2011-09-13 17:44:24 +00:00
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<title>Db::set_h_hash()</title>
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<link rel="up" href="db.html" title="Chapter 2. The Db Handle" />
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<link rel="prev" href="dbset_h_ffactor.html" title="Db::set_h_ffactor()" />
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<p>Library Version 11.2.5.2</p>
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<table width="100%" summary="Navigation header">
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<th colspan="3" align="center">Db::set_h_hash()</th>
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</tr>
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<tr>
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<td width="20%" align="left"><a accesskey="p" href="dbset_h_ffactor.html">Prev</a> </td>
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<th width="60%" align="center">Chapter 2.
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The Db Handle
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</th>
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<td width="20%" align="right"> <a accesskey="n" href="dbset_h_nelem.html">Next</a></td>
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<div>
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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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</div>
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</div>
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</div>
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<pre class="programlisting">#include <db_cxx.h>
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extern "C" {
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typedef u_int32_t (*h_hash_fcn_type)
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(DB *, const void *bytes, u_int32_t length);
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};
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int
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Db::set_h_hash(h_hash_fcn_type h_hash_fcn);</pre>
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<p>
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Set a user-defined hash function; if no hash function is specified, a
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default hash function is used. Because no hash function performs
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equally well on all possible data, the user may find that the built-in
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hash function performs poorly with a particular data set.
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</p>
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<p>
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The <code class="methodname">Db::set_h_hash()</code> method configures operations performed using the
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specified <a class="link" href="db.html" title="Chapter 2. The Db Handle">Db</a> handle, not
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all operations performed on the underlying database.
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</p>
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<p>
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The <code class="methodname">Db::set_h_hash()</code> method may not be called after the
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<a class="xref" href="dbopen.html" title="Db::open()">Db::open()</a> method is called. If
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the database already exists when <a class="xref" href="dbopen.html" title="Db::open()">Db::open()</a>
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is called, the information specified to <code class="methodname">Db::set_h_hash()</code> must be the same as that
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historically used to create the database or corruption can occur.
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</p>
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<p>
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The <code class="methodname">Db::set_h_hash()</code> <span>
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<span>
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method either returns a non-zero error value or throws an
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exception that encapsulates a non-zero error value on
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failure, and returns 0 on success.
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</span>
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</span>
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</p>
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<div class="sect2" lang="en" xml:lang="en">
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<div class="titlepage">
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<div>
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<div>
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2011-12-20 00:07:10 +00:00
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<h3 class="title"><a id="id3577479"></a>Parameters</h3>
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2011-09-13 17:44:24 +00:00
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</div>
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</div>
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</div>
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<div class="sect3" lang="en" xml:lang="en">
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<div class="titlepage">
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<div>
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<div>
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2011-12-20 00:07:10 +00:00
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<h4 class="title"><a id="id3577482"></a>h_hash_fcn</h4>
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2011-09-13 17:44:24 +00:00
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</div>
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</div>
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</div>
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<p>
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The <span class="bold"><strong>h_hash_fcn</strong></span> parameter is the
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application-specified hash function.
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</p>
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<p>
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Application-specified hash functions take a pointer to a byte string
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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
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key values used by the application (possibly including zero-length
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keys).
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</p>
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</div>
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</div>
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<div class="sect2" lang="en" xml:lang="en">
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<div class="titlepage">
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<div>
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<div>
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2011-12-20 00:07:10 +00:00
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<h3 class="title"><a id="id3577175"></a>Errors</h3>
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2011-09-13 17:44:24 +00:00
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</div>
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</div>
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</div>
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<p>
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The <code class="methodname">Db::set_h_hash()</code> <span>
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<span>
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method may fail and throw a <a class="link" href="dbexception.html" title="Chapter 6. The DbException Class">DbException</a>
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exception, encapsulating one of the following non-zero errors, or return one
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of the following non-zero errors:
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</span>
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</span>
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</p>
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<div class="sect3" lang="en" xml:lang="en">
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<div class="titlepage">
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<div>
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<div>
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2011-12-20 00:07:10 +00:00
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<h4 class="title"><a id="id3577600"></a>EINVAL</h4>
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2011-09-13 17:44:24 +00:00
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</div>
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</div>
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</div>
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<p>
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If the method was called after <a class="xref" href="dbopen.html" title="Db::open()">Db::open()</a>
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was called; or if an invalid flag value or parameter was specified.
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</p>
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</div>
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</div>
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<div class="sect2" lang="en" xml:lang="en">
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<div class="titlepage">
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<div>
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<div>
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2011-12-20 00:07:10 +00:00
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<h3 class="title"><a id="id3577622"></a>Class</h3>
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</div>
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</div>
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</div>
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<p>
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<a class="link" href="db.html" title="Chapter 2. The Db Handle">Db</a>
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</p>
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</div>
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<div class="sect2" lang="en" xml:lang="en">
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<div class="titlepage">
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<div>
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<div>
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2011-12-20 00:07:10 +00:00
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<h3 class="title"><a id="id3577369"></a>See Also</h3>
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2011-09-13 17:44:24 +00:00
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</div>
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</div>
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</div>
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<p>
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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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</p>
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<td width="40%" align="left" valign="top">Db::set_h_ffactor() </td>
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<a accesskey="h" href="index.html">Home</a>
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<td width="40%" align="right" valign="top"> Db::set_h_nelem()</td>
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