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Berkeley DB Reference Guide:
  Berkeley DB Transactional Data Store Applications

# Opening the databases

Next, we open three databases ("color" and "fruit" and "cats"), in the database environment. Again, our DB database handles are declared to be free-threaded using the DB_THREAD flag, and so may be used by any number of threads we subsequently create.

>     int
>     main(int argc, char *argv)
>     {
>     	extern char *optarg;
>     	extern int optind;
>     	DB *db_cats, *db_color, *db_fruit;
>     	DB_ENV *dbenv;
>     	pthread_t ptid;
>     	int ch;

>     	while ((ch = getopt(argc, argv, "")) != EOF)
>     		switch (ch) {
>     		case '?':
>     		default:
>     			usage();
>     		}
>     	argc -= optind;
>     	argv += optind;

>     	env_dir_create();
>     	env_open(&dbenv);

>     	/* Open database: Key is fruit class; Data is specific type. */
>     	db_open(dbenv, &db_fruit, "fruit", 0);

>     	/* Open database: Key is a color; Data is an integer. */
>     	db_open(dbenv, &db_color, "color", 0);

>     	/*
>     	 * Open database:
>     	 *	Key is a name; Data is: company name, cat breeds.
>     	 */
>     	db_open(dbenv, &db_cats, "cats", 1);

>     	return (0);
>     }

>     void
>     db_open(DB_ENV *dbenv, DB **dbp, char *name, int dups)
>     {
>     	DB *db;
>     	int ret;

>     	/* Create the database handle. */
>     	if ((ret = db_create(&db, dbenv, 0)) != 0) {
>     		dbenv->err(dbenv, ret, "db_create");
>     		exit (1);
>     	}

>     	/* Optionally, turn on duplicate data items. */
>     	if (dups && (ret = db->set_flags(db, DB_DUP)) != 0) {
>     		dbenv->err(dbenv, ret, "db->set_flags: DB_DUP");
>     		exit (1);
>     	}

>     	/*
>     	 * Open a database in the environment:
>     	 *	create if it doesn't exist
>     	 *	free-threaded handle
>     	 *	read/write owner only
>     	 */
>     	if ((ret = db->open(db, name, NULL,
>     	    DB_BTREE, DB_CREATE | DB_THREAD, S_IRUSR | S_IWUSR)) != 0) {
>     		dbenv->err(dbenv, ret, "db->open: %s", name);
>     		exit (1);
>     	}

>     	*dbp = db;
>     }

There is no reason to wrap database opens inside of transactions. All database creates are transaction-protected internally to Berkeley DB, and applications using transaction-protected environments can simply rely on files either being successfully re-created in a recovered environment or not appearing at all.

After running this initial code, we can use the db_stat utility to display information about a database we have created:

>     prompt> db_stat -h TXNAPP -d color
>     53162   Btree magic number.
>     8       Btree version number.
>     Flags:
>     2       Minimum keys per-page.
>     8192    Underlying database page size.
>     1       Number of levels in the tree.
>     0       Number of unique keys in the tree.
>     0       Number of data items in the tree.
>     0       Number of tree internal pages.
>     0       Number of bytes free in tree internal pages (0% ff).
>     1       Number of tree leaf pages.
>     8166    Number of bytes free in tree leaf pages (0.% ff).
>     0       Number of tree duplicate pages.
>     0       Number of bytes free in tree duplicate pages (0% ff).
>     0       Number of tree overflow pages.
>     0       Number of bytes free in tree overflow pages (0% ff).
>     0       Number of pages on the free list.

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