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	<TITLE>&quot;PTHREAD_RWLOCK_RDLOCK&quot;(P) manual page</TITLE>
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<H4>POSIX Threads for Windows – REFERENCE - <A HREF="http://sources.redhat.com/pthreads-win32">Pthreads-w32</A></H4>
<P><A HREF="index.html">Reference Index</A></P>
<P><A HREF="#toc">Table of Contents</A></P>
<H2><A HREF="#toc0" NAME="sect0">Name</A></H2>
<P>pthread_rwlock_rdlock, pthread_rwlock_tryrdlock - lock a
read-write lock object for reading 
</P>
<H2><A HREF="#toc1" NAME="sect1">Synopsis</A></H2>
<P><B>#include &lt;pthread.h&gt; </B>
</P>
<P><B>int pthread_rwlock_rdlock(pthread_rwlock_t <I>*</I></B><I>rwlock</I><B>);
<BR></B>i<B>nt pthread_rwlock_tryrdlock(pthread_rwlock_t <I>*</I></B><I>rwlock</I><B>);
</B>
</P>
<H2><A HREF="#toc2" NAME="sect2">Description</A></H2>
<P>The <B>pthread_rwlock_rdlock</B> function shall apply a read lock
to the read-write lock referenced by <I>rwlock</I>. The calling
thread acquires the read lock if a writer does not hold the lock and
there are no writers blocked on the lock. 
</P>
<P><B>Pthreads-win32</B> does not prefer either writers or readers in
acquiring the lock – all threads enter a single prioritised FIFO
queue. While this may not be optimally efficient for some
applications, it does ensure that one type does not starve the other.</P>
<P>A thread may hold multiple concurrent read locks on <I>rwlock</I>
(that is, successfully call the <B>pthread_rwlock_rdlock</B> function
<I>n</I> times). If so, the application shall ensure that the thread
performs matching unlocks (that is, it calls the
<A HREF="pthread_rwlock_unlock.html"><B>pthread_rwlock_unlock</B>(3)</A>
function <I>n</I> times). 
</P>
<P>The <B>pthread_rwlock_tryrdlock</B> function shall apply a read
lock as in the <B>pthread_rwlock_rdlock</B> function, with the
exception that the function shall fail if the equivalent
<B>pthread_rwlock_rdlock</B> call would have blocked the calling
thread. In no case shall the <B>pthread_rwlock_tryrdlock</B> function
ever block; it always either acquires the lock or fails and returns
immediately. 
</P>
<P>Results are undefined if any of these functions are called with an
uninitialized read-write lock. 
</P>
<P><B>Pthreads-w32</B> does not detect deadlock if the thread already
owns the lock for writing.</P>
<P><B>Pthreads-w32</B> defines <B>_POSIX_READER_WRITER_LOCKS</B> in
pthread.h as  200112L to indicate that the reader/writer routines are
implemented and may be used.</P>
<H2><A HREF="#toc3" NAME="sect3">Return Value</A></H2>
<P>If successful, the <B>pthread_rwlock_rdlock</B> function shall
return zero; otherwise, an error number shall be returned to indicate
the error. 
</P>
<P>The <B>pthread_rwlock_tryrdlock</B> function shall return zero if
the lock for reading on the read-write lock object referenced by
<I>rwlock</I> is acquired. Otherwise, an error number shall be
returned to indicate the error. 
</P>
<H2><A HREF="#toc4" NAME="sect4">Errors</A></H2>
<P>The <B>pthread_rwlock_tryrdlock</B> function shall fail if: 
</P>
<DL>
	<DT><B>EBUSY</B> 
	</DT><DD STYLE="margin-bottom: 0.5cm">
	The read-write lock could not be acquired for reading because a
	writer holds the lock or a writer with the appropriate priority was
	blocked on it. 
	</DD></DL>
<P>
The <B>pthread_rwlock_rdlock</B> and <B>pthread_rwlock_tryrdlock</B>
functions may fail if: 
</P>
<DL>
	<DT><B>EINVAL</B> 
	</DT><DD>
	The value specified by <I>rwlock</I> does not refer to an
	initialized read-write lock object. 
	</DD><DT>
	<B>EAGAIN</B> 
	</DT><DD STYLE="margin-bottom: 0.5cm">
	The read lock could not be acquired because the maximum number of
	read locks for <I>rwlock</I> has been exceeded. 
	</DD></DL>
<P>
These functions shall not return an error code of [EINTR]. 
</P>
<P><I>The following sections are informative.</I> 
</P>
<H2><A HREF="#toc5" NAME="sect5">Examples</A></H2>
<P>None. 
</P>
<H2><A HREF="#toc6" NAME="sect6">Application Usage</A></H2>
<P>Applications using these functions may be subject to priority
inversion, as discussed in the Base Definitions volume of
IEEE&nbsp;Std&nbsp;1003.1-2001, Section 3.285, Priority Inversion. 
</P>
<H2><A HREF="#toc7" NAME="sect7">Rationale</A></H2>
<P>None. 
</P>
<H2><A HREF="#toc8" NAME="sect8">Future Directions</A></H2>
<P>None. 
</P>
<H2><A HREF="#toc9" NAME="sect9">See Also</A></H2>
<P><A HREF="pthread_rwlock_init.html"><B>pthread_rwlock_destroy</B>(3)</A>
<B>,</B> <A HREF="pthread_rwlock_timedrdlock.html"><B>pthread_rwlock_timedrdlock</B>(3)</A>
<B>,</B> <A HREF="pthread_rwlock_timedwrlock.html"><B>pthread_rwlock_timedwrlock</B>(3)</A>
<B>,</B> <A HREF="pthread_rwlock_wrlock.html"><B>pthread_rwlock_trywrlock</B>(3)</A>
<B>,</B> <A HREF="pthread_rwlock_unlock.html"><B>pthread_rwlock_unlock</B>(3)</A>
<B>,</B> <A HREF="pthread_rwlock_wrlock.html"><B>pthread_rwlock_wrlock</B>(3)</A>
<B>,</B> the Base Definitions volume of IEEE&nbsp;Std&nbsp;1003.1-2001,
<I>&lt;pthread.h&gt;</I> 
</P>
<H2><A HREF="#toc10" NAME="sect10">Copyright</A></H2>
<P>Portions of this text are reprinted and reproduced in electronic
form from IEEE Std 1003.1, 2003 Edition, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The Open
Group Base Specifications Issue 6, Copyright (C) 2001-2003 by the
Institute of Electrical and Electronics Engineers, Inc and The Open
Group. In the event of any discrepancy between this version and the
original IEEE and The Open Group Standard, the original IEEE and The
Open Group Standard is the referee document. The original Standard
can be obtained online at <A HREF="http://www.opengroup.org/unix/online.html">http://www.opengroup.org/unix/online.html</A>
. 
</P>
<P>Modified by Ross Johnson for use with <A HREF="http://sources.redhat.com/pthreads-win32">Pthreads-w32</A>.</P>
<HR>
<P><A NAME="toc"></A><B>Table of Contents</B></P>
<UL>
	<LI><P STYLE="margin-bottom: 0cm"><A HREF="#sect0" NAME="toc0">Name</A>
		</P>
	<LI><P STYLE="margin-bottom: 0cm"><A HREF="#sect1" NAME="toc1">Synopsis</A>
		</P>
	<LI><P STYLE="margin-bottom: 0cm"><A HREF="#sect2" NAME="toc2">Description</A>
		</P>
	<LI><P STYLE="margin-bottom: 0cm"><A HREF="#sect3" NAME="toc3">Return
	Value</A> 
	</P>
	<LI><P STYLE="margin-bottom: 0cm"><A HREF="#sect4" NAME="toc4">Errors</A>
		</P>
	<LI><P STYLE="margin-bottom: 0cm"><A HREF="#sect5" NAME="toc5">Examples</A>
		</P>
	<LI><P STYLE="margin-bottom: 0cm"><A HREF="#sect6" NAME="toc6">Application
	Usage</A> 
	</P>
	<LI><P STYLE="margin-bottom: 0cm"><A HREF="#sect7" NAME="toc7">Rationale</A>
		</P>
	<LI><P STYLE="margin-bottom: 0cm"><A HREF="#sect8" NAME="toc8">Future
	Directions</A> 
	</P>
	<LI><P STYLE="margin-bottom: 0cm"><A HREF="#sect9" NAME="toc9">See
	Also</A> 
	</P>
	<LI><P><A HREF="#sect10" NAME="toc10">Copyright</A> 
	</P>
</UL>
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