DbEnv::memp_register()
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## DbEnv::memp_register()

    #include <db_cxx.h>

    extern "C" {
        typedef int (*pgin_fcn_type)(DB_ENV *dbenv,
    	db_pgno_t pgno, void *pgaddr, DBT *pgcookie);
        typedef int (*pgout_fcn_type)(DB_ENV *dbenv,
    	db_pgno_t pgno, void *pgaddr, DBT *pgcookie);
    };
    int
    DbEnv::memp_register(int ftype,
        pgin_fcn_type pgin_fcn, pgout_fcn_type pgout_fcn);

The `DbEnv::memp_register()` method registers page-in and page-out functions for files of type ftype in the cache.

If the pgin_fcn function is non-NULL, it is called each time a page is read into the cache from a file of type ftype, or a page is created for a file of type ftype (see the DB_MPOOL_CREATE flag for the DbMpoolFile::get() method).

If the pgout_fcn function is non-NULL, it is called each time a page is written to a file of type ftype.

The purpose of the `DbEnv::memp_register()` 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.

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 `DbEnv::memp_register()` 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 Db::open() registers them separately.)

If a thread or process does not call `DbEnv::memp_register()` 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, `DbEnv::memp_register()` 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.

The `DbEnv::memp_register()` 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.

### Parameters

#### ftype

The ftype parameter specifies the type of file for which the page-in and page-out functions will be called.

The ftype 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).

#### pgin_fcn, pgout_fcn

The page-in and page-out functions.

The pgin_fcn and pgout_fcn 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 pgcookie that was specified to the DbMpoolFile::set_pgcookie() method.

The pgin_fcn and pgout_fcn 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.

### Class

DbEnv, DbMpoolFile

### See Also

Memory Pools and Related Methods

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