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<p>
While transactions are frequently used to provide atomicity to
multiple database
operations, it is sometimes necessary to perform
a single database
operation under the control of a transaction.
Rather than force you to obtain a transaction, perform the single
write operation, and then either commit or abort the transaction,
you can automatically group this sequence of events using
<span class="emphasis"><em>auto commit</em></span>.
</p>
<p>
To use auto commit:
</p>
<div class="orderedlist">
<ol type="1">
<li>
<p>
Open your environment and your databases
so that they support
transactions. See <a href="enabletxn.html">Enabling Transactions</a>
for details.
</p>
<p>
Note that frequently auto commit is used for the environment
or database open. To use auto commit for either your
environment or database open, specify
<tt class="literal">DB_AUTO_COMMIT</tt> to the
<tt class="methodname">DB_ENV->set_flags()</tt>
or
<tt class="methodname">DB->open()</tt>
method. If you specify auto commit for the environment
open, then you do not need to also specify auto commit
for the database open.
</p>
</li>
<li>
<p>
Do not provide a transactional handle to the method that is
performing the database
write operation.
</p>
</li>
</ol>
</div>
<p>
Note that auto commit is not available for cursors. You must always
open your cursor using a transaction if you want the cursor's
operations to be transactional protected. See
<a href="txncursor.html">Transactional Cursors</a> for details on using
transactional cursors.
</p>
<p>
For example, the following uses auto commit to perform the database write operation:
</p>
<pre class="programlisting">#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;
DBT key, data;
const char *db_home_dir = "/tmp/myEnvironment";
const char *file_name = "mydb.db";
const char *keystr ="thekey";
const char *datastr = "thedata";
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;
/*
* Open the database. Note that we are using auto commit for the open,
* so the database is able to support transactions.
*/
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;
}
/* Prepare the DBTs */
memset(&key, 0, sizeof(DBT));
memset(&data, 0, sizeof(DBT));
key.data = &keystr;
key.size = strlen(keystr) + 1;
data.data = &datastr;
data.size = strlen(datastr) + 1;
/*
* Perform the database write. A txn handle is not provided, but the
* database support auto commit, so auto commit is used for the write.
*/
ret = dbp->put(dbp, NULL, &key, &data, 0);
if (ret != 0) {
envp->err(envp, ret, "Database put failed.");
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);
} </pre>
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