comparison between signed and unsigned char

Question

I'm pretty much assuming this is a stupid question... but I can't really find the answer for it. So I'm asking this here.

For the purpose of learning about implicit type casting, I'm running the following code on C.

#include <stdio.h>

int main()
{
    unsigned char i;
    char cnt = -1;

    int a[255];

    for (int k = 0; k < 255; k++)
    {
        a[k] = k;
    }
    for (i = cnt - 2; i < cnt; i--)
    {
        a[i] += a[i + 1];
        printf("%d\n", a[i]);
    }

    return 0;
}

When I ran this program, nothing happened.

I was able to found out that the loop condition of for-loop was false at the first iteration, so the program exited the for-loop right away.

However, I don't get the reason why.

As far as I know, C does implicit casting when assigning or comparing variables with different types. So I thought that on i = cnt - 2, the minus operation makes the value -3, and then implicit casting assigns i with a value 253.

Then, shouldn't the condition i < cnt be true since (by another implicit casting of cnt because of comparison of signed and unsigned char) 253 is smaller than 255?

If it isn't, why is this false? Is there something that I missed or is there some exception that I don't know?

Answer 1

Your question is not stupid at all. You were close to the solution: i is assigned the value -3 but the implicit conversion to the type of i, unsigned char, changes the value to 253.

For a more precise explanation, there are multiple issues in your test code:

• char may be signed or unsigned depending on the platform and compiler configuration, so char cnt = -1; may store the value -1 or 255 into cnt, or even some other value if char is unsigned and has more than 8 bits.

• The behavior of for (i = cnt - 2; i < cnt; i--) also depends on whether char is signed or unsigned by default:

  • in all cases, the test i < cnt is evaluated with both operands converted to int (or unsigned int in the rare case where sizeof(int)==1). If int can represent all values of types char and unsigned char, this conversion does not change the values.

  • if char is unsigned and has 8 bits, cnt has the value 255 so i is initialized with the value 253 and the loop runs 254 times with i from 253 down to 0, then i-- stores the value 255 again into i, for which the test i < cnt evaluates to false. The loop prints 507, then 759, ... 32385.

  • if char is signed and has 8 bits, as is probably the case on your system, cnt has the value -1 and i is initialized with the value -3 converted to unsigned char, which is 253. The initial test i < cnt evaluates as 253 < -1, which is false, causing the loop body to be skipped immediately.

You can force char to be unsigned by default by giving the compiler the appropriate flag (eg: gcc -funsigned-char) and test how the behavior changes. Using Godbolt's compiler explorer, you can see that gcc generates just 2 instructions to return 0 in the signed (default) case and the expected output in the unsigned case.