Is a NULL pointer's dereference also equals NULL? [duplicate]

Question

Does the following snippet compile/execute the block in the if-statement?

int* pointer = NULL; int deref = *pointer; if(deref == NULL){ // will execute? }

Since the pointer variable contains NULL does the dereference of this pointer variable also return NULL or will this result in a runtime error?

@LưuVĩnhPhúc The ISO C standard doesn't impose any requirements like "cannot happen" to the dereferencing of a null pointer. It is undefined behavior, and in fact can be done in some environments and can be provided as a documented language extension (though not a very well considered one, obviously). — Kaz
– Kaz, Commented Sep 21, 2016 at 5:39
Historically, it has happened that C programmers exploited dereference-able null pointers to simplify the cases in linked list or tree data structures. you can do things like head_node->next->prev = that without worrying that nead_node->next is the terminating null link. Obviously, that code was found not to be portable to environments that trap null references, oops! It wasn't wrong; it worked fine on the original target platforms, and code was shipped to happy end users. It was nonportable. — Kaz
– Kaz, Commented Sep 21, 2016 at 5:43
@LưuVĩnhPhúc "cannot" isn't entirely correct. It can be done, but the result is undefined. — Johann Gerell
– Johann Gerell, Commented Sep 21, 2016 at 8:08

Brian Bi · Accepted Answer · 2016-09-21 05:18:50Z

3

The result is "undefined behaviour", which may or may not trigger a runtime error, and should in any case always be avoided.

answered Sep 21, 2016 at 5:18

Brian Bi

123k13 gold badges197 silver badges353 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

adisticated · Accepted Answer · 2016-09-21 06:01:31Z

Once you set:

int* pointer = NULL;

pointer points to nothing. Now when you write this:

int deref = *pointer;

deref will try to access what pointer points to, which will lead to an undefined behaviour like segmentation fault.

I hope this explains.

Humam Helfawi · Accepted Answer · 2016-09-21 09:19:01Z

1

The current answers addressed the UB very well. However, I want to add something. If you run this code:

if(0==NULL){ std::cout << "True"; }

It will prints True. So if dereferencing a null pointer on your specific environment leads to returning 0 (which is not a steady case. It is UB), the part inside your if statement will execute.

I just wanted to clarify why it is working on some machines. However, that does not change anything about the fact that it is UB.

edited Sep 21, 2016 at 9:19

answered Sep 21, 2016 at 5:30

Humam Helfawi

20.4k18 gold badges92 silver badges170 bronze badges

4 Comments

Johann Gerell Over a year ago

Just pointing out that your example doesn't dereference NULL, but the following paragraph does talk about that. What was your intent?

Humam Helfawi Over a year ago

@JohannGerell I just wanted to inform the OP that his code might work on some machines and execute the instructions inside the if-statement. This behaviour still undefined but some coders may think that it is defined since it is working and behave like theirs intention. So, I see it worth to tell.

Johann Gerell Over a year ago

What you're addressing then is the case when the '0' in your example if(0==NULL) happens to be the result of dereferencing a null pointer on some platform some time, right? I was confused by how you formulated this.

Humam Helfawi Over a year ago

Yup exactly that what I meant

Collectives™ on Stack Overflow

Is a NULL pointer's dereference also equals NULL? [duplicate]

3 Answers 3

Comments

Comments

4 Comments

Linked

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

Comments

Comments

4 Comments

Linked

Related