Message-ID: <3F25F8B9.32167C0@worldnet.att.net>
From: Paul Cousoulis <paulcsouls AT worldnet DOT att DOT net>
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Newsgroups: comp.os.msdos.djgpp
Subject: Re: integer overflow
References: <3F246120 DOT 63C3753C AT worldnet DOT att DOT net> <bg4dkn$8et$1 AT newsreader1 DOT netway DOT at>
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Date: Tue, 29 Jul 2003 04:32:30 GMT
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To: djgpp AT delorie DOT com
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Reply-To: djgpp AT delorie DOT com

Thanks,

I was trying to get this tgt-edif code which was written for Linux to
compile with DJGPP and this code was in it. It Looked okay to me. My
reference book for C is 'A Book on C' by Ira Pohl and Al Kelley,
Copyright 1998. I guess it's getting a little dated.

Thanks
Paul

News Reader wrote:
> 
> Your compiled code will not result in an overflow.
> You are just getting an overflow warning from the
> compiler, because the sign bit is affected twice during
> the evaluation of your code. It was mentioned in this
> thread that DJGPP defaults to signed 32-bit constants.
> 
> In order to prevent compiler warnings you can
> change your code as follows:
> 
>     unsigned res;
> 
>     res=1; res<<=31; res--;    // was: (1<<31)-1;
> 
> Other examples which will work are:
> 
>     res = (1u<<31)-1;
>     res = ((unsigned)1<<31)-1;
>     res = (unsigned)(1<<31)-1;
> 
> Please note that using 'unsigned res' instead of
> 'unsigned long res' will not change anything because
> DJGPP will produce 32-bit integers in both cases.
> 
> If your intention is to double the size of the variable
> you have to use 'long long' in your declaration.
> This will get you to 64 bits and your maximum
> shift count will therefore be 63 (unsigned).
> 
> Unfortunately this does not help much because
> your code will still be evaluated based on 32-bit
> signed integer constants - unless you use one of
> the following methods:
> 
> long long res;
> 
>      res = (1LL<<31)-1;
> or
>      res = ((long long)1<<31)-1;
> or
>      res=1; res<<=31; res--;
> 
> "Paul Cousoulis" <paulcsouls AT worldnet DOT att DOT net> wrote in message
> news:3F246120 DOT 63C3753C AT worldnet DOT att DOT net...
> >
> > Why does this code result in an integer overflow?
> >
> > unsigned long res;
> >
> > res = (1<<31)-1;
> >
> > Thanks
> > Paul