subtraction between unsigned values - unexpected result

Question

I have two variables (test1 and test2), both unsigned. I need to check which of them is bigger.

I'm trying to understand what happens if overflow occurs.

My first test was done with uint8_t (char) data type:

#include <stdio.h>
#include <stdint.h>
#include <math.h>

int main()
{
    uint8_t test1 = 0;
    printf("test1 = %d\n", test1);

    uint8_t test2 = pow(2, 8 * sizeof(test1)) - 1; //max holdable value of uint8_t
    printf("test2 = %d\n", test2);

    uint8_t test3 = test1 - test2;
    printf("test1 - test2 = %d\n", test3);

    if ((test1 - test2) == 0)
        printf("test1 == test2\n");
    if ((test1 - test2) > 0)
        printf("test1 > test2\n");
    if ((test1 - test2) < 0)
        printf("test1 < test2\n");

    if (test3 == 0)
        printf("test1 == test2\n");
    if (test3 > 0)
        printf("test1 > test2\n");
    if (test3 < 0)
        printf("test1 < test2\n");

    return 0;
}

output:

test1 = 0                                                                                                                                                       
test2 = 255                                                                                                                                                     
test1 - test2 = 1                                                                                                                                               
test1 < test2                                                                                                                                                   
test1 > test2

What? Making the substraction and save it in a variable, then checking it, is different from checking the substraction on the fly?

My second test was done with uint32_t (long) data type:

#include <stdio.h>
#include <stdint.h>
#include <math.h>

int main()
{
    uint32_t test1 = 0;
    printf("test1 = %d\n", test1);

    uint32_t test2 = pow(2, 8 * sizeof(test1)) - 1; //max holdable value of uint32_t
    printf("test2 = %lu\n", test2);

    uint32_t test3 = test1 - test2;
    printf("test1 - test2 = %d\n", test3);

    if ((test1 - test2) == 0)
        printf("test1 == test2\n");
    if ((test1 - test2) > 0)
        printf("test1 > test2\n");
    if ((test1 - test2) < 0)
        printf("test1 < test2\n");

    if (test3 == 0)
        printf("test1 == test2\n");
    if (test3 > 0)
        printf("test1 > test2\n");
    if (test3 < 0)
        printf("test1 < test2\n");

    return 0;
}

output:

test1 = 0                                                                                                                                                       
test2 = 4294967295                                                                                                                                              
test1 - test2 = 1                                                                                                                                               
test1 > test2                                                                                                                                                   
test1 > test2

What??? Now making the substraction and saving it in a variable, then check it, is the same as checking the substraction on the fly?

SO I was expecting that substraction between unsigned values (without an explicit cast) ALWAYS returns a value >= 0. But doing the substraction inside the IF leads to unexpected results.

Now I'm confused. Can someone explain this behaviour to me?

Answer 1

The somewhat arcane type promotion rules apply. In the first example where the operands are uint8_t, for the expression:

test1 - test2

both operands are implicitly promoted to int before subtraction, and the expression itself has type int.