Date: Sun, 16 Jan 2000 14:46:50 -0500 (EST)
Message-Id: <200001161946.OAA18699@indy.delorie.com>
From: Eli Zaretskii <eliz AT delorie DOT com>
To: ams AT ludd DOT luth DOT se
CC: djgpp-workers AT delorie DOT com
In-reply-to: <200001161638.RAA19014@father.ludd.luth.se> (message from Martin
	Str|mberg on Sun, 16 Jan 100 17:38:32 +0100 (MET))
Subject: Re: _lleek
References:  <200001161638 DOT RAA19014 AT father DOT ludd DOT luth DOT se>
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> According to Eli Zaretskii:
> > If whence == SEEK_CUR and offset is positive, you could avoid the double 
> > call, no?
[snip]
> 1: ret = 2147483647.
> 2: ret = -2.
> 3: ret = 2147483645.
> bash$
> 
> So it seems the file pointer wraps around at 2^32. Note particularly
> that I never passed a negative offset, but still the file pointer went
> from 2^32-2 to 2147483645 (-2 is really 2^32-2).

Seems like a consequence of max file size being 4GB - 2, no?  If so,
perhaps we should let DOS have its way here?

> > >   if( whence == SEEK_SET )
> > >   {
> > >     if( offset < 0 )
> > >     {
> > >       offset = 0;
> > >     }
> > >     else if( MAX_FILE_POINTER_POSITION < offset )
> > >     {
> > >       offset = MAX_FILE_POINTER_POSITION;
> > >     }
> > >   }
> > 
> > What happens if you pass the offset unaltered to DOS?  Why should we 
> > silently change the arguments, unless they somehow crash the system or 
> > wipe the disk?
> 
> The total number of bits there are place for in the call (INT21,
> AH=0x42) is 32. So passing bigger values than 2^32 will be the same as
> passing the value modulus 2^32.

But you don't take the modulus, you limit the arguments.  I wonder if
we should do that.

> The biggest size of a file in FAT is
> 2^32-1 <=> biggest offset is 2^32-2. Hence bigger values than 2^32-2
> doesn't make sense.

IMHO it is not a business of a library function to decide what does
and what doesn't make sense.  I say let's pass the arguments unaltered
to DOS.  We can document all the quirks, if you think it's important.

> As a matter of fact I think this is so much better that I'll suggest
> that lseek() calls _llseek() internally.

That was the original intention, at least that's what I thought it
was.

Thanks again for the explanations.