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From: Emiliano <emiliano@example.invalid>
Newsgroups: comp.lang.tcl
Subject: Re: 2GB limitation
Date: Wed, 24 Jul 2024 17:05:19 -0300
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On Wed, 24 Jul 2024 16:22:53 -0000 (UTC)
Andreas Leitgeb <avl@logic.at> wrote:

> Emiliano <emiliano@example.invalid> wrote:
> > In 9.0 the type of the 'length' member of the Tcl_Obj struct (the number of
> > bytes at '*bytes' member, not including the terminating null) has changed
> > from int to ptrdiff_t, so it will remain (1<<31)-1 => 2147483647 bytes on
> > 32 bit platforms (unsurprisingly) and (1<<63)-1 => 9223372036854775807
> > (9,22 exabyte) on 64 bit platforms.
> 
> My hearsay was "generally 64 bit (minus the sign-bit)".
> Are you sure that length-type is *always* ptrdiff_t, and
> that this may be 32bit?

In 9.X, it is ptrdiff_t. In 8.Y is still int. 

See https://core.tcl-lang.org/tcl/file?ci=trunk&name=generic/tcl.h&ln=325-333
and
https://core.tcl-lang.org/tcl/file?ci=trunk&name=generic/tcl.h&ln=740-752

ptrdiff_t can still be a 32 bits wide value. See below.
 
> The "64bit'ness" of a platform is also a bit more complicated...
> There are platforms, where pointers are 64bit, but ints are
> still 32 (despite machine words being all 64bit) - in those
> cases, I'd expect ptrdiff_t to be 64 bit, but on a real old
> 32bit machine, I don't really know for sure...

This is what I mean when say "on 32-bit platforms is still 2GB",
since i386-i686 platform has a 32 bit ptrdiff_t.

On my ancient i686 machine:

$ uname -m
i686
$ tclsh9.0 
% expr {(1 << (8 * $tcl_platform(pointerSize))-1) - 1}
2147483647
% package provide Tcl
9.0b3
% set tcl_platform(pointerSize)
4

> > IIUC that's also the (new) number of elements for a Tcl list.
> > In practice the number will be less, since the length of the
> > string representation of such list will hit the '*bytes' max
> > length first.
> 
> Not all lists are ever turned to string-rep.  While they are
> semantically "just strings", well written programs can avoid
> the actual obtainment of the string rep, at least for those
> really long lists that may be relevant here.

Yes, but that's an optimization. Tcl semantics are still defined
in terms of strings operations. I prefer not to depend on internals.

-- 
Emiliano