In Environments, we introduced environments. While environments are not used in the example built in this book, they are so commonly used for a wide class of DB applications that it is necessary to show their basic usage, if only from a completeness perspective.

To use an environment, you must first create the environment handle using , and then open it. At open time, you must identify the directory in which it resides. This directory must exist prior to the open attempt. You can also identify open properties, such as whether the environment can be created if it does not already exist.

You will also need to initialize the in-memory cache when you open your environment.

For example, to create an environment handle and open an environment:

#include <db.h>
...
DB_ENV *myEnv;            /* Env structure handle */
DB *dbp;                  /* DB structure handle */
u_int32_t db_flags;       /* database open flags */
u_int32_t env_flags;      /* env open flags */
int ret;                  /* function return value */

/* 
   Create an environment object and initialize it for error
   reporting. 
*/
ret = db_env_create(&myEnv, 0);
if (ret != 0) {
    fprintf(stderr, "Error creating env handle: %s\n", db_strerror(ret));
    return -1;
}

/* Open the environment. */
env_flags = DB_CREATE |    /* If the environment does not exist,
                            * create it. */
            DB_INIT_MPOOL; /* Initialize the in-memory cache. */

ret = myEnv->open(myEnv,   /* DB_ENV ptr */
  "/export1/testEnv",      /* env home directory */
  env_flags,               /* Open flags */
  0);                      /* File mode (default) */
if (ret != 0) {
    fprintf(stderr, "Environment open failed: %s", db_strerror(ret));
    return -1;
} 

Once an environment is opened, you can open databases in it. Note that by default databases are stored in the environment's home directory, or relative to that directory if you provide any sort of a path in the database's file name:

/* 
 * Initialize the DB structure. Pass the pointer
 * to the environment in which this DB is opened.
 */
ret = db_create(&dbp, myEnv, 0);
if (ret != 0) {
  /* Error handling goes here */
}

/* Database open flags */
db_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 */
                db_flags,   /* Open flags */
                0);         /* File mode (using defaults) */
if (ret != 0) {
  /* Error handling goes here */
}

When you are done with an environment, you must close it. It is recommended that before closing an environment, you close any open databases.

/* 
* Close the database and environment
*/

if (dbp != NULL) {
    dbp->close(dbp, 0);
}

if (myEnv != NULL) {
    myEnv->close(myEnv, 0);
}