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98 lines
4.3 KiB
HTML
<!--$Id: get.so,v 10.23 2000/12/14 21:42:18 bostic Exp $-->
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<!--Copyright 1997, 1998, 1999, 2000 by Sleepycat Software, Inc.-->
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<!--All rights reserved.-->
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<html>
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<head>
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<title>Berkeley DB Reference Guide: Retrieving elements from a database</title>
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<meta name="description" content="Berkeley DB: An embedded database programmatic toolkit.">
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<meta name="keywords" content="embedded,database,programmatic,toolkit,b+tree,btree,hash,hashing,transaction,transactions,locking,logging,access method,access methods,java,C,C++">
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</head>
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<body bgcolor=white>
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<table><tr valign=top>
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<td><h3><dl><dt>Berkeley DB Reference Guide:<dd>Simple Tutorial</dl></h3></td>
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<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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</td></tr></table>
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<p>
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<h1 align=center>Retrieving elements from a database</h1>
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<p>The simplest way to retrieve elements from a database is the
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<a href="../../api_c/db_get.html">DB->get</a> interface.
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<p>The <a href="../../api_c/db_get.html">DB->get</a> interface takes the same five arguments that the
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<a href="../../api_c/db_put.html">DB->put</a> interface takes:
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<p><dl compact>
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<p><dt>db<dd>The database handle returned by <a href="../../api_c/db_create.html">db_create</a>.
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<p><dt>txnid<dd>A transaction ID. In our simple case, we aren't expecting to recover
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the database after application or system crash, so we aren't using
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transactions, and will leave this argument NULL.
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<p><dt>key<dd>The key item for the key/data pair that we want to retrieve from the
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database.
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<p><dt>data<dd>The data item for the key/data pair that we want to retrieve from the
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database.
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<p><dt>flags<dd>Optional flags modifying the underlying behavior of the <a href="../../api_c/db_get.html">DB->get</a>
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interface.
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</dl>
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<p>Here's what the code to call <a href="../../api_c/db_get.html">DB->get</a> looks like:
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<p><blockquote><pre>#include <sys/types.h>
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#include <stdio.h>
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#include <db.h>
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<p>
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#define DATABASE "access.db"
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<p>
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int
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main()
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{
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DB *dbp;
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DBT key, data;
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int ret;
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<p>
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if ((ret = db_create(&dbp, NULL, 0)) != 0) {
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fprintf(stderr, "db_create: %s\n", db_strerror(ret));
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exit (1);
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}
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if ((ret = dbp->open(
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dbp, DATABASE, NULL, DB_BTREE, DB_CREATE, 0664)) != 0) {
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dbp->err(dbp, ret, "%s", DATABASE);
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goto err;
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}
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<p>
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memset(&key, 0, sizeof(key));
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memset(&data, 0, sizeof(data));
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key.data = "fruit";
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key.size = sizeof("fruit");
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data.data = "apple";
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data.size = sizeof("apple");
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<p>
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if ((ret = dbp->put(dbp, NULL, &key, &data, 0)) == 0)
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printf("db: %s: key stored.\n", (char *)key.data);
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else {
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dbp->err(dbp, ret, "DB->put");
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goto err;
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}
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<p><b> if ((ret = dbp->get(dbp, NULL, &key, &data, 0)) == 0)
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printf("db: %s: key retrieved: data was %s.\n",
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(char *)key.data, (char *)data.data);
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else {
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dbp->err(dbp, ret, "DB->get");
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goto err;
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}
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</b></pre></blockquote>
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<p>It is not usually necessary to clear the <a href="../../api_c/dbt.html">DBT</a> structures passed
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to the Berkeley DB functions between calls. This is not always true, when
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some of the less commonly used flags for <a href="../../api_c/dbt.html">DBT</a> structures are
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used. The <a href="../../api_c/dbt.html">DBT</a> manual page specified the details of those cases.
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<p>It is possible, of course, to distinguish between system errors and the
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key/data pair simply not existing in the database. There are three
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standard returns from <a href="../../api_c/db_get.html">DB->get</a>:
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<p><ol>
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<p><li>The call might be successful and the key found, in which case the return
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value will be 0.
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<li>The call might be successful, but the key not found, in which case the
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return value will be <a href="../../ref/program/errorret.html#DB_NOTFOUND">DB_NOTFOUND</a>.
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<li>The call might not be successful, in which case the return value will
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be a system error.
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</ol>
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<table><tr><td><br></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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</td></tr></table>
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<p><font size=1><a href="http://www.sleepycat.com">Copyright Sleepycat Software</a></font>
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</body>
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</html>
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