PTHREAD_RWLOCKATTR_SETKIND_NP
Section: C Library Functions (3)
Updated: 2020-08-13
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NAME
pthread_rwlockattr_setkind_np, pthread_rwlockattr_getkind_np - set/get
the read-write lock kind of the thread read-write lock attribute object
SYNOPSIS
#include <pthread.h>
int pthread_rwlockattr_setkind_np(pthread_rwlockattr_t *attr,
int pref);
int pthread_rwlockattr_getkind_np(const pthread_rwlockattr_t *attr,
int *pref);
Compile and link with -pthread.
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
read-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
read-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.
VERSIONS
The
pthread_rwlockattr_getkind_np()
and
pthread_rwlockattr_setkind_np()
functions first appeared in glibc 2.1.
CONFORMING TO
These functions are non-standard GNU extensions;
hence the suffix "_np" (nonportable) in the names.
SEE ALSO
pthreads(7)
COLOPHON
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