Re: [Qemu-devel] [RFC PATCH 0/3] tweaks for QEMU's C standard

[Daniel P . Berrangé]

On Mon, Aug 13, 2018 at 11:25:49AM +0200, Thomas Huth wrote:
> On 08/13/2018 11:07 AM, Daniel P. Berrangé wrote:
> > On Fri, Aug 10, 2018 at 06:10:58PM +0100, Alex Bennée wrote:
> >> Hi,
> >>
> >> While I was reviewing Richard's SVE series I found Travis choking on
> >> some perfectly valid c99. It turns out that Travis default image is
> >> old enough that gcc defaults to -std=gnu89 hence the problem. However
> >> switching to c99 isn't enough as we use GNUisms and even gnu99 still
> >> trips up on qemu-secomp.
> >>
> >> Of course we could just jump to C11 already?
> > 
> > We've always required GCC or a compatible compiler (CLang is only viable
> > alternative option really). We use a number of GCC extensions to the C
> > standard and I don't see a compelling reason to stop using them.
> > 
> > From that POV I think we do *NOT* need to care about official C standards
> > (c89, c99, c11, etc), only the GNU C standards (gnu89, gnu99, gnu11, etc).
> > 
> >> This is an RFC because this could descend into a C standards
> >> bike-shedding exercise but I thought I'd at least put it out there on
> >> a Friday afternoon ;-)
> > 
> > I did some archeology to inform our plans...
> > 
> > The default GCC C standards across various versions are:
> > 
> >   8.2.1: gnu17
> >   7.3.1: gnu11
> >   6.4.1: gnu11
> >   5.3.1: gnu11
> >   4.9.1: gnu89
> >   4.4.7: gnu89
> > 
> > Interesting to note that no version of GCC ever defaulted to gnu99. It was
> > not fully implemented initially and by the time the standard was fully
> > implemented, gnu11 was already good enough to be the default. So GCC jumped
> > straight from gnu89 as default to gnu11 as default.
> > 
> > Across the various distros we aim to support we have:
> > 
> >       RHEL-7: 4.8.5
> >       Debian (Stretch): 6.3.0
> >       Debian (Jessie): 4.9.2
> >       OpenBSD (Ports): 4.9.4
> >       FreeBSD (Ports): 8.2.0
> >       OpenSUSE Leap 15: 7.3.1
> >       SLE12-SP2:
> >       Ubuntu (Xenial): 5.4.0
> >       macOS (Homebrew): 8.2.0
> > 
> > IOW plenty of our plaforms are still on 4.x which defaults to gnu89.
> 
> Thanks for the great summary!
> 
> > In GCC 4.x, gnu99 is said to be incomplete (but usable) and gnu11
> > are said to be incomplete and experimental (ie don't use it).
> > 
> > The lowest common denominator supported by all our platforms is thus
> > gnu89.
> > 
> > If we don't mind that gnu99 is not fully complete in 4.x, we could use
> > that standard.
> > 
> > We definitely can't use gnu11 any time soon.
> > 
> > Given that many modern platforms default to gnu11, I think we should
> > set an explicit -std=gnu89, or -std=gnu99, because otherwise we risk
> > accidentally introducing code that relies on gnu11 features.
> 
> Sounds good. What about trying -std=gnu99 first, and if we run into
> problems, switch back to -std=gnu89 later?

I don't have a strong opinion either way - both options would be better
than what we do today by relying on the variable gcc built-in defaults.

Using -std=gnu99 gives slightly weaker protection in that we might still
accidentally use features which are not supported in the limitd gnu99
impl of gcc 4.x. I think that's unlikely, however, hence I don't really
mind either of gnu89 or gnu99

Regards,
Daniel
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