Why are there two ways of expressing NULL in C?

Question

According to §6.3.2.3 ¶3 of the C11 standard, a null pointer constant in C can be defined by an implementation as either the integer constant expression 0 or such an expression cast to void *. In C the null pointer constant is defined by the NULL macro.

My implementation (GCC 9.4.0) defines NULL in stddef.h in the following ways:

#define NULL ((void *)0)
#define NULL 0

Why are both of the above expressions considered semantically equivalent in the context of NULL? More specifically, why do there exist two ways of expressing the same concept rather than one?

Answer 1

Let's consider this example code:

#include <stddef.h>
int *f(void) { return NULL; }
int g(int x) { return x == NULL ? 3 : 4; }

We want f to compile without warnings, and we want g to cause an error or a warning (because an int variable x was compared to a pointer).

In C, #define NULL ((void*)0) gives us both (GCC warning for g, clean compile for f).

However, in C++, #define NULL ((void*)0) causes a compile error for f. Thus, to make it compile in C++, <stddef.h> has #define NULL 0 for C++ only (not for C). Unfortunately, this also prevents the warning from being reported for g. To fix that, C++11 uses built-in nullptr instead of NULL, and with that, C++ compilers report an error for g, and they compile f cleanly.

Answer 2

((void *)0) has stronger typing and could lead to better compiler or static analyser diagnostics. For example since implicit conversions between pointers and plain integers aren't allowed in standard C.

0 is likely allowed for historical reasons, from a pre-standard time when everything in C was pretty much just integers and wild implicit conversions between pointers and integers were allowed, though possibly resulting in undefined behavior.

Ancient K&R 1st edition provides some insight (7.14 the assignment operator):

The compilers currently allow a pointer to be assigned to an integer, an integer to a pointer, and a pointer to a pointer of another type. The assignment is a pure copy operation, with no conversion. This usage is nonportable, and may produce pointers which cause addressing exceptions when used. However, it is guaranteed that assignment of the constant 0 to a pointer will produce a null pointer distinguishable from a pointer to any object.

Answer 3

Few things in C are more confusing than null pointers. The C FAQ list devotes an entire section to the topic, and to the myriad misunderstandings that eternally arise. And we can see that those misunderstandings never go away, as some of them are being recycled even in this thread, in 2022.

The basic facts are these:

1. C has the concept of a null pointer, a distinguished pointer value which points definitively nowhere.
2. The source code construct by which a null pointer is requested — a null pointer constant — fundamentally involves the token 0.
3. Because the token 0 has other uses, ambiguity (not to mention confusion) is possible.
4. To help reduce the confusion and ambiguity, for many years the token 0 as a null pointer constant has been hidden behind the preprocessor macro NULL.
5. To provide some type safety and further reduce errors, it's attractive to have the macro definition of NULL include a pointer cast.
6. However, and most unfortunately, enough confusion crept in along the way that properly mitigating it all has become almost impossible. In particular, there is so very much extant code that says things like strbuf[len] = NULL; (in an obvious but basically wrong attempt to null-terminate a string) that it is believed in some circles to be impossible to actually define NULL with an expansion including either the explicit cast or the hypothetical future (or extant in C++) new keyword nullptr.

See also Why not call nullptr NULL?

Footnote (call this point 3½): It's also possible for a null pointer — despite being represented in C source code as an integer constant 0 — to have an internal value that is not all-bits-0. This fact adds massively to the confusion whenever this topic is discussed, but it doesn't fundamentally change the definition.

Answer 4

There is just one way to express NULL in C, it's a single 4-character token.
But hold on, when going into its definition it gets more interesting.

NULL has to be defined as a null pointer constant, meaning an integer constant with value 0 or such cast to void*.
As an integer constant is just an expression of integer type with a few restrictions to guarantee static evaluation, there are infinite possibilities for any wanted value.

Of all those possibilities, only an integer literal with value 0 is also a null pointer constant in C++, for what it's worth.

The reason for such variation is history and precedent (everyone did it differently, void* was late to the party, and existing code/implementations trumps all), reinforced with backwards-compatibility which preserves it.

6.3.2.3 Pointers

[...] 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. [...]

6.6 Constant expressions

[...] Description
2 A constant expression can be evaluated during translation rather than runtime, and accordingly may be used in any place that a constant may be.
Constraints 3 Constant expressions shall not contain assignment, increment, decrement, function-call, or comma operators, except when they are contained within a subexpression that is not evaluated.117)
4 Each constant expression shall evaluate to a constant that is in the range of representable values for its type.
Semantics
5 An expression that evaluates to a constant is required in several contexts. If a floating expression is evaluated in the translation environment, the arithmetic range and precision shall be at least as great as if the expression were being evaluated in the execution environment.118)
6 An integer constant expression119) shall have integer type and shall only have operands that are integer constants, enumeration constants, character constants, sizeof expressions whose results are integer constants, _Alignof expressions, and floating constants that are the immediate operands of casts.
Cast operators in an integer constant expression shall only convert arithmetic types to integer types, except as part of an operand to the sizeof or _Alignof operator.

Answer 5

C was originally developed on machines where a null pointer constant and the integer constant 0 had the same representation. Later, some vendors ported the language to mainframes where a different special value triggered a hardware trap when used as a pointer, and wanted to use that value for NULL. These companies discovered that so much existing code type-punned between integers and pointers, they had to recognize 0 as a special constant that could implicitly convert to a null pointer constant. ANSI C incorporated this behavior, at the same time as they introduced the void* as a pointer that implicitly converts to any type of object pointer. This allowed NULL to be used as a safer alternative to 0.

I’ve seen some code that (possibly tongue-in-cheek) detected one of these machines by testing if ((char*)1 == 0).

Answer 6

why do there exist two ways of expressing the same concept rather than one?

History.

NULL started as 0 and later better programming practices encouraged ((void *)0).

---

First, there are more than 2 ways:

#define NULL ((void *)0)
#define NULL 0
#define NULL 0L
#define NULL 0LL
#define NULL 0u
...

Before void * (Pre C89)

Before void * and void existed, #define NULL some_integer_type_of_zero was used.

It was useful to have the size of that integer type to match the size of object pointers. Consider the below. With 16-bit int and 32-bit long, it is useful for the type of zero used to match the width of an object pointer.

Consider printing pointers.

double x;
printf("%ld\n", &x);  // On systems where an object pointer was same size as long
printf("%ld\n", NULL);// Would like to use the same specifier for NULL

With 32-bit object pointers, #define NULL 0L is better.

double x;
printf("%d\n", &x);  // On systems where an object pointer was same size as int
printf("%d\n", NULL);// Would like to use the same specifier for NULL

With 16-bit object pointers, #define NULL 0 is better.

---

C89

After the birth of void, void *, it is natural to have the null pointer constant to be a pointer type. This allowed the bit pattern of (void*)0) to be non-zero. This was useful in some architectures.

printf("%p\n", NULL);

With 16-bit object pointers, #define NULL ((void*)0) works above.
With 32-bit object pointers, #define NULL ((void*)0) works.
With 64-bit object pointers, #define NULL ((void*)0) works.
With 16-bit int, #define NULL ((void*)0) works.
With 32-bit int, #define NULL ((void*)0) works.
We now have independence of the int/long/object pointer size. ((void*)0) works in all cases.

Using #define NULL 0 creates issues when passing NULL as a ... argument, hence the irksome need to do printf("%p\n", (void*)NULL); for highly portable code.

With #define NULL ((void*)0), code like char n = NULL; will more likely raise a warning, unlike ``#define NULL 0`

---

C99

With the advent of _Generic, we can distinguish, for better or worse, NULL as a void *, int, long, ...

Answer 7

According to §6.3.2.3 ¶3 of the C11 standard, a null pointer constant in C can be defined by an implementation as either the integer constant expression 0 or such an expression cast to void *.

No, that a misleading paraphrase of the language spec. The actual language of the cited paragraph is

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

Implementations don't get to choose between those alternatives. Both are forms of a null pointer constant in the C language. They can be used interchangeably for the purpose.

Moreover, not only the specific integer constant expression 0 can serve in this role, but any integer constant expression with value 0 can do. For example, 1 + 2 + 3 + 4 - 10 is such an expression.

Additionally, do not confuse null pointer constants generally with the macro NULL. The latter is defined by conforming implementations to expand to a null pointer constant, but that doesn't mean that the replacement text of NULL is the only null pointer constant.

My implementation (GCC 9.4.0) defines NULL in stddef.h in the following ways:

#define NULL ((void *)0)
#define NULL 0

Not both at the same time, of course.

Why are both of the above expressions considered semantically equivalent in the context of NULL?

Again with the reversal. It's not "the context of NULL". It's pointer context. There is nothing particularly special about the macro NULL itself to distinguish contexts in which it appears from contexts where its replacement text appears directly.

And I guess you're asking for rationale for paragraph 6.3.2.3/3, as opposed to "because 6.3.2.3/3". There is no published rationale for C11. There is one for C99, which largely serves for C90 as well, but it does not address this issue.

It should be noted, however, that void (and therefore void *) was an invention of the committee that developed the original C language specification ("ANSI C" / C89 / C90). There was no possibility of an "integer constant expression cast to type void *" before then.

More specifically, why do there exist two ways of expressing the same concept rather than one?

Are there, really?

If we accept an integer constant expression with value 0 as a null pointer constant (a source-code entity), and we want to convert it to a runtime null pointer value, then which pointer type do we choose? Pointers to different object types do not necessarily have the same representation, so this actually matters. Type void * seems the natural choice to me, and that's consistent with the fact that, alone of all pointer types, void * can be converted to other object pointer types without a cast.

But then, in a context where 0 is being interpreted as a null pointer constant, casting it to void * is a no-op, so (void *) 0 expresses exactly the same thing as 0 in such a context.

What's really going on here

At the time the ANSI committee was working, many existing C implementations accepted integer-to-pointer conversions without a cast, and although the meaning of most such conversions was implementation and / or context specific, there was wide acceptance that converting constant 0 to a pointer yielded a null pointer. That use was by far the most common one of converting an integer constant to a pointer. The committee wanted to impose stricter rules on type conversions, but it did not want to break all the existing code that used 0 as a constant representing a null pointer.

So they hacked the spec.

They invented a special kind of constant, the null pointer constant, and provided rules around it that made it compatible with existing use. A null pointer constant, regardless of lexical form, can be implicitly converted to any pointer type, yielding a null pointer (value) of that type. Otherwise, no implicit integer-to-pointer conversions are defined.

But the committee preferred that null pointer constants should actually have pointer type without conversion (which 0 does not, pointer context or no), so they provided for the "cast to type void *" option as part of the definition of a null pointer constant. At the time, that was a forward-looking move, but the general consensus now appears to be that it was the right direction to aim.

And why do we still have the "integer constant expression with value 0"? Backwards compatibility. Consistency with conventional idioms such as {0} as a universal initializer for objects of any type. Resistance to change. Perhaps other reasons as well.

Answer 8

The "why" - it is for historical reasons. NULL was used in various implementations before it was added to a standard. And at the time it was added to a C standard, implementations defined NULL usually as 0, or as 0 cast to some pointer. At that point you wouldn't want to make one of them illegal, because whichever you made illegal, you'd break half the existing code.

Answer 9

The C11 standard allows for a null pointer constant to be defined either as the integer constant expression 0 or as an expression that is cast to void *. The use of the NULL macro makes it easier for programmers to use the null pointer constant in their code, as they don't have to remember which of these definitions the implementation uses.

Using a macro also makes it easier to change the underlying definition of the null pointer constant in the future, if necessary. For example, if the implementation decided to change the definition of NULL to be a different integer constant expression, they could do so by simply modifying the definition of the NULL macro. This would not require any changes to the code that uses the NULL macro, as long as the code uses the NULL macro consistently.

There are two definitions of the NULL macro provided in the example you gave because some systems may define NULL as an expression that is cast to void *, while others may define it as the integer constant expression 0. By providing both definitions, the stddef.h header can be used on a wide range of systems without requiring any modifications.