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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_cxx.h> 39 40extern "C" { 41 typedef int (*bt_compare_fcn_type)(DB *db, const DBT *dbt1, const DBT *dbt2); 42}; 43int 44Db::set_bt_compare(bt_compare_fcn_type bt_compare_fcn);</pre> 45 <p> 46 Set the Btree key comparison function. The comparison function is 47 called whenever it is necessary to compare a key specified by the 48 application with a key currently stored in the tree. 49 </p> 50 <p> 51 If no comparison function is specified, the keys are compared 52 lexically, with shorter keys collating before longer keys. 53 </p> 54 <p> 55 The <code class="methodname">Db::set_bt_compare()</code> method configures operations performed using 56 the specified <a class="link" href="db.html" title="Chapter 2. The Db Handle">Db</a> handle, 57 not all operations performed on the underlying database. 58 </p> 59 <p> 60 The <code class="methodname">Db::set_bt_compare()</code> method may not be called after the 61 <a class="xref" href="dbopen.html" title="Db::open()">Db::open()</a> method is called. If 62 the database already exists when <a class="xref" href="dbopen.html" title="Db::open()">Db::open()</a> 63 is called, the information specified to <code class="methodname">Db::set_bt_compare()</code> must be the same as 64 that historically used to create the database or corruption can occur. 65 </p> 66 <p> 67 The <code class="methodname">Db::set_bt_compare()</code> <span> 68 69 <span> 70 method either returns a non-zero error value or throws an 71 exception that encapsulates a non-zero error value on 72 failure, and returns 0 on success. 73 </span> 74 </span> 75 </p> 76 <div class="sect2" lang="en" xml:lang="en"> 77 <div class="titlepage"> 78 <div> 79 <div> 80 <h3 class="title"><a id="id1643187"></a>Parameters</h3> 81 </div> 82 </div> 83 </div> 84 <div class="sect3" lang="en" xml:lang="en"> 85 <div class="titlepage"> 86 <div> 87 <div> 88 <h4 class="title"><a id="id1643184"></a>bt_compare_fcn</h4> 89 </div> 90 </div> 91 </div> 92 <p> 93 The <span class="bold"><strong>bt_compare_fcn</strong></span> function is the 94 application-specified Btree comparison function. The comparison 95 function takes three parameters: 96 </p> 97 <div class="itemizedlist"> 98 <ul type="disc"> 99 <li> 100 <p> 101 <code class="literal">db</code> 102 </p> 103 <p> 104 The <span class="bold"><strong>db</strong></span> parameter is the enclosing 105 database handle. 106 </p> 107 </li> 108 <li> 109 <p> 110 <code class="literal">dbt1</code> 111 </p> 112 <p> 113 The <span class="bold"><strong>dbt1</strong></span> parameter is the 114 <a class="link" href="dbt.html" title="Chapter 4. The Dbt Handle">Dbt</a> representing the application 115 supplied key. 116 </p> 117 </li> 118 <li> 119 <p> 120 <code class="literal">dbt2</code> 121 </p> 122 <p> 123 The <span class="bold"><strong>dbt2</strong></span> parameter is the 124 <a class="link" href="dbt.html" title="Chapter 4. The Dbt Handle">Dbt</a> representing the current 125 tree's key. 126 </p> 127 </li> 128 </ul> 129 </div> 130 <p> 131 The <span class="bold"><strong>bt_compare_fcn</strong></span> function must return an 132 integer value less than, equal to, or greater than zero if the first key 133 parameter is considered to be respectively less than, equal to, or greater than 134 the second key parameter. In addition, the comparison function must cause the 135 keys in the database to be <span class="emphasis"><em>well-ordered</em></span>. The comparison 136 function must correctly handle any key values used by the application (possibly 137 including zero-length keys). In addition, when Btree key prefix comparison is 138 being performed (see 139 <a class="xref" href="dbset_bt_prefix.html" title="Db::set_bt_prefix()">Db::set_bt_prefix()</a> 140 for more information), the comparison routine may be passed a prefix of any database 141 key. The <span class="bold"><strong>data</strong></span> and <span class="bold"><strong>size</strong></span> 142 fields of the <a class="link" href="dbt.html" title="Chapter 4. The Dbt Handle">Dbt</a> are the only fields 143 that may be used for the purposes of this comparison, and no particular 144 alignment of the memory to which by the <span class="bold"><strong>data</strong></span> 145 field refers may be assumed. 146 </p> 147 </div> 148 </div> 149 <div class="sect2" lang="en" xml:lang="en"> 150 <div class="titlepage"> 151 <div> 152 <div> 153 <h3 class="title"><a id="id1643490"></a>Errors</h3> 154 </div> 155 </div> 156 </div> 157 <p> 158 The <code class="methodname">Db::set_bt_compare()</code> <span> 159 160 <span> 161 method may fail and throw a <a class="link" href="dbexception.html" title="Chapter 6. The DbException Class">DbException</a> 162 exception, encapsulating one of the following non-zero errors, or return one 163 of the following non-zero errors: 164 </span> 165 </span> 166 </p> 167 <div class="sect3" lang="en" xml:lang="en"> 168 <div class="titlepage"> 169 <div> 170 <div> 171 <h4 class="title"><a id="id1643517"></a>EINVAL</h4> 172 </div> 173 </div> 174 </div> 175 <p> 176 If the method was called after <a class="xref" href="dbopen.html" title="Db::open()">Db::open()</a> 177 was called; or if an invalid flag value or parameter was specified. 178 </p> 179 </div> 180 </div> 181 <div class="sect2" lang="en" xml:lang="en"> 182 <div class="titlepage"> 183 <div> 184 <div> 185 <h3 class="title"><a id="id1643518"></a>Class</h3> 186 </div> 187 </div> 188 </div> 189 <p> 190 <a class="link" href="db.html" title="Chapter 2. The Db Handle">Db</a> 191 </p> 192 </div> 193 <div class="sect2" lang="en" xml:lang="en"> 194 <div class="titlepage"> 195 <div> 196 <div> 197 <h3 class="title"><a id="id1643182"></a>See Also</h3> 198 </div> 199 </div> 200 </div> 201 <p> 202 <a class="xref" href="db.html#dblist" title="Database and Related Methods">Database and Related Methods</a> 203 </p> 204 </div> 205 </div> 206 <div class="navfooter"> 207 <hr /> 208 <table width="100%" summary="Navigation footer"> 209 <tr> 210 <td width="40%" align="left"><a accesskey="p" href="dbset_append_recno.html">Prev</a> </td> 211 <td width="20%" align="center"> 212 <a accesskey="u" href="db.html">Up</a> 213 </td> 214 <td width="40%" align="right"> <a accesskey="n" href="dbset_bt_compress.html">Next</a></td> 215 </tr> 216 <tr> 217 <td width="40%" align="left" valign="top">Db::set_append_recno() </td> 218 <td width="20%" align="center"> 219 <a accesskey="h" href="index.html">Home</a> 220 </td> 221 <td width="40%" align="right" valign="top"> Db::set_bt_compress()</td> 222 </tr> 223 </table> 224 </div> 225 </body> 226</html> 227