Assign struct pointer to a struct pointer inside of null struct pointer

Question

This question might be stupid, but I've been wondering what happens if you have:

struct List {
    int x;
    List *next;
};
struct List *start = nullptr;
struct List *tmp = start->next;

What happens to tmp?

I've tried compiling it and I get no errors. If I output address of start I get 0, but if I output tmp I get nothing.

You dereference a null pointer (start), so you have undefined behaviour. After you do `start->next` you can't reason about your program. — , Nov 18 '18 at 17:11
That should be nullpointer access. I dont know how you are not getting run time error! — Mukesh Verma, Nov 18 '18 at 17:14
@Mukesh Undefined behaviour does not guarantee a runtime error. — , Nov 18 '18 at 17:14

score 0 · Answer 1 · answered Nov 18 '18 at 17:32

0

On the line struct List *tmp = start->next; since we know that start == nullptr you are dereferencing a nullptr value. This is UB (undefined behavior) and as such "anything" can happen.

answered Nov 18 '18 at 17:32

Bo R

2,334
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score -1 · Answer 2 · answered Nov 18 '18 at 17:17

-1

It is an undefined behavior. After multiple execusion it may output a different result or maybe a segmentation fault.

answered Nov 18 '18 at 17:17

Ebrahim

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Assign struct pointer to a struct pointer inside of null struct pointer

2 Answers2