Bitwise operator gave me a negative number

Question

I just learned. How Bitwise operators work but when i try to use it in the c code i doesnt work.

#include <stdio.h>

int main()
{
    int a = 7;

    printf("%d\n", (int)~a);

    return 0;
}

The expected output is 8 but it comes -8.

~0111

=

1000

Answer 1

Assuming int is 32 bit on your machine then 7 is 0b00000000000000000000000000000111 and ~7 becomes 0b11111111111111111111111111111000 which is -8.

Background

For signed values the most significant bit is used to determine if the value is negative or not. When the MSB is set, the value is negative.

In addition to that (char) 0b10000000 is -128 and (char) 0b11111111 is -1.

So counting works as follows:

0b10000000 0b10000001 [...]  0b11111111 0b00000000  [...] 0b01111111
   -128       -127    [...]      -1          0      [...]     127

That is the reason why you will get -128 when you count 127+1 and try to store that in a char.