Returning local data from functions in C and C++ via pointer

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

Your friend is wrong.

name is returning a pointer to the call stack. Once you invoke printf, there's no telling how that stack will be overwritten before the data at the pointer is accessed. It may work on his compiler and machine, but it won't work on all of them.

Your friend claims that after name returns, "nothing happens except printing it". printf is itself another function call, with who knows how much complexity inside it. A great deal is happening before the data is printed.

Also, code is never finished, it will be amended and added to. Code the "does nothing" now will do something once it's changed, and your closely-reasoned trick will fall apart.

Returning a pointer to local data is a recipe for disaster.

Answer 2

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);
}

Answer 3

As soon as the scope of the function ends i.e after the closing brace } of function, memory allocated(on stack) for all the local variables will be left. So, returning pointer to some memory which is no longer valid invokes undefined behavior. Also you can say that local variable lifetime is ended when the function finished execution.

Also more details you can read HERE.

Answer 4

My counter-arguments would be:

• it's never OK to write code with undefined behavior,
• how long before somebody else uses that function in different context,
• the language provides facilities to do the same thing legally (and possibly more efficiently)