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In article <27092@netnews.upenn.edu> weisen@eniac.seas.upenn.edu.UUCP (Neil Weisenfeld) writes:
>In article <2008@bnlux0.bnl.gov> reilly@bnlux0.bnl.gov (kevin reilly) writes:
>[NULL ptr stuff deleted]
>>I guess the question is: What memory address do I watch when stepping
>>through the program?
>
>If the program is attempting to write to a NULL pointer, you could
>try watching address 0x0000:0x0000 as this is where the "NULL" pointer
>points.
>
>Neil

If you linked with Microsoft supplied libraries, you probably pulled in their
exit() function definition, and the _nullcheck() function that it calls.
This function actually takes a guess at whether your program changed something
outside of memory using a NULL pointer.  The code for _nullcheck is probably
in your Microsoft compiler/linker software distribution, in a module 
chksum.asm.  The following lines from that module give some insights into
this.  Starting from DS:0, for a length of 0x42, each succesive byte is xor'ed
into the xor'ed result so far.  Then, this value is compared with the value
that is to be expected if none of the bytes were changed when your program ran.
If they are different, the routine assumes that your code accessed data using a
NULL pointer.  This assumption can be incorrect for the following reasons:

1) you accessed a byte within the first 66 bytes of your DS and changed it,
   and also another byte within that same region in such a way that xor'ing
   the new sequence of bytes still gives the same checksum.

2) you changed a byte and the checksum byte as well, so that the change is
   undetectable.

3) you used a NULL pointer as a pointer to a structure and accessed a member
   of the structure that was displaced by more than 66 bytes from the start
   of that structure.  (Fo eg, if struct param { char first[66]; char second; };
   and NULL->second = x;).  Incidentally, this is why you can't just watch
   0x0000:0x0000 as an earlier poster had suggested.

4) you used a different DS and used a NULL pointer.  This can happen if you
   use assembly routines and changed DS somewhere.

Despite these limitations, _nullcheck() does catch quite a few of errant
NULL pointer usage.

/* THIS IS FROM Microsoft's CHKSUM.ASM module */
chkpt	db	8 dup(0)		; for null pointer assignment
	CHKSUM= 11h			; has to be correct or error
	db	'MS Run-Time Library - Copyright (c) 1988, Microsoft Corp'
chkb	db	CHKSUM			; checksum byte
	db	0			; leaves al = 0
chkln=		$ - chkpt		; length to checksum

cProc	_nullcheck,<PUBLIC>,<>
cBegin	nogen				; no arguments - so no frame


	push	si

	xor	si,si			; start at DS:0
	mov	cx,chkln
	xor	ah,ah
	cld

chkloop:				; loop to 1 past end of copyrt. string
	lodsb
	xor	ah,al			; accumulate xor total in AH
	loop	chkloop

	xor	ah,BIAS 		; XOR out the initial BIAS
	jz	setzero

	call	_FF_MSGBANNER		; (1) "\r\n" to stderr
					; (2) FORTRAN $DEBUG file/line
					;     if _Fline is set (not in C)
	                    		; (3) "run-time error" banner
	mov	ax,1			; null pointer assignment message no.
	push	ax
	call	_NMSG_WRITE		; write message out
	mov 	ax,1			; indicate error occurred

;	ax = 0 if the checksum is OK

setzero:
	pop	si



	ret

cEnd	nogen

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D. Venkatrangan
Matrix Computer Systems, Inc.           7 1/2 Harris Rd, Nashua, NH 03062
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