From:	"John M. Aldrich" <fighteer AT NO DOT SPAM DOT cs DOT com>
Newsgroups:	comp.os.msdos.djgpp
Subject:	Re: Try this one on for size
Date:	Wed, 16 Apr 1997 06:05:37 +0000
Organization:	Two pounds of chaos and a pinch of salt
Lines:	34
Message-ID:	<33546C31.6872@NO.SPAM.cs.com>
References:	<199704162029 DOT QAA02720 AT hcst DOT net>
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To:	djgpp AT delorie DOT com
DJ-Gateway:	from newsgroup comp.os.msdos.djgpp

Bryan Murphy wrote:
> 
> Ok, here's something weird...  I have a program that heavily uses pointers.
> 
> However, I've thoroughly checked my pointer routines, and to the best of
> my knowledge, they appear to be working properly.
> 
> If I run the program in Dos, I get a SigSegV right away after running the
> program and it draws the screen.

This symptom is virtually guaranteed to be a NULL pointer dereference of
some kind.  CWSDPMI, which runs in DOS, catches NULL pointer
dereferences, but the Windows DPMI host lets them by without comment,
and thus after a few such dereferences you can really screw up your
program.  As to why it's working in a full screen and not in a window, I
have no idea.

I know from long experience that invalid pointers can be one of the
hardest bugs to find.  Many moons ago, I agonized over an apparent
memory bug in one of my programs, and eventually gave up.  I realize now
that what I probably did was allocate memory improperly.  Imagine the
following:

  struct big_bad_struct *x;
  x = malloc( sizeof(x) );

I leave it up to you to determine why this is wrong.  ;)

-- 
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| John M. Aldrich, aka Fighteer I |     mailto:fighteer AT cs DOT com      |
| Proud owner of what might one   |   http://www.cs.com/fighteer    |
| day be a spectacular MUD...     | Plan: To make Bill Gates suffer |
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