int (*dup_compare_fcn)(DB *, const DBT *, const DBT *));
Set the duplicate data item comparison function. The comparison function
is called when it is necessary to compare a data item specified by the
application with a data item currently stored in the tree. The first
argument to the comparison function is the DBT representing the
application's data item, the second is the current tree's data item.
The comparison function must return an integer value less than, equal
to, or greater than zero if the first data item argument is considered
to be respectively less than, equal to, or greater than the second data
item argument. In addition, the comparison function must cause the data
items in the set to be well-ordered. The comparison function
must correctly handle any data item values used by the application
(possibly including zero-length data items). The data and
size fields of the DBT are the only fields that may be
used for the purposes of this comparison.
If no comparison function is specified, the data items are compared
lexically, with shorter data items collating before longer data items.
The same duplicate data item comparison method must be used each time
a particular Btree is opened.
The DB->set_dup_compare interface may only be used to configure Berkeley DB before
the DB->open interface is called.
The DB->set_dup_compare function returns a non-zero error value on failure and 0 on success.