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<H1>PTHREAD_RWLOCKATTR_SETKIND_NP</H1>
Section: C Library Functions (3)<BR>Updated: 2019-03-06<BR><A HREF="#index">Index</A>
<A HREF="/cgi-bin/man/man2html">Return to Main Contents</A><HR>
<A NAME="lbAB">&nbsp;</A>
<H2>NAME</H2>
pthread_rwlockattr_setkind_np, pthread_rwlockattr_getkind_np - set/get
the read-write lock kind of the thread read-write lock attribute object
<A NAME="lbAC">&nbsp;</A>
<H2>SYNOPSIS</H2>
<PRE>
<B>#include &lt;<A HREF="file:///usr/include/pthread.h">pthread.h</A>&gt;</B>
<B>int pthread_rwlockattr_setkind_np(pthread_rwlockattr_t *</B><I>attr</I><B>,</B>
<B> int </B><I>pref</I><B>);</B>
<B>int pthread_rwlockattr_getkind_np(const pthread_rwlockattr_t *</B><I>attr</I><B>,</B>
<B> int *</B><I>pref</I><B>);</B>
Compile and link with <I>-pthread</I>.
</PRE>
Feature Test Macro Requirements for glibc (see
<B><A HREF="/cgi-bin/man/man2html?7+feature_test_macros">feature_test_macros</A></B>(7)):
<P>
<B>pthread_rwlockattr_setkind_np</B>(),
<B>pthread_rwlockattr_getkind_np</B>():
<BR>
<DL COMPACT><DT id="1"><DD>
_XOPEN_SOURCE&nbsp;&gt;=&nbsp;500 || _POSIX_C_SOURCE &gt;= 200809L
</DL>
<A NAME="lbAD">&nbsp;</A>
<H2>DESCRIPTION</H2>
The
<B>pthread_rwlockattr_setkind_np</B>()
function sets the &quot;lock kind&quot; attribute of the
read-write lock attribute object referred to by
<I>attr</I>
to the value specified in
<I>pref</I>.
The argument
<I>pref</I>
may be set to one of the following:
<DL COMPACT>
<DT id="2"><B>PTHREAD_RWLOCK_PREFER_READER_NP</B>
<DD>
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
<B>PTHREAD_RWLOCK_PREFER_READER_NP</B>,
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.
<DT id="3"><B>PTHREAD_RWLOCK_PREFER_WRITER_NP</B>
<DD>
This is intended as the write lock analog of
<B>PTHREAD_RWLOCK_PREFER_READER_NP</B>.
This is ignored by glibc because the POSIX requirement to support
recursive writer locks would cause this option to create trivial
deadlocks; instead use
<B>PTHREAD_RWLOCK_PREFER_WRITER_NONRECURSIVE_NP</B>
which ensures the application developer will not take recursive
read locks thus avoiding deadlocks.
<DT id="4"><B>PTHREAD_RWLOCK_PREFER_WRITER_NONRECURSIVE_NP</B>
<DD>
Setting the lock kind to this
avoids writer starvation as long as any read locking is not done in a
recursive fashion.
</DL>
<P>
The
<B>pthread_rwlockattr_getkind_np</B>()
function returns the value of the lock kind attribute of the
read-write lock attribute object referred to by
<I>attr</I>
in the pointer
<I>pref</I>.
<A NAME="lbAE">&nbsp;</A>
<H2>RETURN VALUE</H2>
On success, these functions return 0.
Given valid pointer arguments,
<B>pthread_rwlockattr_getkind_np</B>()
always succeeds.
On error,
<B>pthread_rwlockattr_setkind_np</B>()
returns a nonzero error number.
<A NAME="lbAF">&nbsp;</A>
<H2>ERRORS</H2>
<DL COMPACT>
<DT id="5"><B>EINVAL</B>
<DD>
<I>pref</I>
specifies an unsupported value.
</DL>
<A NAME="lbAG">&nbsp;</A>
<H2>VERSIONS</H2>
The
<B>pthread_rwlockattr_getkind_np</B>()
and
<B>pthread_rwlockattr_setkind_np</B>()
functions first appeared in glibc 2.1.
<A NAME="lbAH">&nbsp;</A>
<H2>CONFORMING TO</H2>
These functions are non-standard GNU extensions;
hence the suffix &quot;_np&quot; (nonportable) in the names.
<A NAME="lbAI">&nbsp;</A>
<H2>SEE ALSO</H2>
<B><A HREF="/cgi-bin/man/man2html?7+pthreads">pthreads</A></B>(7)
<A NAME="lbAJ">&nbsp;</A>
<H2>COLOPHON</H2>
This page is part of release 5.05 of the Linux
<I>man-pages</I>
project.
A description of the project,
information about reporting bugs,
and the latest version of this page,
can be found at
<A HREF="https://www.kernel.org/doc/man-pages/.">https://www.kernel.org/doc/man-pages/.</A>
<P>
<HR>
<A NAME="index">&nbsp;</A><H2>Index</H2>
<DL>
<DT id="6"><A HREF="#lbAB">NAME</A><DD>
<DT id="7"><A HREF="#lbAC">SYNOPSIS</A><DD>
<DT id="8"><A HREF="#lbAD">DESCRIPTION</A><DD>
<DT id="9"><A HREF="#lbAE">RETURN VALUE</A><DD>
<DT id="10"><A HREF="#lbAF">ERRORS</A><DD>
<DT id="11"><A HREF="#lbAG">VERSIONS</A><DD>
<DT id="12"><A HREF="#lbAH">CONFORMING TO</A><DD>
<DT id="13"><A HREF="#lbAI">SEE ALSO</A><DD>
<DT id="14"><A HREF="#lbAJ">COLOPHON</A><DD>
</DL>
<HR>
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