How is NULL different from 0 in C?

Question

If these 3 lines are the only time NULL is mentioned in stdio.h, how is NULL different from 0 and most importantly what tells the compiler NULL is actually an invalid address? Talking about C, maybe in other libraries, like in iostream, NULL is defined differently.

0085 #ifndef NULL
0086 #define NULL 0
0087 #endif

What makes you think that the compiler does know that NULL is an invalid address? — Yunnosch, May 13 '18 at 20:35
@Yunnosch The way Clang compiles this function shows that it knows that `NULL` is an invalid address. https://godbolt.org/g/KsydcB — Pascal Cuoq, May 13 '18 at 20:41
@Yunnosch I'm taught NULL and 0 are different, therefore the compiler must see some kind of difference. — Larry Teischwilly, May 13 '18 at 21:44

Angew is no longer proud of SO · Accepted Answer · 2018-05-13T20:43:04.380

2

To the compiler, NULL is indistinguishable from 0 when used as a pointer. The standard actually defines 0 to be a null pointer value, and the standard library just introduces a convenient macro NULL defined to a null pointer value (usually 0 or ((void*)0)), so that you can use it in code for better readability and expressing intent. But there's nothing special about NULL itself; it's the 0 that is relevant.

edited May 13 '18 at 20:43

answered May 13 '18 at 20:35

Angew is no longer proud of SO

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"To the compiler, NULL is indistinguishable from 0" if defined as `(void *)0` the size may differ. I got bitten by that on a 64-bit architecture when I passed 0 to nul terminate a variable length array. – Jean-François Fabre May 13 '18 at 20:38
@Jean-FrançoisFabre Added clarification; is it better? – Angew is no longer proud of SO May 13 '18 at 20:39
@Jean-FrançoisFabre Ah, the classic `execl` problem? :-) – melpomene May 13 '18 at 20:39
@melpomene someone just tried to edit the title as "Excel" instead of "execl" ... – Jean-François Fabre May 13 '18 at 20:40
@Angew yes. it was a cornercase but still... – Jean-François Fabre May 13 '18 at 20:40
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*The standard actually defines 0 to be the null pointer value* The [standard defines `0` to be **a** null pointer value](https://port70.net/~nsz/c/c11/n1570.html#6.3.2.3p3), not **the** null pointer value. "The null pointer value" does not have to be zero. – Andrew Henle May 13 '18 at 20:41

score 1 · Answer 2 · answered May 13 '18 at 20:37

To the compiler, all of 0, NULL and '\0' map to the same value. However, from the readability perspective, you may want to use the appropriate value depending on the context, to make it clear what you're trying to do and to improve the efficacy of code auditing and reviews.

How is NULL different from 0 in C?

2 Answers2