the write lock if no other thread (reader or writer) holds the read-write lock rwlock. Otherwise, the thread shall block until it can acquire the lock. The calling thread may deadlock if at the time the call is made it holds the read-write lock (whether a read or write lock).

> Implementations may favor writers over readers to avoid writer starvation.

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

> If a signal is delivered to a thread waiting for a read-write lock for writing, upon return from the signal handler the thread resumes waiting for the read-write lock for writing as if it was not interrupted.

#### RETURN VALUE

> The pthread_rwlock_trywrlock() function shall return zero if the lock for writing on the read-write lock object referenced by rwlock is acquired. Otherwise, an error number shall be returned to indicate the error.

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

#### ERRORS

> The pthread_rwlock_trywrlock() function shall fail if:

> [EBUSY]
>   The read-write lock could not be acquired for writing because it was already locked for reading or writing.

> The pthread_rwlock_wrlock() function may fail if:

> [EDEADLK]
>   A deadlock condition was detected or the current thread already owns the read-write lock for writing or reading.

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

The following sections are informative.

#### EXAMPLES

> None.

#### APPLICATION USAGE

> Applications using these 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_trywrlock() or pthread_rwlock_wrlock() does not refer to an initialized read-write lock object, it is recommended that the function should fail and report an [EINVAL] error.

#### FUTURE DIRECTIONS

> None.

#### SEE ALSO

> pthread_rwlock_destroy, pthread_rwlock_rdlock, pthread_rwlock_timedrdlock, pthread_rwlock_timedwrlock, pthread_rwlock_unlock

> XBD Priority Inversion, 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 [EDEADLK] error is deleted as a pthread_rwlock_trywrlock() error.

> - The SEE ALSO section is updated.

> IEEE Std 1003.1-2001/Cor 2-2004, item XSH/TC2/D6/104 is applied, updating the ERRORS section so that the [EDEADLK] error includes detection of a deadlock condition.

#### Issue 7

> The pthread_rwlock_trywrlock() and pthread_rwlock_wrlock() functions are 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.

End of informative text.

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