<!--$Id: db_set_h_hash.so,v 10.40 2004/08/13 03:38:56 bostic Exp $--> <!--Copyright (c) 1997,2007 Oracle. All rights reserved.--> <!--See the file LICENSE for redistribution information.--> <html> <head> <title>Berkeley DB: Db::set_h_hash</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> <table width="100%"><tr valign=top> <td> <h3>Db::set_h_hash</h3> </td> <td align=right> <a href="../../db46-devel-4.6.21/api_cxx/api_core.html"><img src="../../db46-devel-4.6.21/images/api.gif" alt="API"></a> <a href="../../db46-devel-4.6.21/ref/toc.html"><img src="../../db46-devel-4.6.21/images/ref.gif" alt="Ref"></a></td> </tr></table> <hr size=1 noshade> <tt> <h3><pre> #include <db_cxx.h> <p> extern "C" { typedef u_int32_t (*h_hash_fcn_type) (DB *, const void *bytes, u_int32_t length); }; int Db::set_h_hash(h_hash_fcn_type h_hash_fcn); </pre></h3> <hr size=1 noshade> <h3>Description: Db::set_h_hash</h3> <p>Set a user-defined hash function; if no hash function is specified, a default hash function is used. Because no hash function performs equally well on all possible data, the user may find that the built-in hash function performs poorly with a particular data set.</p> <p>The Db::set_h_hash method configures operations performed using the specified <a href="../../db46-devel-4.6.21/api_cxx/db_class.html">Db</a> handle, not all operations performed on the underlying database.</p> <p>The Db::set_h_hash method may not be called after the <a href="../../db46-devel-4.6.21/api_cxx/db_open.html">Db::open</a> method is called. If the database already exists when <a href="../../db46-devel-4.6.21/api_cxx/db_open.html">Db::open</a> is called, the information specified to Db::set_h_hash must be the same as that historically used to create the database or corruption can occur.</p> <p>The Db::set_h_hash method either returns a non-zero error value or throws an exception that encapsulates a non-zero error value on failure, and returns 0 on success. </p> <h3>Parameters</h3> <dl compact> <dt><b>h_hash_fcn</b><dd>The <b>h_hash_fcn</b> parameter is the application-specified hash function. <p>Application-specified hash functions take a pointer to a byte string and a length as parameters, and return a value of type <b>u_int32_t</b>. The hash function must handle any key values used by the application (possibly including zero-length keys).</p> </dl> <h3>Errors</h3> <p>The Db::set_h_hash method may fail and throw <a href="../../db46-devel-4.6.21/api_cxx/except_class.html">DbException</a>, encapsulating one of the following non-zero errors, or return one of the following non-zero errors:</p> <dl compact> <dt>EINVAL<dd>If the method was called after <a href="../../db46-devel-4.6.21/api_cxx/db_open.html">Db::open</a> was called; the specified hash function differs from the hash function with which the database was created; or if an invalid flag value or parameter was specified. </dl> <hr size=1 noshade> <h3>Class</h3> <a href="../../db46-devel-4.6.21/api_cxx/db_class.html">Db</a> <h3>See Also</h3> <a href="../../db46-devel-4.6.21/api_cxx/db_list.html">Databases and Related Methods</a> </tt> <table width="100%"><tr><td><br></td><td align=right> <a href="../../db46-devel-4.6.21/api_cxx/api_core.html"><img src="../../db46-devel-4.6.21/images/api.gif" alt="API"></a><a href="../../db46-devel-4.6.21/ref/toc.html"><img src="../../db46-devel-4.6.21/images/ref.gif" alt="Ref"></a> </td></tr></table> <p><font size=1>Copyright (c) 1996,2007 Oracle. All rights reserved.</font> </body> </html>