Complement operator in C

Question

#include<stdio.h>
void main(){
  int i = 3;
  printf("%d", ~i);
}

The output is 2. 3 is 0000 0011. Tilde changes all the bit to their opposite. So how is the answer even 2? As I have read from other posts. 2's complement is (~i)+1 which makes ~ 1's complement operator. Even if it is so how is 2 a possible output?

Answer 1

I doubt the answer it's 2. It should be -4, which is the decimal representation of 11111100.

Online Run, which outputs:

-4

Indeed Two's complement is calculated by inverting the digits and adding one. So -4 + 1 = -3, as @WeatherVane commented.

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PS: Unrelated to your question, but the main method typically returns an int, not void. Read more in What should main() return in C and C++?

Reference: Section 5.1.2.2.1 of the C11 standard (emphasis mine):

It shall be defined with a return type of int and with no parameters:

int main(void) { /* ... */ }

or with two parameters (referred to here as argc and argv, though any names may be used, as they are local to the function in which they are declared):

int main(int argc, char *argv[]) { /* ... */ }

or equivalent;¹⁰⁾ or in some other implementation-defined manner.

¹⁰⁾ Thus, int can be replaced by a typedef name defined as int, or the type of argv can be written as char **argv, and so on.

as @JérômeRichard commented.

Answer 2

1. the main function should be int main(void)
2. The answer is not 2 only -4 https://godbolt.org/z/b1GGv8