160 lines
7.2 KiB
HTML
160 lines
7.2 KiB
HTML
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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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<html>
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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<meta name="Copyright" content="Copyright (c) 2006 by IBM Corporation">
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<title>pthread_rwlock_rdlock()--Get Shared Read Lock</title>
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<!-- All rights reserved. Licensed Materials Property of IBM -->
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<!-- US Government Users Restricted Rights -->
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<!-- Use, duplication or disclosure restricted by -->
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<!-- GSA ADP Schedule Contract with IBM Corp. -->
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<!-- Begin Header Records ========================================== -->
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<!-- NETMG2 SCRIPT A converted by B2H R4.1 (346) (CMS) by HOLTJM at -->
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<!-- RCHVMW2 on 29 Jan 1999 at 10:01:37 -->
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<!--File Edited November 2001 -->
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<!--End Header Records -->
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<link rel="stylesheet" type="text/css" href="../rzahg/ic.css">
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</head>
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<body>
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<!-- Java sync-link -->
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<script language="Javascript" src="../rzahg/synch.js" type="text/javascript">
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</script>
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<a name="Top_Of_Page"></a>
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<h2>pthread_rwlock_rdlock()--Get Shared Read Lock</h2>
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<div class="box" style="width: 60%;">
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<br>
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Syntax:
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<pre> #include <pthread.h>
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int pthread_rwlock_rdlock(pthread_rwlock_t *rwlock); </pre>
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Service Program Name: QP0WPTHR<br>
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<!-- iddvc RMBR -->
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<br>
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Default Public Authority: *USE <br>
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<!-- iddvc RMBR -->
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<br>
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Threadsafe: Yes<br>
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<!-- iddvc RMBR -->
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<br>
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Signal Safe: Yes<br>
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<br>
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</div>
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<p>The <strong>pthread_rwlock_rdlock</strong>() function attempts to acquire a shared read lock on the read/write lock specified by <em>rwlock</em>.</p>
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<p>Any number of threads can hold shared read locks on the same read/write lock object. If any thread holds an exclusive write lock on a read/write lock object, no other threads are allowed to hold a shared read or exclusive write lock.</p>
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<p>If no threads are holding an exclusive write lock on the read/write lock, the calling thread successfully acquires the shared read lock.</p>
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<p>If the calling thread already holds a shared read lock on the read/write lock, another read lock can be successfully acquired by the calling thread. If more than one shared read lock is successfully acquired by a thread on a read/write lock object, that thread is required to successfully call <strong>pthread_rwlock_unlock</strong>() a matching number of times.</p>
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<p>With a large number of readers and relatively few writers, there is the possibility of writer starvation. If threads are waiting for an exclusive write lock on the read/write lock and there are threads that currently hold a shared read lock, the shared read lock request is granted.</p>
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<p>If the read/write lock is destroyed while <strong>pthread_rwlock_rdlock</strong>() is waiting for the shared read lock, the <strong>EDESTROYED</strong> error is returned.</p>
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<p>If a signal is delivered to the thread while it is waiting for the lock, the signal handler (if any) runs, and the thread resumes waiting.</p>
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<br>
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<h3>Read/Write Lock Deadlocks</h3>
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<p>If a thread ends while holding a write lock, the attempt by another thread to acquire a shared read or exclusive write lock will not be successful. In this case, the attempt to acquire the lock will deadlock. If a thread ends while holding a read lock, the system automatically releases the read lock.</p>
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<p>For the <strong>pthread_rwlock_rdlock</strong>() function, the pthreads run-time simulates the deadlock that has occurred in your application. When you are attempting to debug these deadlock scenarios, the CL command WRKJOB, option 20, shows the thread as in a condition wait. Displaying the call stack shows that the function <strong>deadlockOnOrphanedRWLock</strong> is in the call stack.</p>
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<br>
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<h3>Upgrade / Downgrade a Lock</h3>
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<p>If the calling thread currently holds an exclusive write lock on the read/write lock object, the shared read lock request is granted. After the shared read lock request is granted, the calling thread holds <strong>both</strong> the shared read <strong>and</strong> the exclusive write lock for the specified read/write lock object. If the thread calls <strong>pthread_rwlock_unlock</strong>() while holding one or more shared read locks <strong>and</strong> one or more exclusive write locks, the exclusive write locks are unlocked first. If more than one outstanding exclusive write lock was held by the thread, a matching number of successful calls to <strong>pthread_rwlock_unlock</strong>() must be done before all write locks are unlocked. At that time, subsequent calls to <strong>pthread_rwlock_unlock</strong>() unlock the shared read locks.</p>
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<p>This behavior can be used to allow your application to upgrade or downgrade one lock type to another. See <a href="concep26.htm">Read/write locks can be upgraded/downgraded</a>.</p>
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<br>
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<h3>Authorities and Locks</h3>
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<p>None.</p>
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<br>
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<h3>Parameters</h3>
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<dl>
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<dt><strong>rwlock</strong></dt>
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<dd>(Input) The address of the read/write lock</dd>
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</dl>
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<br>
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<h3>Return Value</h3>
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<dl>
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<dt><strong>0</strong></dt>
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<dd><strong>pthread_rwlock_rdlock</strong>() was successful.</dd>
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<dt><em>value</em></dt>
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<dd><strong>pthread_rwlock_rdlock</strong>() was not successful. <em>value</em> is set to indicate the error condition.</dd>
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</dl>
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<br>
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<h3>Error Conditions</h3>
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<p>If <strong>pthread_rwlock_rdlock</strong>() was not successful, the error condition returned usually indicates one of the following errors. Under some conditions, the value returned could indicate an error other than those listed here.</p>
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<dl>
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<dt><em>[EINVAL]</em></dt>
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<dd><p>The value specified for the argument is not correct.</p></dd>
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<dt><em>[EDESTROYED]</em></dt>
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<dd><p>The lock was destroyed while waiting.</p></dd>
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</dl>
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<br>
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<h3>Related Information</h3>
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<ul>
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<li>The <<strong>pthread.h</strong>> header file. See <a href="rzah4hed.htm">Header files for Pthread functions</a>.<br><br></li>
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<li><a href="users_86.htm">pthread_rwlock_init()</a>--Initialize Read/Write Lock<br><br></li>
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<li><a href="users_88.htm">pthread_rwlock_timedrdlock_np()</a>--Get Shared Read Lock with Time-out<br><br></li>
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<li><a href="users_89.htm">pthread_rwlock_timedwrlock_np()</a>--Get Exclusive Write Lock with Time-out<br><br></li>
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<li><a href="users_90.htm">pthread_rwlock_tryrdlock()</a>--Get Shared Read Lock with No Wait<br><br></li>
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<li><a href="users_91.htm">pthread_rwlock_trywrlock()</a>--Get Exclusive Write Lock with No Wait<br><br></li>
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<li><a href="users_92.htm">pthread_rwlock_unlock()</a>--Unlock Exclusive Write or Shared Read Lock<br><br></li>
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<li><a href="users_93.htm">pthread_rwlock_wrlock()</a>--Get Exclusive Write Lock</li>
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</ul>
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<br>
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<h3>Example</h3>
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<p>See <a href="../apiref/aboutapis.htm#codedisclaimer">Code disclaimer information</a>
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for information pertaining to code examples.</p>
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<p>See the <a href="users_86.htm#372485">pthread_rwlock_init() example</a>.</p>
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<hr>
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API introduced: V4R3
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<hr>
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<center>
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<table cellpadding="2" cellspacing="2">
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<tr align="center">
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<td valign="middle" align="center"><a href="#Top_Of_Page">Top</a> |
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<a href="rzah4mst.htm">Pthread APIs</a> |
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<a href="aplist.htm">APIs by category</a></td>
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</tr>
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</table>
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</center>
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</body>
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</html>
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