NULLl Mean -1?

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

Add your voice to the tech community where 5M+ people just like you are talking about what matters.

  • Help others & share knowledge
  • Earn cash & points
  • Learn & ask questions
  • 3
  • 3
  • 2
  • +7
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 25 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)

Percona Live Europe 2017 | Sep 25 - 27, 2017

The Percona Live Open Source Database Conference Europe 2017 is the premier event for the diverse and active European open source database community, as well as businesses that develop and use open source database software.

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 25 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 25 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 25 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
You have asked many questions and only graded 4 of them, and they were all  C and D grades, hard to understand.  ADMINISTRATION WILL BE CONTACTING YOU SHORTLY.  Moderators Computer101 or Netminder will return to finalize these if they are still open in 14 days.  Experts, please post closing recommendations before that time.

Below are your open questions as of today.  Questions which have been inactive for 21 days or longer are considered to be abandoned and for those, your options are:
1. Accept a Comment As Answer (use the button next to the Expert's name).
2. Close the question if the information was not useful to you, but may help others. You must tell the participants why you wish to do this, and allow for Expert response.  This choice will include a refund to you, and will move this question to our PAQ (Previously Asked Question) database.  If you found information outside this question thread, please add it.
3. Ask Community Support to help split points between participating experts, or just comment here with details and we'll respond with the process.
4. Delete the question (if it has no potential value for others).
   --> Post comments for expert of your intention to delete and why
   --> YOU CANNOT DELETE A QUESTION with comments; special handling by a Moderator is required.

For special handling needs, please post a zero point question in the link below and include the URL (question QID/link) that it regards with details.
Please click this link for Help Desk, Guidelines/Member Agreement and the Question/Answer process.

Click you Member Profile to view your question history and please keep them updated. If you are a KnowledgePro user, use the Power Search option to find them.  

Questions which are LOCKED with a Proposed Answer but do not help you, should be rejected with comments added.  When you grade the question less than an A, please comment as to why.  This helps all involved, as well as others who may access this item in the future.  PLEASE DO NOT AWARD POINTS TO ME.

To view your open questions, please click the following link(s) and keep them all current with updates.

To view your locked questions, please click the following link(s) and evaluate the proposed answer.

*****  E X P E R T S    P L E A S E  ******  Leave your closing recommendations.
If you are interested in the cleanup effort, please click this link 
POINTS FOR EXPERTS awaiting comments are listed in the link below
Moderators will finalize this question if in @14 days Asker has not responded.  This will be moved to the PAQ (Previously Asked Questions) at zero points, deleted or awarded.
Thanks everyone.
Moderator @ Experts Exchange
LVL 23

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

Featured Post

Want Experts Exchange at your fingertips?

With Experts Exchange’s latest app release, you can now experience our most recent features, updates, and the same community interface while on-the-go. Download our latest app release at the Android or Apple stores today!

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

Here is how to use MFC's automatic Radio Button handling in your dialog boxes and forms.  Beginner programmers usually start with a OnClick handler for each radio button and that's just not the right way to go.  MFC has a very cool system for handli…
In this article, I'll describe -- and show pictures of -- some of the significant additions that have been made available to programmers in the MFC Feature Pack for Visual C++ 2008.  These same feature are in the MFC libraries that come with Visual …
This video will show you how to get GIT to work in Eclipse.   It will walk you through how to install the EGit plugin in eclipse and how to checkout an existing repository.
There are cases when e.g. an IT administrator wants to have full access and view into selected mailboxes on Exchange server, directly from his own email account in Outlook or Outlook Web Access. This proves useful when for example administrator want…

628 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question