If I'm not mistaken, there is no mention of FFI in the luatex reference manual (at least in 1.16).

It would be really nice to have some kind of minimal documentation about it. I know it would be better if I could write a patch myself but I'm not fluent enough in English nor technically competent, sorry.

Many thanks for the hard work done with luatex.

All the best

-- 

Christophe