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From: wes@obie.UUCP (Barnacle Wes)
Newsgroups: news.admin,news.software.b
Subject: Re: bugfix for inews (control messages cause coredump)
Summary: How about error AND no error on the SAME system?
Message-ID: <252@obie.UUCP>
Date: 4 Nov 88 06:58:34 GMT
References: <5686@fluke.COM> <5695@fluke.COM> <6266@claris.com> <25995@tut.cis.ohio-state.edu>
Organization: the Well of Souls
Lines: 48

In article <25995@tut.cis.ohio-state.edu>, karl@triceratops.cis.ohio-state.edu (Karl Kleinpaste) writes:
> Tut == Pyramid 98x:
> 
> [135] [1:59pm] tut:/dino0/karl/tmp> cat > z.c
> main()
> {
>         char *p = (char *)0;
>         p++;
>         *p = 'p';
>         printf("done\n");
> }
> [136] [1:59pm] tut:/dino0/karl/tmp> cc z.c -o z
> [137] [2:00pm] tut:/dino0/karl/tmp> ./z
> Bus error (core dumped)

Here are the results for System V/AT ('286):
obie!wes[44] cat z.c
main()
{
	char *p = (char *)0;
	p++;
	*p = 'p';
	printf("done\n");
}
obie!wes[45] time cc -o z z.c
2.4u 4.1s 0:10 64%
obie!wes[45] ./z
done
obie!wes[46] time cc -o z -Ml z.c	# use "large memory model"
2.8u 3.9s 0:11 60%
obie!wes[47] ./z
./z: Segmentation violation -- Core dumped
obie!wes[48] rm core z z.c

Neat, huh?  This is why: using the small memory model (the default),
pointers are 16-bit offsets into the data segments.  Addresses DS:0 and
DS:1 are valid, writable addresses, so the program runs correctly in
small model.  In the large model, however, pointers are 32 bits, and
include the segment selector.  This makes (char *)0 equal address 0:0,
which may or may not be a valid address in the process' space.  If it is
a valid address, it is probably in the code segment, which CANNOT be
writable on the '286.  This system can really be a nightmare to port to!

-- 
"The whole problem with the world is that fools and fanatics are always so
certain of themselves, but wiser people so full of doubts." - Bertrand Russell

"How come he didn't put `I think' at the end of it?" - James P. Hogan