manipulating HeapTuples in a libpq client

I've got a server function that returns a set of HeapTuples. The
server function is invoked in a query sent by a libpq client.However,
I haven't spotted a way to decompose the HeapTuple returned to the
client. Attempts to call GetAttributeByNum on the client result in the
linker complaining about an undefined reference.

Any ideas appreciated.

Thank you.
Eric.