segmentation fault 11 in C++ on Mac

You’ve exceeded your stack space given by the OS. If you need more memory, the easiest way is to allocate it dynamically:

int N=1000000;
short* res = new short[N];

However, std::vector is preferred in this context, because the above requires you to free the memory by hand.

int N = 1000000;
std::vector<short> res (N);

If you can use C++11, you can possibly save some fraction of time by using unique_ptr array specialization, too:

std::unique_ptr<short[]> res (new short[N]);

Both of the automatic methods above can still be used with familiar res[index] syntax thanks to overloaded operator[], but to get the raw pointer for memory operations you’d need res.data() with vector or res.get() with unique_ptr.