Unsigned long with negative value

Question

Please see the simple code below:

#include <iostream>
#include <stdlib.h>

using namespace std;


int main(void)
{
    unsigned long currentTrafficTypeValueDec;
    long input;
    input=63;
    currentTrafficTypeValueDec = (unsigned long) 1LL << input; 
    cout << currentTrafficTypeValueDec << endl;
    printf("%u \n", currentTrafficTypeValueDec);
    printf("%ld \n", currentTrafficTypeValueDec);
    return 0;
}

Why printf() displays the currentTrafficTypeValueDec (unsigned long) with negative value?

The output is:

9223372036854775808
0
-9223372036854775808 

Answer 1

%d is a signed formatter. Reinterpreting the bits of currentTrafficTypeValueDec (2 to the 63rd power) as a signed long gives a negative value. So printf() prints a negative number.

Maybe you want to use %lu?

Answer 2

You lied to printf by passing it an unsigned value while the format spec said it would be a signed one.