Opening a Transactional Environment and Database
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## Opening a Transactional Environment and Database

To enable transactions for your environment, you must initialize the transactional subsystem. Note that doing this also initializes the logging subsystem. In addition, you must initialize the memory pool (in-memory cache). Frequently, but not always, you will also initialize the locking subsystem. For example:

Notice in the following example that you create your environment handle using the `db_env_create()` function before you open the environment:

    #include <stdio.h>
    #include <stdlib.h>

    #include "db.h"

    int
    main(void)
    {
        int ret, ret_c;
        u_int32_t env_flags;
        DB_ENV *envp;
        const char *db_home_dir = "/tmp/myEnvironment";

        envp = NULL;

        /* Open the environment */
        ret = db_env_create(&envp, 0);
        if (ret != 0) {
            fprintf(stderr, "Error creating environment handle: %s\n",
                db_strerror(ret));
            return (EXIT_FAILURE);
        }

        env_flags = DB_CREATE |    /* Create the environment if it does
                                    * not already exist. */
                    DB_INIT_TXN  | /* Initialize transactions */
                    DB_INIT_LOCK | /* Initialize locking. */
                    DB_INIT_LOG  | /* Initialize logging */
                    DB_INIT_MPOOL; /* Initialize the in-memory cache. */

        ret = envp->open(envp, db_home_dir, env_flags, 0);
        if (ret != 0) {
            fprintf(stderr, "Error opening environment: %s\n",
                db_strerror(ret));
            goto err;
        }

    err:
        /* Close the environment */
        if (envp != NULL) {
            ret_c = envp->close(envp, 0);
            if (ret_c != 0) {
                fprintf(stderr, "environment close failed: %s\n",
                    db_strerror(ret_c));
                ret = ret_c;
            }
        }

        return (ret == 0 ? EXIT_SUCCESS : EXIT_FAILURE);
    }

You then create and open your database(s) as you would for a non-transactional system. The only difference is that you must pass the environment handle to the `db_create()` function, and you must open the database within a transaction. Typically auto commit is used for this purpose. To do so, pass `DB_AUTO_COMMIT` to the database open command. Also, make sure you close all your databases before you close your environment. For example:

    #include <stdio.h>
    #include <stdlib.h>

    #include "db.h"

    int
    main(void)
    {
        int ret, ret_c;
        u_int32_t db_flags, env_flags;
        DB *dbp;
        DB_ENV *envp;
        const char *db_home_dir = "/tmp/myEnvironment";
        const char *file_name = "mydb.db";

        dbp = NULL;
        envp = NULL;

        /* Open the environment */
        ret = db_env_create(&envp, 0);
        if (ret != 0) {
            fprintf(stderr, "Error creating environment handle: %s\n",
                db_strerror(ret));
            return (EXIT_FAILURE);
        }

        env_flags = DB_CREATE |    /* Create the environment if it does
                                    * not already exist. */
                    DB_INIT_TXN  | /* Initialize transactions */
                    DB_INIT_LOCK | /* Initialize locking. */
                    DB_INIT_LOG  | /* Initialize logging */
                    DB_INIT_MPOOL; /* Initialize the in-memory cache. */

        ret = envp->open(envp, db_home_dir, env_flags, 0);
        if (ret != 0) {
            fprintf(stderr, "Error opening environment: %s\n",
                db_strerror(ret));
            goto err;
        }

        /* Initialize the DB handle */
        ret = db_create(&dbp, envp, 0);
        if (ret != 0) {
            envp->err(envp, ret, "Database creation failed");
            goto err;
        }

        db_flags = DB_CREATE | DB_AUTO_COMMIT;
        ret = dbp->open(dbp,        /* Pointer to the database */
                        NULL,       /* Txn pointer */
                        file_name,  /* File name */
                        NULL,       /* Logical db name */
                        DB_BTREE,   /* Database type (using btree) */
                        db_flags,   /* Open flags */
                        0);         /* File mode. Using defaults */
        if (ret != 0) {
            envp->err(envp, ret, "Database '%s' open failed",
                file_name);
            goto err;
        }

    err:
        /* Close the database */
        if (dbp != NULL) {
            ret_c = dbp->close(dbp, 0);
            if (ret_c != 0) {
                envp->err(envp, ret_c, "Database close failed.");
                ret = ret_c
            }
        }

        /* Close the environment */
        if (envp != NULL) {
            ret_c = envp->close(envp, 0);
            if (ret_c != 0) {
                fprintf(stderr, "environment close failed: %s\n",
                    db_strerror(ret_c));
                ret = ret_c;
            }
        }

        return (ret == 0 ? EXIT_SUCCESS : EXIT_FAILURE);
    }

### Note

Never close a database that has active transactions. Make sure all transactions are resolved (either committed or aborted) before closing the database.

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