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<h2 class="title" style="clear: both"><a id="dbset_bt_compress"></a>DB->set_bt_compress()</h2>
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<pre class="programlisting">#include <db.h>
int
DB->set_bt_compress(DB *db,
int (*bt_compress_fcn)(DB *db, const DBT *prevKey,
const DBT *prevData, const DBT *key, const DBT *data, DBT *dest),
int (*bt_decompress_fcn)(DB *db, const DBT *prevKey,
const DBT *prevData, DBT *compressed, DBT *destKey,
DBT *destData)); </pre>
<p>
Set the Btree compression and decompression functions. The
compression function is called whenever a key/data pair is added to
the tree and the decompression function is called whenever data is
requested from the tree.
</p>
<p>
If NULL function pointers are specified, then default compression
and decompression functions are used. Berkeley DB's default
compression function performs prefix compression on all keys and
prefix compression on data values for duplicate keys. If using
default compression, both the default compression and decompression
functions must be used.
</p>
<p>
The <code class="methodname">DB->set_bt_compress()</code> method
configures operations performed using the specified <a class="link" href="db.html" title="Chapter 2. The DB Handle">DB</a> handle, not all operations
performed on the underlying database.
</p>
<p>
The <code class="methodname">DB->set_bt_compress()</code> method may not be called after the
<a class="xref" href="dbopen.html" title="DB->open()">DB->open()</a> method is called. If
the database already exists when <a class="xref" href="dbopen.html" title="DB->open()">DB->open()</a>
is called, the information specified to <code class="methodname">DB->set_bt_compress()</code> must be the same as
that historically used to create the database or corruption can occur.
</p>
<p>
The <code class="methodname">DB->set_bt_compress()</code> <span>
<span>
method returns a non-zero error value on failure and 0 on success.
</span>
</span>
</p>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="id1643149"></a>Parameters</h3>
</div>
</div>
</div>
<div class="sect3" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="id1643216"></a>bt_compress_fcn</h4>
</div>
</div>
</div>
<p>
The <span class="bold"><strong>bt_compress_fcn</strong></span> function is the
application-specified Btree compression function. The compression
function takes six parameters:
</p>
<div class="itemizedlist">
<ul type="disc">
<li>
<p>
<code class="literal">db</code>
</p>
<p>
The <span class="bold"><strong>db</strong></span> parameter is the enclosing
database handle.
</p>
</li>
<li>
<p>
<code class="literal">prevKey</code>
</p>
<p>
The <span class="bold"><strong>prevKey</strong></span>
parameter is the <a class="link" href="dbt.html" title="Chapter 4. The DBT Handle">DBT</a>
representing the key immediately preceding
the application supplied key.
</p>
</li>
<li>
<p>
<code class="literal">prevData</code>
</p>
<p>
The <span class="bold"><strong>prevData</strong></span>
parameter is the <a class="link" href="dbt.html" title="Chapter 4. The DBT Handle">DBT</a>
representing the data associated with
<span class="bold"><strong>prevKey</strong></span>.
</p>
</li>
<li>
<p>
<code class="literal">key</code>
</p>
<p>
The <span class="bold"><strong>key</strong></span>
parameter is the
<a class="link" href="dbt.html" title="Chapter 4. The DBT Handle">DBT</a>
representing the application supplied key.
</p>
</li>
<li>
<p>
<code class="literal">data</code>
</p>
<p>
The <span class="bold"><strong>data</strong></span> parameter is the
<a class="link" href="dbt.html" title="Chapter 4. The DBT Handle">DBT</a> representing the application
supplied data.
</p>
</li>
<li>
<p>
<code class="literal">dest</code>
</p>
<p>
The <span class="bold"><strong>dest</strong></span>
parameter is the
<a class="link" href="dbt.html" title="Chapter 4. The DBT Handle">DBT</a>
representing the data stored in the tree,
where the function should write the
compressed data.
</p>
</li>
</ul>
</div>
<p>
The <span class="bold"><strong>bt_compress_fcn</strong></span>
function must return 0 on success and a non-zero value
on failure. If the compressed data cannot fit in
<span class="bold"><strong>dest->data</strong></span>
(the size of which is
<span>
stored in
<span class="bold"><strong>dest->ulen</strong></span>),
</span>
the function should identify the required buffer size in
<span class="bold"><strong>dest->size</strong></span>
and return <code class="literal">DB_BUFFER_SMALL</code>.
</p>
</div>
<div class="sect3" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="id1643380"></a>bt_decompress_fcn</h4>
</div>
</div>
</div>
<p>
The <span class="bold"><strong>bt_decompress_fcn</strong></span> function is the
application-specified Btree decompression function. The decompression
function takes six parameters:
</p>
<div class="itemizedlist">
<ul type="disc">
<li>
<p>
<code class="literal">db</code>
</p>
<p>
The <span class="bold"><strong>db</strong></span> parameter is the enclosing
database handle.
</p>
</li>
<li>
<p>
<code class="literal">prevKey</code>
</p>
<p>
The <span class="bold"><strong>prevKey</strong></span>
parameter is the
<a class="link" href="dbt.html" title="Chapter 4. The DBT Handle">DBT</a>
representing the key immediately preceding
the key being decompressed.
</p>
</li>
<li>
<p>
<code class="literal">prevData</code>
</p>
<p>
The <span class="bold"><strong>prevData</strong></span>
parameter is the
<a class="link" href="dbt.html" title="Chapter 4. The DBT Handle">DBT</a>
representing the data associated with
<span class="bold"><strong>prevKey</strong></span>.
</p>
</li>
<li>
<p>
<code class="literal">compressed</code>
</p>
<p>
The
<span class="bold"><strong>compressed</strong></span>
parameter is the
<a class="link" href="dbt.html" title="Chapter 4. The DBT Handle">DBT</a>
representing the data stored in the tree,
that is, the compressed data.
</p>
</li>
<li>
<p>
<code class="literal">key</code>
</p>
<p>
The <span class="bold"><strong>key</strong></span>
parameter is the <a class="link" href="dbt.html" title="Chapter 4. The DBT Handle">DBT</a> where
the decompression function should store the
decompressed key.
</p>
</li>
<li>
<p>
<code class="literal">data</code>
</p>
<p>
The <span class="bold"><strong>data</strong></span>
parameter is the
<a class="link" href="dbt.html" title="Chapter 4. The DBT Handle">DBT</a> where
the decompression function should store the
decompressed key.
</p>
</li>
</ul>
</div>
<p>
The <span class="bold"><strong>bt_decompress_fcn</strong></span>
function must return 0 on success and a non-zero value
on failure.
If the decompressed data cannot fit in
<span class="bold"><strong>key->data</strong></span>
or
<span class="bold"><strong>data->data</strong></span>
(the size of which is
<span>
available in the <a class="link" href="dbt.html" title="Chapter 4. The DBT Handle">DBT</a>'s
<span class="bold"><strong>ulen</strong></span> field),
</span>
the function should identify the required buffer size
<span>
using the <a class="link" href="dbt.html" title="Chapter 4. The DBT Handle">DBT</a>'s
<span class="bold"><strong>size</strong></span>
field
</span>
and return <code class="literal">DB_BUFFER_SMALL</code>.
</p>
</div>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="id1643321"></a>Errors</h3>
</div>
</div>
</div>
<p>
The <code class="methodname">DB->set_bt_compress()</code> <span>
<span>
method may fail and return one of the following non-zero errors:
</span>
</span>
</p>
<div class="sect3" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="id1643552"></a>EINVAL</h4>
</div>
</div>
</div>
<p>
If the method was called after <a class="xref" href="dbopen.html" title="DB->open()">DB->open()</a>
was called; or if an invalid flag value or parameter was specified.
</p>
</div>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="id1643711"></a>Class</h3>
</div>
</div>
</div>
<p>
<a class="link" href="db.html" title="Chapter 2. The DB Handle">DB</a>
</p>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="id1643210"></a>See Also</h3>
</div>
</div>
</div>
<p>
<a class="xref" href="db.html#dblist" title="Database and Related Methods">Database and Related Methods</a>
</p>
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