Why this form is correct in C?

Question

I have this piece of code in C:

void f(void *x) {
    printf(">>> %p\n", x);
}

int main() {

  f NULL;
    return 0;
}

I think is for the definition of NULL, but I'd like an explanation to clarify my doubt.

iirc NULL is a #define'd to ((void*)0), so when its copied in you have f((void*)0) which works as a parameter for the function — Daboyzuk, May 31 '13 at 08:10
If you're runing gcc, try gcc -E and see what happens. I think you'll understand — Xaqq, May 31 '13 at 08:12

score 9 · Accepted Answer · answered May 31 '13 at 08:09

9

If NULL is defined as ((void *)0) or (0), then this expands to f ((void *)0) or f (0), which are proper function calls. The code errors out for anything non-parenthesized during compilation.

answered May 31 '13 at 08:09

I downvoted, and removed my downvote after noticing that it was a "monday morning" moment... – Johannes Schaub - litb May 31 '13 at 08:17
Probably this is consider a stupid question for programmers in C. I'm a beginner of C (but I love it!!). ;) – Kyrol May 31 '13 at 08:23

jbr · Answer 2 · 2013-05-31T08:18:05.367

2

In C NULL is often defined as follows:

#if !defined(NULL)
    #define NULL ((void*)0)
#endif

If this is the case then NULL is just a special pointer, and you example works.

edited May 31 '13 at 08:18

answered May 31 '13 at 08:10

jbr

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No, the C standard doesn't define how NULL should be defined. – May 31 '13 at 08:10
5

Also, `NULL` is not a pointer *to the address 0*. Rather, the *literal value 0*, when encountered in a pointer context, is the null pointer. The *actual* value of the null pointer is implementation-specific and can be anything. – Jon May 31 '13 at 08:12
2

Fair enough, the standard says that the definition of NULL is implementation specific. However in section 6.3.2.3 part 3, in http://www.open-std.org/jtc1/sc22/WG14/www/docs/n1256.pdf. It says that a `0` cast to (void *) is called a null pointer constant. – jbr May 31 '13 at 08:17
3

@jbr: Exactly, a *literal* `0` gives a null ptr. But for example, this is not (technically) a null ptr: `int a = 0; void* p = (void*)a;`. – Jon May 31 '13 at 08:20

Why this form is correct in C?

2 Answers2