http://ideone.com/8k2jMb

It's very easy to implement a 0-indexed BIT, all you need is to increase pos by 1 in the function add and sum.

        const int MAX_N = 100000;
        int s[MAX_N], n;
	#define lowbit(x) ((x) & -(x))
	inline void add(int pos, int d) {
		for (int i = pos + 1; i <= n; i += lowbit(i))
                        s[i] += d;
	}
	inline int sum(int x) {
		int res = 0;
		for (int i = pos + 1; i > 0; i -= lowbit(i))
                        res += s[i];
		return res;
	}