Re: [RFC PATCH] libxfs: compile with a C++ compiler

[Sam James <sam@gentoo.org>]

"Darrick J. Wong" <djwong@kernel.org> writes:

> On Wed, Aug 28, 2024 at 01:01:31AM +0100, Sam James wrote:
>> "Darrick J. Wong" <djwong@kernel.org> writes:
>> 
>> > From: Darrick J. Wong <djwong@kernel.org>
>> >
>> > Apparently C++ compilers don't like the implicit void* casts that go on
>> > in the system headers.  Compile a dummy program with the C++ compiler to
>> > make sure this works, so Darrick has /some/ chance of figuring these
>> > things out before the users do.
>> 
>> Thanks, this is a good idea. Double thanks for the quick fix.
>> 
>> 1) yes, it finds the breakage:
>> Tested-by: Sam James <sam@gentoo.org>
>> 
>> 2) with the fix below (CC -> CXX):
>> Reviewed-by: Sam James <sam@gentoo.org>
>> 
>> 3) another thing to think about is:
>> * -pedantic?
>
> -pedantic won't build because C++ doesn't support flexarrays:
>
> In file included from ../include/xfs.h:61:
> ../include/xfs/xfs_fs.h:523:33: error: ISO C++ forbids flexible array member ‘bulkstat’ [-Werror=pedantic]
>   523 |         struct xfs_bulkstat     bulkstat[];
>       |                                 ^~~~~~~~
>
> even if you wrap it in extern "C" { ... };

Doh. So the ship has kind of sailed already anyway.

>
>> * maybe do one for a bunch of standards? (I think systemd does every
>> possible value [1])
>
> That might be overkill since xfsprogs' build system doesn't have a good
> mechanism for detecting if a compiler supports a particular standard.
> I'm not even sure there's a good "reference" C++ standard to pick here,
> since the kernel doesn't require a C++ compiler.
>
>> * doing the above for C as well
>
> Hmm, that's a good idea.
>
> I think the only relevant standard here is C11 (well really gnu11),
> because that's what the kernel compiles with since 5.18.  xfsprogs
> doesn't specify any particular version of C, but perhaps we should match
> the kernel every time they bump that up?

Projects are often (IMO far too) conservative with the features they use
in their headers and I don't think it's unreasonable to match the kernel
here.

>
> IOWs, should we build xfsprogs with -std=gnu11?  The commit changing the
> kernel to gnu11 (e8c07082a810) remarks that gcc 5.1 supports it just
> fine.  IIRC RHEL 7 only has 4.8.5 but it's now in extended support so
> ... who cares?  The oldest supported Debian stable has gcc 8.

so, I think we should match whatever linux-headers / the uapi rules are,
and given I've seen flexible array members in there, it's at least C99.

I did some quick greps and am not sure if we're using any C11 features
in uapi at least.

Just don't blame me if someone yells ;)

(kees, any idea if I'm talking rubbish?)

tl;dr: let's try gnu11?

> [...]

sam