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<h2 class="title" style="clear: both"><a id="dbset_h_hash"></a>Db::set_h_hash()</h2>
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<pre class="programlisting">#include <db_cxx.h>
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>
<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 <code class="methodname">Db::set_h_hash()</code> method configures operations performed using the
specified <a class="link" href="db.html" title="Chapter 2. The Db Handle">Db</a> handle, not
all operations performed on the underlying database.
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<p>
The <code class="methodname">Db::set_h_hash()</code> method may not be called after the
<a class="xref" href="dbopen.html" title="Db::open()">Db::open()</a> method is called. If
the database already exists when <a class="xref" href="dbopen.html" title="Db::open()">Db::open()</a>
is called, the information specified to <code class="methodname">Db::set_h_hash()</code> must be the same as that
historically used to create the database or corruption can occur.
</p>
<p>
The <code class="methodname">Db::set_h_hash()</code> <span>
<span>
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.
</span>
</span>
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<h3 class="title"><a id="id1648629"></a>Parameters</h3>
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<h4 class="title"><a id="id1648714"></a>h_hash_fcn</h4>
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<p>
The <span class="bold"><strong>h_hash_fcn</strong></span> parameter is the
application-specified hash function.
</p>
<p>
Application-specified hash functions take a pointer to a byte string
and a length as parameters, and return a value of type <span class="bold"><strong>u_int32_t</strong></span>. The hash function must handle any
key values used by the application (possibly including zero-length
keys).
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<h3 class="title"><a id="id1648804"></a>Errors</h3>
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<p>
The <code class="methodname">Db::set_h_hash()</code> <span>
<span>
method may fail and throw a <a class="link" href="dbexception.html" title="Chapter 6. The DbException Class">DbException</a>
exception, encapsulating one of the following non-zero errors, or return one
of the following non-zero errors:
</span>
</span>
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<h4 class="title"><a id="id1648603"></a>EINVAL</h4>
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<p>
If the method was called after <a class="xref" href="dbopen.html" title="Db::open()">Db::open()</a>
was called; or if an invalid flag value or parameter was specified.
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<h3 class="title"><a id="id1648604"></a>Class</h3>
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<a class="link" href="db.html" title="Chapter 2. The Db Handle">Db</a>
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<h3 class="title"><a id="id1648664"></a>See Also</h3>
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<a class="xref" href="db.html#dblist" title="Database and Related Methods">Database and Related Methods</a>
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