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<h2 class="title"><a id="databases"></a>Chapter 2. Databases</h2>
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<div class="toc">
<p>
<b>Table of Contents</b>
</p>
<dl>
<dt>
<span class="sect1">
<a href="databases.html#DBOpen">Opening Databases</a>
</span>
</dt>
<dt>
<span class="sect1">
<a href="coredbclose.html">Closing Databases</a>
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<a href="DBOpenFlags.html">Database Open Flags</a>
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<a href="CoreDbUsage.html">Database Example</a>
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<p>In Berkeley DB, a database is a collection of <span class="emphasis"><em>records</em></span>. Records,
in turn, consist of key/data pairings.
</p>
<p>
Conceptually, you can think of a
<span>database</span>
as containing a two-column table where column 1 contains a key and column 2
contains data. Both the key and the data are managed using
<code class="classname">DBT</code>
<span>structures</span>
(see <a class="xref" href="DBEntry.html" title="Chapter 3. Database Records">Database Records</a> for details on this
<span>structure</span>).
So, fundamentally, using a DB
<span>database</span>
involves putting, getting, and deleting database records, which in turns involves efficiently
managing information
<span>contained in </span>
<code class="classname">DBT</code>
<span>structures.</span>
The next several chapters of this book are dedicated to those activities.
</p>
<div class="sect1" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h2 class="title" style="clear: both"><a id="DBOpen"></a>Opening Databases</h2>
</div>
</div>
</div>
<p>
To open a database, you must first use the <code class="function">db_create()</code> function to
initialize a <code class="classname">DB</code> handle.
Once you have initialized the <code class="classname">DB</code>
handle, you use its <code class="methodname">open()</code> method to open the database.
</p>
<p>
Note that by default, DB does not create databases if they do not already exist.
To override this behavior, specify the
<a class="link" href="DBOpenFlags.html" title="Database Open Flags"><code class="literal">DB_CREATE</code></a> flag on the
<code class="methodname">open()</code> method.
</p>
<p>
The following code fragment illustrates a database open:
</p>
<a id="c_db1"></a>
<pre class="programlisting">#include <db.h>
...
DB *dbp; /* DB structure handle */
u_int32_t flags; /* database open flags */
int ret; /* function return value */
/* Initialize the structure. This
* database is not opened in an environment,
* so the environment pointer is NULL. */
ret = db_create(&dbp, NULL, 0);
if (ret != 0) {
/* Error handling goes here */
}
/* Database open flags */
flags = DB_CREATE; /* If the database does not exist,
* create it.*/
/* open the database */
ret = dbp->open(dbp, /* DB structure pointer */
NULL, /* Transaction pointer */
"my_db.db", /* On-disk file that holds the database. */
NULL, /* Optional logical database name */
DB_BTREE, /* Database access method */
flags, /* Open flags */
0); /* File mode (using defaults) */
if (ret != 0) {
/* Error handling goes here */
}</pre>
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