Returning ptrs to local function variables

[RealityVelJackson]

I was under the impression that one shouldn't return pointers to variables that are created inside of a function, unless the variable was static or allocated on the heap. Why does the following code work fine ? In foo1() since
str points to a string-literal, I guess the string-literal is allocated in a memory location outside of the stack frame, so it won't be deallocated when the function returns.
If this is true then I have no problem here.
I'm confused by the fact that the value in int x is not wiped-out when foo2() returns. My understanding is that int x is created on the stack-frame and deallocated by the time control is passed back to main(). How could a pointer to a deallocated auto variable still retrieve the value ?

#include <stdio.h>

char* foo1();
int* foo2();

int main(){

	char* s;
	int* d;

	s = foo1();
	printf("%s\n", s);

	d = foo2();
	printf("%d\n", *d);

	return 0;
}

char* foo1(){

	char* str = "hello";
	return str;
}

int* foo2(){

	int x = 8;
	return &x;
}

 

[TrojanWarBlade]

Because you're returning "by value" effectively.
The literal "hello" is statically allocated and will be available for the duraction of the life of the code.
Th char* str get's assigned the address of the start of this string.
The "return str" returns the VALUE of "str", not a reference to it. The VALUE of "str" is the ADDRESS of the literal string "hello" which, as I said earlier, is safe.

Does that make sense?



Trojan.

 

[columb]

My understanding is that, although the memory pointed to by &x has been deallocated it hasn't been overwritten by the time you come to print it. I would bet that

  d = foo2();
  /*Lots of other stuff*/
  .......
  .......
  printf("%d\n", *d);

would give random results.

Columb Healy

 

[TrojanWarBlade]

With foo2 I agree completely.
You are at serious risk if you return pointers to released stack space.
Bad idea.


Trojan.

 

[jstreich]

In regards to foo1, sting literals are static memory.

In regards to foo2, it lived on the stack, and because C/C++ isn't a "safe" language you can get the old data untill it is overwritten (in this case, by another function call).

try adding a foo3, like this

void foo3()
{
  int x = 300, y = 200, z = 500;
  for(int i = 0; i < 10; i++){ //only here to stop optimizer 
    int a = x+y+z+i;           // may not be needed.
    printf("%d\n", a);         // but better safe then sorry
  } 
}

and change main to:

int main(){

    char* s;
    int* d;

    s = foo1();
    printf("%s\n", s);

    d = foo2();
    foo3()
    printf("%d\n", *d);

    return 0;
}

And when I compile it I get 300 from printing out d... no suprise there, as the new x is alloced at the same address (time in function) as the old x.

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