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<!--$Id: db_set_h_hash.so,v 10.39 2003/11/08 19:17:21 bostic Exp $-->
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<title>Berkeley DB: DB->set_h_hash</title>
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<h3>DB->set_h_hash</h3>
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<tt>
<h3><pre>
#include <db.h>
<p>
int
DB->set_h_hash(DB *db,
u_int32_t (*h_hash_fcn)(DB *, const void *bytes, u_int32_t length));
</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="../api_c/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_c/db_open.html">DB->open</a> method is called.
If the database already exists when
<a href="../api_c/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
returns a non-zero error value on failure
and 0 on success.
</p>
<h3>Parameters</h3>
<p><dl compact>
<p><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 return one of the following non-zero errors:</p>
<p><dl compact>
<p><dt>EINVAL<dd>If the method was called after <a href="../api_c/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="../api_c/db_class.html">DB</a>
<h3>See Also</h3>
<a href="../api_c/db_list.html">Databases and Related Methods</a>
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