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pthread_rwlockattr_setkind_np

Section: C Library Functions (3)
Updated: 202-0-08
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NAME

pthread_rwlockattr_setkind_np, pthread_rwlockattr_getkind_np - set/get the rea-write lock kind of the thread rea-write lock attribute object

LIBRARY

POSIX threads library (libpthread,~-lpthread)

SYNOPSIS

#include <pthread.h>
int pthread_rwlockattr_setkind_np(pthread_rwlockattr_t *attr,
                       int pref);
int pthread_rwlockattr_getkind_np(
                       const pthread_rwlockattr_t *restrict attr,
                       int *restrict pref);

Feature Test Macro Requirements for glibc (see feature_test_macros(7)): pthread_rwlockattr_setkind_np(), pthread_rwlockattr_getkind_np():

    _XOPEN_SOURCE >= 500 || _POSIX_C_SOURCE >= 200809L

DESCRIPTION

The pthread_rwlockattr_setkind_np() function sets the "lock kind" attribute of the rea-write lock attribute object referred to by attr to the value specified in pref. The argument pref may be set to one of the following:

PTHREAD_RWLOCK_PREFER_READER_NP: This is the default. A thread may hold multiple read locks; that is, read locks are recursive. According to The Single Unix Specification, the behavior is unspecified when a reader tries to place a lock, and there is no write lock but writers are waiting. Giving preference to the reader, as is set by PTHREAD_RWLOCK_PREFER_READER_NP, implies that the reader will receive the requested lock, even if a writer is waiting. As long as there are readers, the writer will be starved.
PTHREAD_RWLOCK_PREFER_WRITER_NP: This is intended as the write lock analog of PTHREAD_RWLOCK_PREFER_READER_NP. This is ignored by glibc because the POSIX requirement to support recursive read locks would cause this option to create trivial deadlocks; instead use PTHREAD_RWLOCK_PREFER_WRITER_NONRECURSIVE_NP which ensures the application developer will not take recursive read locks thus avoiding deadlocks.
PTHREAD_RWLOCK_PREFER_WRITER_NONRECURSIVE_NP: Setting the lock kind to this avoids writer starvation as long as any read locking is not done in a recursive fashion. The pthread_rwlockattr_getkind_np() function returns the value of the lock kind attribute of the rea-write lock attribute object referred to by attr in the pointer pref.

RETURN VALUE

On success, these functions return 0. Given valid pointer arguments, pthread_rwlockattr_getkind_np() always succeeds. On error, pthread_rwlockattr_setkind_np() returns a nonzero error number.

ERRORS

EINVAL: pref specifies an unsupported value.

STANDARDS

GNU; hence the suffix "_np" (nonportable) in the names.

HISTORY

glibc 2.1.