How are opaque(ish) types normally handled in UFFI (or FFI)?

E.g. if I want to use libstuff which defines the following structure:

struct stuffstruct {
�� �� typeA a;
�� �� typeB b;
�� �� } stufftype;


I might know they're integers, but not the size, signed vs. unsigned,
etc.

The definitions of typeA and typeB are buried in multiple layers of
header files, dependent on #if tests, can be different sizes on
different platforms (e.g. Linux vs. MacOS), etc.?

Is this documented somewhere?

Thanks,
Alistair