C Programming – Detecting Unsigned Integer Overflow When Adding Two Numbers

c++integer-overflowunsignedunsigned-integer

This is my implementation to detect if an unsigned int overflow has occurred when trying to add two numbers.

The max value of unsigned int (UINT_MAX) on my system is 4294967295.

void check_addition_overflow(unsigned int a, unsigned int b) {
   if (b > (UINT_MAX - a)) {
     printf("overflow has occured\n");
   }
}

This seems to work with the values I've tried.

Any rogue cases? What do you think are the pros and cons?

Best Answer

You could use

if((a + b) < a)

The point is that if a + b is overflowing, the result will be trimmed and must be lower then a.

Consider the case with hypothetical bound range of 0 -> 9 (overflows at 10):

b can be 9 at the most. For any value a such that a + b >= 10, (a + 9) % 10 < a.
For any values a, b such that a + b < 10, since b is not negative, a + b >= a.

Related Solutions

Detecting Signed Overflow in C/C++ – Methods and Best Practices

No, your 2nd code isn't correct, but you are close: if you set

int half = INT_MAX/2;
int half1 = half + 1;

the result of an addition is INT_MAX. (INT_MAX is always an odd number). So this is valid input. But in your routine you will have INT_MAX - half == half1 and you would abort. A false positive.

This error can be repaired by putting < instead of <= in both checks.

But then also your code isn't optimal. The following would do:

int add(int lhs, int rhs)
{
 if (lhs >= 0) {
  if (INT_MAX - lhs < rhs) {
   /* would overflow */
   abort();
  }
 }
 else {
  if (rhs < INT_MIN - lhs) {
   /* would overflow */
   abort();
  }
 }
 return lhs + rhs;
}

To see that this is valid, you have to symbolically add lhs on both sides of the inequalities, and this gives you exactly the arithmetical conditions that your result is out of bounds.

Note in 2023: C23 will have the <stdckdint.h> header that implements such overflow checks in the same way as the gcc builtins that are mentioned in other answers.

Best Answer

Related Solutions

Detecting Signed Overflow in C/C++ – Methods and Best Practices

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