Correct way of defining NULL and NULL_POINTER?

Question

As far as I know, C defines NULL like this:

#define NULL ( (void *) 0)

Then, how should we define NULL_POINTER ? I defined it the same in my program and it worked, but I suppose that is just a coincidence:

#define NULL_POINTER ( (void *) 0)

What would be the logical definition, if any ?

`NULL` **is** a null pointer, why do you want a separate define for that? — Bernhard Barker, Sep 13 '13 at 08:21
You should leave the definition for the implementation. Defining your own version of NULL can only make things worse. — Michael Foukarakis, Sep 13 '13 at 08:26

score 17 · Accepted Answer · edited Jun 20 '20 at 09:12

17

#define NULL ( (void *) 0)

and

#define NULL 0

are both valid. If you need to implement your own macro for null pointer, the same rule applies.

C11(ISO/IEC 9899:201x) §6.3.2.3 Pointers Section 3

An integer constant expression with the value 0, or such an expression cast to type void *, is called a null pointer constant

edited Jun 20 '20 at 09:12

Community

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answered Sep 13 '13 at 08:20

Yu Hao

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@PaulR According to http://port70.net/~nsz/c/c99/n1256.html#6.3.2.3, it's the same in C99. Although I do agree that the first definition is better. – Yu Hao Sep 13 '13 at 08:28
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The first definition seems better to me since in some compilers comparing a numeric value to NULL results in a warning (e.g. _"[Warning] comparison between pointer and integer"_) while the second definition doesn't. Thus, the first definition provides better semantics. – Piovezan Dec 11 '14 at 17:14

score 5 · Answer 2 · edited Sep 13 '13 at 08:28

5

It is only in pointer contexts that NULL and 0 are equivalent. NULL should not be used when another kind of 0 is required, even though it might work, because doing so sends the wrong stylistic message. (Furthermore, ANSI allows the definition of NULL to be ((void *)0), which will not work at all in non-pointer contexts.) In particular, do not use NULL when the ASCII null character (NUL) is desired. Provide your own definition

#define NUL '\0'

NULL should be used only as a pointer constant.

edited Sep 13 '13 at 08:28

Paul R

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answered Sep 13 '13 at 08:27

Dayal rai

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Just to be clear, this is referencing the [c-faq](http://c-faq.com/null/nullor0.html). – Freddie Sep 13 '13 at 08:29

Correct way of defining NULL and NULL_POINTER?

2 Answers2

C11(ISO/IEC 9899:201x) §6.3.2.3 Pointers Section 3

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