Table of Contents
In Berkeley DB, a database is a collection of records. Records, in turn, consist of key/data pairings.
Conceptually, you can think of a
database
as containing a two-column table where column 1 contains a key and column 2
contains data. Both the key and the data are managed using
DBT
structures
(see Database Records for details on this
structure).
So, fundamentally, using a DB
database
involves putting, getting, and deleting database records, which in turns involves efficiently
managing information
contained in
DBT
structures.
The next several chapters of this book are dedicated to those activities.
To open a database, you must first use the db_create() function to
initialize a DB handle.
Once you have initialized the DB
handle, you use its open() method to open the database.
Note that by default, DB does not create databases if they do not already exist.
To override this behavior, specify the
DB_CREATE flag on the
open() method.
The following code fragment illustrates a database open:
#include <db.h>
...
DB *dbp; /* DB structure handle */
u_int32_t flags; /* database open flags */
int ret; /* function return value */
/* Initialize the structure. This
* database is not opened in an environment,
* so the environment pointer is NULL. */
ret = db_create(&dbp, NULL, 0);
if (ret != 0) {
/* Error handling goes here */
}
/* Database open flags */
flags = DB_CREATE; /* If the database does not exist,
* create it.*/
/* open the database */
ret = dbp->open(dbp, /* DB structure pointer */
NULL, /* Transaction pointer */
"my_db.db", /* On-disk file that holds the database. */
NULL, /* Optional logical database name */
DB_BTREE, /* Database access method */
flags, /* Open flags */
0); /* File mode (using defaults) */
if (ret != 0) {
/* Error handling goes here */
}