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<h1>DB_ENV->memp_register</h1>
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<h3><pre>
#include <db.h>
<p>
int
DB_ENV->memp_register(DB_ENV *env, int ftype,
int (*pgin_fcn)(DB_ENV *, db_pgno_t pgno, void *pgaddr, DBT *pgcookie),
int (*pgout_fcn)(DB_ENV *, db_pgno_t pgno, void *pgaddr, DBT *pgcookie));
</pre></h3>
<h1>Description</h1>
<p>The DB_ENV->memp_register method registers page-in and page-out
functions for files of type <b>ftype</b> in the specified pool.
<p>If the <b>pgin_fcn</b> function is non-NULL, it is called each time
a page is read into the memory pool from a file of type <b>ftype</b>, or
a page is created for a file of type <b>ftype</b> (see the
DB_MPOOL_CREATE flag for the <a href="../api_c/memp_fget.html">DB_MPOOLFILE->get</a> method).
<p>If the <b>pgout_fcn</b> function is non-NULL, it is called each time
a page is written to a file of type <b>ftype</b>.
<p>Both the <b>pgin_fcn</b> and <b>pgout_fcn</b> functions are called
with a reference to the current environment, the page number, a pointer
to the page being read or written, and any argument <b>pgcookie</b>
that was specified to the <a href="../api_c/memp_set_pgcookie.html">DB_MPOOLFILE->set_pgcookie</a> method. The
<b>pgin_fcn</b> and <b>pgout_fcn</b> functions should return 0 on
success, and an applicable non-zero <b>errno</b> value on failure, in
which case the shared memory pool interface routine (and, by extension,
any Berkeley DB library function) calling it will also fail, returning that
<b>errno</b> value.
<p>The purpose of the DB_ENV->memp_register function is to support processing
when pages are entered into, or flushed from, the pool. A file type must
be specified to make it possible for unrelated threads or processes that
are sharing a pool, to evict each other's pages from the pool. During
initialization, applications should call DB_ENV->memp_register for each
type of file requiring input or output processing that will be sharing
the underlying pool. (No registry is necessary for the standard Berkeley DB
access method types because <a href="../api_c/db_open.html">DB->open</a> registers them separately.)
<p>If a thread or process does not call DB_ENV->memp_register for a file
type, it is impossible for it to evict pages for any file requiring input
or output processing from the pool. For this reason,
DB_ENV->memp_register should always be called by each application sharing
a pool for each type of file included in the pool, regardless of whether
or not the application itself uses files of that type.
<p>There are no standard values for <b>ftype</b>, <b>pgin_fcn</b>,
<b>pgout_fcn</b>, and <b>pgcookie</b>, except that the <b>ftype</b>
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). For this reason, applications sharing a pool must coordinate
the values among themselves.
<p>The DB_ENV->memp_register method returns a non-zero error value on failure and 0 on success.
<h1>Errors</h1>
<p>The DB_ENV->memp_register method may fail and return a non-zero error for errors specified for other Berkeley DB and C library or system functions.
If a catastrophic error has occurred, the DB_ENV->memp_register method may fail and
return <a href="../ref/program/errorret.html#DB_RUNRECOVERY">DB_RUNRECOVERY</a>,
in which case all subsequent Berkeley DB calls will fail in the same way.
<h1>Class</h1>
<a href="../api_c/env_class.html">DB_ENV</a>, <a href="../api_c/mempfile_class.html">DB_MPOOLFILE</a>
<h1>See Also</h1>
<a href="../api_c/memp_list.html">Memory Pools and Related Methods</a>
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