How can Autoconf help with the transition to stricter compilation defaults?

[Zack Weinberg via cfe-commits]

Florian Weimer <fweimer at redhat.com> writes:
> based on a limited attempt to get this fixed about three years
> ago, I expect that many of the problematic packages have not had their
> configure scripts regenerated using autoconf for a decade or more.  This
> means that as an autoconf maintainer, you unfortunately won't be able to
> help us much.

I’m sadly not surprised.

This is definitely more work than I can see myself doing on a volunteer
basis, but a 2.69.1 patch release — nothing that’s not already on trunk,
cherry pick the changes needed to support the newer compilers (and
also newer Perl and Bash and M4) is a thing that could happen.

> Thanks, these changes are going to be helpful to get a clean run from
> our Fedora tester.

Autoconf’s own test suite is sadly not very thorough.  If you find more
problems I will prioritize them.

> Once you include the header, you also need to know function parameters,
> otherwise you won't be able to form a valid call.

You can assign to a function pointer variable if you know the complete
type signature, which is desirable for other reasons (see reply to Rich).
Needing to know how to form argument *values* could be much more trouble,
but I don’t think it should be necessary.

>> p.s. GCC and Clang folks: As long as you’re changing the defaults out
>> from under people,
>
> Hmph, I wouldn't frame it this way.  We are aware of GCC's special role
> as the system compiler.  We're trying to upstream the changes to sources
> before flipping the compiler default.  (The burden of being a system
> compiler and all that.)  A 25-year transition period evidently wasn't
> enough, so some effort is still needed.  We may conclude that removing
> these extensions is too costly even in 2024.

I didn’t mean to imply that I disliked any of the changes.  In fact,
with my day job (CS professor) hat on, I am quite looking forward to not
having to warn the kids about these legacy features anymore (we don’t
_teach_ them, but they inevitably use them by accident, particularly
implicit function declarations, and then get confused because ‘cc’ with
no -W options doesn’t catch the mistake).

>> can you please also remove the last few predefined
>> user-namespace macros (-Dlinux, -Dunix, -Darm, etc) from all the
>> -std=gnuXX modes?
>
> That's a good point, I'll think about how we can instrument GCC to
> support tracking that.  We won't be able help with -Darm on the Fedora
> side (the AArch64 port doesn't have that, and there's no longer a Fedora
> 32-bit Arm port), but -Dlinux and -Dunix we can help with.

These are also a trip hazard for novices, and the only way to turn them
off is with -std=cXX, which also turns another trip hazard (trigraphs)
*on*… so yeah, anything you can do to help speed up their removal, I
think it’d be worthwhile.

zw