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HTML
191 lines
9.2 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>Dbstl memory management</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_efficienct_use.html" title="Using dbstl efficiently" />
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<link rel="next" href="stl_misc.html" title="Dbstl miscellaneous notes" />
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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 memory management</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_efficienct_use.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_misc.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_memory_mgmt"></a>Dbstl memory management</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_memory_mgmt.html#id3893347">Freeing memory</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_memory_mgmt.html#id3893430">Type specific notes</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="id3893347"></a>Freeing memory</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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When using dbstl, make sure memory allocated in the heap is
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released after use. The rules for this are:
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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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dbstl will free/delete any memory allocated by dbstl itself.
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</p>
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</li>
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<li>
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<p>
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You are responsible for freeing/deleting any memory allocated by your code outside of dbstl.
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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 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="id3893430"></a>Type specific notes</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="id3894010"></a>DbEnv/Db</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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When you open a <code class="classname">DbEnv</code> or <code class="classname">Db</code> object using
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<code class="methodname">dbstl::open_env()</code> or <code class="methodname">dbstl::open_db()</code>, you
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do not need to delete that object. However, if you new'd that object and then opened it
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without using the <code class="methodname">dbstl::open_env()</code> or
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<code class="methodname">dbstl::open_db()</code> methods, you are responsible for deleting the
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object.
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</p>
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<p>
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Note that you must <code class="literal">new</code> the <code class="classname">Db</code> or
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<code class="classname">DbEnv</code> object, which allocates it on the heap. You can not allocate it
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on the stack. If you do, the order of destruction is uncontrollable, which makes dbstl
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unable to work properly.
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</p>
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<p>
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You can call <code class="function">dbstl_exit()</code> before the process exits, to release any
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memory allocated by dbstl that has to live during the entire process lifetime. Releasing the
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memory explicitly will not make much difference, because the process is about to exit and so
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all memory allocated on the heap is going to be returned to the operating system anyway. The
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only real difference is that your memory leak checker will not report false memory leaks.
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</p>
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<p>
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<code class="function">dbstl_exit()</code> releases any memory allocated by dbstl on the heap. It
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also performs other required shutdown operations, such as closing any databases and
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environments registered to dbstl and shared across the process.
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</p>
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<p>
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If you are calling the <code class="function">dbstl_exit()</code> function, and your
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<code class="classname">DbEnv</code> or <code class="classname">Db</code> objects are new'd by your code,
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the <code class="function">dbstl_exit()</code> function should be called before deleting the
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<code class="classname">DbEnv</code> or <code class="classname">Db</code> objects, because they need to be
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closed before being deleted. Alternatively, you can call the
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<code class="methodname">dbstl::close_env()</code> or <code class="methodname">dbstl::close_db()</code>
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functions before deleting the <code class="classname">DbEnv</code> or <code class="classname">Db</code>
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objects in order to explicitly close the databases or environments. If you do this,
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can then delete these objects, and then call <code class="function">dbstl_exit()</code>.
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</p>
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<p>
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In addition, before exiting a thread that uses dbstl API, you can call the <code class="function">dbstl_thread_exit() </code>function to release any Berkeley DB handles if they are not used by other threads.
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If you do not call the <code class="function">dbstl_thread_exit() </code>function or call this function only in some threads, all open Berkeley DB handles will be closed by the <code class="function">dbstl_exit()</code>function.
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You must call the <code class="function">dbstl_exit() </code>function before the process exits, to avoid memory leak and database update loss, if you do not have transactions and persistent log files.
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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="id3893773"></a>DbstlDbt</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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Only when you are storing raw bytes (such as a bitmap) do you have to
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store and retrieve data by using the <code class="classname">DbstlDbt</code> helper class. Although you
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also can do so simply by using the Berkeley DB <code class="classname">Dbt</code> class, the
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<code class="classname">DbstlDbt</code> class offers more convenient memory management behavior.
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</p>
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<p>
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When you are storing <code class="classname">DbstlDbt</code> objects (such as
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<code class="classname">db_vector<DbstlDbt></code>), you <span class="emphasis"><em>must</em></span> allocate
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heap memory explicitly using the <code class="function">malloc()</code> function for the
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<code class="classname">DbstlDbt</code> object to reference, but you do not need to free the memory
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– it is automatically freed by the <code class="classname">DbstlDbt</code> object that owns it
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by calling the standard C library <code class="function">free()</code> function.
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</p>
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<p>
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However, because dbstl supports storing any type of object or primitive data, it is rare
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that you would have to store data using <code class="classname">DbstlDbt</code> objects while using
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dbstl. Examples of storing <code class="classname">DbstlDbt</code> objects can be found in the
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<code class="methodname">StlAdvancedFeaturesExample::arbitrary_object_storage()</code> and
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<code class="methodname">StlAdvancedFeaturesExample::char_star_string_storage()</code> methods.
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</p>
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</div>
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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_efficienct_use.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_misc.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">Using dbstl efficiently </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 miscellaneous notes</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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