Question

Related question: How to detect integer overflow?

In C code, should integer overflow be addressed whenever integers are added? It seems like pointers and array indexes should be checked at all. When should integer overflow be checked for?

When numbers are added in C without type explicitly mentioned, or printed with printf, when will overflow occur?

Is there a way to automatically detect integer arithmetic overflow?

Answer 1

I’ve heard about setjmp()- or longjmp()-based exception handling in C, but I think there’s no native way of doing this. What I usually do is just make sure the types used are long enough to contain all the additions/multiplications I’ll need to make.

The whole point of using C, as opposed to managed languages such as C#, which will throw an OverflowException, is precisely the fact that no CPU power is wasted on safety checks. C will simply turn the counter around, and go from FFFFFFFF to 00000000, so you can check for that (if a>b and such), but other than that I can just recommend using longer types. 64 bits (long long) should address all your needs.

Overflow won’t occur when you print a number with printf, or at least I haven’t heard of such a possibility. For additions, I’d just use adequate types and tell the compiler how to interpret the values so that you can avoid unnecessary casts (like, the literal “123” will be interpreted as 32 bit, but “123LL” will be 64 bit – same as with “.1f” vs. “.1”).

For array indices – you should always make sure you don’t read/write out of your array, as C in many cases will happily corrupt your data without causing an error.

As for when integer overflow should be checked for… Well, whenever it may occur and you don’t want it to occur :).