How to check for variable overflow?

[Gat Pelsinger]

I most programming languages (C in my case) how do you check if a value being assigned to a variable doesn't overflow it and truncates its value? For example, if I have an unsigned integer called num, and I want to add a safety check that it is not less than 0, there is no way to do that is the value just wraps around and the conditional statement will never result in true (not). Maybe checking the number in string format?

[Sauron]

I don't think you can in C. Once the value has overflowed or underflowed its impossible to tell the current value is not correct.

 

The only way to prevent overflows is to check for them before running the operation or assignment.

Instead of this:

unsigned int a;

//some code here

a -= 1;

do this:

unsigned int a;

//some code here

if (a > 0) {
  a -= 1;
}

 

C will not prevent you from writing bad code. Some languages, like Rust, have built-in utilities to avoid this problem: https://doc.rust-lang.org/std/primitive.i32.html#method.checked_add but fundamentally the logic is the same; check that the operation would not cause an overflow before running it.

[BobVonBob]

Most string to numeric converting functions have bounds checking, and the ones that don't shouldn't be used on values that could be outside of the acceptable range. In the case of C with an unsigned integer you would generally use strtoul(), which sets errno if the value is out of bounds. You can check that as follows:

 

#include <errno.h>
#include <stdlib.h>
 
int main(void) {
    const char *strNum = "-40";
    char *end = NULL;
    unsigned int num = strtoul(strNum, &end, 10); // overflows
    if (errno == ERANGE) {
        // You've just had a range error
        errno = 0;
    }
}