libpq: NULL in binary parameters

[john heasley <heas(at)shrubbery(dot)net>]

Using binary parameters with libpq (PQexecParams) I am having trouble
figuring out how NULL is represented since there does not appear to be
a NULLOID.

For example, if a table has a varchar column and it IS NULL for a given
row, then what I received is a zero-length string ("").  Also, while I
can insert a NULL varchar simply by setting the value to NULL (or 0), it
is not clear how to acheive this with say an integer column.  And, is it
possible to match NULL using a parameter.

I suspect this boils down to, how is NULL represented as a binary parameter?

clues?