Frontend and backend communication

[Dainius (GreatEmerald)]

I have one program design problem and I wonder if anyone here could
give any suggestions about it. The situation is like this: I am
splitting a game into a frontend (a library) and a backend (an
executable). The backend is going to handle all the game mechanics,
while the frontend is going to handle I/O. But there are certain
problems. For example, now I have a function Shuffle() that calls
PlaySound(SHUFFLE). Shuffle() is a backend function, while PlaySound()
is a frontend one, so obviously it won't work that way after the
split. It would be ideal if there was a way to create hooks - say an
empty PlaySound() function that the frontend could receive calls to.
But I can't see a way to do that. Another way to do that that was
suggested to me was to use an event loop - set a global variable, then
have the frontend monitor it for changes and then respond as
necessary, but that just isn't a very clean way to do it.

And then there is the fact that the backend is going to be written in
D (right now it's a mix of C and D), while the frontend will be in C
(one of the frontends, anyway - the second one will also be in D). Any
suggestions about this?