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

["Daniel P. Berrangé" <berrange@redhat.com>]

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
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