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.TH PTHREAD_RWLOCKATTR_SETKIND_NP 3 2016-10-08 "Linux Programmer's Manual"
.SH NAME
pthread_rwlockattr_setkind_np, pthread_rwlockattr_getkind_np \- set/get
the read-write lock kind of the thread read-write lock attribute object
.SH SYNOPSIS
.nf
.B #include <pthread.h>

.BI "int pthread_rwlockattr_setkind_np(pthread_rwlockattr_t *" attr ,
.BI "                                   int " pref );
.BI "int pthread_rwlockattr_getkind_np(const pthread_rwlockattr_t *" attr ,
.BI "                                   int *" pref );
.sp
Compile and link with \fI\-pthread\fP.
.sp
.fi
.in -4n
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.in
.sp
.BR pthread_rwlockattr_setkind_np (),
.BR pthread_rwlockattr_getkind_np ():
.br
.RS 4
.ad l
_XOPEN_SOURCE\ >=\ 500 || _POSIX_C_SOURCE >= 200809L
.RE
.ad
.SH DESCRIPTION
The
.BR pthread_rwlockattr_setkind_np ()
function sets the "lock kind" attribute of the
read-write lock attribute object referred to by
.I attr
to the value specified in
.IR pref .
The argument
.I pref
may be set to one of the following:
.TP
.B 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
.BR 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.
.TP
.B PTHREAD_RWLOCK_PREFER_WRITER_NP
This is intended as the write lock analog of
.BR PTHREAD_RWLOCK_PREFER_READER_NP .
But see BUGS.
.TP
.B 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.
.PP
The
.BR pthread_rwlockattr_getkind_np ()
function returns the value of the lock kind attribute of the
read-write lock attribute object referred to by
.IR attr
in the pointer
.IR pref .
.SH RETURN VALUE
On success, these functions return 0.
Given valid pointer arguments,
.BR pthread_rwlockattr_getkind_np ()
always succeeds.
On error,
.BR pthread_rwlockattr_setkind_np ()
returns a non-zero error number.
.SH ERRORS
.TP
.BR EINVAL
.I pref
specifies an unsupported value.
.SH VERSIONS
The
.BR pthread_rwlockattr_getkind_np ()
and
.BR pthread_rwlockattr_setkind_np ()
functions first appeared in glibc 2.1.
.SH CONFORMING TO
These functions are non-standard GNU extensions;
hence the suffix "_np" (non-portable) in the names.
.SH BUGS
Setting the value read-write lock kind to
.BR  PTHREAD_RWLOCK_PREFER_WRITER_NP
results in the same behavior as setting the value to
.BR PTHREAD_RWLOCK_PREFER_READER_NP .
As long as a reader thread holds the lock, the thread holding a
write lock will be starved.
Setting the lock kind to
.BR PTHREAD_RWLOCK_PREFER_WRITER_NONRECURSIVE_NP
allows writers to run, but, as the name implies a writer
may not lock recursively.
.\" http://sourceware.org/bugzilla/show_bug.cgi?id=7057
.SH SEE ALSO
.BR pthreads (7)
.SH COLOPHON
This page is part of release 4.10 of the Linux
.I man-pages
project.
A description of the project,
information about reporting bugs,
and the latest version of this page,
can be found at
\%https://www.kernel.org/doc/man\-pages/.