Re: [libc-coord] Re: regression in man pages for interfaces using loff_t

Jonathan Wakely
2023-06-30T08:16:01Z

On Fri, 30 Jun 2023 at 09:02, Jonathan Wakely wrote:
>
> On Fri, 30 Jun 2023 at 08:11, Paul Eggert wrote:
>>
>> 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.
>
>
> For APIs like copy_file_range(2) and splice(2) the arguments are loff_t* so you can't just "pass arguments that fit in off_t" to them. You have to get the pointer type correct, because writing 64-bits through a 32-bit off_t would be bad. And in C++ it won't even compile unless you get the pointer types exactly right (C compilers will typically allow the mismatch with just a warning).
>
> People miss footnotes. I would really prefer if the signature shown in the man page used a type that will actually compile. If it shows off_t, that won't compile for 32-bit systems without LFS support enabled.

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