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<title>Berkeley DB Reference Guide: Key/data pairs</title>
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<td><h3><dl><dt>Berkeley DB Reference Guide:<dd>Simple Tutorial</dl></h3></td>
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<h1 align=center>Key/data pairs</h1>
<p>Berkeley DB uses key/data pairs to identify elements in the database.
That is, in the general case, whenever you call a Berkeley DB interface,
you present a key to identify the key/data pair on which you intend
to operate.
<p>For example, you might store some key/data pairs as follows:
<p><table border=1>
<tr><th>Key:</th><th>Data:</th></tr>
<tr><td>fruit</td><td>apple</td></tr>
<tr><td>sport</td><td>cricket</td></tr>
<tr><td>drink</td><td>water</td></tr>
</table>
<p>In each case, the first element of the pair is the key, and the second is
the data. To store the first of these key/data pairs into the database,
you would call the Berkeley DB interface to store items, with <b>fruit</b> as
the key, and <b>apple</b> as the data. At some future time, you could
then retrieve the data item associated with <b>fruit</b>, and the Berkeley DB
retrieval interface would return <b>apple</b> to you. While there are
many variations and some subtleties, all accesses to data in Berkeley DB come
down to key/data pairs.
<p>Both key and data items are stored in simple structures (called
<a href="../../api_c/dbt_class.html">DBT</a>s) that contain a reference to memory and a length, counted
in bytes. (The name <a href="../../api_c/dbt_class.html">DBT</a> is an acronym for <i>database
thang</i>, chosen because nobody could think of a sensible name that wasn't
already in use somewhere else.) Key and data items can be arbitrary
binary data of practically any length, including 0 bytes. There is a
single data item for each key item, by default, but databases can be
configured to support multiple data items for each key item.
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