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