Question

Why this code is giving unexpected output.

int * func(int *xp)
{
    int y = 10 + *xp;
    return (&y);
}
void main()
{
    int x = 10;
    int *xp = func(&x);
    printf("%d\n", x);
    printf("%d\n", *xp);
}

expected output:

10
20

Real Output:

10
1074194112

Answer 1

You are returning a pointer to a variable (y) that goes out of scope the moment you leave func. This is undefined behaviour.

It is not entirely clear what you’re trying to achieve, but there are several ways to fix this:

1. return the int by value;
2. modify *xp in place and don’t return anything;
3. take an additional int* parameter and store the result in there (again returning void);
4. allocate memory on the heap using malloc or new and return that (the caller would be responsible for correctly freeing that memory).

It is impossible to say which of the above are applicable to your problem without knowing the broader context.

Lastly, main() should return int (thanks @pmg for spotting this). This is not related to the problem you’re having but is worthing pointing out.