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_h_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_flags.html" title="DB->set_flags()" /> 12 <link rel="next" href="dbset_h_ffactor.html" title="DB->set_h_ffactor()" /> 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_h_compare()</th> 19 </tr> 20 <tr> 21 <td width="20%" align="left"><a accesskey="p" href="dbset_flags.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_h_ffactor.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_h_compare"></a>DB->set_h_compare()</h2> 35 </div> 36 </div> 37 </div> 38 <pre class="programlisting">#include <db.h> 39 40int 41DB->set_h_compare(DB *db, 42 int (*compare_fcn)(DB *db, const DBT *dbt1, const DBT *dbt2)); </pre> 43 <p> 44 Set the Hash 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 database. 47 </p> 48 <p> 49 If no comparison function is specified, a byte-by-byte comparison is 50 performed. 51 </p> 52 <p> 53 The <code class="methodname">DB->set_h_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_h_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_h_compare()</code> 62 must be the same as that historically used to create the database or corruption can occur. 63 </p> 64 <p> 65 The <code class="methodname">DB->set_h_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="id1646156"></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="id1647450"></a>compare_fcn</h4> 85 </div> 86 </div> 87 </div> 88 <p> 89 The <span class="bold"><strong>compare_fcn</strong></span> function is the 90 application-specified Hash 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 <a class="link" href="dbt.html" title="Chapter��4.�� The DBT Handle">DBT</a> 110 representing the application supplied key. 111 </p> 112 </li> 113 <li> 114 <p> 115 <code class="literal">dbt2</code> 116 </p> 117 <p> 118 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> 119 representing the current database's key. 120 </p> 121 </li> 122 </ul> 123 </div> 124 <p> 125 The <span class="bold"><strong>compare_fcn</strong></span> function must return an integer 126 value less than, equal to, or greater than zero if the first key parameter is 127 considered to be respectively less than, equal to, or greater than the second 128 key parameter. The comparison function must correctly handle any key values used 129 by the application (possibly including zero-length keys). The 130 <span class="bold"><strong>data</strong></span> and <span class="bold"><strong>size</strong></span> 131 fields of the <a class="link" href="dbt.html" title="Chapter��4.�� The DBT Handle">DBT</a> are the only fields 132 that may be used for the purposes of this comparison, and no particular 133 alignment of the memory to which by the <span class="bold"><strong>data</strong></span> 134 field refers may be assumed. 135 </p> 136 </div> 137 </div> 138 <div class="sect2" lang="en" xml:lang="en"> 139 <div class="titlepage"> 140 <div> 141 <div> 142 <h3 class="title"><a id="id1647476"></a>Errors</h3> 143 </div> 144 </div> 145 </div> 146 <p> 147 The <code class="methodname">DB->set_h_compare()</code> <span> 148 <span> 149 method may fail and return one of the following non-zero errors: 150 </span> 151 152 </span> 153 </p> 154 <div class="sect3" lang="en" xml:lang="en"> 155 <div class="titlepage"> 156 <div> 157 <div> 158 <h4 class="title"><a id="id1647428"></a>EINVAL</h4> 159 </div> 160 </div> 161 </div> 162 <p> 163 If the method was called after <a class="xref" href="dbopen.html" title="DB->open()">DB->open()</a> 164 was called; or if an invalid flag value or parameter was specified. 165 </p> 166 </div> 167 </div> 168 <div class="sect2" lang="en" xml:lang="en"> 169 <div class="titlepage"> 170 <div> 171 <div> 172 <h3 class="title"><a id="id1647449"></a>Class</h3> 173 </div> 174 </div> 175 </div> 176 <p> 177 <a class="link" href="db.html" title="Chapter��2.�� The DB Handle">DB</a> 178 </p> 179 </div> 180 <div class="sect2" lang="en" xml:lang="en"> 181 <div class="titlepage"> 182 <div> 183 <div> 184 <h3 class="title"><a id="id1647550"></a>See Also</h3> 185 </div> 186 </div> 187 </div> 188 <p> 189 <a class="xref" href="db.html#dblist" title="Database and Related Methods">Database and Related Methods</a> 190 </p> 191 </div> 192 </div> 193 <div class="navfooter"> 194 <hr /> 195 <table width="100%" summary="Navigation footer"> 196 <tr> 197 <td width="40%" align="left"><a accesskey="p" href="dbset_flags.html">Prev</a>��</td> 198 <td width="20%" align="center"> 199 <a accesskey="u" href="db.html">Up</a> 200 </td> 201 <td width="40%" align="right">��<a accesskey="n" href="dbset_h_ffactor.html">Next</a></td> 202 </tr> 203 <tr> 204 <td width="40%" align="left" valign="top">DB->set_flags()��</td> 205 <td width="20%" align="center"> 206 <a accesskey="h" href="index.html">Home</a> 207 </td> 208 <td width="40%" align="right" valign="top">��DB->set_h_ffactor()</td> 209 </tr> 210 </table> 211 </div> 212 </body> 213</html> 214