free() same heap memory with different pointers

Question

int main()
{
    int* Pointer;

    Pointer = (int*) malloc(sizeof(int));
    *Pointer = 33;

    int* Pointer2 = Pointer;

    printf("%d\n", *Pointer);

    free(Pointer);
    free(Pointer2);

    return 0;
}

The output is 33 with no errors or warnings. I declared two pointers that are pointing to the same heap address. I know it's wrong to free them both and it is sufficient to only free one. Is it undefined if I free them both and will it do anything wrong if I free the same heap area from different pointers (Pointer and Pointer2)?

It's not OK. Maybe nothing will happen or maybe the system will crash. — Fiddling Bits, Mar 02 '17 at 15:11
@StoryTeller, the question isn't whether the code performs a double free, but what the effect of that is. — John Bollinger, Mar 02 '17 at 15:14
@JohnBollinger - The dup I suggested asks the same question in different words. I see no difference between asking "is this undefined" and "is this as bad as directly causing UB". But by all means, suggest a better dup. I doubt there's a shortage of them. — StoryTeller - Unslander Monica, Mar 02 '17 at 15:16
For safety you should null the second pointer. Depending on the system you could access undefined memory. — Alex, Mar 02 '17 at 15:17
@StoryTeller, evidently you and I interpret your proposed dupe differently. As far as I can tell, your proposed dupe and all the answers to it are focused tightly on the issue of whether a double free is defined in terms of the *value* of the pointer that is freed, or in terms of the *variable* holding that value. I don't see that as being more than tangentially related to this question, which acknowledges that there is double free, and asks about its *effect*. — John Bollinger, Mar 02 '17 at 15:26
@JohnBollinger - *"evidently you and I interpret your proposed dupe differently"* Yes it's obvious we are. But I reiterate: please propose a better one, seeing as this question is asked often enough that I found several options and chose poorly. Assuming of course the purpose of this entire debate is to find a good dup, and not to make sure a low hanging fruit is left up for grabs. — StoryTeller - Unslander Monica, Mar 02 '17 at 15:30

score 1 · Accepted Answer · answered Mar 02 '17 at 15:17

As per the C11 standard draft 7.22.3.3p2

The free function causes the space pointed to by ptr to be deallocated, that is, made available for further allocation. If ptr is a null pointer, no action occurs. Otherwise, if the argument does not match a pointer earlier returned by a memory management function, or if the space has been deallocated by a call to free or realloc, the behavior is undefined.

(The emphasis is mine..)

free() same heap memory with different pointers

1 Answers1