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HTML
201 lines
8.2 KiB
HTML
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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>Dbstl miscellaneous notes</title>
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<link rel="stylesheet" href="gettingStarted.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 Programmer's Reference Guide" />
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<link rel="up" href="stl.html" title="Chapter 7. Standard Template Library API" />
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<link rel="prev" href="stl_memory_mgmt.html" title="Dbstl memory management" />
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<link rel="next" href="stl_known_issues.html" title="Dbstl known issues" />
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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">Dbstl miscellaneous notes</th>
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</tr>
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<tr>
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<td width="20%" align="left"><a accesskey="p" href="stl_memory_mgmt.html">Prev</a> </td>
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<th width="60%" align="center">Chapter 7. Standard Template Library API</th>
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<td width="20%" align="right"> <a accesskey="n" href="stl_known_issues.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="stl_misc"></a>Dbstl miscellaneous notes</h2>
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</div>
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</div>
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</div>
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<div class="toc">
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<dl>
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<dt>
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<span class="sect2">
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<a href="stl_misc.html#id3893652">Special notes about trivial methods</a>
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</span>
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</dt>
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<dt>
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<span class="sect2">
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<a href="stl_misc.html#id3893947">Using correct container and iterator public types</a>
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</span>
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</dt>
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</dl>
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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="id3893652"></a>Special notes about trivial methods</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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There are some standard STL methods which are meaningless in dbstl, but they are kept in
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dbstl as no-ops so as to stay consistent with the standard. These are:
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</p>
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<table class="simplelist" border="0" summary="Simple list">
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<tr>
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<td>
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<code class="methodname">db_vecter::reserve();</code>
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</td>
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</tr>
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<tr>
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<td>
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<code class="methodname">db_vector::max_size();</code>
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</td>
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</tr>
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<tr>
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<td>
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<code class="methodname">db_vector::capacity();</code>
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</td>
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</tr>
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<tr>
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<td>
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<code class="methodname">db_map::reserve();</code>
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</td>
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</tr>
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<tr>
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<td>
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<code class="methodname">db_map::max_size();</code>
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</td>
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</tr>
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</table>
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<p>
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<code class="methodname">db_vector<>::max_size()</code> and
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<code class="methodname">db_map<>::max_size()</code> both return 2^30. This does not mean
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that Berkeley DB can only hold that much data. This value is returned to conform to some
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compilers' overflow rules — if we set bigger numbers like 2^32 or 2^31, some compilers
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complain that the number has overflowed.
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</p>
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<p>
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See the Berkeley DB documentation for information about limitations on how much data a
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database can store.
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</p>
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<p>
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There are also some read-only functions. You set the configuration
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for these using the Berkeley DB API. You access them using the container's methods. Again,
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this is to keep consistent with C++ standard STL containers, such as:
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</p>
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<table class="simplelist" border="0" summary="Simple list">
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<tr>
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<td>
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<code class="methodname">db_map::key_comp();</code>
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</td>
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</tr>
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<tr>
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<td>
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<code class="methodname">db_map::value_comp();</code>
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</td>
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</tr>
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<tr>
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<td>
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<code class="methodname">db_map::hash_funct();</code>
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</td>
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</tr>
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<tr>
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<td>
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<code class="methodname">db_map::key_eq();</code>
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</td>
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</tr>
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</table>
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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="id3893947"></a>Using correct container and iterator public types</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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All public types defined by the C++ STL specification are present in dbstl. One thing to
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note is the <span class="bold"><strong>value_type</strong></span>. dbstl defines the <span class="bold"><strong>value_type</strong></span> for each iterator and container class to be the raw
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type without the <code class="classname">ElementRef</code>/<code class="classname">ElementHolder</code>
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wrapper, so this type of variable can not be used to store data in a database. There is a
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<span class="bold"><strong>value_type_wrap</strong></span> type for each container and iterator type,
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with the raw type wrapped by the
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<code class="classname">ElementRef</code>/<code class="classname">ElementHolder</code>.
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</p>
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<p>
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For example, when type <code class="literal">int_vector_t</code> is defined as
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</p>
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<pre class="programlisting">db_vector<int, ElementHolder<int> ></pre>
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<p>
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its <span class="bold"><strong>value_type</strong></span> is <code class="literal">int</code>, its
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<span class="bold"><strong>value_type_wrap</strong></span> is <code class="literal">ElementHolder<int></code>,
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and its reference and pointer types are <code class="literal">ElementHolder<int>&</code> and
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<code class="literal">ElementHolder<int>*</code> respectively. If you need to store data, use
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<span class="bold"><strong>value_type_wrap</strong></span> to make use of the wrapper to store data
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into database.
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</p>
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<p>
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The reason we leave <span class="bold"><strong>value_type</strong></span> as the raw type is that we
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want the existing algorithms in the STL library to work with dbstl because we have seen that
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without doing so, a few tests will fail.
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</p>
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<p>
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You need to use the same type as the return type of the data element retrieval functions to
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hold a value in order to properly manipulate the data element. For example, when calling
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</p>
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<pre class="programlisting">db_vector<T>::operator[]</pre>
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<p>
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check that the return type for this function is
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</p>
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<pre class="programlisting">db_vector<T>::datatype_wrap</pre>
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<p>
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Then, hold the return value using an object of the same type:
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</p>
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<pre class="programlisting">db_vector<T>::datatype_wrap refelem = vctr[3];</pre>
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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="stl_memory_mgmt.html">Prev</a> </td>
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<td width="20%" align="center">
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<a accesskey="u" href="stl.html">Up</a>
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</td>
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<td width="40%" align="right"> <a accesskey="n" href="stl_known_issues.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">Dbstl memory management </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"> Dbstl known issues</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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