You must have been told before, it's NOT a void! That's just the return type (of the lack of it), its just a function.
And why give it a parameter r and do nothing with it?
And using meaningless variable names is already not smart, giving globals a single letter name is just plain stupid...
And why define something as int while it doesn't go over 255 nor negative?
But this does the trick
void matrixx(int in[]){
}
Only thing, you can NOT use sizeof() inside the function.