246 lines
5.9 KiB
HTML
246 lines
5.9 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>sem_trywait()-Try to Decrement Semaphore</title>
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<!-- Begin Header Records ========================================== -->
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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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<!-- Change History: -->
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<!-- Direct1 SCRIPT J converted by B2H R4.1 (346) (CMS) by V2KEA304 -->
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<!-- at RCHVMW2 on 17 Feb 1999 at 11:05:09 -->
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<!-- Edited by Kersten Feb 02 -->
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<!-- This file has undergone html cleanup on 05/01/02 by JET -->
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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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<!--End Header Records -->
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<!-- Java sync-link -->
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<script type="text/javascript" language="Javascript" src="../rzahg/synch.js">
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</script>
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<a name="Top_Of_Page"></a>
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<h2>sem_trywait()--Try to Decrement Semaphore</h2>
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<div class="box" style="width: 60%;">
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<br>
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Syntax<br>
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<pre>
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#include <semaphore.h>
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int sem_trywait(sem_t * <em>sem</em>);
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</pre>
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<br>
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Service Program Name: QP0ZPSEM<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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</div>
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<p>The <strong>sem_trywait()</strong> function attempts to decrement the value
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of the semaphore. The semaphore will be decremented if its value is greater
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than zero. If the value of the semaphore is zero, then <strong>
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sem_trywait()</strong> will return -1 and set <em>errno</em> to EAGAIN.</p>
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<br>
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<h3>Parameters</h3>
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<dl>
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<dt><strong>sem</strong></dt>
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<dd>(Input) A pointer to an initialized unnamed semaphore or opened named
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semaphore.</dd>
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</dl>
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<br>
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<h3>Authorities</h3>
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<p>None</p>
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<br>
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<h3>Return Value</h3>
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<table cellpadding="5">
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<!-- cols="5 95" -->
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<tr>
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<td align="left" valign="top"><em>0</em></td>
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<td align="left" valign="top"><strong>sem_trywait()</strong> was
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successful.</td>
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</tr>
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<tr>
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<td align="left" valign="top" nowrap><em>-1</em></td>
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<td align="left" valign="top"><strong>sem_trywait()</strong> was not
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successful. The <em>errno</em> variable is set to indicate the error.</td>
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</tr>
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</table>
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<br>
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<br>
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<h3>Error Conditions</h3>
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<p>If <strong>sem_trywait()</strong> is not successful, <em>errno</em>
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usually indicates one of the following errors. Under some conditions, <em>
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errno</em> could indicate an error other than those listed here.</p>
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<dl>
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<dt><em>[EAGAIN]</em></dt>
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<dd>
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<p>Operation would have caused the process to be suspended.</p>
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<p>The value of the semaphore is currently zero and cannot be decremented.</p>
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</dd>
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<dt><em>[EINTR]</em></dt>
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<dd>
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<p>Interrupted function call.</p>
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</dd>
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<dt><em>[EINVAL]</em></dt>
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<dd>
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<p>The value specified for the argument is not correct.</p>
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<p>A function was passed incorrect argument values, or an operation was
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attempted on an object and the operation specified is not supported for that
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type of object.</p>
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<p>An argument value is not valid, out of range, or NULL.</p>
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</dd>
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</dl>
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<br>
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<h3>Error Messages</h3>
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<p>None.</p>
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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>semaphore.h</strong>> file (see <a href="unix13.htm">
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Header Files for UNIX-Type Functions</a>)<br>
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<br>
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</li>
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<li><a href="ipcsemcl.htm">sem_close()</a>--Close Named Semaphore<br>
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<br>
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</li>
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<li><a href="ipcsemde.htm">sem_destroy()</a>--Destroy Unnamed Semaphore<br>
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<br>
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</li>
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<li><a href="ipcsemge.htm">sem_getvalue()</a>--Get Semaphore Value<br>
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<br>
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</li>
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<li><a href="ipcsemi.htm">sem_init()</a>--Initialize Unnamed Semaphore<br>
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<br>
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</li>
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<li><a href="ipcsemin.htm">sem_init_np()</a>--Initialize Unnamed Semaphore with
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Maximum Value<br>
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<br>
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</li>
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<li><a href="ipcsemo.htm">sem_open()</a>--Open Named Semaphore<br>
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<br>
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</li>
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<li><a href="ipcsemon.htm">sem_open_np()</a>--Open Named Semaphore with Maximum
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Value<br>
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<br>
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</li>
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<li><a href="ipcsemp.htm">sem_post()</a>--Post to Semaphore<br>
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<br>
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</li>
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<li><a href="ipcsempn.htm">sem_post_np()</a>--Post Value to Semaphore<br>
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<br>
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</li>
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<li><a href="ipcsemun.htm">sem_unlink()</a>--Unlink Named Semaphore<br>
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<br>
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</li>
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<li><a href="ipcsemw.htm">sem_wait()</a>--Wait for Semaphore<br>
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<br>
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</li>
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<li><a href="ipcsemwn.htm">sem_wait_np()</a>--Wait for Semaphore with
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Timeout</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>The following example attempts to decrement a semaphore with a current value
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of zero.</p>
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<pre>
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#include <stdio.h>
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#include <errno.h>
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#include <semaphore.h>
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main() {
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sem_t my_semaphore;
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int value;
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int rc;
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sem_init(&my_semaphore, 0, 1);
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sem_getvalue(&my_semaphore, &value);
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printf("The initial value of the semaphore is %d\n", value);
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sem_wait(&my_semaphore);
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sem_getvalue(&my_semaphore, &value);
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printf("The value of the semaphore after the wait is %d\n", value);
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rc = sem_trywait(&my_semaphore);
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if ((rc == -1) && (errno == EAGAIN)) {
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printf("sem_trywait did not decrement the semaphore\n");
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}
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}
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</pre>
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<br>
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<h3>Output:</h3>
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<pre>
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The initial value of the semaphore is 1
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The value of the semaphore after the wait is 0
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sem_trywait did not decrement the semaphore
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</pre>
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<br>
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<hr>
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API introduced: V4R4
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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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"unix.htm">UNIX-Type APIs</a> | <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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