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<td><dl><dt>Berkeley DB Reference Guide:<dd>Access Methods</dl></td>
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Btree comparison
The Btree data structure is a sorted, balanced tree structure storing
associated key/data pairs. By default, the sort order is lexicographical,
with shorter keys collating before longer keys. The user can specify the
sort order for the Btree by using the <a href="../../api_c/db_set_bt_compare.html">DB-&gt;set_bt_compare</a> method.
Sort routines are passed pointers to keys as arguments. The keys are
represented as <a href="../../api_c/dbt_class.html">DBT</a> structures. The routine must return an integer
less than, equal to, or greater than zero if the first argument is
considered to be respectively less than, equal to, or greater than the
second argument. The only fields that the routines may examine in the
<a href="../../api_c/dbt_class.html">DBT</a> structures are data and size fields.
An example routine that might be used to sort integer keys in the database
is as follows:
<blockquote><pre>int
compare_int(dbp, a, b)
	DB *dbp;
	const DBT *a, *b;
{
	int ai, bi;

	/*
	 * Returns:
	 *	&lt; 0 if a &lt; b
	 *	= 0 if a = b
	 *	&gt; 0 if a &gt; b
	 */
	memcpy(&ai, a-&gt;data, sizeof(int));
	memcpy(&bi, b-&gt;data, sizeof(int));
	return (ai - bi);
}</pre></blockquote>
Note that the data must first be copied into memory that is appropriately
aligned, as Berkeley DB does not guarantee any kind of alignment of the
underlying data, including for comparison routines. When writing
comparison routines, remember that databases created on machines of
different architectures may have different integer byte orders, for which
your code may need to compensate.
An example routine that might be used to sort keys based on the first
five bytes of the key (ignoring any subsequent bytes) is as follows:
<blockquote><pre>int
compare_dbt(dbp, a, b)
	DB *dbp;
	const DBT *a, *b;
{
	int len;
	u_char *p1, *p2;

	/*
	 * Returns:
	 *	&lt; 0 if a &lt; b
	 *	= 0 if a = b
	 *	&gt; 0 if a &gt; b
	 */
	for (p1 = a-&gt;data, p2 = b-&gt;data, len = 5; len--; ++p1, ++p2)
		if (*p1 != *p2)
			return ((long)*p1 - (long)*p2);
	return (0);
}</pre></blockquote>
All comparison functions must cause the keys in the database to be
well-ordered. The most important implication of being well-ordered is
that the key relations must be transitive, that is, if key A is less
than key B, and key B is less than key C, then the comparison routine
must also return that key A is less than key C.
It is reasonable for a comparison function to not examine an entire key
in some applications, which implies partial keys may be specified to the
Berkeley DB interfaces. When partial keys are specified to Berkeley DB, interfaces
which retrieve data items based on a user-specified key (for example,
<a href="../../api_c/db_get.html">DB-&gt;get</a> and <a href="../../api_c/dbc_get.html">DBcursor-&gt;get</a> with the <a href="../../api_c/dbc_get.html#DB_SET">DB_SET</a> flag), will
modify the user-specified key by returning the actual key stored in the
database.
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