Global MDB_rel_func )(MDB_val *item, void *oldptr, void *newptr, void *relctx): This feature is currently unimplemented.
Global mdb_set_relfunc (MDB_txn *txn, MDB_dbi dbi, MDB_rel_func *rel): The relocation function is called whenever it is necessary to move the data of an item to a different position in the database (e.g. through tree balancing operations, shifts as a result of adds or deletes, etc.). It is intended to allow address/position-dependent data items to be stored in a database in an environment opened with the MDB_FIXEDMAP option. Currently the relocation feature is unimplemented and setting this function has no effect.