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<h2 class="title" style="clear: both"><a id="dbset_bt_compare"></a>Db::set_bt_compare()</h2>
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<pre class="programlisting">#include <db_cxx.h>
extern "C" {
typedef int (*bt_compare_fcn_type)(DB *db, const DBT *dbt1,
const DBT *dbt2);
};
int
Db::set_bt_compare(bt_compare_fcn_type bt_compare_fcn);</pre>
<p>
Set the Btree key comparison function. The comparison function is
called whenever it is necessary to compare a key specified by the
application with a key currently stored in the tree.
</p>
<p>
If no comparison function is specified, the keys are compared
lexically, with shorter keys collating before longer keys.
</p>
<p>
The <code class="methodname">Db::set_bt_compare()</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_compare()</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_compare()</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_compare()</code> <span>
<span>
method either returns a non-zero error value or throws an
exception that encapsulates a non-zero error value on
failure, and returns 0 on success.
</span>
</span>
</p>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="idp3192720"></a>Parameters</h3>
</div>
</div>
</div>
<div class="sect3" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="idp3675224"></a>bt_compare_fcn</h4>
</div>
</div>
</div>
<p>
The <span class="bold"><strong>bt_compare_fcn</strong></span> function is the
application-specified Btree comparison function. The comparison
function takes three 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">dbt1</code>
</p>
<p>
The <span class="bold"><strong>dbt1</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">dbt2</code>
</p>
<p>
The <span class="bold"><strong>dbt2</strong></span> parameter is the
<a class="link" href="dbt.html" title="Chapter 4. The Dbt Handle">Dbt</a> representing the current
tree's key.
</p>
</li>
</ul>
</div>
<p>
The <span class="bold"><strong>bt_compare_fcn</strong></span> function must return an
integer value less than, equal to, or greater than zero if the first key
parameter is considered to be respectively less than, equal to, or greater than
the second key parameter. In addition, the comparison function must cause the
keys in the database to be <span class="emphasis"><em>well-ordered</em></span>. The comparison
function must correctly handle any key values used by the application (possibly
including zero-length keys). In addition, when Btree key prefix comparison is
being performed (see
<a class="xref" href="dbset_bt_prefix.html" title="Db::set_bt_prefix()">Db::set_bt_prefix()</a>
for more information), the comparison routine may be passed a prefix of any database
key. The <span class="bold"><strong>data</strong></span> and <span class="bold"><strong>size</strong></span>
fields of the <a class="link" href="dbt.html" title="Chapter 4. The Dbt Handle">Dbt</a> are the only fields
that may be used for the purposes of this comparison, and no particular
alignment of the memory to which by the <span class="bold"><strong>data</strong></span>
field refers may be assumed.
</p>
</div>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="idp1532656"></a>Errors</h3>
</div>
</div>
</div>
<p>
The <code class="methodname">Db::set_bt_compare()</code> <span>
<span>
method may fail and throw a <a class="link" href="dbexception.html" title="Chapter 6. The DbException Class">DbException</a>
exception, encapsulating one of the following non-zero errors, or 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="idp2300360"></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="idp2539376"></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="idp2699552"></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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