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 -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|
