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<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
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<title>DB_ENV->memp_register()</title>
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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="memp.html" title="Chapter 8. The DB_MPOOLFILE Handle" />
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<link rel="prev" href="mempfcreate.html" title="DB_ENV->memp_fcreate()" />
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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">DB_ENV->memp_register()</th>
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</tr>
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<tr>
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<td width="20%" align="left"><a accesskey="p" href="mempfcreate.html">Prev</a> </td>
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<th width="60%" align="center">Chapter 8.
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The DB_MPOOLFILE Handle
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</th>
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<td width="20%" align="right"> <a accesskey="n" href="mempstat.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="mempregister"></a>DB_ENV->memp_register()</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_ENV->memp_register(DB_ENV *env, int ftype,
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int (*pgin_fcn)(DB_ENV *env, db_pgno_t pgno, void *pgaddr,
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DBT *pgcookie), int (*pgout_fcn)(DB_ENV *env, db_pgno_t pgno,
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void *pgaddr, DBT *pgcookie)); </pre>
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<p>
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The <code class="methodname">DB_ENV->memp_register()</code> method registers page-in and page-out
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functions for files of type <span class="bold"><strong>ftype</strong></span> in the cache.
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</p>
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<p>
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If the <span class="bold"><strong>pgin_fcn</strong></span> function is non-NULL,
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it is called each time a page is read into the cache from a file
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of type <span class="bold"><strong>ftype</strong></span>, or a page is created
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for a file of type <span class="bold"><strong>ftype</strong></span> (see the
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DB_MPOOL_CREATE flag for the
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<a class="xref" href="mempfget.html" title="DB_MPOOLFILE->get()">DB_MPOOLFILE->get()</a> method).
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</p>
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<p>
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If the <span class="bold"><strong>pgout_fcn</strong></span> function is
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non-NULL, it is called each time a page is written to a file of type
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<span class="bold"><strong>ftype</strong></span>.
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</p>
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<p>
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The purpose of the <code class="methodname">DB_ENV->memp_register()</code> function is to support
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processing when pages are entered into, or flushed from, the cache. For example, this
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functionality might be used to do byte-endian conversion as pages are read from, or written
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to, the underlying file.
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</p>
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<p>
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A file type must be specified to make it possible for unrelated
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threads or processes that are sharing a cache, to evict each other's
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pages from the cache. During initialization, applications should call
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<code class="methodname">DB_ENV->memp_register()</code> for each type of file requiring input or
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output processing that will be sharing the underlying cache. (No
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registry is necessary for the standard Berkeley DB access method types
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because <a class="xref" href="dbopen.html" title="DB->open()">DB->open()</a>
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registers them separately.)
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</p>
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<p>
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If a thread or process does not call <code class="methodname">DB_ENV->memp_register()</code> for a
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file type, it is impossible for it to evict pages for any file
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requiring input or output processing from the cache. For this reason,
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<code class="methodname">DB_ENV->memp_register()</code> should always be called by each application
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sharing a cache for each type of file included in the cache, regardless
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of whether or not the application itself uses files of that type.
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</p>
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<p>
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The <code class="methodname">DB_ENV->memp_register()</code> <span>
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<span>
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method returns a non-zero error value on failure and 0 on success.
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</span>
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</span>
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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="id3733630"></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="id3733620"></a>ftype</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>ftype</strong></span> parameter specifies the
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type of file for which the page-in and page-out functions will be
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called.
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</p>
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<p>
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The <span class="bold"><strong>ftype</strong></span> value for a file must be a
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non-zero positive number less than 128 (0 and negative numbers are
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reserved for internal use by the Berkeley DB library).
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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="id3733544"></a>pgin_fcn, pgout_fcn</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 page-in and page-out functions.
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</p>
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<p>
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The <span class="bold"><strong>pgin_fcn</strong></span> and <span class="bold"><strong>pgout_fcn</strong></span> functions are called with a reference
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to the current database environment, the page number being read or
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written, a pointer to the page being read or written, and any
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parameter <span class="bold"><strong>pgcookie</strong></span> that was specified
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to the <a class="xref" href="mempset_pgcookie.html" title="DB_MPOOLFILE->set_pgcookie()">DB_MPOOLFILE->set_pgcookie()</a>
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method.
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</p>
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<p>
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The <span class="bold"><strong>pgin_fcn</strong></span> and <span class="bold"><strong>pgout_fcn</strong></span> functions should return 0 on success,
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and a non-zero value on failure, in which case the shared Berkeley DB
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library function calling it will also fail, returning that non-zero
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value. The non-zero value should be selected from values outside of
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the Berkeley DB library namespace.
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</p>
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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="id3733436"></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="env.html" title="Chapter 5. The DB_ENV Handle">DB_ENV</a>, <a class="link" href="memp.html" title="Chapter 8. The DB_MPOOLFILE Handle">DB_MPOOLFILE</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="id3733760"></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="memp.html#memplist" title="Memory Pools and Related Methods">Memory Pools 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="mempfcreate.html">Prev</a> </td>
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<td width="20%" align="center">
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<a accesskey="u" href="memp.html">Up</a>
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</td>
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<td width="40%" align="right"> <a accesskey="n" href="mempstat.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_ENV->memp_fcreate() </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_ENV->memp_stat()</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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