So unsigned long long is the same as uint64_t in the 32-bit compilation but not in 64-bit compilation?
Yes.
In 32-bit mode, most likely long is 32 bits and long long is 64 bits. In 64-bit mode, both are probably 64 bits.
In 32-bit mode, the compiler (more precisely the uint64_t as unsigned long long, because unsigned long isn't wide enough.
In 64-bit mode, it defines uint64_t as unsigned long.
It could have defined it as unsigned long long in both modes. The choice is arbitrary; all that's required is that it has to be a 64-bit type.
In general, each of the integer types defined in
