NAME¶

pthread_rwlockattr_setkind_np, pthread_rwlockattr_getkind_np - set/get the read-write lock kind of the thread read-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);

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.

STANDARDS¶

These functions are non-standard GNU extensions; hence the suffix "_np" (nonportable) in the names.

SEE ALSO¶

pthreads(7)