Strange overflow assigning 2 * 1024 * 1024 * 1024

Question

void main()
{
    long long ll = 2 * 1024 * 1024 * 1024; 
    unsigned long long ull = (2 * 1024 * 1024 * 1024);
    std::cout << ll << "\n" << ull;
}

My output with Visual Studio 2019 is

 -2147483648
18446744071562067968

I have no idea why overflow is happening here, please help

In both cases you are doing the calculation as an int and overflowing. All the numbers on the right hand side are integers. Try `std::cout << 2 * 1024 * 1024 * 1024ull << std::endl;` — drescherjm, Dec 13 '20 at 23:22
See also [Why write 1,000,000,000 as 1000*1000*1000](https://stackoverflow.com/a/40637622/2410359). — chux - Reinstate Monica, Dec 14 '20 at 00:05
Tip: Use things like `uint64_t ull` from [``](https://en.cppreference.com/w/cpp/types/integer) instead of madness like `unsigned long long long long plz be long ull` where it's unclear what you'll actually get. — tadman, Dec 14 '20 at 00:31

drescherjm · Accepted Answer · 2020-12-14T00:41:39.623

In both cases the calculation is done using integers because all the values on the right are integer literals. 2 * 1024 * 1024 * 1024 is 2,147,483,648 which 1 larger than the max 32 bit int so this part of the calculation is overflowing. To fix do the first calculation as long long using long long ll = 2LL * 1024 * 1024 * 1024; and the second calculation as an unsigned long long using unsigned long long ull = 2ULL * 1024 * 1024 * 1024;

You can learn more about integer literals here: https://en.cppreference.com/w/cpp/language/integer_literal

Here is the fixed code:

#include <iostream>
#include <limits>

using namespace std;

int main()
{
    long long ll = 2LL * 1024 * 1024 * 1024; 
    unsigned long long ull = 2ULL * 1024 * 1024 * 1024;
    std::cout << ll << "\n" << ull;
}

I put an online version of this fixed code here: https://ideone.com/5QkEls

Makes more sense to _lead_ the multiplication chain with `LL` so `2LL * 1024 * 1024 * 1024`. Now the product is correct even if `int` was 16-bit. — chux - Reinstate Monica, Dec 14 '20 at 00:06

Strange overflow assigning 2 * 1024 * 1024 * 1024

1 Answers1