<!--$Id: bt_compare.so,v 10.26 2008/01/10 16:02:56 bostic Exp $--> <!--Copyright (c) 1997,2008 Oracle. All rights reserved.--> <!--See the file LICENSE for redistribution information.--> <html> <head> <title>Berkeley DB Reference Guide: Btree comparison</title> <meta name="description" content="Berkeley DB: An embedded database programmatic toolkit."> <meta name="keywords" content="embedded,database,programmatic,toolkit,btree,hash,hashing,transaction,transactions,locking,logging,access method,access methods,Java,C,C++"> </head> <body bgcolor=white> <a name="2"><!--meow--></a> <table width="100%"><tr valign=top> <td><b><dl><dt>Berkeley DB Reference Guide:<dd>Access Methods</dl></b></td> <td align=right><a href="../am_conf/malloc.html"><img src="../../images/prev.gif" alt="Prev"></a><a href="../toc.html"><img src="../../images/ref.gif" alt="Ref"></a><a href="../am_conf/bt_prefix.html"><img src="../../images/next.gif" alt="Next"></a> </td></tr></table> <p align=center><b>Btree comparison</b></p> <p>The Btree data structure is a sorted, balanced tree structure storing associated key/data pairs. By default, the sort order is lexicographical, with shorter keys collating before longer keys. The user can specify the sort order for the Btree by using the <a href="../../api_c/db_set_bt_compare.html">DB->set_bt_compare</a> method.</p> <p>Sort routines are passed pointers to keys as arguments. The keys are represented as <a href="../../api_c/dbt_class.html">DBT</a> structures. The routine must return an integer less than, equal to, or greater than zero if the first argument is considered to be respectively less than, equal to, or greater than the second argument. The only fields that the routines may examine in the <a href="../../api_c/dbt_class.html">DBT</a> structures are <b>data</b> and <b>size</b> fields.</p> <p>An example routine that might be used to sort integer keys in the database is as follows:</p> <blockquote><pre>int compare_int(dbp, a, b) DB *dbp; const DBT *a, *b; { int ai, bi; <p> /* * Returns: * < 0 if a < b * = 0 if a = b * > 0 if a > b */ memcpy(&ai, a->data, sizeof(int)); memcpy(&bi, b->data, sizeof(int)); return (ai - bi); }</pre></blockquote> <p>Note that the data must first be copied into memory that is appropriately aligned, as Berkeley DB does not guarantee any kind of alignment of the underlying data, including for comparison routines. When writing comparison routines, remember that databases created on machines of different architectures may have different integer byte orders, for which your code may need to compensate.</p> <p>An example routine that might be used to sort keys based on the first five bytes of the key (ignoring any subsequent bytes) is as follows:</p> <blockquote><pre>int compare_dbt(dbp, a, b) DB *dbp; const DBT *a, *b; { int len; u_char *p1, *p2; <p> /* * Returns: * < 0 if a < b * = 0 if a = b * > 0 if a > b */ for (p1 = a->data, p2 = b->data, len = 5; len--; ++p1, ++p2) if (*p1 != *p2) return ((long)*p1 - (long)*p2); return (0); }</pre></blockquote> <p>All comparison functions must cause the keys in the database to be well-ordered. The most important implication of being well-ordered is that the key relations must be transitive, that is, if key A is less than key B, and key B is less than key C, then the comparison routine must also return that key A is less than key C.</p> <p>It is reasonable for a comparison function to not examine an entire key in some applications, which implies partial keys may be specified to the Berkeley DB interfaces. When partial keys are specified to Berkeley DB, interfaces which retrieve data items based on a user-specified key (for example, <a href="../../api_c/db_get.html">DB->get</a> and <a href="../../api_c/dbc_get.html">DBcursor->get</a> with the <a href="../../api_c/dbc_get.html#DB_SET">DB_SET</a> flag), will modify the user-specified key by returning the actual key stored in the database.</p> <table width="100%"><tr><td><br></td><td align=right><a href="../am_conf/malloc.html"><img src="../../images/prev.gif" alt="Prev"></a><a href="../toc.html"><img src="../../images/ref.gif" alt="Ref"></a><a href="../am_conf/bt_prefix.html"><img src="../../images/next.gif" alt="Next"></a> </td></tr></table> <p><font size=1>Copyright (c) 1996,2008 Oracle. All rights reserved.</font> </body> </html>