Re: [musl] _GNU_SOURCE and _LARGEFILE_SOURCE

[Markus Wichmann <nullplan@gmx.net>]

Am Tue, Sep 17, 2024 at 07:29:17PM -0500 schrieb Brian Cain:
> In the "WHATSNEW" text, there's an item from 0.9.2 that states "- make
> _GNU_SOURCE imply _LARGEFILE64_SOURCE".  Is that intended to be the case
> generally?  I ask because in 25e6fee2 (remove LFS64 programming interfaces
> (macro-only) from _GNU_SOURCE, 2022-09-27) it stated "portable software
> should be prepared for them not to exist" and "the intent is that this be a
> very short-term measure and that the macros be removed entirely in the next
> release cycle."
>
> This comes up because in the QEMU project, there's a linux multiarch test
> case that uses readdir64 and it does define _GNU_SOURCE but does not define
> _LARGEFILE64_SOURCE and as such the cross compiler complains that there's no
> declaration of readdir64 before the call site.  I suppose that the test case
> would be more portable if it defined _LARGEFILE64_SOURCE.  But I'd also be
> happy to send a patch to musl that could have _GNU_SOURCE imply
> _LARGEFILE64_SOURCE (again?) if that's desired.  But - I gather that
> defining the macros is not what we want.  Instead of macros I should add
> declarations for readdir64() and its LFS64 friends, but only when
> _LARGEFILE64_SOURCE is defined?
>
> -Brian
>

The LFS64 interfaces are not in POSIX, so you cannot assume they exist.
Rather, you can test for their existance with a configure test, and if
it fails fall back to the portable interface. Or just use the portable
interface in the first place.

If you want to compile portable code on glibc, then define
_FILE_OFFSET_BITS to 64 and you get the exact same interface!

Ciao,
Markus