<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"> <HTML> <HEAD> <META HTTP-EQUIV="CONTENT-TYPE" CONTENT="text/html; charset=utf-8"> <TITLE>"PTHREAD_RWLOCK_RDLOCK"(P) manual page</TITLE> <META NAME="GENERATOR" CONTENT="OpenOffice.org 1.1.3 (Linux)"> <META NAME="CREATED" CONTENT="20050505;10352600"> <META NAME="CHANGED" CONTENT="20050505;12554200"> <!-- manual page source format generated by PolyglotMan v3.2, --> <!-- available at http://polyglotman.sourceforge.net/ --> </HEAD> <BODY LANG="en-GB" BGCOLOR="#ffffff" DIR="LTR"> <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 <pthread.h> </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 Std 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 Std 1003.1-2001, <I><pthread.h></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> </BODY> </HTML>