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<?xml version="1.0" encoding="UTF-8" standalone="no"?>
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<th width="60%" align="center">Chapter 8.
The DB_MPOOLFILE Handle
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<h2 class="title" style="clear: both"><a id="mempregister"></a>DB_ENV->memp_register()</h2>
</div>
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<pre class="programlisting">#include <db.h>
int
DB_ENV->memp_register(DB_ENV *env, int ftype,
int (*pgin_fcn)(DB_ENV *env, db_pgno_t pgno, void *pgaddr,
DBT *pgcookie), int (*pgout_fcn)(DB_ENV *env, db_pgno_t pgno,
void *pgaddr, DBT *pgcookie)); </pre>
<p>
The <code class="methodname">DB_ENV->memp_register()</code> method registers page-in and page-out
functions for files of type <span class="bold"><strong>ftype</strong></span> in the cache.
</p>
<p>
If the <span class="bold"><strong>pgin_fcn</strong></span> function is non-NULL,
it is called each time a page is read into the cache from a file
of type <span class="bold"><strong>ftype</strong></span>, or a page is created
for a file of type <span class="bold"><strong>ftype</strong></span> (see the
DB_MPOOL_CREATE flag for the
<a class="xref" href="mempfget.html" title="DB_MPOOLFILE->get()">DB_MPOOLFILE->get()</a> method).
</p>
<p>
If the <span class="bold"><strong>pgout_fcn</strong></span> function is
non-NULL, it is called each time a page is written to a file of type
<span class="bold"><strong>ftype</strong></span>.
</p>
<p>
The purpose of the <code class="methodname">DB_ENV->memp_register()</code> function is to support
processing when pages are entered into, or flushed from, the cache. For example, this
functionality might be used to do byte-endian conversion as pages are read from, or written
to, the underlying file.
</p>
<p>
A file type must be specified to make it possible for unrelated
threads or processes that are sharing a cache, to evict each other's
pages from the cache. During initialization, applications should call
<code class="methodname">DB_ENV->memp_register()</code> for each type of file requiring input or
output processing that will be sharing the underlying cache. (No
registry is necessary for the standard Berkeley DB access method types
because <a class="xref" href="dbopen.html" title="DB->open()">DB->open()</a>
registers them separately.)
</p>
<p>
If a thread or process does not call <code class="methodname">DB_ENV->memp_register()</code> for a
file type, it is impossible for it to evict pages for any file
requiring input or output processing from the cache. For this reason,
<code class="methodname">DB_ENV->memp_register()</code> should always be called by each application
sharing a cache for each type of file included in the cache, regardless
of whether or not the application itself uses files of that type.
</p>
<p>
The <code class="methodname">DB_ENV->memp_register()</code> <span>
<span>
method returns a non-zero error value on failure and 0 on success.
</span>
</span>
</p>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="idp53048424"></a>Parameters</h3>
</div>
</div>
</div>
<div class="sect3" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="idp53047776"></a>ftype</h4>
</div>
</div>
</div>
<p>
The <span class="bold"><strong>ftype</strong></span> parameter specifies the
type of file for which the page-in and page-out functions will be
called.
</p>
<p>
The <span class="bold"><strong>ftype</strong></span> value for a file must be a
non-zero positive number less than 128 (0 and negative numbers are
reserved for internal use by the Berkeley DB library).
</p>
</div>
<div class="sect3" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="idp53042856"></a>pgin_fcn, pgout_fcn</h4>
</div>
</div>
</div>
<p>
The page-in and page-out functions.
</p>
<p>
The <span class="bold"><strong>pgin_fcn</strong></span> and <span class="bold"><strong>pgout_fcn</strong></span> functions are called with a reference
to the current database environment, the page number being read or
written, a pointer to the page being read or written, and any
parameter <span class="bold"><strong>pgcookie</strong></span> that was specified
to the <a class="xref" href="mempset_pgcookie.html" title="DB_MPOOLFILE->set_pgcookie()">DB_MPOOLFILE->set_pgcookie()</a>
method.
</p>
<p>
The <span class="bold"><strong>pgin_fcn</strong></span> and <span class="bold"><strong>pgout_fcn</strong></span> functions should return 0 on success,
and a non-zero value on failure, in which case the shared Berkeley DB
library function calling it will also fail, returning that non-zero
value. The non-zero value should be selected from values outside of
the Berkeley DB library namespace.
</p>
</div>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="idp53036568"></a>Class</h3>
</div>
</div>
</div>
<p>
<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>
</p>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="idp53055712"></a>See Also</h3>
</div>
</div>
</div>
<p>
<a class="xref" href="memp.html#memplist" title="Memory Pools and Related Methods">Memory Pools and Related Methods</a>
</p>
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