> So what do you do when there isn't a predefined conversion function
> between Scheme type and C type?

You write it in C.  I'm not sure what you are getting at here...  I
could give you the details, but the way this is done is Gambit
specific and I'm not advocating it for a portable FFI.  The purpose of
my previous message is to suggest that an FFI should allow the definition
of new types.  This way the conversion code can be isolated in a
couple of conversion functions and the API to foreign functions is
high-level (i.e. you're not always passing Scheme objects and having
to convert them "by hand" to whatever a particular function expects).

Marc