1<?xml version="1.0" encoding="UTF-8" standalone="no"?> 2<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> 3<html xmlns="http://www.w3.org/1999/xhtml"> 4 <head> 5 <meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /> 6 <title>DB->set_bt_compare()</title> 7 <link rel="stylesheet" href="apiReference.css" type="text/css" /> 8 <meta name="generator" content="DocBook XSL Stylesheets V1.73.2" /> 9 <link rel="start" href="index.html" title="Berkeley DB C API Reference" /> 10 <link rel="up" href="db.html" title="Chapter��2.�� The DB Handle" /> 11 <link rel="prev" href="dbset_append_recno.html" title="DB->set_append_recno()" /> 12 <link rel="next" href="dbset_bt_compress.html" title="DB->set_bt_compress()" /> 13 </head> 14 <body> 15 <div class="navheader"> 16 <table width="100%" summary="Navigation header"> 17 <tr> 18 <th colspan="3" align="center">DB->set_bt_compare()</th> 19 </tr> 20 <tr> 21 <td width="20%" align="left"><a accesskey="p" href="dbset_append_recno.html">Prev</a>��</td> 22 <th width="60%" align="center">Chapter��2.�� 23 The DB Handle 24 </th> 25 <td width="20%" align="right">��<a accesskey="n" href="dbset_bt_compress.html">Next</a></td> 26 </tr> 27 </table> 28 <hr /> 29 </div> 30 <div class="sect1" lang="en" xml:lang="en"> 31 <div class="titlepage"> 32 <div> 33 <div> 34 <h2 class="title" style="clear: both"><a id="dbset_bt_compare"></a>DB->set_bt_compare()</h2> 35 </div> 36 </div> 37 </div> 38 <pre class="programlisting">#include <db.h> 39 40int 41DB->set_bt_compare(DB *db, 42 int (*bt_compare_fcn)(DB *db, const DBT *dbt1, const DBT *dbt2)); </pre> 43 <p> 44 Set the Btree key comparison function. The comparison function is 45 called whenever it is necessary to compare a key specified by the 46 application with a key currently stored in the tree. 47 </p> 48 <p> 49 If no comparison function is specified, the keys are compared 50 lexically, with shorter keys collating before longer keys. 51 </p> 52 <p> 53 The <code class="methodname">DB->set_bt_compare()</code> method configures operations performed using 54 the specified <a class="link" href="db.html" title="Chapter��2.�� The DB Handle">DB</a> handle, 55 not all operations performed on the underlying database. 56 </p> 57 <p> 58 The <code class="methodname">DB->set_bt_compare()</code> method may not be called after the 59 <a class="xref" href="dbopen.html" title="DB->open()">DB->open()</a> method is called. If 60 the database already exists when <a class="xref" href="dbopen.html" title="DB->open()">DB->open()</a> 61 is called, the information specified to <code class="methodname">DB->set_bt_compare()</code> must be the same as 62 that historically used to create the database or corruption can occur. 63 </p> 64 <p> 65 The <code class="methodname">DB->set_bt_compare()</code> <span> 66 <span> 67 method returns a non-zero error value on failure and 0 on success. 68 </span> 69 70 </span> 71 </p> 72 <div class="sect2" lang="en" xml:lang="en"> 73 <div class="titlepage"> 74 <div> 75 <div> 76 <h3 class="title"><a id="id1642390"></a>Parameters</h3> 77 </div> 78 </div> 79 </div> 80 <div class="sect3" lang="en" xml:lang="en"> 81 <div class="titlepage"> 82 <div> 83 <div> 84 <h4 class="title"><a id="id1642898"></a>bt_compare_fcn</h4> 85 </div> 86 </div> 87 </div> 88 <p> 89 The <span class="bold"><strong>bt_compare_fcn</strong></span> function is the 90 application-specified Btree comparison function. The comparison 91 function takes three parameters: 92 </p> 93 <div class="itemizedlist"> 94 <ul type="disc"> 95 <li> 96 <p> 97 <code class="literal">db</code> 98 </p> 99 <p> 100 The <span class="bold"><strong>db</strong></span> parameter is the enclosing 101 database handle. 102 </p> 103 </li> 104 <li> 105 <p> 106 <code class="literal">dbt1</code> 107 </p> 108 <p> 109 The <span class="bold"><strong>dbt1</strong></span> parameter is the 110 <a class="link" href="dbt.html" title="Chapter��4.�� The DBT Handle">DBT</a> representing the application 111 supplied key. 112 </p> 113 </li> 114 <li> 115 <p> 116 <code class="literal">dbt2</code> 117 </p> 118 <p> 119 The <span class="bold"><strong>dbt2</strong></span> parameter is the 120 <a class="link" href="dbt.html" title="Chapter��4.�� The DBT Handle">DBT</a> representing the current 121 tree's key. 122 </p> 123 </li> 124 </ul> 125 </div> 126 <p> 127 The <span class="bold"><strong>bt_compare_fcn</strong></span> function must return an 128 integer value less than, equal to, or greater than zero if the first key 129 parameter is considered to be respectively less than, equal to, or greater than 130 the second key parameter. In addition, the comparison function must cause the 131 keys in the database to be <span class="emphasis"><em>well-ordered</em></span>. The comparison 132 function must correctly handle any key values used by the application (possibly 133 including zero-length keys). In addition, when Btree key prefix comparison is 134 being performed (see 135 <a class="xref" href="dbset_bt_prefix.html" title="DB->set_bt_prefix()">DB->set_bt_prefix()</a> 136 for more information), the comparison routine may be passed a prefix of any database 137 key. The <span class="bold"><strong>data</strong></span> and <span class="bold"><strong>size</strong></span> 138 fields of the <a class="link" href="dbt.html" title="Chapter��4.�� The DBT Handle">DBT</a> are the only fields 139 that may be used for the purposes of this comparison, and no particular 140 alignment of the memory to which by the <span class="bold"><strong>data</strong></span> 141 field refers may be assumed. 142 </p> 143 </div> 144 </div> 145 <div class="sect2" lang="en" xml:lang="en"> 146 <div class="titlepage"> 147 <div> 148 <div> 149 <h3 class="title"><a id="id1643163"></a>Errors</h3> 150 </div> 151 </div> 152 </div> 153 <p> 154 The <code class="methodname">DB->set_bt_compare()</code> <span> 155 <span> 156 method may fail and return one of the following non-zero errors: 157 </span> 158 159 </span> 160 </p> 161 <div class="sect3" lang="en" xml:lang="en"> 162 <div class="titlepage"> 163 <div> 164 <div> 165 <h4 class="title"><a id="id1643231"></a>EINVAL</h4> 166 </div> 167 </div> 168 </div> 169 <p> 170 If the method was called after <a class="xref" href="dbopen.html" title="DB->open()">DB->open()</a> 171 was called; or if an invalid flag value or parameter was specified. 172 </p> 173 </div> 174 </div> 175 <div class="sect2" lang="en" xml:lang="en"> 176 <div class="titlepage"> 177 <div> 178 <div> 179 <h3 class="title"><a id="id1643232"></a>Class</h3> 180 </div> 181 </div> 182 </div> 183 <p> 184 <a class="link" href="db.html" title="Chapter��2.�� The DB Handle">DB</a> 185 </p> 186 </div> 187 <div class="sect2" lang="en" xml:lang="en"> 188 <div class="titlepage"> 189 <div> 190 <div> 191 <h3 class="title"><a id="id1643006"></a>See Also</h3> 192 </div> 193 </div> 194 </div> 195 <p> 196 <a class="xref" href="db.html#dblist" title="Database and Related Methods">Database and Related Methods</a> 197 </p> 198 </div> 199 </div> 200 <div class="navfooter"> 201 <hr /> 202 <table width="100%" summary="Navigation footer"> 203 <tr> 204 <td width="40%" align="left"><a accesskey="p" href="dbset_append_recno.html">Prev</a>��</td> 205 <td width="20%" align="center"> 206 <a accesskey="u" href="db.html">Up</a> 207 </td> 208 <td width="40%" align="right">��<a accesskey="n" href="dbset_bt_compress.html">Next</a></td> 209 </tr> 210 <tr> 211 <td width="40%" align="left" valign="top">DB->set_append_recno()��</td> 212 <td width="20%" align="center"> 213 <a accesskey="h" href="index.html">Home</a> 214 </td> 215 <td width="40%" align="right" valign="top">��DB->set_bt_compress()</td> 216 </tr> 217 </table> 218 </div> 219 </body> 220</html> 221