Weird "function does not exist" error.

Hi,

I ran into a "function .... does not exist" error when invoking a function
created by myself. What's weird is that the function works perfectly if I
invoke it through the command line tool "psql" but fails if I invoke it
through the C API.

The SQL to create the function is:
CREATE OR REPLACE FUNCTION "lo_select_page"(uid_ BIGINT, lottery_type_ INT,
type_ INT, time_start_ BIGINT, time_end_ BIGINT,
page_number_ BIGINT, prize_status_ INT)
RETURNS SETOF REFCURSOR AS $$
...
END $$ LANGUAGE plpgsql;

The SQL to invoke the function:
BEGIN;SELECT lo_select_page(4163481095, 0, 0, 0, 1893477600000, 0, 0);FETCH
ALL IN order_list_cursor; FETCH ALL IN page_count_cursor;COMMIT;

Server version is 9.1, both 9.1 & 9.2 psql works, C API version is 9.2

Any ideas ?

Thanks in advance
Yao