From: "Michael Farnham" <MCFARNHAM AT prodigy DOT net>
Newsgroups: comp.os.msdos.djgpp
Subject: Re: Lack of overflow problem
Date: Fri, 7 Jan 2000 04:38:34 -0600
Organization: Prodigy Internet http://www.prodigy.com
Lines: 46
Message-ID: <854fv0$a7bo$1@newssvr03-int.news.prodigy.com>
References: <dhwa4.274$Gh2 DOT 1735 AT newsfeed DOT slurp DOT net> <851dtd$bfv$1 AT news DOT luth DOT se> <854cnj$453 AT romeo DOT logica DOT co DOT uk>
NNTP-Posting-Host: dll2a010-0115.splitrock.net
X-Trace: newssvr03-int.news.prodigy.com 947241760 3966267 209.254.212.115 (7 Jan 2000 10:42:40 GMT)
X-Complaints-To: abuse AT prodigy DOT net
NNTP-Posting-Date: 7 Jan 2000 10:42:40 GMT
X-Newsreader:  Microsoft Outlook Express 5.00.2314.1300
X-MSMail-Priority:  Normal
X-Priority:  3
X-MimeOLE:  Produced By Microsoft MimeOLE V5.00.2314.1300
To: djgpp AT delorie DOT com
DJ-Gateway: from newsgroup comp.os.msdos.djgpp
Reply-To: djgpp AT delorie DOT com



Chris Underwood <underwoodc AT logica DOT nospam DOT com> wrote in message
news:854cnj$453 AT romeo DOT logica DOT co DOT uk...
>
> >Rob McCrea (r_mccrea AT hotmail DOT com) wrote:
> >: My expression won't recognize the overflow...
> >:
> >: while ((s+1) > s) s++;
> >:
> >: is an infinite loop.  What's up with that?
> >: Apparently the condition is being evaluted as (1>0).
> >: That's not right is it?  I'm compiling with gpp -Wall testsize.cpp -o
> >: testsize.exe.
> >:
> >: Please, can someone explain why its not acting like a computer?
>
>
> I'm not sure exactly what you're claiming is wrong here, but you might
not
> realise that sooner or later, given that 's' is likely an integer of some
> size or another, it'll wrap around, and your loop will exit. For example,
if
> 's' is an unsigned char you will loop until s = 255, then s+1 will be 256
> which overflows and wraps round to 0. (0 > 255) is false, and you'll exit
> the loop.
>
> Chris
>
>

I think that is what was expected.

It is possible that the compiler has optimized this loop to while(true)
s++;.
In the real world s + 1 > s is always true. Sometimes compilers make
optimizations that a human being would not make.

I have not tested the code and therefore do not know that this is
happening.
But it is an explanation that fits the observed facts.


Regards
Mike