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6.1 KiB
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<!--$Id: put.so,v 10.31 2000/12/18 21:05:15 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: Adding elements to 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/open.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/get.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>Adding elements to a database</h1>
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<p>The simplest way to add elements to a database is the <a href="../../api_c/db_put.html">DB->put</a>
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interface.
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<p>The <a href="../../api_c/db_put.html">DB->put</a> interface takes five arguments:
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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 handle. In our simple case, we aren't expecting to
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recover the database after application or system crash, so we aren't
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using 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 add to the database.
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<p><dt>data<dd>The data item for the key/data pair that we want to add to the database.
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<p><dt>flags<dd>Optional flags modifying the underlying behavior of the <a href="../../api_c/db_put.html">DB->put</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_put.html">DB->put</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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<b>DBT key, data;</b>
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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><b> 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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</b></pre></blockquote>
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<p>The first thing to notice about this new code is that we clear the
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<a href="../../api_c/dbt.html">DBT</a> structures that we're about to pass as arguments to Berkeley DB
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functions. This is very important, and being careful to do so will
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result in fewer errors in your programs. All Berkeley DB structures
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instantiated in the application and handed to Berkeley DB should be cleared
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before use, without exception. This is necessary so that future
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versions of Berkeley DB may add additional fields to the structures. If
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applications clear the structures before use, it will be possible for
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Berkeley DB to change those structures without requiring that the applications
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be rewritten to be aware of the changes.
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<p>Notice also that we're storing the trailing nul byte found in the C
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strings <b>"fruit"</b> and <b>"apple"</b> in both the key and data
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items, that is, the trailing nul byte is part of the stored key, and
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therefore has to be specified in order to access the data item. There is
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no requirement to store the trailing nul byte, it simply makes it easier
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for us to display strings that we've stored in programming languages that
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use nul bytes to terminate strings.
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<p>In many applications, it is important not to overwrite existing
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data. For example, we might not want to store the key/data pair
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<b>fruit/apple</b> if it already existed, e.g., if someone had
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previously stored the key/data pair <b>fruit/cherry</b> into the
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database.
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<p>This is easily accomplished by adding the <a href="../../api_c/db_put.html#DB_NOOVERWRITE">DB_NOOVERWRITE</a> flag to
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the <a href="../../api_c/db_put.html">DB->put</a> call:
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<p><blockquote><pre><b>if ((ret =
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dbp->put(dbp, NULL, &key, &data, DB_NOOVERWRITE)) == 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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}</b></pre></blockquote>
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<p>This flag causes the underlying database functions to not overwrite any
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previously existing key/data pair. (Note that the value of the previously
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existing data doesn't matter in this case. The only question is if a
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key/data pair already exists where the key matches the key that we are
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trying to store.)
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<p>Specifying <a href="../../api_c/db_put.html#DB_NOOVERWRITE">DB_NOOVERWRITE</a> opens up the possibility of a new
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Berkeley DB return value from the <a href="../../api_c/db_put.html">DB->put</a> function, <a href="../../api_c/dbc_put.html#DB_KEYEXIST">DB_KEYEXIST</a>,
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which means we were unable to add the key/data pair to the database
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because the key already existed in the database. While the above sample
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code simply displays a message in this case:
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<p><blockquote><pre>DB->put: DB_KEYEXIST: Key/data pair already exists</pre></blockquote>
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<p>The following code shows an explicit check for this possibility:
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<p><blockquote><pre><b>switch (ret =
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dbp->put(dbp, NULL, &key, &data, DB_NOOVERWRITE)) {
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case 0:
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printf("db: %s: key stored.\n", (char *)key.data);
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break;
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case DB_KEYEXIST:
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printf("db: %s: key previously stored.\n",
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(char *)key.data);
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break;
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default:
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dbp->err(dbp, ret, "DB->put");
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goto err;
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}</b></pre></blockquote>
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<table><tr><td><br></td><td width="1%"><a href="../../ref/simple_tut/open.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/get.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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