<?xml version="1.0" encoding="UTF-8" standalone="no"?> <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> <html xmlns="http://www.w3.org/1999/xhtml"> <head> <meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /> <title>Db::set_bt_compare()</title> <link rel="stylesheet" href="apiReference.css" type="text/css" /> <meta name="generator" content="DocBook XSL Stylesheets V1.73.2" /> <link rel="start" href="index.html" title="Berkeley DB C++ API Reference" /> <link rel="up" href="db.html" title="Chapter 2. The Db Handle" /> <link rel="prev" href="dbset_append_recno.html" title="Db::set_append_recno()" /> <link rel="next" href="dbset_bt_compress.html" title="Db::set_bt_compress()" /> </head> <body> <div xmlns="" class="navheader"> <div class="libver"> <p>Library Version 11.2.5.3</p> </div> <table width="100%" summary="Navigation header"> <tr> <th colspan="3" align="center">Db::set_bt_compare()</th> </tr> <tr> <td width="20%" align="left"><a accesskey="p" href="dbset_append_recno.html">Prev</a> </td> <th width="60%" align="center">Chapter 2. The Db Handle </th> <td width="20%" align="right"> <a accesskey="n" href="dbset_bt_compress.html">Next</a></td> </tr> </table> <hr /> </div> <div class="sect1" lang="en" xml:lang="en"> <div class="titlepage"> <div> <div> <h2 class="title" style="clear: both"><a id="dbset_bt_compare"></a>Db::set_bt_compare()</h2> </div> </div> </div> <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> </div> </div> <div class="navfooter"> <hr /> <table width="100%" summary="Navigation footer"> <tr> <td width="40%" align="left"><a accesskey="p" href="dbset_append_recno.html">Prev</a> </td> <td width="20%" align="center"> <a accesskey="u" href="db.html">Up</a> </td> <td width="40%" align="right"> <a accesskey="n" href="dbset_bt_compress.html">Next</a></td> </tr> <tr> <td width="40%" align="left" valign="top">Db::set_append_recno() </td> <td width="20%" align="center"> <a accesskey="h" href="index.html">Home</a> </td> <td width="40%" align="right" valign="top"> Db::set_bt_compress()</td> </tr> </table> </div> </body> </html>