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<!--$Id: partial.so,v 10.18 2000/12/18 21:05:14 bostic Exp $-->
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<title>Berkeley DB Reference Guide: Partial record storage and retrieval</title>
<meta name="description" content="Berkeley DB: An embedded database programmatic toolkit.">
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<td><h3><dl><dt>Berkeley DB Reference Guide:<dd>Access Methods</dl></h3></td>
<td width="1%"><a href="../../ref/am/stability.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/am/verify.html"><img src="../../images/next.gif" alt="Next"></a>
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<p>
<h1 align=center>Partial record storage and retrieval</h1>
<p>It is possible to both store and retrieve parts of data items in all
Berkeley DB access methods. This is done by setting the
<a href="../../api_c/dbt.html#DB_DBT_PARTIAL">DB_DBT_PARTIAL</a> flag in the <a href="../../api_c/dbt.html">DBT</a> structure passed to the
Berkeley DB interface.
<p>The <a href="../../api_c/dbt.html#DB_DBT_PARTIAL">DB_DBT_PARTIAL</a> flag is based on the values of two fields
of the <a href="../../api_c/dbt.html">DBT</a> structure, <b>dlen</b> and <b>doff</b>. The value
of <b>dlen</b> is the number of bytes of the record in which the
application is interested. The value of <b>doff</b> is the offset from
the beginning of the data item where those bytes start.
<p>For example, if the data item were <b>ABCDEFGHIJKL</b>, a <b>doff</b>
value of 3 would indicate that the bytes of interest started at
<b>D</b>, and a <b>dlen</b> value of 4 would indicate that the bytes
of interest were <b>DEFG</b>.
<p>When retrieving a data item from a database, the <b>dlen</b> bytes
starting <b>doff</b> bytes from the beginning of the record are
returned, as if they comprised the entire record. If any or all of the
specified bytes do not exist in the record, the retrieval is still
successful and any existing bytes (and nul bytes for any non-existent
bytes) are returned.
<p>When storing a data item into the database, the <b>dlen</b> bytes
starting <b>doff</b> bytes from the beginning of the specified key's
data record are replaced by the data specified by the <b>data</b> and
<b>size</b> fields. If <b>dlen</b> is smaller than <b>size</b>, the
record will grow, and if <b>dlen</b> is larger than <b>size</b>, the
record will shrink. If the specified bytes do not exist, the record will
be extended using nul bytes as necessary, and the store call will still
succeed.
<p>The following are various examples of the put case for the
<a href="../../api_c/dbt.html#DB_DBT_PARTIAL">DB_DBT_PARTIAL</a> flag. In all examples, the initial data item is 20
bytes in length:
<p><b>ABCDEFGHIJ0123456789</b>
<p><ol>
<p><li><p><blockquote><pre>size = 20
doff = 0
dlen = 20
data = abcdefghijabcdefghij
<p>
Result: The 20 bytes at offset 0 are replaced by the 20 bytes of data,
i.e., the entire record is replaced.
<p>
ABCDEFGHIJ0123456789 -&gt; abcdefghijabcdefghij
</pre></blockquote>
<p><li><p><blockquote><pre>size = 10
doff = 20
dlen = 0
data = abcdefghij
<p>
Result: The 0 bytes at offset 20 are replaced by the 10 bytes of data,
i.e., the record is extended by 10 bytes.
<p>
ABCDEFGHIJ0123456789 -&gt; ABCDEFGHIJ0123456789abcdefghij
</pre></blockquote>
<p><li><p><blockquote><pre>size = 10
doff = 10
dlen = 5
data = abcdefghij
<p>
Result: The 5 bytes at offset 10 are replaced by the 10 bytes of data.
<p>
ABCDEFGHIJ0123456789 -&gt; ABCDEFGHIJabcdefghij56789
</pre></blockquote>
<p><li><p><blockquote><pre>size = 10
doff = 10
dlen = 0
data = abcdefghij
<p>
Result: The 0 bytes at offset 10 are replaced by the 10 bytes of data,
i.e., 10 bytes are inserted into the record.
<p>
ABCDEFGHIJ0123456789 -&gt; ABCDEFGHIJabcdefghij0123456789
</pre></blockquote>
<p><li><p><blockquote><pre>size = 10
doff = 2
dlen = 15
data = abcdefghij
<p>
Result: The 15 bytes at offset 2 are replaced by the 10 bytes of data.
<p>
ABCDEFGHIJ0123456789 -&gt; ABabcdefghij789
</pre></blockquote>
<p><li><p><blockquote><pre>size = 10
doff = 0
dlen = 0
data = abcdefghij
<p>
Result: The 0 bytes at offset 0 are replaced by the 10 bytes of data,
i.e., the 10 bytes are inserted at the beginning of the record.
<p>
ABCDEFGHIJ0123456789 -&gt; abcdefghijABCDEFGHIJ0123456789
</pre></blockquote>
<p><li><p><blockquote><pre>size = 0
doff = 0
dlen = 10
data = ""
<p>
Result: The 10 bytes at offset 0 are replaced by the 0 bytes of data,
i.e., the first 10 bytes of the record are discarded.
<p>
ABCDEFGHIJ0123456789 -&gt; 0123456789
</pre></blockquote>
<p><li><p><blockquote><pre>size = 10
doff = 25
dlen = 0
data = abcdefghij
<p>
Result: The 0 bytes at offset 25 are replaced by the 10 bytes of data,
i.e., 10 bytes are inserted into the record past the end of the current
data (\0 represents a nul byte).
<p>
ABCDEFGHIJ0123456789 -&gt; ABCDEFGHIJ0123456789\0\0\0\0\0abcdefghij
</pre></blockquote>
</ol>
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