168 lines
7.1 KiB
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168 lines
7.1 KiB
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<META HTTP-EQUIV="CONTENT-TYPE" CONTENT="text/html; charset=utf-8">
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<TITLE>"PTHREAD_RWLOCK_RDLOCK"(P) manual page</TITLE>
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<META NAME="CREATED" CONTENT="20050505;10352600">
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<BODY LANG="en-GB" BGCOLOR="#ffffff" DIR="LTR">
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<H4>POSIX Threads for Windows – REFERENCE - <A HREF="http://sources.redhat.com/pthreads-win32">Pthreads-w32</A></H4>
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<P><A HREF="index.html">Reference Index</A></P>
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<P><A HREF="#toc">Table of Contents</A></P>
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<H2><A HREF="#toc0" NAME="sect0">Name</A></H2>
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<P>pthread_rwlock_rdlock, pthread_rwlock_tryrdlock - lock a
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read-write lock object for reading
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</P>
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<H2><A HREF="#toc1" NAME="sect1">Synopsis</A></H2>
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<P><B>#include <pthread.h> </B>
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</P>
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<P><B>int pthread_rwlock_rdlock(pthread_rwlock_t <I>*</I></B><I>rwlock</I><B>);
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<BR></B>i<B>nt pthread_rwlock_tryrdlock(pthread_rwlock_t <I>*</I></B><I>rwlock</I><B>);
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</B>
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</P>
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<H2><A HREF="#toc2" NAME="sect2">Description</A></H2>
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<P>The <B>pthread_rwlock_rdlock</B> function shall apply a read lock
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to the read-write lock referenced by <I>rwlock</I>. The calling
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thread acquires the read lock if a writer does not hold the lock and
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there are no writers blocked on the lock.
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</P>
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<P><B>Pthreads-win32</B> does not prefer either writers or readers in
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acquiring the lock – all threads enter a single prioritised FIFO
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queue. While this may not be optimally efficient for some
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applications, it does ensure that one type does not starve the other.</P>
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<P>A thread may hold multiple concurrent read locks on <I>rwlock</I>
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(that is, successfully call the <B>pthread_rwlock_rdlock</B> function
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<I>n</I> times). If so, the application shall ensure that the thread
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performs matching unlocks (that is, it calls the
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<A HREF="pthread_rwlock_unlock.html"><B>pthread_rwlock_unlock</B>(3)</A>
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function <I>n</I> times).
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</P>
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<P>The <B>pthread_rwlock_tryrdlock</B> function shall apply a read
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lock as in the <B>pthread_rwlock_rdlock</B> function, with the
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exception that the function shall fail if the equivalent
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<B>pthread_rwlock_rdlock</B> call would have blocked the calling
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thread. In no case shall the <B>pthread_rwlock_tryrdlock</B> function
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ever block; it always either acquires the lock or fails and returns
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immediately.
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</P>
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<P>Results are undefined if any of these functions are called with an
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uninitialized read-write lock.
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</P>
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<P><B>Pthreads-w32</B> does not detect deadlock if the thread already
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owns the lock for writing.</P>
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<P><B>Pthreads-w32</B> defines <B>_POSIX_READER_WRITER_LOCKS</B> in
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pthread.h as 200112L to indicate that the reader/writer routines are
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implemented and may be used.</P>
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<H2><A HREF="#toc3" NAME="sect3">Return Value</A></H2>
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<P>If successful, the <B>pthread_rwlock_rdlock</B> function shall
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return zero; otherwise, an error number shall be returned to indicate
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the error.
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</P>
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<P>The <B>pthread_rwlock_tryrdlock</B> function shall return zero if
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the lock for reading on the read-write lock object referenced by
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<I>rwlock</I> is acquired. Otherwise, an error number shall be
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returned to indicate the error.
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</P>
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<H2><A HREF="#toc4" NAME="sect4">Errors</A></H2>
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<P>The <B>pthread_rwlock_tryrdlock</B> function shall fail if:
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</P>
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<DL>
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<DT><B>EBUSY</B>
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</DT><DD STYLE="margin-bottom: 0.5cm">
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The read-write lock could not be acquired for reading because a
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writer holds the lock or a writer with the appropriate priority was
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blocked on it.
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</DD></DL>
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<P>
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The <B>pthread_rwlock_rdlock</B> and <B>pthread_rwlock_tryrdlock</B>
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functions may fail if:
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</P>
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<DL>
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<DT><B>EINVAL</B>
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</DT><DD>
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The value specified by <I>rwlock</I> does not refer to an
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initialized read-write lock object.
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</DD><DT>
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<B>EAGAIN</B>
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</DT><DD STYLE="margin-bottom: 0.5cm">
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The read lock could not be acquired because the maximum number of
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read locks for <I>rwlock</I> has been exceeded.
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</DD></DL>
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<P>
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These functions shall not return an error code of [EINTR].
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</P>
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<P><I>The following sections are informative.</I>
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</P>
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<H2><A HREF="#toc5" NAME="sect5">Examples</A></H2>
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<P>None.
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</P>
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<H2><A HREF="#toc6" NAME="sect6">Application Usage</A></H2>
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<P>Applications using these functions may be subject to priority
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inversion, as discussed in the Base Definitions volume of
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IEEE Std 1003.1-2001, Section 3.285, Priority Inversion.
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</P>
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<H2><A HREF="#toc7" NAME="sect7">Rationale</A></H2>
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<P>None.
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</P>
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<H2><A HREF="#toc8" NAME="sect8">Future Directions</A></H2>
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<P>None.
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</P>
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<H2><A HREF="#toc9" NAME="sect9">See Also</A></H2>
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<P><A HREF="pthread_rwlock_init.html"><B>pthread_rwlock_destroy</B>(3)</A>
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<B>,</B> <A HREF="pthread_rwlock_timedrdlock.html"><B>pthread_rwlock_timedrdlock</B>(3)</A>
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<B>,</B> <A HREF="pthread_rwlock_timedwrlock.html"><B>pthread_rwlock_timedwrlock</B>(3)</A>
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<B>,</B> <A HREF="pthread_rwlock_wrlock.html"><B>pthread_rwlock_trywrlock</B>(3)</A>
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<B>,</B> <A HREF="pthread_rwlock_unlock.html"><B>pthread_rwlock_unlock</B>(3)</A>
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<B>,</B> <A HREF="pthread_rwlock_wrlock.html"><B>pthread_rwlock_wrlock</B>(3)</A>
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<B>,</B> the Base Definitions volume of IEEE Std 1003.1-2001,
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<I><pthread.h></I>
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</P>
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<H2><A HREF="#toc10" NAME="sect10">Copyright</A></H2>
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<P>Portions of this text are reprinted and reproduced in electronic
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form from IEEE Std 1003.1, 2003 Edition, Standard for Information
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Technology -- Portable Operating System Interface (POSIX), The Open
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Group Base Specifications Issue 6, Copyright (C) 2001-2003 by the
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Institute of Electrical and Electronics Engineers, Inc and The Open
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Group. In the event of any discrepancy between this version and the
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original IEEE and The Open Group Standard, the original IEEE and The
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Open Group Standard is the referee document. The original Standard
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can be obtained online at <A HREF="http://www.opengroup.org/unix/online.html">http://www.opengroup.org/unix/online.html</A>
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.
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</P>
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<P>Modified by Ross Johnson for use with <A HREF="http://sources.redhat.com/pthreads-win32">Pthreads-w32</A>.</P>
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<HR>
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<P><A NAME="toc"></A><B>Table of Contents</B></P>
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<UL>
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<LI><P STYLE="margin-bottom: 0cm"><A HREF="#sect0" NAME="toc0">Name</A>
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</P>
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<LI><P STYLE="margin-bottom: 0cm"><A HREF="#sect1" NAME="toc1">Synopsis</A>
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</P>
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<LI><P STYLE="margin-bottom: 0cm"><A HREF="#sect2" NAME="toc2">Description</A>
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</P>
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<LI><P STYLE="margin-bottom: 0cm"><A HREF="#sect3" NAME="toc3">Return
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Value</A>
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</P>
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<LI><P STYLE="margin-bottom: 0cm"><A HREF="#sect4" NAME="toc4">Errors</A>
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</P>
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<LI><P STYLE="margin-bottom: 0cm"><A HREF="#sect5" NAME="toc5">Examples</A>
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</P>
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<LI><P STYLE="margin-bottom: 0cm"><A HREF="#sect6" NAME="toc6">Application
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Usage</A>
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</P>
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<LI><P STYLE="margin-bottom: 0cm"><A HREF="#sect7" NAME="toc7">Rationale</A>
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</P>
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<LI><P STYLE="margin-bottom: 0cm"><A HREF="#sect8" NAME="toc8">Future
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Directions</A>
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</P>
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<LI><P STYLE="margin-bottom: 0cm"><A HREF="#sect9" NAME="toc9">See
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Also</A>
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</P>
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<LI><P><A HREF="#sect10" NAME="toc10">Copyright</A>
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</P>
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</UL>
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