I don't understand why the behavior of the 1's complement in C is different when acting on a number vs acting on a variable storing a number

Question

Output of the following code:

 test1: 0x00000002 0b00000010 (1 bytes)                               
 test2: 0x000000fd 0b11111101 (1 bytes)                                
~test1: 0xfffffffd 0b4294967285 (4 bytes)

I don't understand why doing ~(test1) is different from ~(0x02) since test1=0x02 and everything is unsigned. It appears that ~(test1) does the proper complement but then adds 3 bytes of ones to the left.

#include <stdio.h>

int binConv(int num)
{
    if (num == 0)
    {
        return 0;
    }
    else
    {
        return (num % 2) + 10 * binConv(num / 2);
    }
}

int main()
{
    unsigned char test1;
    unsigned char test2;

    test1=0x02;
    test2=~(0x02);

    printf(" test1: 0x%08x 0b%08u (%d bytes)\n",test1,binConv(test1),sizeof(test1));
    printf(" test2: 0x%08x 0b%08u (%d bytes)\n",test2,binConv(test2),sizeof(test2));
    printf("~test1: 0x%08x 0b%08u (%d bytes)",~test1,binConv(~test1),sizeof(~test1));
    return 0;
}

Code on onlinegdb.com

Because they're different types: `test1` and `test2` are type `char`. `0x02` is type `int`. — Lee Daniel Crocker, Jan 05 '18 at 22:37
Also, your `printf` formats are wrong, you should use `%zu` for `size_t` and `%hhu` for `unsigned char`. — Jens Gustedt, Jan 05 '18 at 22:39
"The operand of ~ shall have integral or unscoped enumeration type; the result is the one’s complement of　its operand. **Integral promotions are performed**." — Jean-François Fabre, Jan 05 '18 at 22:39
Although `test1` has size 1, it is promoted to `int` for `~test1` and because the result is not assigned to any other type of variable, `sizeof(~test1)` is the same as `sizeof(int)`. — Weather Vane, Jan 05 '18 at 22:42
Thanks for the quick responses. I didn't know the term "integer promotion" to search for but now that I see there are other questions on SO about it I've marked this one as duplicate. — Yelneerg, Jan 05 '18 at 23:08

score 2 · Answer 1 · edited Jan 05 '18 at 23:04

2

This has nothing (in particular) to do with one's complement.

Your problem is in the binConv function.

You are giving it a 32 bit value, and converting each bit to a base 10 digit. That's 10³². That value will not fit in an int.

You should also be passing unsigned values to and from binConv.

edited Jan 05 '18 at 23:04

chux - Reinstate Monica

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answered Jan 05 '18 at 22:56

Doug Currie

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While you are correct that there is an issue with passing that to binConv, that wasn't the real issue. The comments gave the answer I was looking for about integer promotion – Yelneerg Jan 05 '18 at 23:06

I don't understand why the behavior of the 1's complement in C is different when acting on a number vs acting on a variable storing a number

1 Answers1