GForth callbacks

I'm trying to use the C callbacks available with GForth 0.7.9 but 
somehow it doesn't seem to work.

I have a callback that takes two arguments and one return value.

I define it like this:


c-callback cs-renderer n a -- a

' render>buf cs-renderer CONSTANT c_render_to_buf


When passing c_render_to_buf to the function which requires the callback 
the callback gets invoked but when it returns things go wrong.

If I place a .s in the callback it prints <-34288843> so it seems the 
stack isn't set up properly. But I don't know what to do different to 
make it work.

I've yet to create a complete example I can post.

--
René Hartmann