How to subtract two unsigned ints with wrap around or overflow

Question

There are two unsigned ints (x and y) that need to be subtracted. x is always larger than y. However, both x and y can wrap around; for example, if they were both bytes, af

Answer 1

To echo everyone else replying, if you just subtract the two and interpret the result as unsigned you'll be fine.

Unless you have an explicit counterexample.

Your example of x = 0x2, y= 0x14 would not result in 0x4, it would result in 0xEE, unless you have more constraints on the math that are unstated.

Answer 2

Just to put the already correct answer into code:

If you know that x is the smaller value, the following calculation just works:

int main()
{
    uint8_t x = 0xff;
    uint8_t y = x + 20;
    uint8_t res = y - x;
    printf("Expect 20: %d\n", res); // res is 20

    return 0;
}

If you do not know which one is smaller:

int main()
{
    uint8_t x = 0xff;
    uint8_t y = x + 20;
    int8_t res1 = (int8_t)x - y;
    int8_t res2 = (int8_t)y - x;
    printf("Expect -20 and 20: %d and %d\n", res1, res2);

    return 0;
}

Where the difference must be inside the range of uint8_t in this case.

The code experiment helped me to understand the solution better.