Interfacing with C - calling member function of D struct from C?

[unleashy via Digitalmars-d-learn]

Hello! If I have a D struct like:

struct Foo
{
     int bar;

     void addToBar(int what) {
         bar += what;
     }
}

How would I call `addToBar` from C code? Would I need to put the 
`addToBar` function outside of the struct and mark it as `extern 
(C)` and in normal D code take advantage of UFCS or is there some 
magic C incantation?

I've scoured the forums and other places for anything about this 
but couldn't find any information whatsoever... so yeah. (or my 
Google-fu is terrible)

Thanks!