mirror of
https://github.com/yasuhirokimura/db18
synced 2026-06-08 18:29:23 +00:00
Yasuhiro KIMURA
279 lines
12 KiB
HTML
279 lines
12 KiB
HTML
<?xml version="1.0" encoding="UTF-8" standalone="no"?>
|
||
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
|
||
<html xmlns="http://www.w3.org/1999/xhtml">
|
||
<head>
|
||
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
|
||
<title>Db::set_slice_callback()</title>
|
||
<link rel="stylesheet" href="apiReference.css" type="text/css" />
|
||
<meta name="generator" content="DocBook XSL Stylesheets V1.73.2" />
|
||
<link rel="start" href="index.html" title="Berkeley DB C++ API Reference" />
|
||
<link rel="up" href="db.html" title="Chapter 2. The Db Handle" />
|
||
<link rel="prev" href="dbset_re_source.html" title="Db::set_re_source()" />
|
||
<link rel="next" href="dbslice_lookup.html" title="Db::slice_lookup()" />
|
||
</head>
|
||
<body>
|
||
<div xmlns="" class="navheader">
|
||
<div class="libver">
|
||
<p>Library Version 18.1.40</p>
|
||
</div>
|
||
<table width="100%" summary="Navigation header">
|
||
<tr>
|
||
<th colspan="3" align="center">
|
||
|
||
<span xmlns="http://www.w3.org/1999/xhtml">Db::set_slice_callback()</span>
|
||
</th>
|
||
</tr>
|
||
<tr>
|
||
<td width="20%" align="left"><a accesskey="p" href="dbset_re_source.html">Prev</a> </td>
|
||
<th width="60%" align="center">Chapter 2.
|
||
The Db Handle
|
||
</th>
|
||
<td width="20%" align="right"> <a accesskey="n" href="dbslice_lookup.html">Next</a></td>
|
||
</tr>
|
||
</table>
|
||
<hr />
|
||
</div>
|
||
<div class="sect1" lang="en" xml:lang="en">
|
||
<div class="titlepage">
|
||
<div>
|
||
<div>
|
||
<h2 class="title" style="clear: both"><a id="dbset_slice_callback"></a>
|
||
|
||
<span>Db::set_slice_callback()</span>
|
||
</h2>
|
||
</div>
|
||
</div>
|
||
</div>
|
||
<pre class="programlisting">#include <db_cxx.h>
|
||
|
||
Db->set_slice_callback(int (*func)(const Db *db, const Dbt *key,
|
||
Dbt *slice)); </pre>
|
||
<p>
|
||
The <code class="methodname">Db::set_slice_callback()</code> method
|
||
is optionally used to identify slices for applications using a
|
||
sliced environment. The callback identifies the portion of a
|
||
record's key that is used to allocate records across slices. This
|
||
is the <span class="emphasis"><em>slice-relevant</em></span> portion of the key. If
|
||
no callback is specified, then the entire key is used to allocate
|
||
records across slices.
|
||
</p>
|
||
<p>
|
||
The <code class="methodname">Db::set_slice_callback()</code> method
|
||
must be called on an opened database handle. Changing this callback
|
||
after the database has been created results in undefined behavior.
|
||
</p>
|
||
<p>
|
||
The <code class="methodname">Db::set_backup_callbacks()</code> <span>
|
||
|
||
<span>
|
||
method either returns a non-zero error value or throws an
|
||
exception that encapsulates a non-zero error value on
|
||
failure, and returns 0 on success.
|
||
</span>
|
||
</span>
|
||
</p>
|
||
<div class="sect2" lang="en" xml:lang="en">
|
||
<div class="titlepage">
|
||
<div>
|
||
<div>
|
||
<h3 class="title"><a id="idm139755617805808"></a>Parameters</h3>
|
||
</div>
|
||
</div>
|
||
</div>
|
||
<div class="sect3" lang="en" xml:lang="en">
|
||
<div class="titlepage">
|
||
<div>
|
||
<div>
|
||
<h4 class="title"><a id="idm139755617806128"></a>func</h4>
|
||
</div>
|
||
</div>
|
||
</div>
|
||
<p>
|
||
The <span class="bold"><strong>func</strong></span> parameter
|
||
is the function used to determine which part of a key is
|
||
slice relevant.
|
||
</p>
|
||
<p>
|
||
This function takes three parameters:
|
||
</p>
|
||
<div class="itemizedlist">
|
||
<ul type="disc">
|
||
<li>
|
||
<p>
|
||
<code class="literal">db</code>
|
||
</p>
|
||
<p>
|
||
The <span class="bold"><strong>db</strong></span>
|
||
parameter is the sliced database for which the
|
||
callback is set.
|
||
</p>
|
||
</li>
|
||
<li>
|
||
<p>
|
||
<code class="literal">key</code>
|
||
</p>
|
||
<p>
|
||
The <span class="bold"><strong>key</strong></span> parameter is the
|
||
source of the slice specification fields. That is,
|
||
this is the key portion of the database's records. Some
|
||
portion of this key is used to determine slice
|
||
location, and that portion of the key is determined
|
||
by the function implemented by this callback.
|
||
</p>
|
||
</li>
|
||
<li>
|
||
<p>
|
||
<code class="literal">slice</code>
|
||
</p>
|
||
<p>
|
||
The <span class="bold"><strong>slice</strong></span> parameter is the
|
||
<a class="link" href="dbt.html" title="Chapter 4. The Dbt Handle">Dbt</a>
|
||
into which the slice-relevant portion of the key
|
||
will be written. This
|
||
<a class="link" href="dbt.html" title="Chapter 4. The Dbt Handle">Dbt</a>
|
||
is user-owned.
|
||
</p>
|
||
<p>
|
||
You should zero this
|
||
<a class="link" href="dbt.html" title="Chapter 4. The Dbt Handle">Dbt</a>, with
|
||
the exception of the <code class="literal">size</code> and
|
||
<code class="literal">data</code> fields. When initializing
|
||
the structure, set
|
||
<code class="literal">size</code> to <code class="literal">DB_SLICE_BUFLEN</code>, and
|
||
<code class="literal">data</code> to the address of a 32-bit
|
||
word aligned buffer of <code class="literal">DB_SLICE_BUFLEN</code>
|
||
bytes.
|
||
</p>
|
||
<p>
|
||
Within the context of the callback, you can set
|
||
these two fields in the following ways:
|
||
</p>
|
||
<div class="itemizedlist">
|
||
<ul type="circle">
|
||
<li>
|
||
<p>
|
||
If the slice-relevant portion of the key is one contiguous
|
||
region, the set <code class="literal">data</code> to the first
|
||
relevant byte in that region, and <code class="literal">size</code>
|
||
to the number of significant bytes. Change no other
|
||
fields.
|
||
</p>
|
||
</li>
|
||
<li>
|
||
<p>
|
||
If the slice-relevant portion of the key is more than one
|
||
contiguous region, then calculate the total size required
|
||
to hold the slice-relevant data. If this total size is
|
||
less than <code class="literal">DB_SLICE_BUFLEN</code>, then leave
|
||
the initialized buffer unchanged. If the total size is
|
||
greater than <code class="literal">DB_SLICE_BUFLEN</code>, then:
|
||
</p>
|
||
<div class="orderedlist">
|
||
<ol type="1">
|
||
<li>
|
||
<p>
|
||
Allocate enough memory to contain all of the
|
||
slice-relevant data. By default,
|
||
<code class="literal">malloc()</code> is used to allocate
|
||
memory. However, if your application configured a
|
||
custom memory allocator using
|
||
|
||
|
||
<span>
|
||
<a class="xref" href="envset_alloc.html" title="DbEnv::set_alloc()">
|
||
|
||
<span>DbEnv::set_alloc()</span>
|
||
</a>,
|
||
</span>
|
||
|
||
then use that function.
|
||
</p>
|
||
</li>
|
||
<li>
|
||
<p>
|
||
Set the <code class="literal">flags</code> parameter to
|
||
<code class="literal">DB_DBT_APPMALLOC</code>. DB will
|
||
deallocate the memory before returning from the
|
||
API call, using the free function corresponding
|
||
to the allocator just used.
|
||
</p>
|
||
</li>
|
||
</ol>
|
||
</div>
|
||
<p>
|
||
Once you have created a large enough buffer, set the slice
|
||
<a class="link" href="dbt.html" title="Chapter 4. The Dbt Handle">Dbt</a>
|
||
handle's size to the new buffer's size.
|
||
</p>
|
||
<p>
|
||
After that, concatenate all of the slice-relevant data
|
||
together into the newly created buffer.
|
||
</p>
|
||
<p>
|
||
Be aware that the slice
|
||
<a class="link" href="dbt.html" title="Chapter 4. The Dbt Handle">Dbt</a>
|
||
handle's size can be zero, in which case the record will
|
||
be placed in slice 0. Also, this size may be larger than
|
||
the key's size, if it is appropriate for your application.
|
||
</p>
|
||
</li>
|
||
</ul>
|
||
</div>
|
||
</li>
|
||
</ul>
|
||
</div>
|
||
</div>
|
||
</div>
|
||
<div class="sect2" lang="en" xml:lang="en">
|
||
<div class="titlepage">
|
||
<div>
|
||
<div>
|
||
<h3 class="title"><a id="idm139755617760112"></a>Class</h3>
|
||
</div>
|
||
</div>
|
||
</div>
|
||
<p>
|
||
<a class="link" href="db.html" title="Chapter 2. The Db Handle">Db</a>
|
||
</p>
|
||
</div>
|
||
<div class="sect2" lang="en" xml:lang="en">
|
||
<div class="titlepage">
|
||
<div>
|
||
<div>
|
||
<h3 class="title"><a id="idm139755617799376"></a>See Also</h3>
|
||
</div>
|
||
</div>
|
||
</div>
|
||
<p>
|
||
<a class="xref" href="db.html#dblist" title="Database and Related Methods">Database and Related Methods</a>
|
||
</p>
|
||
</div>
|
||
</div>
|
||
<div class="navfooter">
|
||
<hr />
|
||
<table width="100%" summary="Navigation footer">
|
||
<tr>
|
||
<td width="40%" align="left"><a accesskey="p" href="dbset_re_source.html">Prev</a> </td>
|
||
<td width="20%" align="center">
|
||
<a accesskey="u" href="db.html">Up</a>
|
||
</td>
|
||
<td width="40%" align="right"> <a accesskey="n" href="dbslice_lookup.html">Next</a></td>
|
||
</tr>
|
||
<tr>
|
||
<td width="40%" align="left" valign="top">
|
||
|
||
<span>Db::set_re_source()</span>
|
||
</td>
|
||
<td width="20%" align="center">
|
||
<a accesskey="h" href="index.html">Home</a>
|
||
</td>
|
||
<td width="40%" align="right" valign="top">
|
||
|
||
<span>Db::slice_lookup()</span>
|
||
</td>
|
||
</tr>
|
||
</table>
|
||
</div>
|
||
</body>
|
||
</html>
|