KnowledgeBase Archive

An Archive of Early Microsoft KnowledgeBase Articles

View on GitHub

Q71252: Null Pointer Assignment

Article: Q71252
Product(s): Microsoft Programming Utilities
Version(s): 2.2,3.0,3.11,3.12,3.14,3.5,4.0,4.01,4.05,4.1
Operating System(s): 
Keyword(s): kb16bitonly
Last Modified: 08-MAY-2002

-------------------------------------------------------------------------------
The information in this article applies to:

- Microsoft CodeView for MS-DOS, versions 2.2, 3.0, 3.11, 3.14, 4.0, 4.01, 4.05, 4.1 
- Microsoft CodeView for OS/2, versions 2.2, 3.0, 3.11, 3.12, 3.5 
-------------------------------------------------------------------------------

SYMPTOMS
========



RESOLUTION
==========



STATUS
======



MORE INFORMATION
================

The location of the null segment can be observed in a link map. It starts at
DS:0 and is 42H bytes long. The Microsoft copyright notice is written there at
program startup and if this area is written to during the course of the program,
the run-time error R6001 will be generated upon program termination.

The most common cause of this error is using a pointer that has not been
initialized to point to a memory area. Pointers that have not had space
allocated for them [for example, with malloc()] or that have not been assigned
to point to a specific data element (for example, arrays or structures) are
considered uninitialized. Writing to one of these pointers generally results in
an overwrite of the null data segment.

With CodeView versions 1.0, 2.0, 2.1, 2.2, 2.3, and 2.35, the procedure described
above should be used for locating the source of an R6001 error. With CodeView
versions 3.0 and above, you may also use the following menu and keyboard
commands to set the breakpoint for tracking down a null pointer assignment:

1. Press the F8 or F10 key to step into main().

2. Select Set Breakpoint from the Watch menu.

3. Choose "Break When Expression Has Changed" from the Set Breakpoint dialog
  box.

4. In the Expression field, enter DS:0.

5. In the Length field, enter 0n66.

6. Choose OK, or press ENTER.

7. Press the F5 key to run the program.

As described above, the program will then break immediately after the errant
pointer has written to the null segment.

Note that it may be necessary to clear any breakpoints on the null data segment
and restart the program before entering the above commands in order for these
procedures to work correctly.

In addition, any breakpoint on DS:0 may stop execution if DS changes. For
example, if the above breakpoint is set and the program is restarted under
CodeView, the program will break during the C start-up code [before reaching the
main() function], since the C start-up sets DS to the default data segment.
[This is also the reason you must step into main() before setting the breakpoint
in the first place, otherwise you would not be setting the breakpoint on the
correct segment.

Additional query words: kbinf 3.00 4.00 4.10

======================================================================
Keywords          : kb16bitonly 
Technology        : kbAudDeveloper kbCodeView kbZNotKeyword3 kbCodeView220DOS kbCodeView300DOS kbCodeView311DOS kbCodeView314DOS kbCodeView400DOS kbCodeView401DOS kbCodeView405DOS kbCodeView410DOS kbCodeView220OS2 kbCodeView300OS2 kbCodeView311OS2 kbCodeView312OS2 kbCodeView350OS2
Version           : :2.2,3.0,3.11,3.12,3.14,3.5,4.0,4.01,4.05,4.1
Issue type        : kbprb

=============================================================================

THE INFORMATION PROVIDED IN THE MICROSOFT KNOWLEDGE BASE IS PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND. MICROSOFT DISCLAIMS ALL WARRANTIES, EITHER EXPRESS OR IMPLIED, INCLUDING THE WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. IN NO EVENT SHALL MICROSOFT CORPORATION OR ITS SUPPLIERS BE LIABLE FOR ANY DAMAGES WHATSOEVER INCLUDING DIRECT, INDIRECT, INCIDENTAL, CONSEQUENTIAL, LOSS OF BUSINESS PROFITS OR SPECIAL DAMAGES, EVEN IF MICROSOFT CORPORATION OR ITS SUPPLIERS HAVE BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES. SOME STATES DO NOT ALLOW THE EXCLUSION OR LIMITATION OF LIABILITY FOR CONSEQUENTIAL OR INCIDENTAL DAMAGES SO THE FOREGOING LIMITATION MAY NOT APPLY.