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8.4 KiB
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198 lines
8.4 KiB
HTML
<?xml version="1.0" encoding="UTF-8" standalone="no"?>
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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
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<html xmlns="http://www.w3.org/1999/xhtml">
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
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<title>DB->exists()</title>
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<link rel="stylesheet" href="apiReference.css" type="text/css" />
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<meta name="generator" content="DocBook XSL Stylesheets V1.73.2" />
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<link rel="start" href="index.html" title="Berkeley DB C API Reference" />
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<link rel="up" href="db.html" title="Chapter 2. The DB Handle" />
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<link rel="prev" href="dberr.html" title="DB->err()" />
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</head>
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<body>
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<div xmlns="" class="navheader">
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<div class="libver">
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<p>Library Version 11.2.5.3</p>
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</div>
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<table width="100%" summary="Navigation header">
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<tr>
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<th colspan="3" align="center">DB->exists()</th>
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</tr>
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<tr>
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<td width="20%" align="left"><a accesskey="p" href="dberr.html">Prev</a> </td>
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<th width="60%" align="center">Chapter 2.
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The DB Handle
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</th>
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<td width="20%" align="right"> <a accesskey="n" href="dbfd.html">Next</a></td>
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</tr>
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</table>
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<hr />
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</div>
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<div class="sect1" lang="en" xml:lang="en">
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<div class="titlepage">
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<div>
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<div>
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<h2 class="title" style="clear: both"><a id="dbexists"></a>DB->exists()</h2>
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</div>
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</div>
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</div>
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<pre class="programlisting">#include <db.h>
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int
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DB->exists(DB *db, DB_TXN *txnid, DBT *key, u_int32_t flags); </pre>
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<p>
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The <code class="methodname">DB->exists()</code> method returns whether the specified key appears in the
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database.
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</p>
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<p>
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The <code class="methodname">DB->exists()</code> method will return <a href="../../programmer_reference/program_errorret.html#program_errorret.DB_NOTFOUND" class="olink">DB_NOTFOUND</a> if the specified key
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is not in the database. The <code class="methodname">DB->exists()</code> method will return
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<a href="../../programmer_reference/program_errorret.html#program_errorret.DB_KEYEMPTY" class="olink">DB_KEYEMPTY</a> if the database
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is a Queue or Recno database and the specified key exists, but was never explicitly created
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by the application or was later deleted.
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</p>
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<div class="sect2" lang="en" xml:lang="en">
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<div class="titlepage">
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<div>
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<div>
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<h3 class="title"><a id="idp57425248"></a>Parameters</h3>
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</div>
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</div>
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</div>
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<div class="sect3" lang="en" xml:lang="en">
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<div class="titlepage">
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<div>
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<div>
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<h4 class="title"><a id="idp57488696"></a>txnid</h4>
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</div>
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</div>
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</div>
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<p>
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If the operation is part of an application-specified transaction, the <span class="bold"><strong>txnid</strong></span> parameter is a transaction handle returned from
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<a class="xref" href="txnbegin.html" title="DB_ENV->txn_begin()">DB_ENV->txn_begin()</a>; if the operation is part of a
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Berkeley DB Concurrent Data Store group, the <span class="bold"><strong>txnid</strong></span>
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parameter is a handle returned from
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<a class="xref" href="envcdsgroup_begin.html" title="DB_ENV->cdsgroup_begin()">DB_ENV->cdsgroup_begin()</a>; otherwise NULL.
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If no transaction handle is specified, but the
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operation occurs in a transactional database, the operation will be
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implicitly transaction protected.
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</p>
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</div>
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<div class="sect3" lang="en" xml:lang="en">
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<div class="titlepage">
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<div>
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<div>
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<h4 class="title"><a id="idp57488400"></a>key</h4>
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</div>
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</div>
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</div>
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<p>
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The key <a class="link" href="dbt.html" title="Chapter 4. The DBT Handle">DBT</a> operated on.
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</p>
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</div>
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<div class="sect3" lang="en" xml:lang="en">
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<div class="titlepage">
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<div>
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<div>
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<h4 class="title"><a id="idp57491552"></a>flags</h4>
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</div>
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</div>
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</div>
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<p>
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The <span class="bold"><strong>flags</strong></span> parameter must be set to zero or
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by bitwise inclusively <span class="bold"><strong>OR</strong></span>'ing together one
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or more of the following values:
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</p>
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<div class="itemizedlist">
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<ul type="disc">
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<li>
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<p><a id="exists_DB_READ_COMMITTED"></a>
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<code class="literal">DB_READ_COMMITTED</code>
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</p>
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<p>
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Configure a transactional read operation to have degree 2 isolation
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(the read is not repeatable).
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</p>
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</li>
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<li>
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<p><a id="exists_DB_READ_UNCOMMITTED"></a>
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<code class="literal">DB_READ_UNCOMMITTED</code>
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</p>
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<p>
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Configure a transactional read operation to have degree 1 isolation,
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reading modified but not yet committed data. Silently ignored if the
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<a class="link" href="dbopen.html#dbopen_DB_READ_UNCOMMITTED">DB_READ_UNCOMMITTED</a>
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flag was not specified when the underlying database was opened.
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</p>
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</li>
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<li>
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<p><a id="exists_DB_RMW"></a>
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<code class="literal">DB_RMW</code>
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</p>
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<p>
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Acquire write locks instead of read locks when doing the read, if locking is
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configured. Setting this flag can eliminate deadlock during a read-modify-write
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cycle by acquiring the write lock during the read part of the cycle so that
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another thread of control acquiring a read lock for the same item, in its own
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read-modify-write cycle, will not result in deadlock.
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</p>
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<p>
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Because the <code class="methodname">DB->exists()</code> method will not hold locks
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across Berkeley DB calls in non-transactional operations, the
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<a class="link" href="dbcget.html#dbcget_DB_RMW">DB_RMW</a> flag to the
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<code class="methodname">DB->exists()</code> call is meaningful only in the presence
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of transactions.
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</p>
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</li>
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</ul>
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</div>
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</div>
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</div>
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<div class="sect2" lang="en" xml:lang="en">
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<div class="titlepage">
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<div>
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<div>
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<h3 class="title"><a id="idp57504192"></a>Class</h3>
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</div>
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</div>
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</div>
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<p>
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<a class="link" href="db.html" title="Chapter 2. The DB Handle">DB</a>
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</p>
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</div>
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<div class="sect2" lang="en" xml:lang="en">
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<div class="titlepage">
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<div>
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<div>
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<h3 class="title"><a id="idp57498936"></a>See Also</h3>
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</div>
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</div>
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</div>
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<p>
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<a class="xref" href="db.html#dblist" title="Database and Related Methods">Database and Related Methods</a>
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</p>
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</div>
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</div>
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<div class="navfooter">
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<hr />
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<table width="100%" summary="Navigation footer">
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<tr>
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<td width="40%" align="left"><a accesskey="p" href="dberr.html">Prev</a> </td>
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<td width="20%" align="center">
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<a accesskey="u" href="db.html">Up</a>
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</td>
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<td width="40%" align="right"> <a accesskey="n" href="dbfd.html">Next</a></td>
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</tr>
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<tr>
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<td width="40%" align="left" valign="top">DB->err() </td>
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<td width="20%" align="center">
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<a accesskey="h" href="index.html">Home</a>
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</td>
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<td width="40%" align="right" valign="top"> DB->fd()</td>
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</tr>
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</table>
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</div>
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</body>
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</html>
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