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From: forrest@sybase.com
Newsgroups: comp.sys.ibm.pc
Subject: How Turbo C Handles Null Pointer Assignments
Message-ID: <4178@sybase.sybase.com>
Date: 10 May 89 00:34:41 GMT
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Organization: Sybase, Inc.
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I've just figured out some interesting facts about how Turbo C 2.0
detects null pointer assignments. Since this doesn't appear to be
documented I thought I'd pass along what I've learned.

All this started when I saw the message "Null Pointer Assignment"
when running a program. This was surprising to me at first because
I didn't know how a computer could detect null pointer assignments
without using some kind of hardware memory protection scheme.
After looking at the c0m.asm source code supplied with Turbo C I
figured out what was going happening. Turbo C stores a check sum
of the memory area starting with address 0 and continuing through
where the message is stored that you see when you run Turbo C.
As part of the _exit() procedure, this checksum is recomputed and
compared to the original checksum.
If you accidently put something in address 0 using a null pointer
assignment these two values won't be the same a Turbo C prints
the "Null Pointer Assignment" message.

Although I haven't tried this yet, I think it would be possible to
detect null pointer assignments at run time by setting a watchpoint
on address 0 (I use the Turbo Debugger. I don't know if you can do this
with the debugger that comes with Turbo C). Then, whenever the value
at address 0 changes you will be notified.

Jon Forrest
forrest@sybase.com
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