27

We know CPython promotes integers to long integers (which allow arbitrary-precision arithmetic) silently when the number gets bigger.

How can we detect overflow of int and long long in pure C?

Improve this question
4
  • 4
    It's very tricky since you just can't add two numbers and check if the value is above some threshold (because signed integer arithmetic overflow and such). A simple solution might be to check if x (the value you want to check) is above a specific threshold, or if adding one goes above a threshold. If it does and the other number you want to add is larger than one, then you have an overflow situation. Commented Apr 2, 2019 at 7:11
  • 2
    Nitpick, but, it was CPython 2.7 that did this. CPython 3 doesn't "promote" anything, even internally there is just one type. Commented Apr 2, 2019 at 12:49
  • 1
    there are a lot of duplicates depending on what you want to do with the values (add/sub/mul/div/...?) How to check if A+B exceed long long? (both A and B is long long), Detecting signed overflow in C/C++, Test for overflow in integer addition Commented Apr 2, 2019 at 14:00
  • and add 1 more codereview.stackexchange.com/questions/37177/… Commented Apr 2, 2019 at 14:36

3 Answers 3

Reset to default
33

You can predict signed int overflow but attempting to detect it after the summation is too late. You have to test for possible overflow before you do a signed addition.

It's not possible to avoid undefined behaviour by testing for it after the summation. If the addition overflows then there is already undefined behaviour.

If it were me, I'd do something like this:

#include <limits.h>

int safe_add(int a, int b) 
{
    if (a >= 0) {
        if (b > (INT_MAX - a)) {
            /* handle overflow */
        }
    } else {
        if (b < (INT_MIN - a)) {
            /* handle underflow */
        }
    }
    return a + b;
}

Refer this paper for more information. You can also find why unsigned integer overflow is not undefined behaviour and what could be portability issues in the same paper.

EDIT:

GCC and other compilers have some provisions to detect the overflow. For example, GCC has following built-in functions allow performing simple arithmetic operations together with checking whether the operations overflowed.

bool __builtin_add_overflow (type1 a, type2 b, type3 *res)
bool __builtin_sadd_overflow (int a, int b, int *res)
bool __builtin_saddl_overflow (long int a, long int b, long int *res)
bool __builtin_saddll_overflow (long long int a, long long int b, long long int *res)
bool __builtin_uadd_overflow (unsigned int a, unsigned int b, unsigned int *res)
bool __builtin_uaddl_overflow (unsigned long int a, unsigned long int b, unsigned long int *res)
bool __builtin_uaddll_overflow (unsigned long long int a, unsigned long long int b, unsigned long long int *res)

Visit this link.

EDIT:

Regarding the question asked by someone

I think, it would be nice and informative to explain why signed int overflow undefined, whereas unsigned apperantly isn't..

The answer depends upon the implementation of the compiler. Most C implementations (compilers) just used whatever overflow behaviour was easiest to implement with the integer representation it used.

In practice, the representations for signed values may differ (according to the implementation): one's complement, two's complement, sign-magnitude. For an unsigned type there is no reason for the standard to allow variation because there is only one obvious binary representation (the standard only allows binary representation).

Improve this answer
Sign up to request clarification or add additional context in comments.

11 Comments

Why extra parentheses? Also you could save one test on average with if (a >= 0) { test overflow } else { test underflow } return a + b;
@chqrlie that is not sufficient because there is no possibility of overflow when a == 0.
It is not necessary to test overflow if a == 0 but testing a just once saves one comparison if a < 0, which is half the cases.
Also, both are technically called overflow. Underflow means that the value is too small in magnitude to be representable in a floating point variable.
@AnttiHaapala it does not ignore the case a == 0 where there is no possible overflow, it just handles it differently.
|
32

You cannot detect signed int overflow. You have to write your code to avoid it.

Signed int overflow is Undefined Behaviour and if it is present in your program, the program is invalid and the compiler is not required to generate any specific behaviour.

Improve this answer

8 Comments

You can check you input values before doing a calculation to prevent overflow.
I think, it would be nice and informative to explain why signed int overflow undefined, whereas unsigned apperantly isn't..
@hetepeperfan It's because that's what the language standard says.
@sneftel thats an authoritative argument lacking an authoritative source, despise it is probably correct. On top of that, standards make more sense to people, once they start to understand the language, which is perhaps a reason they visit stackoverflow in the first place.
@hetepeperfan the reason for why the standard is written as it is, is for the most part outside the scope of Stack Overflow.
|
11

Signed operands must be tested before the addition is performed. Here is a safe addition function with 2 comparisons in all cases:

#include <limits.h>

int safe_add(int a, int b) {
    if (a >= 0) {
        if (b > INT_MAX - a) {
            /* handle overflow */
        } else {
            return a + b;
        }
    } else {
        if (b < INT_MIN - a) {
            /* handle negative overflow */
        } else {
            return a + b;
        }
    }
}

If the type long long is known to have a larger range than type int, you could use this approach, which might prove faster:

#include <limits.h>

int safe_add(int a, int b) {
    long long res = (long long)a + b;
    if (res > INT_MAX || res < INT_MIN) {
        /* handle overflow */
    } else {
        return (int)res;
    }
}
Improve this answer

Comments

Start asking to get answers

Find the answer to your question by asking.

Ask question

Explore related questions

See similar questions with these tags.