Using a composite SQL type in C

I have a type defined in SQL:

CREATE TYPE text_info AS (str text, id integer);

I'd like to use this type as the aggregate type in an aggregate function:

CREATE FUNCTION foobar(text_info,integer,text,text) RETURNS text_info AS 
'$libdir/plugins/mylib.dll', 'foobar' LANGUAGE C;

CREATE AGGREGATE foobar(integer,text,text) (SFUNC = foobar, STYPE = 
text_info);

My problem is, how do I access this type in my function (in C), and how 
do I create a new object of this type to return from the function?  I 
need to store both a text and an int, to avoid doing an extra query 
around the result of this aggregate (to get the corresponding text 
value), which is what I'm currently doing.  Any ideas?

Thanks,
Dan