Can a conforming C implementation #define NULL to be something wacky

Question

I'm asking because of the discussion that's been provoked in this thread.

Trying to have a serious back-and-forth discussion using comments under other people's replies is not easy or fun. So I'd like to hear what our C experts think without being restricted to 500 characters at a time.

The C standard has precious few words to say about NULL and null pointer constants. There's only two relevant sections that I can find. First:

3.2.2.3 Pointers

An integral constant expression with the value 0, or such an expression cast to type void * , is called a null pointer constant. If a null pointer constant is assigned to or compared for equality to a pointer, the constant is converted to a pointer of that type. Such a pointer, called a null pointer, is guaranteed to compare unequal to a pointer to any object or function.

and second:

4.1.5 Common definitions

The macros are

NULL

which expands to an implementation-defined null pointer constant;

The question is, can NULL expand to an implementation-defined null pointer constant that is different from the ones enumerated in 3.2.2.3?

In particular, could it be defined as:

#define NULL __builtin_magic_null_pointer

Or even:

#define NULL ((void*)-1)

My reading of 3.2.2.3 is that it specifies that an integral constant expression of 0, and an integral constant expression of 0 cast to type void* must be among the forms of null pointer constant that the implementation recognizes, but that it isn't meant to be an exhaustive list. I believe that the implementation is free to recognize other source constructs as null pointer constants, so long as no other rules are broken.

So for example, it is provable that

#define NULL (-1)

is not a legal definition, because in

if (NULL) 
   do_stuff(); 

do_stuff() must not be called, whereas with

if (-1)
   do_stuff();

do_stuff() must be called; since they are equivalent, this cannot be a legal definition of NULL.

But the standard says that integer-to-pointer conversions (and vice-versa) are implementation-defined, therefore it could define the conversion of -1 to a pointer as a conversion that produces a null pointer. In which case

if ((void*)-1) 

would evaluate to false, and all would be well.

So what do other people think?

I'd ask for everybody to especially keep in mind the "as-if" rule described in 2.1.2.3 Program execution. It's huge and somewhat roundabout, so I won't paste it here, but it essentially says that an implementation merely has to produce the same observable side-effects as are required of the abstract machine described by the standard. It says that any optimizations, transformations, or whatever else the compiler wants to do to your program are perfectly legal so long as the observable side-effects of the program aren't changed by them.

So if you are looking to prove that a particular definition of NULL cannot be legal, you'll need to come up with a program that can prove it. Either one like mine that blatantly breaks other clauses in the standard, or one that can legally detect whatever magic the compiler has to do to make the strange NULL definition work.

Steve Jessop found an example of way for a program to detect that NULL isn't defined to be one of the two forms of null pointer constants in 3.2.2.3, which is to stringize the constant:

#define stringize_helper(x) #x
#define stringize(x) stringize_helper(x) 

Using this macro, one could

puts(stringize(NULL));

and "detect" that NULL does not expand to one of the forms in 3.2.2.3. Is that enough to render other definitions illegal? I just don't know.

Thanks!

Answer 1

Well, I've found a way to prove that

#define NULL ((void*)-1)

is not a legal definition of NULL.

int main(void) 
{ 
   void (*fp)() = NULL;   
}

Initializing a function pointer with NULL is legal and correct, whereas...

int main(void) 
{ 
   void (*fp)() = (void*)-1;   
}

...is a constraint violation that requires a diagnostic. So that's out.

But the __builtin_magic_null_pointer definition of NULL wouldn't suffer that problem. I'd still like to know if anybody can come up with a reason why it can't be.

Answer 2

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

NULL which expands to an implementation-defined null pointer constant;

therefore either

NULL == 0

or

NULL == (void *)0

Answer 3

In the C99 standard, 7.17.3 states that NULL “expands to an implementation defined null pointer constant”. Meanwhile 6.3.2.3.3 defines null pointer constant as “an integer constant expression with the value 0, or such an expression cast to type void *”. As there is no other definition for a null pointer constant, a conforming definition of NULL must expand to an integer constant expression with the value zero (or this cast to void *).

Further quoting from the C FAQ question 5.5 (emphasis added):

Section 4.1.5 of the C Standard states that NULL “expands to an implementation-defined null pointer constant,” which means that the implementation gets to choose which form of 0 to use and whether to use a `void *` cast; see questions 5.6 and 5.7. “Implementation-defined” here does not mean that NULL might be #defined to match some implementation-specific nonzero internal null pointer value.

It makes perfect sense; since the standard requires a zero integer constant in pointer contexts to compile into a null pointer (regardless of whether or not the machine's internal representation of that has a value of zero), the case where NULL is defined as zero must be handled anyhow. The programmer is not required to type NULL to obtain null pointers; it's just a stylistic convention (and may help catch errors e.g. when a NULL defined as (void *)0 is used in a non-pointer context).

Edit: One source of confusion here seems to be the concise language used by the standard, i.e. it does not explicitly say that there is no other value that might be considered a null pointer constant. However, when the standard says “…is called a null pointer constant”, it means that exactly the given definitions are called null pointer constants. It does not need to explicitly follow every definition by stating what is non-conforming when (by definition) the standard defines what is conforming.

Answer 4

The null pointer constant must evaluate 0, otherwise expressions like !ptr would not work as expected.

The NULL macro expands to a 0-valued expression; AFAIK, it always has.