Berkeley DB Reference Guide: Access Methods
Record padding byte value (re_pad, DB_PAD)
The Recno access method can be used to store both variable and fixed-length data items. By default, the data items are expected to be variable length. If the DB_FIXEDLEN flag is set in the DB_INFO structure during the db_open call to open the database, the data items are expected to be fixed-length.
When storing fixed length records in the database, a pad character may be specified by setting the DB_PAD flag and the re_pad element of the DB_INFO structure. Any record read from the backing source file or otherwise stored in the database that is shorter than the expected length will automatically be padded with this character.
If fixed-length records are specified but no pad character is specified, a space character (i.e., 0x20 in the ASCII character set) will be used.
