251 lines
7.4 KiB
HTML
251 lines
7.4 KiB
HTML
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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<html>
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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_init()--Initialize Mutex</title>
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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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<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_init()--Initialize 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_init(pthread_mutex_t *mutex,
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const pthread_mutexattr_t *attr);
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pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
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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_init</strong>() function initializes a mutex with
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the specified attributes for use. The new mutex may be used immediately for
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serializing critical resources. If <em>attr</em> is specified as <strong>
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NULL</strong>, all attributes are set to the default mutex attributes for the
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newly created mutex.</p>
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<p>With these declarations and initialization:</p>
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<pre>
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pthread_mutex_t mutex2;
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pthread_mutex_t mutex3;
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pthread_mutexattr_t mta;
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pthread_mutexattr_init(&mta);
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</pre>
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<p>The following three mutex initialization mechanisms have equivalent
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function.</p>
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<pre>
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pthread_mutex_t mutex1 = PTHREAD_MUTEX_INITIALIZER;
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pthread_mutex_init(&mutex2, NULL);
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pthread_mutex_init(&mutex3, &mta);
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</pre>
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<p>All three mutexes are created with the default mutex attributes.</p>
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<p>Every mutex must eventually be destroyed with <strong>
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pthread_mutex_destroy</strong>(). The machine eventually detects the error if a
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mutex is not destroyed. Large numbers of these entries can affect system
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performance. Always use <strong>pthread_mutex_destroy</strong>() before freeing
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or reusing mutex storage.</p>
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<p>Once a mutex is created, it cannot be validly copied or moved to a new
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location. If the mutex is copied or moved to a new location, the new object is
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not valid and cannot be used. Attempts to use the new object result in the
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EINVAL error.</p>
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<p><strong>Note:</strong> Mutex initialization using the <strong>
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PTHREAD_MUTEX_INITIALIZER</strong> does not immediately initialize the mutex.
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Instead, on first use, the <strong>pthread_mutex_lock</strong>() or <strong>
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pthread_mutex_trylock</strong>() functions branch into a slow path and cause
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the initialization of the mutex. Because a mutex is not just a simple memory
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object and requires that some resources be allocated by the system, an attempt
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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_INITIALER</strong> and was not yet locked causes an
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<strong>EINVAL</strong> error.</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 variable to contain a mutex object.</dd>
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<dt><strong>attr</strong></dt>
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<dd>(Input) The address of the variable containing the mutex attributes
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object.</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_init</strong>() was successful.</dd>
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<dt><strong>value</strong></dt>
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<dd><strong>pthread_mutex_init</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_init</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>[ENOMEM]</em></dt>
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<dd><p>The system cannot allocate the resources required to create the mutex.</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_62.htm">pthread_mutex_lock()</a>--Lock 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_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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#define _MULTI_THREADED
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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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pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
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pthread_mutex_t mutex2;
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pthread_mutex_t mutex3;
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int main(int argc, char **argv)
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{
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int rc=0;
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pthread_mutexattr_t mta;
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printf("Enter Testcase - %s\n", argv[0]);
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printf("Create a default mutex attribute\n");
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rc = pthread_mutexattr_init(&mta);
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checkResults("pthread_mutexattr_init\n", rc);
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printf("Create the mutexes using the default mutex attributes\n");
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printf("First mutex created via static PTHREAD_MUTEX_INITIALIZER\n");
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printf("Create the mutex using the NULL attributes (default)\n");
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rc = pthread_mutex_init(&mutex3, NULL);
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checkResults("pthread_mutex_init(NULL)\n", rc);
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printf("Create the mutex using a mutex attributes object\n");
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rc = pthread_mutex_init(&mutex2, &mta);
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checkResults("pthread_mutex_init(mta)\n", rc);
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printf("- At this point, all mutexes can be used with their\n");
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printf("- default attributes from any threads that want to\n");
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printf("- use them\n");
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printf("Destroy all mutexes\n");
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pthread_mutex_destroy(&mutex);
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pthread_mutex_destroy(&mutex2);
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pthread_mutex_destroy(&mutex3);
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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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Enter Testcase - QP0WTEST/TPMTXINI0
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Create a default mutex attribute
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Create the mutexes using the default mutex attributes
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First mutex created via static PTHREAD_MUTEX_INITIALIZER
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Create the mutex using the NULL attributes (default)
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Create the mutex using a mutex attributes object
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- At this point, all mutexes can be used with their
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- default attributes from any threads that want to
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- use them
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Destroy all mutexes
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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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"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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