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). You must 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);
}