Lock inside trigger

Hello list,

I have pl/pgsql trigger that before insert a details table it must
update a column in other table, the trigger have this code:

CREATE OR REPLACE FUNCTION public.tg_ecproc_insert()
RETURNS trigger AS
' DECLARE
salproc numeric(12,2);
BEGIN
select coalesce(prvsalproc,0) into salproc from
prvdor where prvcode=new.ecp_provcode;
-- How do I prevent someone else to change
-- prvdor.prvsalproc during this transaction
if new.ecp_dc=''C'' then
new.ecp_saldo = salproc - new.ecp_valor;
else
new.ecp_saldo = salproc + new.ecp_valor;
end if;
update prvdor set prvsalproc = new.ecp_saldo
where prvcode=new.ecp_provcode ;
-- Here I should be able to unlock the row on
-- prvdor table
return new;
END;
'
LANGUAGE 'plpgsql' VOLATILE;

Does postgresql automatically handle the lock on tables updated from
within a trigger? or what's the must appropiate way to get this done?

Thanks in advance,

--

Josué Maldonado