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Berkeley DB Reference Guide:
  Access Methods

# 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 DB->set_bt_compare function.

Sort routines are passed pointers to keys as arguments. The keys are represented as DBT 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 DBT structures are data and size fields.

An example routine that might be used to sort integer keys in the database is as follows:

>     int
>     compare_int(a, b)
>     	const DBT *a, *b;
>     {
>     	int ai, bi;

>     	/*
>     	 * Returns:
>     	 *	< 0 if a < b
>     	 *	= 0 if a = b
>     	 *	> 0 if a > b
>     	 */
>     	memcpy(&ai, a->data, sizeof(int));
>     	memcpy(&bi, b->data, sizeof(int));
>     	return (ai - bi);
>     }

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:

>     int
>     compare_dbt(a, b)
>     	const DBT *a, *b;
>     {
>     	u_char *p1, *p2;

>     	/*
>     	 * Returns:
>     	 *	< 0 if a < b
>     	 *	= 0 if a = b
>     	 *	> 0 if a > b
>     	 */
>     	for (p1 = a->data, p2 = b->data, len = 5; len--; ++p1, ++p2)
>     		if (*p1 != *p2)
>     			return ((long)*p1 - (long)*p2);
>     	return (0);
>     }

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