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From: thj@mtcchi.uucp (XT0221000-Tom Hjellming(ZG90210)0000)
Newsgroups: comp.os.msdos.programmer
Subject: Re: NULL pointer error
Keywords: Null dos error
Message-ID: <1990Jul16.145157.4105@mtcchi.uucp>
Date: 16 Jul 90 14:51:57 GMT
References: <2008@bnlux0.bnl.gov>
Organization: Memorex Telex Corporation NSBG/STP
Lines: 37

reilly@bnlux0.bnl.gov (kevin reilly) writes:

>After exiting my program DOS writes an error to the screen
>NULL pointer error R6001
>I understand the meaning of the error message but how do I go about
>debugging the program to find what is causing it.
>From reading the documentation from MicroSoft (QC2.5) the program will
>compile and link error free. But when run and then exited that is when
>the error will appear.
>The program ran error free up until I linked in the menu.obj file that
>came with the compiler.
>I guess the question is: What memory address do I watch when stepping
>through the program?
>Thank you in advance!
>reilly@bnlux0.bnl.gov

If you have codeview or some other debugger that supports watchpoints,
set it to break when someone overwrites the first 4 or so bytes of the
data segment (ds:0).  Microsoft uses this area as the NULL segment.  When
a program terminates, the exit code will check to see if this area has
been modified.  I think the NULL segment is 16 bytes long, but you can
probably get away with just watching the first 4 bytes.

Hope this helps.


Tom Hjellming
Software Consultant
Analysts International Corp. (AiC)
Schaumburg, IL
...!uunet!tellab5!mtcchi!thj

-- 
Tom Hjellming
Software Consultant
Analysts International Corp. (AiC)
Schaumburg, IL