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What is the difference between NULL, '\0' and 0?


Question

In C, there appear to be differences between various values of zero -- NULL, NUL and 0.

I know that the ASCII character '0' evaluates to 48 or 0x30.

The NULL pointer is usually defined as:

#define NULL 0

Or

#define NULL (void *)0

In addition, there is the NUL character '\0' which seems to evaluate to 0 as well.

Are there times when these three values can not be equal?

Is this also true on 64 bit systems?

2020/05/10
1
315
5/10/2020 1:40:04 PM

Accepted Answer

Note: This answer applies to the C language, not C++.


Null Pointers

The integer constant literal 0 has different meanings depending upon the context in which it's used. In all cases, it is still an integer constant with the value 0, it is just described in different ways.

If a pointer is being compared to the constant literal 0, then this is a check to see if the pointer is a null pointer. This 0 is then referred to as a null pointer constant. The C standard defines that 0 cast to the type void * is both a null pointer and a null pointer constant.

Additionally, to help readability, the macro NULL is provided in the header file stddef.h. Depending upon your compiler it might be possible to #undef NULL and redefine it to something wacky.

Therefore, here are some valid ways to check for a null pointer:

if (pointer == NULL)

NULL is defined to compare equal to a null pointer. It is implementation defined what the actual definition of NULL is, as long as it is a valid null pointer constant.

if (pointer == 0)

0 is another representation of the null pointer constant.

if (!pointer)

This if statement implicitly checks "is not 0", so we reverse that to mean "is 0".

The following are INVALID ways to check for a null pointer:

int mynull = 0;
<some code>
if (pointer == mynull)

To the compiler this is not a check for a null pointer, but an equality check on two variables. This might work if mynull never changes in the code and the compiler optimizations constant fold the 0 into the if statement, but this is not guaranteed and the compiler has to produce at least one diagnostic message (warning or error) according to the C Standard.

Note that what is a null pointer in the C language. It does not matter on the underlying architecture. If the underlying architecture has a null pointer value defined as address 0xDEADBEEF, then it is up to the compiler to sort this mess out.

As such, even on this funny architecture, the following ways are still valid ways to check for a null pointer:

if (!pointer)
if (pointer == NULL)
if (pointer == 0)

The following are INVALID ways to check for a null pointer:

#define MYNULL (void *) 0xDEADBEEF
if (pointer == MYNULL)
if (pointer == 0xDEADBEEF)

as these are seen by a compiler as normal comparisons.

Null Characters

'\0' is defined to be a null character - that is a character with all bits set to zero. This has nothing to do with pointers. However you may see something similar to this code:

if (!*string_pointer)

checks if the string pointer is pointing at a null character

if (*string_pointer)

checks if the string pointer is pointing at a non-null character

Don't get these confused with null pointers. Just because the bit representation is the same, and this allows for some convenient cross over cases, they are not really the same thing.

Additionally, '\0' is (like all character literals) an integer constant, in this case with the value zero. So '\0' is completely equivalent to an unadorned 0 integer constant - the only difference is in the intent that it conveys to a human reader ("I'm using this as a null character.").

References

See Question 5.3 of the comp.lang.c FAQ for more. See this pdf for the C standard. Check out sections 6.3.2.3 Pointers, paragraph 3.

2016/10/24
357
10/24/2016 8:27:24 AM

It appears that a number of people misunderstand what the differences between NULL, '\0' and 0 are. So, to explain, and in attempt to avoid repeating things said earlier:

A constant expression of type int with the value 0, or an expression of this type, cast to type void * is a null pointer constant, which if converted to a pointer becomes a null pointer. It is guaranteed by the standard to compare unequal to any pointer to any object or function.

NULL is a macro, defined in as a null pointer constant.

\0 is a construction used to represent the null character, used to terminate a string.

A null character is a byte which has all its bits set to 0.

2020/03/13

All three define the meaning of zero in different context.

  • pointer context - NULL is used and means the value of the pointer is 0, independent of whether it is 32bit or 64bit (one case 4 bytes the other 8 bytes of zeroes).
  • string context - the character representing the digit zero has a hex value of 0x30, whereas the NUL character has hex value of 0x00 (used for terminating strings).

These three are always different when you look at the memory:

NULL - 0x00000000 or 0x00000000'00000000 (32 vs 64 bit)
NUL - 0x00 or 0x0000 (ascii vs 2byte unicode)
'0' - 0x20

I hope this clarifies it.

2017/09/22

If NULL and 0 are equivalent as null pointer constants, which should I use? in the C FAQ list addresses this issue as well:

C programmers must understand that NULL and 0 are interchangeable in pointer contexts, and that an uncast 0 is perfectly acceptable. Any usage of NULL (as opposed to 0) should be considered a gentle reminder that a pointer is involved; programmers should not depend on it (either for their own understanding or the compiler's) for distinguishing pointer 0's from integer 0's.

It is only in pointer contexts that NULL and 0 are equivalent. NULL should not be used when another kind of 0 is required, even though it might work, because doing so sends the wrong stylistic message. (Furthermore, ANSI allows the definition of NULL to be ((void *)0), which will not work at all in non-pointer contexts.) In particular, do not use NULL when the ASCII null character (NUL) is desired. Provide your own definition

#define NUL '\0'

if you must.

2009/08/18

What is the difference between NULL, ‘\0’ and 0

"null character (NUL)" is easiest to rule out. '\0' is a character literal. In C, it is implemented as int, so, it's the same as 0, which is of INT_TYPE_SIZE. In C++, character literal is implemented as char, which is 1 byte. This is normally different from NULL or 0.

Next, NULL is a pointer value that specifies that a variable does not point to any address space. Set aside the fact that it is usually implemented as zeros, it must be able to express the full address space of the architecture. Thus, on a 32-bit architecture NULL (likely) is 4-byte and on 64-bit architecture 8-byte. This is up to the implementation of C.

Finally, the literal 0 is of type int, which is of size INT_TYPE_SIZE. The default value of INT_TYPE_SIZE could be different depending on architecture.

Apple wrote:

The 64-bit data model used by Mac OS X is known as "LP64". This is the common data model used by other 64-bit UNIX systems from Sun and SGI as well as 64-bit Linux. The LP64 data model defines the primitive types as follows:

  • ints are 32-bit
  • longs are 64-bit
  • long-longs are also 64-bit
  • pointers are 64-bit

Wikipedia 64-bit:

Microsoft's VC++ compiler uses the LLP64 model.

64-bit data models
Data model short int long  long long pointers Sample operating systems
LLP64      16    32  32    64        64       Microsoft Win64 (X64/IA64)
LP64       16    32  64    64        64       Most Unix and Unix-like systems (Solaris, Linux, etc.)
ILP64      16    64  64    64        64       HAL
SILP64     64    64  64    64        64       ?

Edit: Added more on the character literal.

#include <stdio.h>

int main(void) {
    printf("%d", sizeof('\0'));
    return 0;
}

The above code returns 4 on gcc and 1 on g++.

2009/08/19

A one-L NUL, it ends a string.

A two-L NULL points to no thing.

And I will bet a golden bull

That there is no three-L NULLL.

How do you deal with NUL?

2017/05/23

Source: https://stackoverflow.com/questions/1296843
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