From: Martin Str|mberg <ams AT ludd DOT luth DOT se>
Message-Id: <200110100001.CAA17010@father.ludd.luth.se>
Subject: Re: Resend: DJGPP and files > 2GB
In-Reply-To: <3BC38161.C1FBC8F1@phekda.freeserve.co.uk> from Richard Dawe at "Oct 9, 2001 11:59:45 pm"
To: djgpp-workers AT delorie DOT com
Date: Wed, 10 Oct 2001 02:01:08 +0200 (MET DST)
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According to Richard Dawe:
> > > I don't think the library's current behaviour is good - who knows what
> > > effect silently returning bogus sizes for files >= 2GB will have?
> > 
> > As the possible values are -2^31 - 2^31-1 there no ambiguity (if
> > you're aware of that a negative value != -1 is really a big positive
> > one).
> 
> Sure, there's no ambiguity for stat alone. But if you report the file size
> as > 2GB in stat, then you may not be able to manipulate some portions of
> the file. E.g. you may want to use relative seeks to get to the top 2GB of
> a file, but you can't, because off_t is a signed value and cannot be used
> to represent a seek to > +2GB of the current position. How would you
> interpret the seek to > +2GB, when you can't represent > +2GB, because
> negative values are used for backwards seeking?

Easily. You just send in 2GiB + 10 (e. g.) which is a very
magnitudeally big number (but negative) and it works! Because every
seek operation works mod 2^32 and/or x86 uses two's complement for
signed numbers. This is a conclusion I've made after trying to
understand what was happening when I called the DOZE (INT 21 driven)
seek function with SEEK_SET and offset -2^30 (e. g.).

> > How does the type offset_t fit into the ways of addressing the
> > problem (if you know)?
> 
> (I presume you are mean off_t rather than offset_t.)

No. I mean the type offset_t that I added for supporting FAT32 at the
same time I added llseek(). It's defined in include/sys/djtypes.h:

#define __DJ_offset_t   typedef long long offset_t;


Right,

						MartinS