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<title>Berkeley DB: Db::set_h_hash</title>
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Db::set_h_hash
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#include &lt;db_cxx.h&gt;

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);
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Description: Db::set_h_hash
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.
The Db::set_h_hash method configures operations performed using the specified
<a href="../../db4-devel-4.7.25/api_cxx/db_class.html">Db</a> handle, not all operations performed on the underlying
database.
The Db::set_h_hash method may not be called after the <a href="../../db4-devel-4.7.25/api_cxx/db_open.html">Db::open</a> method is called.
If the database already exists when
<a href="../../db4-devel-4.7.25/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.
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.

Parameters 
 h_hash_fcn<ul compact><li>The h_hash_fcn parameter is the application-specified hash function.
Application-specified hash functions take a pointer to a byte string and
a length as parameters, and return a value of type u_int32_t.
The hash function must handle any key values used by the application
(possibly including zero-length keys).</ul>
 
 Errors
The Db::set_h_hash method
may fail and throw
<a href="../../db4-devel-4.7.25/api_cxx/except_class.html">DbException</a>,
encapsulating one of the following non-zero errors, or return one of
the following non-zero errors:
 
EINVAL<ul compact><li>If the method was called after <a href="../../db4-devel-4.7.25/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>
 
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 Class
<a href="../../db4-devel-4.7.25/api_cxx/db_class.html">Db</a>
 See Also
<a href="../../db4-devel-4.7.25/api_cxx/db_list.html">Databases and Related Methods</a>
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