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From: Michael S <already5chosen@yahoo.com>
Newsgroups: comp.arch
Subject: Re: streams and file locks, ancient OS history, ARM is sort of
 channeling
Date: Mon, 1 Jul 2024 11:23:03 +0300
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On Mon, 1 Jul 2024 02:10:43 -0000 (UTC)
John Levine <johnl@taugh.com> wrote:

> According to MitchAlsup1 <mitchalsup@aol.com>:
> >>>> In fact, you can ftruncate arbitrarily large at the start -
> >>>> backing sectors for the mapped pages will only be allocated when
> >>>> a page is referenced for the first time. =20
> >>>
> >>>Can you ftruncate( 0x7FFFFFFFFFFFFFFFF ); ?? =C2=BD of the address
> >>>space =20
> >>
> >> Every filesystem has a maximum file size. Exceed that and you'll
> >> get an EFBIG error. =20
> >
> >Then it is not capable of ftruncate arbitrarily large in a hardware's
> >view of arbitrarily large which tends to be 1-bit smaller than the
> >largest container. =20
>=20
> The goal here is to map a file into the address space. What's your use
> case for mapping a file-like thing that is bigger than any real file
> and can't be written out to the disk?
>=20

Consider that configurations in which maximal file size is bigger than
half of address space are quite common. And not just on 32-bit HW.

Of course, what is considered "half of address space" is not a very
simple question by itself. Take x86-64. Is its address space 2**64 or
2*48? Take aarch64. Is its address space 2**64 or 2**49? Or, may be,
2**56?