Question

I saw a buggy code in C which was used to check whether addition results in overflow or not. It works fine with char, but gives incorrect answer when arguments are int and I couldn’t figure why .
Here’s the code with short arguments.

short add_ok( short x, short y ){
    short sum = x+y;
    return (sum-x==y) && (sum-y==x);
}

This version works fine, problem arise when you change arguments to int ( you can check it with INT_MAX )
Can you see what’s wrong in here ?

Answer 1

Because in 2s complement, the integers can be arranged into a circle (in the sense of modulo arithmetic). Adding y and then subtracting y always gets you back where you started (undefined behaviour notwithstanding).