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