Q42791: QuickC: Tracking Down Null Pointer Assignments

Article: Q42791
Product(s): See article
Version(s): 2.00
Operating System(s): MS-DOS
Keyword(s): ENDUSER | | mspl13_c
Last Modified: 2-MAY-1989

When a program modifies memory at location 0 (NULL) in the small or
medium memory models, the following error message is reported at
program termination:

   run-time error R6001
   - null pointer assignment

You can find the location of null pointer assignments in the small and
medium memory models using either the debugger in QuickC Versions 2.00
and later or the CodeView debugger.

To detect null pointer assignments in QuickC 2.00 and later, do the
following:

1. Select the Watchpoint option under the Debug menu.

2. Input the following:

      * (long *) 0

This watchpoint will cause QuickC to stop executing your program when
the memory location 0 (NULL) is modified. QuickC will immediately
display the following pop-up and will highlight the line of code
directly following the line that modified memory location 0:

   Watchpoint reached:
       * (long *) 0

The null segment is 66 bytes long, and only four (4) bytes are watched
with the above watchpoint. However, most cases of null pointer
assignment occur when the first four bytes of the null segment are
modified. Those cases will be caught with this watchpoint.

Note that this technique does not apply to the compact, large, and
huge memory models because their data pointers are far, by default.

Information for using the CodeView debugger is provided in the
"Microsoft CodeView and Utilities Software Development Tools for the
MS-DOS Operating System" manual.

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