<!--$Id: get.html,v 1.2 2004/03/30 01:22:58 jtownsen Exp $--> <!--Copyright 1997-2003 by Sleepycat Software, Inc.--> <!--All rights reserved.--> <!--See the file LICENSE for redistribution information.--> <html> <head> <title>Berkeley DB Reference Guide: Retrieving elements from a database</title> <meta name="description" content="Berkeley DB: An embedded database programmatic toolkit."> <meta name="keywords" content="embedded,database,programmatic,toolkit,b+tree,btree,hash,hashing,transaction,transactions,locking,logging,access method,access methods,Java,C,C++"> </head> <body bgcolor=white> <table width="100%"><tr valign=top> <td><h3><dl><dt>Berkeley DB Reference Guide:<dd>Simple Tutorial</dl></h3></td> <td align=right><a href="../simple_tut/put.html"><img src="../../images/prev.gif" alt="Prev"></a><a href="../toc.html"><img src="../../images/ref.gif" alt="Ref"></a><a href="../simple_tut/del.html"><img src="../../images/next.gif" alt="Next"></a> </td></tr></table> <p> <h3 align=center>Retrieving elements from a database</h3> <p>The simplest way to retrieve elements from a database is the <a href="../../api_c/db_get.html">DB->get</a> method.</p> <p>The <a href="../../api_c/db_get.html">DB->get</a> method takes the same five arguments that the <a href="../../api_c/db_put.html">DB->put</a> interface takes:</p> <p><dl compact> <p><dt>db<dd>The database handle returned by <a href="../../api_c/db_class.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> method. </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 <stdlib.h> #include <string.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, NULL, 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_class.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_class.html">DBT</a> structures are used. The <a href="../../api_c/dbt_class.html">DBT</a> manual page specified the details of those cases.</p> <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> <table width="100%"><tr><td><br></td><td align=right><a href="../simple_tut/put.html"><img src="../../images/prev.gif" alt="Prev"></a><a href="../toc.html"><img src="../../images/ref.gif" alt="Ref"></a><a href="../simple_tut/del.html"><img src="../../images/next.gif" alt="Next"></a> </td></tr></table> <p><font size=1><a href="../../sleepycat/legal.html">Copyright (c) 1996-2003</a> <a href="http://www.sleepycat.com">Sleepycat Software, Inc.</a> - All rights reserved.</font> </body> </html>