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<h2 class="title" style="clear: both"><a id="envopen"></a>Opening a Transactional Environment and
<span>Database</span>
</h2>
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<p>
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.
<span>
For example:
</span>
</p>
<p>
Notice in the following example that you create your environment
handle using the <tt class="function">db_env_create()</tt> function before you open
the environment:
</p>
<pre class="programlisting">#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);
} </pre>
<p>
You then create and open your database(s) as you would for a non-transactional system.
<span>
The only difference is that you must pass the environment handle to
the
<span>
<tt class="function">db_create()</tt> function,
</span>
and you must open the database within a transaction.
Typically auto commit is used for this purpose. To do so, pass
<tt class="literal">DB_AUTO_COMMIT</tt> to the database open command.
Also, make sure you close all your databases before you close
your environment.
For example:
</span>
</p>
<pre class="programlisting">#include <stdio.h>
#include <stdlib.h>
#include "db.h"
int
main(void)
{
int ret, ret_c;
u_int32_t <b class="userinput"><tt>db_flags,</tt></b> env_flags;
<b class="userinput"><tt>DB *dbp;</tt></b>
DB_ENV *envp;
const char *db_home_dir = "/tmp/myEnvironment";
<b class="userinput"><tt>const char *file_name = "mydb.db";
dbp = NULL;</tt></b>
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;
}
<b class="userinput"><tt>/* 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;
}</tt></b>
err:
<b class="userinput"><tt>/* 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
}
}</tt></b>
/* 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);
} </pre>
<div class="note" style="margin-left: 0.5in; margin-right: 0.5in;">
<h3 class="title">Note</h3>
<p>
Never close a database that has active transactions. Make sure
all transactions are resolved (either committed or aborted)
before closing the database.
</p>
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