[SaswatPadhi]

(First of all, note that you have to define the _ARGS_ macro and ANSI_PROTO yourself. It's not predefined.)

Now, you can define the sum function this way :

Declaration :

int sum _ARGS_((int a, int b));

Definition :

int sum
#ifdef ANSI_PROTO
    (int a, int b)
#else
    (a, b)
    int a;
    int b;
#endif

I hope it's clear to you now.