docs/api_reference/C/dbset_bt_compare.html

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  21           <th colspan="3" align="center">DB-&gt;set_bt_compare()</th>
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  23         <tr>
  24           <td width="20%" align="left"><a accesskey="p" href="dbset_append_recno.html">Prev</a> </td>
  25           <th width="60%" align="center">Chapter 2.
  26                 The DB Handle
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  33     <div class="sect1" lang="en" xml:lang="en">
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  35         <div>
  36           <div>
  37             <h2 class="title" style="clear: both"><a id="dbset_bt_compare"></a>DB-&gt;set_bt_compare()</h2>
  38           </div>
  39         </div>
  40       </div>
  41       <pre class="programlisting">#include &lt;db.h&gt;
  42
  43 int
  44 DB-&gt;set_bt_compare(DB *db,
  45     int (*bt_compare_fcn)(DB *db, const DBT *dbt1, const DBT *dbt2));  </pre>
  46       <p>
  47          Set the Btree key comparison function.  The comparison function is
  48          called whenever it is necessary to compare a key specified by the
  49          application with a key currently stored in the tree.
  50     </p>
  51       <p>
  52          If no comparison function is specified, the keys are compared
  53          lexically, with shorter keys collating before longer keys.
  54     </p>
  55       <p>
  56          The <code class="methodname">DB-&gt;set_bt_compare()</code> method configures operations performed using
  57          the specified <a class="link" href="db.html" title="Chapter 2.  The DB Handle">DB</a>  handle,
  58          not all operations performed on the underlying database.
  59     </p>
  60       <p>
  61          The <code class="methodname">DB-&gt;set_bt_compare()</code> method may not be called after the
  62          <a class="xref" href="dbopen.html" title="DB-&gt;open()">DB-&gt;open()</a>  method is called. If
  63          the database already exists when <a class="xref" href="dbopen.html" title="DB-&gt;open()">DB-&gt;open()</a>
  64          is called, the information specified to <code class="methodname">DB-&gt;set_bt_compare()</code> must be the same as
  65          that historically used to create the database or corruption can occur.
  66     </p>
  67       <p>
  68          The <code class="methodname">DB-&gt;set_bt_compare()</code> <span>
  69             <span>
  70                   method returns a non-zero error value on failure and 0 on success.
  71             </span>
  72
  73         </span>
  74     </p>
  75       <div class="sect2" lang="en" xml:lang="en">
  76         <div class="titlepage">
  77           <div>
  78             <div>
  79               <h3 class="title"><a id="idp58023488"></a>Parameters</h3>
  80             </div>
  81           </div>
  82         </div>
  83         <div class="sect3" lang="en" xml:lang="en">
  84           <div class="titlepage">
  85             <div>
  86               <div>
  87                 <h4 class="title"><a id="idp58064680"></a>bt_compare_fcn</h4>
  88               </div>
  89             </div>
  90           </div>
  91           <p>
  92                           The <span class="bold"><strong>bt_compare_fcn</strong></span> function is the
  93                           application-specified Btree comparison function.  The comparison
  94                           function takes three parameters:
  95                      </p>
  96           <div class="itemizedlist">
  97             <ul type="disc">
  98               <li>
  99                 <p>
 100                                 <code class="literal">db</code>
 101                             </p>
 102                 <p>
 103                                  The <span class="bold"><strong>db</strong></span> parameter is the enclosing
 104                                  database handle.
 105                             </p>
 106               </li>
 107               <li>
 108                 <p>
 109                                 <code class="literal">dbt1</code>
 110                             </p>
 111                 <p>
 112                                  The <span class="bold"><strong>dbt1</strong></span> parameter is the
 113                                  <a class="link" href="dbt.html" title="Chapter 4.  The DBT Handle">DBT</a>  representing the application
 114                                  supplied key.
 115                             </p>
 116               </li>
 117               <li>
 118                 <p>
 119                                 <code class="literal">dbt2</code>
 120                                 </p>
 121                 <p>
 122                                  The <span class="bold"><strong>dbt2</strong></span> parameter is the
 123                                  <a class="link" href="dbt.html" title="Chapter 4.  The DBT Handle">DBT</a>  representing the current
 124                                  tree's key.
 125                             </p>
 126               </li>
 127             </ul>
 128           </div>
 129           <p>
 130                     The <span class="bold"><strong>bt_compare_fcn</strong></span> function must return an
 131                     integer value less than, equal to, or greater than zero if the first key
 132                     parameter is considered to be respectively less than, equal to, or greater than
 133                     the second key parameter. In addition, the comparison function must cause the
 134                     keys in the database to be <span class="emphasis"><em>well-ordered</em></span>. The comparison
 135                     function must correctly handle any key values used by the application (possibly
 136                     including zero-length keys). In addition, when Btree key prefix comparison is
 137                     being performed (see
 138                     <a class="xref" href="dbset_bt_prefix.html" title="DB-&gt;set_bt_prefix()">DB-&gt;set_bt_prefix()</a>
 139                     for more information), the comparison routine may be passed a prefix of any database
 140                     key. The <span class="bold"><strong>data</strong></span> and <span class="bold"><strong>size</strong></span>
 141                     fields of the <a class="link" href="dbt.html" title="Chapter 4.  The DBT Handle">DBT</a> are the only fields
 142                     that may be used for the purposes of this comparison, and no particular
 143                     alignment of the memory to which by the <span class="bold"><strong>data</strong></span>
 144                     field refers may be assumed.
 145             </p>
 146         </div>
 147       </div>
 148       <div class="sect2" lang="en" xml:lang="en">
 149         <div class="titlepage">
 150           <div>
 151             <div>
 152               <h3 class="title"><a id="idp58043144"></a>Errors</h3>
 153             </div>
 154           </div>
 155         </div>
 156         <p>
 157                          The <code class="methodname">DB-&gt;set_bt_compare()</code> <span>
 158             <span>
 159                  method may fail and return one of the following non-zero errors:
 160             </span>
 161
 162         </span>
 163                     </p>
 164         <div class="sect3" lang="en" xml:lang="en">
 165           <div class="titlepage">
 166             <div>
 167               <div>
 168                 <h4 class="title"><a id="idp58084576"></a>EINVAL</h4>
 169               </div>
 170             </div>
 171           </div>
 172           <p>
 173                 If the method was called after <a class="xref" href="dbopen.html" title="DB-&gt;open()">DB-&gt;open()</a>
 174                 was called; or if an invalid flag value or parameter was specified.
 175             </p>
 176         </div>
 177       </div>
 178       <div class="sect2" lang="en" xml:lang="en">
 179         <div class="titlepage">
 180           <div>
 181             <div>
 182               <h3 class="title"><a id="idp58085656"></a>Class</h3>
 183             </div>
 184           </div>
 185         </div>
 186         <p>
 187                 <a class="link" href="db.html" title="Chapter 2.  The DB Handle">DB</a>
 188             </p>
 189       </div>
 190       <div class="sect2" lang="en" xml:lang="en">
 191         <div class="titlepage">
 192           <div>
 193             <div>
 194               <h3 class="title"><a id="idp58081424"></a>See Also</h3>
 195             </div>
 196           </div>
 197         </div>
 198         <p>
 199                      <a class="xref" href="db.html#dblist" title="Database and Related Methods">Database and Related Methods</a>
 200                 </p>
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