C: Implicit casting and integer overflowing in the evaluation of expressions

Question

Let's take the code

int a, b, c;
...
if ((a + b) > C)

If we add the values of a and b and the sum exceeds the maximum value of an int, will the integrity of the comparison be compromised? I was thinking that there might be an implicit up cast or overflow bit check and that will be factored into the evaluation of this expression.

Answer 1

C will do no such thing. It will silently overflow and lead to a possibly incorrect comparison. You can up-cast yourself, but it will not be done automatically.

Answer 2

A test confirms that GCC 4.2.3 will simply compare with the overflowed result:

#include <stdio.h>

int main()
{
    int a, b, c;

    a = 2000000000;
    b = 2000000000;
    c = 2100000000;

    printf("%d + %d = %d\n", a, b, a+b);
    if ((a + b) > c)
    {
        printf("%d + %d > %d\n", a, b, c);
    }
    else
    {
        printf("%d + %d < %d\n", a, b, c);
    }
    return 0;
}

Displays the following:

2000000000 + 2000000000 = -294967296
2000000000 + 2000000000 < 2100000000