On Mon, Aug 13, 2018 at 10:07:05AM +0100, 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. > > 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.
Also, we should ensure the min required GCC version via biuld time check of some kind. eg something like #if !(__GNUC_PREREQ(4, 4) || defined(__clang__)) # error "QEMU requires GCC >= 4.4, or CLang" #endif We can even check the C standard at build time if desired. eg I see these symbols defined for various -std=xxx args: gnu89: #undef __STDC_VERSION__ gnu99: #define __STDC_VERSION__ 199901 gnu11: #define __STDC_VERSION__ 201112L gnu17: #define __STDC_VERSION__ 201710L (See "gcc -std=XXX -dM -E - < /dev/null") 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 :|
