Why do `(char)~0` and `(unsigned char)~0` return values of different widths?

Question

I bumped into this while writing a program trying to print the constituent byte values of UTF-8 characters.

This is the program that I wrote to test the various ~0 operations:

#include <stdio.h>

int main()
{
    printf("%x\n", (char)~0); // ffffffff
    printf("%x\n", (unsigned char)~0); // ff
    printf("%d\n", sizeof(char) == sizeof(unsigned char)); // 1
    printf("%d\n", sizeof(char) == sizeof(unsigned int)); // 0
    printf("%d\n", (char)~0 == (unsigned int)~0); // 1
}

I'm struggling to understand why char would produce an int-sized value, when unsigned char produces a char-sized value.

Answer 1

When passing a type smaller than int to a variadic function like printf, it get promoted to type int.

In the first case, you're passing char with value -1 whose representation (assuming 2's complement) is 0xff. This is promoted to an int with value -1 and representation 0xffffffff, so this is what is printed.

In the second case, you're passing an unsigned char with value 255 whose representation is 0xff. This is promoted to an int with value 255 and representation 0x000000ff, so this is what is printed (without the leading zeros).