Chapter 2. Databases

Table of Contents

Opening Databases
Closing Databases
Database Open Flags
Administrative Methods
Error Reporting Functions
Managing Databases in Environments
Database Example

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.

Opening Databases

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 */