c - How undefined is undefined behavior?

Question

I'm not sure I quite understand the extent to which undefined behavior can jeopardize a program.

Let's say I have this code:

#include <stdio.h>

int main()
{
    int v = 0;
    scanf("%d", &v);
    if (v != 0)
    {
        int *p;
        *p = v;  // Oops
    }
    return v;
}

Is the behavior of this program undefined for only those cases in which v is nonzero, or is it undefined even if v is zero?

Answer 1

I'd say that the behavior is undefined only if the users inserts any number different from 0. After all, if the offending code section is not actually run the conditions for UB aren't met (i.e. the non-initialized pointer is not created neither dereferenced).

A hint of this can be found into the standard, at 3.4.3:

behavior, upon use of a nonportable or erroneous program construct or of erroneous data, for which this International Standard imposes no requirements

This seems to imply that, if such "erroneous data" was instead correct, the behavior would be perfectly defined - which seems pretty much applicable to our case.

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Additional example: integer overflow. Any program that does an addition with user-provided data without doing extensive check on it is subject to this kind of undefined behavior - but an addition is UB only when the user provides such particular data.