Is every null pointer constant a null pointer?

 

Is every null pointer constant a null pointer?

From the C17 draft (6.3.2.3 ¶3):

An integer constant expression with the value 0, or such an expression cast to type void *, is called a null pointer constant.⁶⁷⁾ If a null pointer constant is converted to a pointer type, the resulting pointer, called a null pointer, is guaranteed to compare unequal to a pointer to any object or function.

⁶⁷⁾The macro NULL is defined in <stddef.h> (and other headers) as a null pointer constant [...].

From this, it follows that the following are null pointer constants: 0, 0UL, (void *)0, (void *)0UL, NULL.

It further follows that the following are null pointers: (int *)0, (int *)0UL, (int *)(void *)0, (int *)(void *)0UL, (int *)NULL. Interestingly, none of these are "null pointer constants"; see here.

The following null pointer constants are null pointers (because void * is a pointer type and 0 and 0UL are null pointer constants): (void *)0, (void *)0UL. In this regard, according to the C17 draft (6.2.5 ¶19-20):

The void type comprises an empty set of values; it is an incomplete object type that cannot be completed.
[...]
A pointer type may be derived from a function type or an object type, called the referenced type. [...] A pointer type is a complete object type.

void is not a pointer type itself, and it is an incomplete object type. But void * is a pointer type.

But it seems that the following are null pointer constants which are not null pointers (because there is no cast to a pointer type): 0, 0UL, NULL. (To be precise, while the standard only requires that NULL be defined as "a null pointer constant", it would be permissible to define it as a null pointer constant which is also a null pointer. But it seems that the standard doesn't require NULL to be defined in such a way that it is simultaneously a null pointer.)

Is every null pointer constant a null pointer? (Is NULL really not a null pointer?)

Finally (and somewhat tongue-in-cheek): In case certain null pointer constants are not null pointers, would they technically be a kind of "non-null pointer"? (This wording appears in some places in the standard.) Note that linguistically we have a so-called bracketing paradox; we can read this as "[non-null] pointer" or "non-[null pointer]".

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No. In fact, no null pointer constant is a null pointer! This is because constants and pointers are different kinds of entities.

A null pointer constant is a constant expression which has a particular form. An expression is a sequence of tokens, and null pointer constants are defined as sequences of tokens that have a particular form.

A null pointer is a value. In C, each type has its set of potential values. For each pointer type, one or more value in that set is a null pointer. The C standard does not define the concept of value formally. A formal semantics would need to do this (and formally defining values of pointers gets rather complicated, which is why the C standard, an English document written without mathematics, doesn't try).

An expression evaluates to a value in a context (possibly causing side effects). All null pointer constants whose type is a pointer type evaluate to a null pointer. Some null pointer constants (e.g. 0, 1L - 'z' / 'z') have an integer type, and those do not evaluate to a null pointer: they evaluate to a null integer (i.e. an integer with the value 0 — the C standard does not use the expression “null integer” because it isn't anything remarkable that would need a specific name).

The C standard guarantees that if e is a constant expression with an integer type and the value 0, then any expression that converts this value to a pointer type evaluates to a null pointer. Note that this guarantee is not given for arbitrary expressions: (void*) f() might not be a null pointer even if f is defined as int f(void) { return 0; }.

The C standard allows NULL to have either an integer type or a pointer type. If it has a pointer type, the expression NULL evaluates to a null pointer. If it has an integer type, it doesn't.

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Is every null pointer constant a null pointer?

TL;DR: no.

As you have already observed, integer constant expressions with value 0 are null pointer constants, despite not having pointer type. You have also quoted the specification's definition of null pointer: "a null pointer constant [] converted to pointer type". That means that null pointer constants of this general form ...

(void *)(<integer constant expression with value 0>)

... satisfy the definition of "null pointer". The integer constant expression is a null pointer constant itself, so the cast makes the overall expression a null pointer (in addition to being a null pointer constant).

On the other hand, null pointer constants that take the form of integer constant expressions with value 0 do not satisfy the definition of "null pointer", and there is no other provision in the language spec that would make them null pointers. Examples: 0, 0x00UL, 1 + 2 + 3 - 6.

it seems that the standard doesn't require NULL to be defined in such a way that it is simultaneously a null pointer.

Correct.

Is every null pointer constant a null pointer?

Definitely not (see above), but for most purposes, it does not matter.

(Is NULL really not a null pointer?)

It depends on your C implementation. The language spec allows either answer. In practice, it is a null pointer in most implementations you're likely to meet.

In case certain null pointer constants are not null pointers, would they technically be a kind of "non-null pointer"?

No. Null pointer constants that are not null pointers are not pointers at all. They are integers.