Question

On left shift of (char) 0xff by 8 and casting it to int we get -256 or 0xffffff00.
Can somebody explain why this should happen?

#include <stdio.h>
int main (void)
{   
    char c = 0xff;
    printf("%d %x\n", (int)(c<<8),(int)(c<<8));
    return 0;
}

Output is

-256 ffffff00

Answer 1

char can be signed or unsigned – it’s implementation-defined. You see these results because char is signed by default on your compiler.

For the signed char 0xFF corresponds to −1 (that’s how two’s complement work). When you try to shift it it is first promoted to an int and then shifted – you effectively get multiplication by 256.

So it is this code:

char c = 0xFF; // -1
int shifted = c << 8; //-256 (-1 * 256)
printf( "%d, %x", shifted, shifted );