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<title>Berkeley DB: DB->set_h_hash</title>
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<b>DB->set_h_hash</b>
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<b><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));
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<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_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>
<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>
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<br><b>Errors</b>
<p>The DB->set_h_hash method
may fail and 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_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.</ul>
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<br><b>Class</b>
<a href="../api_c/db_class.html">DB</a>
<br><b>See Also</b>
<a href="../api_c/db_list.html">Databases and Related Methods</a>
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