postgres functions(.so)

Hello

Can anybody help me?
The question is: Is it possible to write
postgres function on C/C++ that selects
some tables of a database ?

More detailed description.
Suppose I have database with tables A, B.
I've written program on C++ with 'main' function
that reads tables A, B and returns some value, e.g. integer.
Now I'd like to implement the same actions as postgres
function (e.g. as .so module).

Is it possible ?

I know:
1) How compile C/C++ code to .so
2) How to CREATE FUNCTION .....
I don't know:
1) Is it possible to postges function
to read tables of the database
2) If possible, how it should be done?
3) Which restrictions applicable to .so
functions ?

If you know examples those do something similar,
please let me know.

Any help will be appreciated.

Thanks

--
Vladimir Zolotych gsmith(at)eurocom(dot)od(dot)ua