Question about accessing current row data inside trigger

Hello,

I have a trigger function written in C.
The trigger function is called via:

CREATE TRIGGER after_update AFTER UPDATE ON some_table
    FOR EACH ROW EXECUTE PROCEDURE  my_trigger_function();

    Since the trigger is called after each row update the actual row data
should be available in some way to the trigger.
    What functionality (SPI ?) do I use to use the column values from
the current row in the actual trigger?

thanks for any insight,

Peter