Berkeley DB Reference Guide: Database open

The DB->open method opens a database, and takes five arguments:

file: The name of the file to be opened.
database: An optional database name.
type: The type of database to open. This value will be one of the four access methods Berkeley DB supports: DB_BTREE, DB_HASH, DB_QUEUE or DB_RECNO, or the special value DB_UNKNOWN, which allows you to open an existing file without knowing its type.
mode: The permissions to give to any created file.

There are a few flags that you can set to customize open:

DB_CREATE: Create the underlying database and any necessary physical files.
DB_NOMMAP: Do not map this database into process memory.
DB_RDONLY: Treat the data base as read-only.
DB_THREAD: The returned handle is free-threaded, that is, it can be used simultaneously by multiple threads within the process.
DB_TRUNCATE: Physically truncate the underlying database file, discarding all databases it contained. Underlying filesystem primitives are used to implement this flag. For this reason it is only applicable to the physical file and cannot be used to discard individual databases from within physical files.
DB_UPGRADE: Upgrade the database format as necessary.