create trigger

I am trying to use "triggers". I get an error after "create trigger".
Does anybody know why?

 create function insert_into_db2 RETURNS int AS
'/work/posgresql/lib/libpq.so' language 'C';

 create trigger trial before insert or update on db1user for each row
execute procedure  insert_into_db2();

 ERROR:  CreateTrigger: function insert_into_db2() must return OPAQUE

Does anybody know why?

Also, if a function is enforced to return OPAQUE then  what is the
"RETURNS" clause?


One more question,
     If a trigger does not succeed then I want to ABORT transaction, is
it possible? how?


- Sandeep