Q24270: Tracking Down a Null Pointer Assignment Error

Article: Q24270
Product(s): See article
Version(s): 1.00 1.10 2.00 2.10 2.20 2.30 | 2.20 2.30
Operating System(s): MS-DOS | OS/2
Keyword(s): ENDUSER | S_C | mspl13_basic
Last Modified: 21-AUG-1989

Question:

I consistently get the error number R6001 "null pointer assignment"
when I run my program. How can I use CodeView to determine the point
at which the null pointer assignment is occurring?

Response:

Use a tracepoint, which can be set to watch a range of memory up to
128 bytes. It will halt the execution of the program when any value in
this range is changed. If you set a tracepoint over the entire range
of the null segment, the program will halt immediately after the
instruction that wrote over the null segment.

The location of the null segment is available in the link maps of
Microsoft compilers. 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
error r6001 is generated. 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 (using
malloc for example) or that have not been assigned to a specific
data element (arrays or structures for example) are considered
uninitialized.

An example of using CodeView to determine where an unitialized pointer
is being used follows:

g main                  /* go to the beginning of main() */
n16                     /* switch to hexadecimal (base 16) */
tpb DS:0 DS:42          /* set a Trace Point of type Byte starting
                              at address DS:0 and extending to DS:42 */

When any value in the specified range changes, CodeView will stop the
execution of your program. The previously executed line was probably
the line that caused the R6001 error.

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