1<!--$Id: db_set_bt_compare.so,v 10.50 2006/10/26 20:59:24 bostic Exp $--> 2<!--Copyright (c) 1997,2008 Oracle. All rights reserved.--> 3<!--See the file LICENSE for redistribution information.--> 4<html> 5<head> 6<title>Berkeley DB: Db::set_bt_compare</title> 7<meta name="description" content="Berkeley DB: An embedded database programmatic toolkit."> 8<meta name="keywords" content="embedded,database,programmatic,toolkit,btree,hash,hashing,transaction,transactions,locking,logging,access method,access methods,Java,C,C++"> 9</head> 10<body bgcolor=white> 11<table width="100%"><tr valign=top> 12<td> 13<b>Db::set_bt_compare</b> 14</td> 15<td align=right> 16<a href="../api_cxx/api_core.html"><img src="../images/api.gif" alt="API"></a> 17<a href="../ref/toc.html"><img src="../images/ref.gif" alt="Ref"></a></td> 18</tr></table> 19<hr size=1 noshade> 20<tt> 21<b><pre> 22#include <db_cxx.h> 23<p> 24extern "C" { 25 typedef int (*bt_compare_fcn_type)(DB *db, const DBT *dbt1, const DBT *dbt2); 26}; 27int 28Db::set_bt_compare(bt_compare_fcn_type bt_compare_fcn); 29</pre></b> 30<hr size=1 noshade> 31<b>Description: Db::set_bt_compare</b> 32<p>Set the Btree key comparison function. The comparison function is 33called whenever it is necessary to compare a key specified by the 34application with a key currently stored in the tree.</p> 35<p>If no comparison function is specified, the keys are compared lexically, 36with shorter keys collating before longer keys.</p> 37<p>The Db::set_bt_compare method configures operations performed using the specified 38<a href="../api_cxx/db_class.html">Db</a> handle, not all operations performed on the underlying 39database.</p> 40<p>The Db::set_bt_compare method may not be called after the <a href="../api_cxx/db_open.html">Db::open</a> method is called. 41If the database already exists when 42<a href="../api_cxx/db_open.html">Db::open</a> is called, the information specified to Db::set_bt_compare must 43be the same as that historically used to create the database or 44corruption can occur.</p> 45<p>The Db::set_bt_compare method 46either returns a non-zero error value 47or throws an exception that encapsulates a non-zero error value on 48failure, and returns 0 on success. 49</p> 50<b>Parameters</b> <br> 51 <b>bt_compare_fcn</b><ul compact><li>The <b>bt_compare_fcn</b> function is the application-specified Btree 52comparison function. The comparison function takes three parameters: 53<br> 54<b><b>db</b></b><ul compact><li>The <b>db</b> parameter is the enclosing database handle.</ul> 55<b><b>dbt1</b></b><ul compact><li>The <b>dbt1</b> parameter is the <a href="../api_cxx/dbt_class.html">Dbt</a> representing the 56application supplied key.</ul> 57<b><b>dbt2</b></b><ul compact><li>The <b>dbt2</b> parameter is the <a href="../api_cxx/dbt_class.html">Dbt</a> representing the 58current tree's key.</ul> 59<br></ul> 60<p>The <b>bt_compare_fcn</b> function must return an integer value less 61than, equal to, or greater than zero if the first key parameter is 62considered to be respectively less than, equal to, or greater than the 63second key parameter. In addition, the comparison function must cause 64the keys in the database to be <i>well-ordered</i>. The comparison 65function must correctly handle any key values used by the application 66(possibly including zero-length keys). In addition, when Btree key 67prefix comparison is being performed (see <a href="../api_cxx/db_set_bt_prefix.html">Db::set_bt_prefix</a> for 68more information), the comparison routine may be passed a prefix of any 69database key. The <b>data</b> and <b>size</b> fields of the 70<a href="../api_cxx/dbt_class.html">Dbt</a> are the only fields that may be used for the purposes of 71this comparison, and no particular alignment of the memory to which by 72the <b>data</b> field refers may be assumed.</p> 73<br> 74<br><b>Errors</b> 75<p>The Db::set_bt_compare method 76may fail and throw 77<a href="../api_cxx/except_class.html">DbException</a>, 78encapsulating one of the following non-zero errors, or return one of 79the following non-zero errors:</p> 80<br> 81<b>EINVAL</b><ul compact><li>If the method was called after <a href="../api_cxx/db_open.html">Db::open</a> was called; or if an 82invalid flag value or parameter was specified.</ul> 83<br> 84<hr size=1 noshade> 85<br><b>Class</b> 86<a href="../api_cxx/db_class.html">Db</a> 87<br><b>See Also</b> 88<a href="../api_cxx/db_list.html">Databases and Related Methods</a> 89</tt> 90<table width="100%"><tr><td><br></td><td align=right> 91<a href="../api_cxx/api_core.html"><img src="../images/api.gif" alt="API"></a><a href="../ref/toc.html"><img src="../images/ref.gif" alt="Ref"></a> 92</td></tr></table> 93<p><font size=1>Copyright (c) 1996,2008 Oracle. All rights reserved.</font> 94</body> 95</html> 96
