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 the value of a null pointer in the C language 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. '\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.").
'\0' has nothing to do with pointers. However, you may see something similar to this code:
if (!*char_pointer)
checks if the char pointer is pointing at a null character.
if (*char_pointer)
checks if the char 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.
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.
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 the value of a null pointer in the C language 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. '\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.").
'\0' has nothing to do with pointers. However, you may see something similar to this code:
if (!*char_pointer)
checks if the char pointer is pointing at a null character.
if (*char_pointer)
checks if the char 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.
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.
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.
I know that NULL is guaranteed to point to nothing, and cause a segfault when accessed, but is NULL guaranteed to be 0 on all hardware? What about false?
#include <stddef.h>
int main() {
if (!NULL) {
printf("%d\n", NULL == 0); /* Is this line always run, and is the value printed always 1? */
}
return 0;
}Videos
In C, NULL is a macro that expands either to 0 or (void*)0 (or something that has a similar effect).
In the first case, you can not differentiate between NULL and 0, because they are literally the same.
In the second case, your code will cause a compile error, because you can't compare an integer variable with a pointer.
First some background ...
The macros are
NULLwhich expands to an implementation-defined null pointer constant; C11 §7.19 3
NULL typically is an integer constant 0 or (void*)0 or the like. It may have a different implementation or type - It could be ((int*) 0xDEADBEEF) as strange as that may be.
NULL might be type int. It might be type void * or something else. The type of NULL is not defined.
When the null pointer constant NULL is cast to any pointer, is is a null pointer. An integer 0 cast to a pointer is also a null pointer. A system could have many different (bit-wise) null pointers. They all compare equally to each other. They all compare unequally to any valid object/function. Recall this compare is done as pointers, not integers.
An integer 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 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. C11 §6.3.2.3 3
int x;
if (&x == NULL) ... // this is false
So after all that chapter and verse how to distinguish NULL from 0?
If the macro NULL is defined as an int 0 - it is game over - there is no difference between 0 and NULL.
If NULL is not an int, then code can use _Generic() to differentiate NULL and 0. This does not help OP's "Any change made can only be made within the function itself." requirement as that function accepts an int augment.
If NULL is an int that has a different bit-pattern than 0, then a simple memcmp() can differentiate.
I suspect the whole reason for this exercise is to realize there is no portable method to distinguish NULL from 0.
0 being an int like other integers, sizeof(0) will yield 4 bytes.
sizeof(NULL) will yield 8 bytes. In binary system, it is 8x8=64 bits, all bits with 0.
Pointers have 8 bytes allocated against characters with 1 bytes and integers 4 bytes. Is 8 bytes the maximum bytes for any datatype? I believe so as NULL is set to 8 bytes apparently for that reason to take care NULL denotes 0 for all datatypes.
Actually, you can use a literal 0 anyplace you would use NULL.
Section 6.3.2.3p3 of the C standard states:
An integer 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 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.
And section 7.19p3 states:
The macros are:
NULLwhich expands to an implementation-defined null pointer constant
So 0 qualifies as a null pointer constant, as does (void *)0 and NULL. The use of NULL is preferred however as it makes it more evident to the reader that a null pointer is being used and not the integer value 0.
NULL is used to make it clear it is a pointer type.
Ideally, the C implementation would define NULL as ((void *) 0) or something equivalent, and programmers would always use NULL when they want a null pointer constant.
If this is done, then, when a programmer has, for example, an int *x and accidentally writes *x = NULL;, then the compiler can recognize that a mistake has been made, because the left side of = has type int, and the right side has type void *, and this is not a proper combination for assignment.
In contrast, if the programmer accidentally writes *x = 0; instead of x = 0;, then the compiler cannot recognize this mistake, because the left side has type int, and the right side has type int, and that is a valid combination.
Thus, when NULL is defined well and is used, mistakes are detected earlier.
In particular answer to your question “Is there a context in which just plain literal 0 would not work exactly the same?”:
- In correct code,
NULLand0may be used interchangeably as null pointer constants. 0will function as an integer (non-pointer) constant, butNULLmight not, depending on how the C implementation defines it.- For the purpose of detecting errors,
NULLand0do not work exactly the same; usingNULLwith a good definition serves to help detect some mistakes that using0does not.
The C standard allows 0 to be used for null pointer constants for historic reasons. However, this is not beneficial except for allowing previously written code to compile in compilers using current C standards. New code should avoid using 0 as a null pointer constant.
I'm assuming you mean the null pointer. It is guaranteed to compare equal to 0.1 But it doesn't have to be represented with all-zero bits.2
See also the comp.lang.c FAQ on null pointers.
- See C99, 6.3.2.3.
- There's no explicit claim; but see the footnote for C99, 7.20.3 (thanks to @birryree in the comments).
§ 6.3.2.3 of the C99 standard says
An integer 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 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.
§ 7.17 also says
[...] NULL which expands to an implementation-defined null pointer constant [...]
The address of the NULL pointer might be different from 0, while it will behave like it was in most cases.
(This should be the same as in older C standards, which I don't have at hand right now)