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<h2 class="title"><a id="DBEntry"></a>Chapter 3. Database Records</h2>
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<b>Table of Contents</b>
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<dt>
<span class="sect1">
<a href="DBEntry.html#usingDbEntry">Using Database Records</a>
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<dt>
<span class="sect1">
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<span class="sect2">
<a href="usingDbt.html#databaseWrite">Writing Records to the Database</a>
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<span class="sect2">
<a href="usingDbt.html#CoreDatabaseRead">Getting Records from the Database</a>
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<dt>
<span class="sect2">
<a href="usingDbt.html#recordDelete">Deleting Records</a>
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<span class="sect2">
<a href="usingDbt.html#datapersist">Data Persistence</a>
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<p>
DB records contain two parts — a key and some data. Both the key
and its corresponding data are
encapsulated in
<span><tt class="classname">Dbt</tt> class objects.</span>
Therefore, to access a DB record, you need two such
<span>objects,</span> one for the key and
one for the data.
</p>
<p>
<tt class="classname">Dbt</tt> objects provide a <tt class="literal">void *</tt>
data member that you use to point to your data, and another member that identifies
the data length. They can therefore be used to store anything from simple
primitive data to complex class objects so long as the information you want to
store resides in a single contiguous block of memory.
</p>
<p>
This chapter describes
<tt class="classname">Dbt</tt>
usage. It also
introduces storing and retrieving key/value pairs from a database.
</p>
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<h2 class="title" style="clear: both"><a id="usingDbEntry"></a>Using Database Records</h2>
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<p>
Each database record is comprised of two
<span><tt class="classname">Dbt</tt> objects</span>
— one for the key and another for the data.
</p>
<a id="cxx_dbt1"></a>
<pre class="programlisting">#include <db_cxx.h>
#include <string.h>
...
float money = 122.45;
char *description = "Grocery bill.";
Dbt key(&money, sizeof(float));
Dbt data(description, strlen(description)+1); </pre>
<p>
Note that in the following example we do not allow DB to assign the
memory for the retrieval of the money value. The reason why is that some
systems may require float values to have a specific alignment, and the
memory as returned by DB
may not be properly aligned (the same problem may exist for structures
on some systems). We tell DB to use our memory instead of its
own by specifying the <tt class="literal">DB_DBT_USERMEM</tt> flag. Be aware that
when we do this, we must also identify how much user memory is available
through the use of the <tt class="literal">ulen</tt> field.
</p>
<a id="cxx_dbt2"></a>
<pre class="programlisting">#include <db_cxx.h>
#include <string.h>
...
Dbt key, data;
float money;
char *description;
key.set_data(&money);
key.set_ulen(sizeof(float));
key.set_flags(DB_DBT_USERMEM);
// Database retrieval code goes here
// Money is set into the memory that we supplied.
description = (char *)data.get_data();</pre>
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