From: Paul Eggert <eggert@cs.ucla.edu>
To: Rich Felker <dalias@libc.org>
Cc: linux-man@vger.kernel.org, musl@lists.openwall.com,
libc-alpha@sourceware.org, libc-coord@lists.openwall.com
Subject: [musl] Re: regression in man pages for interfaces using loff_t
Date: Fri, 30 Jun 2023 00:11:20 -0700 [thread overview]
Message-ID: <8e65a459-a933-38b4-5f82-f7016c107d91@cs.ucla.edu> (raw)
In-Reply-To: <20230628191525.GS20050@brightrain.aerifal.cx>
On 2023-06-28 12:15, Rich Felker wrote:
> There's also the problem that off64_t is "exactly 64-bit" which makes
> it unsuitable as an interface type for cross-platform functions where
> one could imagine the native type being larger (rather horrifying but
> possible).
Although we won't have files with 2**63 bytes any time soon, this is the
best argument for preferring "loff_t" to "off64_t".
But come to think of it, it'd be better to document the type simply as
"off_t", with a footnote saying the equivalent of "this assumes that on
32-bit glibc platforms you compile with -DFILE_OFFSET_BITS=64 like any
sane person would." The intent really is off_t here, and that will
remain true even if off_t ever widens past 64 bits.
All the apps I know that use the syscalls in question simply pass
values that fit in off_t to these functions, and this will work
regardless of whether these apps are compiled with 64- or (horrors!)
32-bit off_t. Admittedly the footnote solution would not be perfect, but
it's good enough, and it would sidestep the loff_t vs off64_t confusion.
> As for why off64_t is not an appropriate type, it's defined by and
> associated with the LFS64 summit and the related intefaces, and
> governed by them. Using it makes these interfaces non-standardizable,
> because no standard is going to adopt a function whose public
> interface depends on another optional thing they don't want to
> mandate.
I don't see why not. For example, POSIX-2018 requires int32_t even
though C17 says it's optional. So there's precedent for POSIX adopting a
type that's optional elsewhere.
Also, to POSIX loff_t is just as optional as off64_t is. glibc defines
neither type if the app #defines _POSIX_C_SOURCE as POSIX requires. So
from a standardization viewpoint there's no reason to prefer one type
over the other.
> This is exactly the problem why ISO C is stuck with the
> broken and unusable fseek/ftell that take long, and hasn't adopted
> fseeko/ftello from POSIX -- their public interfaces use the
> POSIX-governed type off_t, and as such, ISO C adopting them without
> adopting the whole POSIX off_t is out of the question.
I'm not sure what the point is here, as far as standardization goes.
Neither ISO C nor POSIX use loff_t, and neither is likely to ever use
it: ISO C won't even adopt off_t much less loff_t, and POSIX works just
fine with off_t and doesn't need loff_t. (The same goes for off64_t of
course.)
> As a particular practical concern, applications performing
> configure-like tests may use the existence of an off64_t type to
> conclude that the LFS64 API is supported on the system they're being
> built on.
Which apps do that?
But anyway this is all moot if we simply document the arguments as off_t
with a footnote.
next prev parent reply other threads:[~2023-06-30 7:11 UTC|newest]
Thread overview: 29+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-06-28 17:53 [musl] " Rich Felker
2023-06-28 18:21 ` [musl] " Paul Eggert
2023-06-28 19:15 ` Rich Felker
2023-06-30 7:11 ` Paul Eggert [this message]
2023-06-30 8:02 ` [musl] Re: [libc-coord] " Jonathan Wakely
2023-06-30 8:14 ` Jonathan Wakely
2023-06-30 8:30 ` Sam James
2023-06-30 19:44 ` Paul Eggert
2023-07-02 1:18 ` A. Wilcox
2023-07-02 19:21 ` Paul Eggert
2023-07-03 18:16 ` Jakub Wilk
2023-07-03 21:35 ` Paul Eggert
2023-07-08 17:03 ` Alejandro Colomar
2023-07-09 6:07 ` [musl] [PATCH v4] off64_t: prefer off_t for splice, etc Paul Eggert
2023-07-09 6:16 ` [musl] Re: [libc-coord] " Sam James
2023-07-15 15:08 ` Alejandro Colomar
2023-07-15 18:35 ` Rich Felker
2023-07-15 20:01 ` Paul Eggert
2023-07-16 0:35 ` Alejandro Colomar
2023-07-16 0:39 ` Alejandro Colomar
2023-06-30 23:37 ` [musl] Re: regression in man pages for interfaces using loff_t Rich Felker
2023-07-01 7:24 ` [musl] Re: [libc-coord] " Paul Eggert
2023-07-01 13:36 ` Szabolcs Nagy
2023-07-01 23:02 ` Paul Eggert
2023-07-01 14:32 ` Rich Felker
2023-07-01 18:45 ` Alejandro Colomar
2023-07-01 23:06 ` Paul Eggert
2023-06-28 19:19 ` Szabolcs Nagy
2023-06-28 19:28 ` Rich Felker
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