Here is how stdint.h on Linux conjures up the int64_t type.

  #if __WORDSIZE == 64
  typedef long int int64_t;
  #else
  typedef long long int int64_t;
  #endif

You're whooshing.

You don't want to use "long long" because that's not necessarily 64-bits. You want to use int64_t which guarantees it is 64-bits.

And then, the correct format specifier for that is PRIi64, and not "%ld" or "%lld" which will break in different platforms.

Please don't be disingenuous. We both know that using #defines, you can get a type which is exactly 64 bits on any modern architecture. The fact that long long and long int are a builtin types and int64_t is implemented in terms of them, rather than the other way around, is just an implementation detail.