Calling C is trivial if you understand the calling convention, can do your own fixups, etc. etc. You prepare the stack and go. You can hand link but most FORTHs of any richness do all this for you - see for example [link|http://www.tinyboot.com/appc.html|http://www.tinyboot.com/appc.html].
Also in general:
[link|http://www.equi4.com/minotaur/minotaur.html|http://www.equi4.com...aur/minotaur.html]