# 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);

# Description

The DbEnv::memp_register method registers page-in and page-out functions for files of type ftype in the specified pool.

If the pgin_fcn function is non-NULL, it is called each time a page is read into the memory pool 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.

Both the pgin_fcn and pgout_fcn 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 pgcookie that was specified to the DbMpoolFile::open function when the file was opened. The pgin_fcn and pgout_fcn functions should return 0 on success, and an applicable non-zero errno 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 errno value.

The purpose of the DbEnv::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. Applications should call DbEnv::memp_register, during initialization, 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, as 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 pool. For this reason, DbEnv::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.

There are no standard values for ftype, pgin_fcn, pgout_fcn and pgcookie, except that the ftype value for a file must be a non-zero positive number, as negative numbers are reserved for internal use by the Berkeley DB library. For this reason, applications sharing a pool must coordinate their values amongst themselves.

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.

# Errors

The DbEnv::memp_register method may fail and throw an exception or return a non-zero error for errors specified for other Berkeley DB and C library or system methods. If a catastrophic error has occurred, the DbEnv::memp_register method may fail and either return DB_RUNRECOVERY or throw an exception encapsulating DB_RUNRECOVERY, in which case all subsequent Berkeley DB calls will fail in the same way.

### Classes

DbEnv, DbMpoolFile

# See Also

DbEnv::set_mp_mmapsize, DbMpoolFile::close, DbMpoolFile::get, DbMpoolFile::open, DbMpoolFile::put, DbMpoolFile::set, DbMpoolFile::sync, DbEnv::memp_register, DbEnv::memp_stat, DbEnv::memp_sync and DbEnv::memp_trickle.`

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