347 lines
11 KiB
HTML
347 lines
11 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_mutex_lock()--Lock Mutexe</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_mutex_lock()--Lock Mutex</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>
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#include <pthread.h>
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int pthread_mutex_lock(pthread_mutex_t *mutex);
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</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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<!-- iddvc RMBR -->
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<br>
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</div>
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<p>The <strong>pthread_mutex_lock</strong>() function acquires ownership of the
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mutex specified. If the mutex currently is locked by another thread, the call
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to <strong>pthread_mutex_lock</strong>() blocks until that thread relinquishes
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ownership by a call to <strong>pthread_mutex_unlock</strong>().</p>
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<p>If a signal is delivered to a thread while that thread is waiting for a
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mutex, when the signal handler returns, the wait resumes. <strong>
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pthread_mutex_lock</strong>() does not return <strong>EINTR</strong> like some
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other blocking function calls.</p>
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<p>Use the CL command WRKJOB, option 20, to help you debug mutex deadlocks.</p>
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<p>Destroying a held mutex is a common way to serialize destruction of objects
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that are protected by that mutex. This action is allowed. The call to <strong>
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pthread_mutex_lock</strong>() may fail with the <strong>EDESTROYED</strong>
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error if the mutex is destroyed by the thread that was currently holding
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it.</p>
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<p>Mutex initialization using the <strong>PTHREAD_MUTEX_INITIALIZER</strong>
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does not immediately initialize the mutex. Instead, on first use, <strong>
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pthread_mutex_timedlock_np</strong>() or <strong>pthread_mutex_lock</strong>()
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or <strong>pthread_mutex_trylock</strong>() branches into a slow path and
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causes the initialization of the mutex. Because a mutex is not just a simple
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memory object and requires that some resources be allocated by the system, an
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attempt to call <strong>pthread_mutex_destroy</strong>() or <strong>
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pthread_mutex_unlock</strong>() on a mutex that was statically initialized
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using <strong>PTHREAD_MUTEX_INITIALIZER</strong> and was not yet locked causes an
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<strong>EINVAL</strong> error.</p>
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<p>A pthread mutex is a structure of type pthread_mutex_t that implement a
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behavior based on the Pthread mutexes. An MI mutex is a structure built into
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the machine that implement a similar sort of serialization construct.</p>
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<p>The maximum number of recursive locks by the owning thread is 32,767. When
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this number is exceeded, attempts to lock the mutex return the <strong>
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ERECURSE</strong> error.</p>
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<br>
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<h3>Mutex Types</h3>
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<p>A normal mutex cannot be locked repeatedly by the owner. Attempts by a
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thread to relock an already held mutex, or to lock a mutex that was held by
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another thread when that thread terminated, result in a deadlock condition.</p>
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<p>A recursive mutex can be locked repeatedly by the owner. The mutex does not
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become unlocked until the owner has called <strong>
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pthread_mutex_unlock</strong>() for each successful lock request that it has
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outstanding on the mutex.</p>
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<p>An errorcheck mutex checks for deadlock conditions that occur when a thread
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relocks an already held mutex. If a thread attempts to relock a mutex that it
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already holds, the lock request fails with the <strong>EDEADLK</strong>
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error.</p>
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<p>An ownerterm mutex is an i5/OS extension to the errorcheck mutex type. An
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ownerterm mutex checks for deadlock conditions that occur when a thread relocks
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an already held mutex. If a thread attempts to relock a mutex that it already
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holds, the lock request fails with the <strong>EDEADLK</strong> error. An
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ownerterm mutex also checks for deadlock conditions that occur when a thread
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attempts to lock a mutex that was held by another thread when that thread
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terminated (an orphaned mutex). If a thread attempts to lock an orphaned mutex,
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the lock request fails with the <strong>EOWNERTERM</strong> error.</p>
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<p>When a thread terminates while holding a mutex lock on a normal or
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errorcheck mutex, other threads that wait for that mutex will block forever.
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The pthreads run-time simulates the deadlock that has occurred in your
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application. When you are attempting to debug these deadlock scenarios, the CL
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command WRKJOB, option 20 shows the thread as in a condition wait. Displaying
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the call stack shows that the function
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<strong>deadlockOnOrphanedMutex</strong> is in the call stack.</p>
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<p>When a thread attempts to acquire a normal mutex that it already holds, the
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thread will block forever. The pthreads run-time simulates the deadlock that
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has occurred in your application. When you are attempting to debug these
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deadlock scenarios, the CL command WRKJOB, option 20, shows the thread as in a
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condition wait. Displaying the call stack will show that the function <strong>
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deadlockOnAlreadyHeldMutex</strong> is in the call stack.</p>
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<p>To change these behaviors, use an errorcheck or ownerterm mutex type.</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>mutex</strong></dt>
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<dd>(Input) The address of the mutex to 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_mutex_lock</strong>() was successful.</dd>
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<dt><strong>value</strong></dt>
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<dd><strong>pthread_mutex_lock</strong>() was not successful. <em>value</em> is
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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_mutex_lock</strong>() was not successful, the error
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condition returned usually indicates one of the following errors. Under some
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conditions, the value returned could indicate an error other than those listed
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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>While waiting for the mutex lock to be satisfied, the mutex was
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destroyed.</p></dd>
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<dt><em>[EOWNERTERM]</em></dt>
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<dd><p>A thread terminated the holding of the mutex, and the mutex is an ownerterm
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mutex type.</p></dd>
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<dt><em>[EDEADLK]</em></dt>
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<dd><p></p>A thread attempted to relock an already held mutex, and the mutex is an
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errorcheck mutex type. <a name="336837"></a><p></p></dd>
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<dt><em>[ERECURSE]</em></dt>
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<dd><p>The recursive mutex cannot be recursively locked again.</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=
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"rzah4hed.htm">Header files for Pthread functions</a>.<br><br></li>
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<li><a href="users_60.htm">pthread_mutex_destroy()</a>--Destroy Mutex<br><br></li>
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<li><a href="users_61.htm">pthread_mutex_init()</a>--Initialize Mutex<br><br></li>
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<li><a href="users_64.htm">pthread_mutex_trylock()</a>--Lock Mutex with No
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Wait<br><br></li>
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<li><a href="users_63.htm">pthread_mutex_timedlock_np()</a>--Lock Mutex with Time-Out<br><br></li>
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<li><a href="users_65.htm">pthread_mutex_unlock()</a>--Unlock Mutex</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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<pre>
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#include <pthread.h>
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#include <stdio.h>
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#include "check.h"
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/*
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This example shows the corruption that can result if no
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serialization is done and also shows the use of
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pthread_mutex_lock(). Call it with no parameters
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to use pthread_mutex_lock() to protect the critical section,
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or 1 or more parameters to show data corruption that occurs
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without locking.
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*/
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#define LOOPCONSTANT 100000
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#define THREADS 10
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pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
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int i,j,k,l;
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int uselock=1;
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void *threadfunc(void *parm)
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{
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int loop = 0;
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int rc;
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for (loop=0; loop<LOOPCONSTANT; ++loop) {
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if (uselock) {
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rc = pthread_mutex_lock(&mutex);
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checkResults("pthread_mutex_lock()\n", rc);
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}
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++i; ++j; ++k; ++l;
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if (uselock) {
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rc = pthread_mutex_unlock(&mutex);
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checkResults("pthread_mutex_unlock()\n", rc);
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}
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}
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return NULL;
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}
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int main(int argc, char **argv)
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{
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pthread_t threadid[THREADS];
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int rc=0;
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int loop=0;
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pthread_attr_t pta;
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printf("Entering testcase\n");
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printf("Give any number of parameters to show data corruption\n");
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if (argc != 1) {
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printf("A parameter was specified, no serialization is being done!\n");
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uselock = 0;
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}
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pthread_attr_init(&pta);
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pthread_attr_setdetachstate(&pta, PTHREAD_CREATE_JOINABLE);
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printf("Creating %d threads\n", THREADS);
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for (loop=0; loop<THREADS; ++loop) {
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rc = pthread_create(&threadid[loop], &pta, threadfunc, NULL);
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checkResults("pthread_create()\n", rc);
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}
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printf("Wait for results\n");
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for (loop=0; loop<THREADS; ++loop) {
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rc = pthread_join(threadid[loop], NULL);
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checkResults("pthread_join()\n", rc);
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}
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printf("Cleanup and show results\n");
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pthread_attr_destroy(&pta);
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pthread_mutex_destroy(&mutex);
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printf("\nUsing %d threads and LOOPCONSTANT = %d\n",
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THREADS, LOOPCONSTANT);
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printf("Values are: (should be %d)\n", THREADS * LOOPCONSTANT);
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printf(" ==>%d, %d, %d, %d\n", i, j, k, l);
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printf("Main completed\n");
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return 0;
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}
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</pre>
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<p><strong>Output:</strong></p>
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<pre>
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Entering testcase
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Give any number of parameters to show data corruption
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Creating 10 threads
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Wait for results
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Cleanup and show results
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Using 10 threads and LOOPCONSTANT = 100000
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Values are: (should be 1000000)
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==>1000000, 1000000, 1000000, 1000000
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Main completed
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</pre>
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<br>
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<p><strong>Output:</strong></p>
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<a name="336938"></a>
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<h3>(data corruption without locking example)</h3>
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<pre>
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Entering testcase
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Give any number of parameters to show data corruption
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A parameter was specified, no serialization is being done!
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Creating 10 threads
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Wait for results
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Cleanup and show results
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Using 10 threads and LOOPCONSTANT = 100000
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Values are: (should be 1000000)
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==>883380, 834630, 725131, 931883
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Main completed
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</pre>
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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> | <a href=
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||
|
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"rzah4mst.htm">Pthread APIs</a> | <a href="aplist.htm">APIs by
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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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