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<h2 class="title"><a id="DB"></a>Chapter 2. Databases</h2>
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<b>Table of Contents</b>
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<dl>
<dt>
<span class="sect1">
<a href="DB.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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<span class="sect1">
<a href="CoreDBAdmin.html">Administrative Methods</a>
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<a href="dbErrorReporting.html">Error Reporting Functions</a>
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<a href="CoreEnvUsage.html">Managing Databases in Environments</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
<tt class="classname">DBT</tt>
<span>structures</span>
(see <a href="DBEntry.html">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>
<tt class="classname">DBT</tt>
<span>structures.</span>
The next several chapters of this book are dedicated to those activities.
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<h2 class="title" style="clear: both"><a id="DBOpen"></a>Opening Databases</h2>
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<p>
To open a database, you must first use the <tt class="function">db_create()</tt> function to
initialize a <tt class="classname">DB</tt> handle.
Once you have initialized the <tt class="classname">DB</tt>
handle, you use its <tt class="methodname">open()</tt> 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 href="DBOpenFlags.html" title="Database Open Flags"><tt class="literal">DB_CREATE</tt></a> flag on the
<tt class="methodname">open()</tt> 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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