1<!--$Id: db_set_h_compare.so,v 1.1 2006/10/27 00:28:44 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_h_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_h_compare</b> 14</td> 15<td align=right> 16<a href="/api_c/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.h> 23<p> 24int 25DB->set_h_compare(DB *db, 26 int (*compare_fcn)(DB *db, const DBT *dbt1, const DBT *dbt2)); 27</pre></b> 28<hr size=1 noshade> 29<b>Description: DB->set_h_compare</b> 30<p>Set the Hash key comparison function. The comparison function is called 31whenever it is necessary to compare a key specified by the application 32with a key currently stored in the database.</p> 33<p>If no comparison function is specified, a byte-by-byte comparison is 34performed.</p> 35<p>The DB->set_h_compare method configures operations performed using the specified 36<a href="/api_c/db_class.html">DB</a> handle, not all operations performed on the underlying 37database.</p> 38<p>The DB->set_h_compare method may not be called after the <a href="/api_c/db_open.html">DB->open</a> method is called. 39If the database already exists when 40<a href="/api_c/db_open.html">DB->open</a> is called, the information specified to DB->set_h_compare must 41be the same as that historically used to create the database or 42corruption can occur.</p> 43<p>The DB->set_h_compare method 44returns a non-zero error value on failure 45and 0 on success. 46</p> 47<b>Parameters</b> <br> 48 <b>compare_fcn</b><ul compact><li>The <b>compare_fcn</b> function is the application-specified Hash 49comparison function. The comparison function takes three parameters: 50<br> 51<b><b>db</b></b><ul compact><li>The <b>db</b> parameter is the enclosing database handle.</ul> 52<b><b>dbt1</b></b><ul compact><li>The <b>dbt1</b> parameter is the <a href="/api_c/dbt_class.html">DBT</a> representing the 53application supplied key.</ul> 54<b><b>dbt2</b></b><ul compact><li>The <b>dbt2</b> parameter is the <a href="/api_c/dbt_class.html">DBT</a> representing the 55current database's key.</ul> 56<br></ul> 57<p>The <b>compare_fcn</b> function must return an integer value less 58than, equal to, or greater than zero if the first key parameter is 59considered to be respectively less than, equal to, or greater than the 60second key parameter. The comparison function must correctly handle any 61key values used by the application (possibly including zero-length 62keys). The <b>data</b> and <b>size</b> fields of the <a href="/api_c/dbt_class.html">DBT</a> are 63the only fields that may be used for the purposes of this comparison, 64and no particular alignment of the memory to which by the <b>data</b> 65field refers may be assumed.</p> 66<br> 67<br><b>Errors</b> 68<p>The DB->set_h_compare method 69may fail and return one of the following non-zero errors:</p> 70<br> 71<b>EINVAL</b><ul compact><li>If the method was called after <a href="/api_c/db_open.html">DB->open</a> was called; or if an 72invalid flag value or parameter was specified.</ul> 73<br> 74<hr size=1 noshade> 75<br><b>Class</b> 76<a href="/api_c/db_class.html">DB</a> 77<br><b>See Also</b> 78<a href="/api_c/db_list.html">Databases and Related Methods</a> 79</tt> 80<table width="100%"><tr><td><br></td><td align=right> 81<a href="/api_c/api_core.html"><img src="/images/api.gif" alt="API"></a><a href="/ref/toc.html"><img src="/images/ref.gif" alt="Ref"></a> 82</td></tr></table> 83<p><font size=1>Copyright (c) 1996,2008 Oracle. All rights reserved.</font> 84</body> 85</html> 86
