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<!--$Id: get.so,v 10.23 2000/12/14 21:42:18 bostic Exp $-->
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<title>Berkeley DB Reference Guide: Retrieving elements from a database</title>
<meta name="description" content="Berkeley DB: An embedded database programmatic toolkit.">
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<td><h3><dl><dt>Berkeley DB Reference Guide:<dd>Simple Tutorial</dl></h3></td>
<td width="1%"><a href="../../ref/simple_tut/put.html"><img src="../../images/prev.gif" alt="Prev"></a><a href="../../ref/toc.html"><img src="../../images/ref.gif" alt="Ref"></a><a href="../../ref/simple_tut/del.html"><img src="../../images/next.gif" alt="Next"></a>
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<p>
<h1 align=center>Retrieving elements from a database</h1>
<p>The simplest way to retrieve elements from a database is the
<a href="../../api_c/db_get.html">DB->get</a> interface.
<p>The <a href="../../api_c/db_get.html">DB->get</a> interface takes the same five arguments that the
<a href="../../api_c/db_put.html">DB->put</a> interface takes:
<p><dl compact>
<p><dt>db<dd>The database handle returned by <a href="../../api_c/db_create.html">db_create</a>.
<p><dt>txnid<dd>A transaction ID. In our simple case, we aren't expecting to recover
the database after application or system crash, so we aren't using
transactions, and will leave this argument NULL.
<p><dt>key<dd>The key item for the key/data pair that we want to retrieve from the
database.
<p><dt>data<dd>The data item for the key/data pair that we want to retrieve from the
database.
<p><dt>flags<dd>Optional flags modifying the underlying behavior of the <a href="../../api_c/db_get.html">DB->get</a>
interface.
</dl>
<p>Here's what the code to call <a href="../../api_c/db_get.html">DB->get</a> looks like:
<p><blockquote><pre>#include <sys/types.h>
#include <stdio.h>
#include <db.h>
<p>
#define DATABASE "access.db"
<p>
int
main()
{
DB *dbp;
DBT key, data;
int ret;
<p>
if ((ret = db_create(&dbp, NULL, 0)) != 0) {
fprintf(stderr, "db_create: %s\n", db_strerror(ret));
exit (1);
}
if ((ret = dbp->open(
dbp, DATABASE, NULL, DB_BTREE, DB_CREATE, 0664)) != 0) {
dbp->err(dbp, ret, "%s", DATABASE);
goto err;
}
<p>
memset(&key, 0, sizeof(key));
memset(&data, 0, sizeof(data));
key.data = "fruit";
key.size = sizeof("fruit");
data.data = "apple";
data.size = sizeof("apple");
<p>
if ((ret = dbp->put(dbp, NULL, &key, &data, 0)) == 0)
printf("db: %s: key stored.\n", (char *)key.data);
else {
dbp->err(dbp, ret, "DB->put");
goto err;
}
<p><b> if ((ret = dbp->get(dbp, NULL, &key, &data, 0)) == 0)
printf("db: %s: key retrieved: data was %s.\n",
(char *)key.data, (char *)data.data);
else {
dbp->err(dbp, ret, "DB->get");
goto err;
}
</b></pre></blockquote>
<p>It is not usually necessary to clear the <a href="../../api_c/dbt.html">DBT</a> structures passed
to the Berkeley DB functions between calls. This is not always true, when
some of the less commonly used flags for <a href="../../api_c/dbt.html">DBT</a> structures are
used. The <a href="../../api_c/dbt.html">DBT</a> manual page specified the details of those cases.
<p>It is possible, of course, to distinguish between system errors and the
key/data pair simply not existing in the database. There are three
standard returns from <a href="../../api_c/db_get.html">DB->get</a>:
<p><ol>
<p><li>The call might be successful and the key found, in which case the return
value will be 0.
<li>The call might be successful, but the key not found, in which case the
return value will be <a href="../../ref/program/errorret.html#DB_NOTFOUND">DB_NOTFOUND</a>.
<li>The call might not be successful, in which case the return value will
be a system error.
</ol>
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