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Import BSDDB 4.7.25 (as of svn r89086)

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Zachary Ware
2017-09-04 13:40:25 -05:00
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<!--$Id: db_set_h_hash.html 63573 2008-05-23 21:43:21Z trent.nelson $-->
<!--Copyright (c) 1997,2008 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>
<b>Db::set_h_hash</b>
</td>
<td align=right>
<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></td>
</tr></table>
<hr size=1 noshade>
<tt>
<b><pre>
#include &lt;db_cxx.h&gt;
<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></b>
<hr size=1 noshade>
<b>Description: Db::set_h_hash</b>
<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="../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="../api_cxx/db_open.html">Db::open</a> method is called.
If the database already exists when
<a href="../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>
<b>Parameters</b> <br>
<b>h_hash_fcn</b><ul compact><li>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></ul>
<br>
<br><b>Errors</b>
<p>The Db::set_h_hash method
may fail and throw
<a href="../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>
<br>
<b>EINVAL</b><ul compact><li>If the method was called after <a href="../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.</ul>
<br>
<hr size=1 noshade>
<br><b>Class</b>
<a href="../api_cxx/db_class.html">Db</a>
<br><b>See Also</b>
<a href="../api_cxx/db_list.html">Databases and Related Methods</a>
</tt>
<table width="100%"><tr><td><br></td><td align=right>
<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>
</td></tr></table>
<p><font size=1>Copyright (c) 1996,2008 Oracle. All rights reserved.</font>
</body>
</html>