In mainstream compilers, even on x64, sizeof(long) == sizeof(int). You need long long instead. Of course, in the general case there's exactly zero guarantee that there even exists an integral type wider than int.

The C99 language standard requires support for long long.

But an implementation where sizeof(int) == sizeof(long) == sizeof(long long) is allowed by the specification.

On a (slightly) more practical level: there exists some largest integer type. You may someday need to sort it. If you want your sort to always work, you can't afford to overflow.