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NULLl Mean -1?

Posted on 2001-09-14
Medium Priority
Last Modified: 2013-11-20
we know handle means integer.
 if X_handle <> NULL {NULL means -1 or 0?}
Question by:johngodzbi
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LVL 32

Expert Comment

ID: 6482242
Your questions is quite ambiguous but the SPECIFIC value of NULL is 0 on a Microsoft Windows system.  But note that to assume that NULL always is 0 is not wise as the C and C++ language specifications do not require this.

Expert Comment

ID: 6482275
If u want the definition of NULL, place the text cursor above it and press F12.

Accepted Solution

ShaunWilde earned 100 total points
ID: 6482296
in a lot of the header files you will see the following definition

/* Define NULL pointer value */

#ifndef NULL
#ifdef  __cplusplus
#define NULL    0
#define NULL    ((void *)0)

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LVL 10

Expert Comment

ID: 6482315
always refer to null as 'NULL' and not 0 or -1 etc this way it dont matter what the headers define it as.

Expert Comment

ID: 6482383
Yes - eventhough NULL will usually be 0 in C and C++, in C# there is a keyword 'null' and in Visual Basic Scripting there is one called Null

eg the definition for null

The null keyword represents a special value that indicates a variable does not refer to any object. The value null may be assigned to any class or interface variable to aid garbage collection. (Assigning the value null to any of the items above indicates to the garbage collection system that the object or variable is no longer in use.) It cannot be cast to any other type, and should not be considered to have a known numeric value.

While you are here - also remember that BOOL is not the same as bool and is not the same as VARIANT_BOOL
which I like to right like this


Expert Comment

ID: 6482384
eek for 'right' read 'write' :)

Assisted Solution

peterchen092700 earned 100 total points
ID: 6482447
#define NULL 0

is perfect except when compiler can't deduce it's meant as a pointer (calling undeclared c-style or vararg - in this case, the ((void *)0) is necessary (I guess even when calling from C++ and sizeof(int)<sizeof(void *) )

i.a.W. NULL is just that you don't have 0's in your source...

However, some functions (like CreateFile) return INVALID_HANDLE_VALUE rather than 0 in case of error,
and INVALID_HANDLE_VALUE happens to be ((HANDLE)-1).


Expert Comment

ID: 6482454
jhance: null pointers are always denoted by the constant 0 in C and C++, even when the underlying machine uses another representation (which is not uncommon). The conversion from "x bits 0" to the internal null pointer representation must be done by the compiler - that's why the compiler needs to know when a 0 is meant as pointer, hence the ((void *)0)


Expert Comment

ID: 6482465
oh, johngodzbi, just another comment:

where do you know HANDLE means integer?
wtypes.h says
typedef void __RPC_FAR *HANDLE;

Expert Comment

ID: 6482725

Actually NULL is the null-pointer value used with many pointer operations and functions. A null character is defined as ASCII character 0, Chr$(0), so u can make use of NULL in comparison statement. By this statement we can say NULL == 0.


MRN Murthy
LVL 49

Assisted Solution

DanRollins earned 100 total points
ID: 6484385
Unless you are a C-religion purist, NULL means 0.  Period.
It *never* means -1.  Period.

Most handles *are* integers.  But there is no reason to make that assuumption.

Now, what is your question?

-- Dan

LVL 32

Assisted Solution

jhance earned 100 total points
ID: 6484498
I'm not a C-religion purist but I've looked at the definition of NULL in the ANSI C specification and I see ONLY the following:

1) Assigning a pointer to 0 is the same as assigning it to NULL.

2) No valid pointer will ever compare as equal to NULL.

3) NULL will compare as equal to a NULL pointer.

Note, however that the above rules and in fact real world implementations of C and C++ do not require that NULL == 0.  The specific value that is stored to represent NULL is arbitrary and I know that there are some machine architectures out there that lend themselves to using a value for NULL other than 0.

This is one of those issues (that I'm sure you are "religious" about) like data type size.

Assume only that NULL is what it is.  Namely a NULL or invalid pointer.  If you need to test for NULL, use NULL itself.  Anytime you are tempted to assume that NULL == 0, check yourself and ask WHY?

Expert Comment

ID: 6520207
Well, there is a difference in the meaning of NULL in C and C++. In C it is defined as a pointer that does not reference any object. In C++ it is an expression that evaluates to 0 (zero). In other words:

void* p = TRUE ? NULL : NULL;

should compile on any compliant ANSI-C-compiler; on any compliant C++-compiler an error should be issued since there is no implicit conversion from 0 to a void*. ShaunWild's post includes the #define's that you might want to check out. I deliberately used 'should' instead of 'do' as the real world is not perfect. VC++6.0 will compile the above line of code, even though it is erroneous.

Btw. the only constant you can be sure of in the C/C++ is sizeof( char ) == 1, even if it uses more than one byte in memory like some Alpha-chips.

-- .:fl0yd:.

Expert Comment

ID: 6976086
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Expert Comment

by:Roshan Davis
ID: 9532247
No comment has been added lately, so it's time to clean up this TA.
I will leave a recommendation in the Cleanup topic area that this question is:

Answered by: ShaunWilde, peterchen, DanRollins, jhance (points to be split)

Please leave any comments here within the next seven days.


Roshan Davis
EE Cleanup Volunteer

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