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HTML
212 lines
9.6 KiB
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<?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>DbSequence::get()</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="seq.html" title="Chapter 12. The DbSequence Handle" />
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<link rel="prev" href="seqclose.html" title="DbSequence::close()" />
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<link rel="next" href="seqget_cachesize.html" title="DbSequence::get_cachesize()" />
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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 11.2.5.2</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">DbSequence::get()</th>
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</tr>
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<tr>
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<td width="20%" align="left"><a accesskey="p" href="seqclose.html">Prev</a> </td>
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<th width="60%" align="center">Chapter 12.
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The DbSequence Handle
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</th>
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<td width="20%" align="right"> <a accesskey="n" href="seqget_cachesize.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="seqget"></a>DbSequence::get()</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_cxx.h>
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int
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DbSequence::get(DbTxn *txnid, int32_t delta, db_seq_t *retp,
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u_int32_t flags);</pre>
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<p>
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The <code class="methodname">DbSequence::get()</code> method returns the next available element in
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the sequence and changes the sequence value by <span class="bold"><strong>delta</strong></span>. The value of <span class="bold"><strong>delta</strong></span> must be greater than zero. If there are
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enough cached values in the sequence handle then they will be
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returned. Otherwise the next value will be fetched from the database
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and incremented (decremented) by enough to cover the <span class="bold"><strong>delta</strong></span> and the next batch of cached values.
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</p>
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<p>
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For maximum concurrency a non-zero cache size should be specified
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prior to opening the sequence handle and
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<a class="link" href="envset_flags.html#envset_flags_DB_TXN_NOSYNC">DB_TXN_NOSYNC</a>
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should be specified for each <code class="methodname">DbSequence::get()</code> method call.
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</p>
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<p>
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By default, sequence ranges do not wrap; to cause the sequence to wrap
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around the beginning or end of its range, specify the
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<a class="link" href="seqset_flags.html#seqset_flags_DB_SEQ_WRAP">DB_SEQ_WRAP</a>
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flag to the <a class="xref" href="seqset_flags.html" title="DbSequence::set_flags()">DbSequence::set_flags()</a>
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method.
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</p>
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<p>
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The <code class="methodname">DbSequence::get()</code> method will return EINVAL if the record in the
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database is not a valid sequence record, or the sequence has reached
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the beginning or end of its range and is not configured to wrap.
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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="id3667374"></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="id3667477"></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
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<span class="bold"><strong>txnid</strong></span> parameter is a transaction
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handle returned from <a class="xref" href="txnbegin.html" title="DbEnv::txn_begin()">DbEnv::txn_begin()</a>; if the
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operation is part of a Berkeley DB Concurrent Data Store group, the
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<span class="bold"><strong>txnid</strong></span> parameter is a handle returned
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from <a class="xref" href="envcdsgroup_begin.html" title="DbEnv::cdsgroup_begin()">DbEnv::cdsgroup_begin()</a>;
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otherwise NULL. 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. No <span class="bold"><strong>txnid</strong></span> handle may be specified if the sequence
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handle was opened with a non-zero cache size.
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</p>
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<p>
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If the underlying database handle was opened in a transaction, calling
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<code class="methodname">DbSequence::get()</code> may result in changes to the sequence object;
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these changes will be automatically committed in a transaction
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internal to the Berkeley DB library. If the thread of control calling
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<code class="methodname">DbSequence::get()</code> has an active transaction, which holds locks on
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the same database as the one in which the sequence object is stored,
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it is possible for a thread of control calling <code class="methodname">DbSequence::get()</code> to
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self-deadlock because the active transaction's locks conflict with the
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internal transaction's locks. For this reason, it is often preferable
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for sequence objects to be stored in their own database.
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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="id3667160"></a>delta</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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Specifies the amount to increment or decrement the sequence.
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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="id3667841"></a>retp</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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<span class="bold"><strong>retp</strong></span> points to the memory to hold the
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return value from the sequence.
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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="id3654998"></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 0
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or by bitwise inclusively <span class="bold"><strong>OR</strong></span>'ing
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together one 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>
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<code class="literal">DB_TXN_NOSYNC</code>
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</p>
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<p>
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If the operation is implicitly transaction protected (the <span class="bold"><strong>txnid</strong></span> argument is NULL but the operation occurs
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to a transactional database), do not synchronously flush the log when
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the transaction commits.
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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="id3667769"></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="seq.html" title="Chapter 12. The DbSequence Handle">DbSequence</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="id3667703"></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="seq.html#seqlist" title="Sequences and Related Methods">Sequences 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="seqclose.html">Prev</a> </td>
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<td width="20%" align="center">
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<a accesskey="u" href="seq.html">Up</a>
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</td>
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<td width="40%" align="right"> <a accesskey="n" href="seqget_cachesize.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">DbSequence::close() </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"> DbSequence::get_cachesize()</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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