How to check for signed integer overflow in C without undefined behaviour?

Question

There\'s (1):

// assume x,y are non-negative
if(x > max - y) error;

And (2):

// assume x,y are non-negative
int sum = x          


        

Answer 1

You only have to check one of them. If x + y overflows, it will be less than both x and y. Hence:

int sum = x + y;
if (sum < x) error;

should be sufficient.

The following site has a bunch of stuff about integer overflow:

http://www.fefe.de/intof.html

If you want to handle negative numbers, it can be expanded:

int sum = x + y;
if (y >= 0) {
   if (sum < x) error;
} else {
   if (sum > x) error;
}