Question

I have argument with my friend. He says that I can return a pointer to local data from a function. This is not what I have learned but I can’t find a counterargument for him to prove my knowledge.

Here is illustrated case:

char *name() {
    char n[10] = "bodacydo!";
    return n;
}

And it’s used as:

int main() {
    char *n = name();
    printf("%s\n", n);
}

He says this is perfectly OK because after a program calls name, it returns a pointer to n, and right after that it just prints it. Nothing else happens in the program meanwhile, because it’s single threaded and execution is serial.

I can’t find a counter-argument. I would never write code like that, but he’s stubborn and says this is completely ok. If I was his boss, I would fire him for being a stubborn idiot, but I can’t find a counter argument.

Another example:

int *number() {
    int n = 5;
    return &n;
}

int main() {
    int *a = number();
    int b = 9;
    int c = *a * b;
    printf("%d\n", c);
}

I will send him this link after I get some good answers, so he at least learns something.

Answer 1

you will get a problem, when you call another function between name() and printf(), which itself uses the stack

char *fun(char *what) {
   char res[10];
   strncpy(res, what, 9);
   return res;
}

main() {
  char *r1 = fun("bla");
  char *r2 = fun("blubber");
  printf("'%s' is bla and '%s' is blubber", r1, r2);
}