Re: Moving to C11? (was Re: Redefinition of typedefs (C11 feature))

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

On Mon, Sep 14, 2020 at 09:46:36AM -0400, Eduardo Habkost wrote:
> On Mon, Sep 14, 2020 at 07:39:09AM +0200, Thomas Huth wrote:
> > On 13/09/2020 04.51, Eduardo Habkost wrote:
> > > On Sat, Sep 12, 2020 at 08:45:19AM +0200, Thomas Huth wrote:
> > >> On 11/09/2020 22.06, Eduardo Habkost wrote:
> > >>> On Fri, Sep 11, 2020 at 08:06:10PM +0100, Peter Maydell wrote:
> > >>>> On Fri, 11 Sep 2020 at 19:49, Eduardo Habkost <ehabkost@redhat.com> wrote:
> > >>>>>
> > >>>>> I'm wondering: do our supported build host platforms all include
> > >>>>> compilers that are new enough to let us redefine typedefs?
> > >>>>>
> > >>>>> The ability to redefine typedefs is a C11 feature which would be
> > >>>>> very useful for simplifying our QOM boilerplate code.  The
> > >>>>> feature is supported by GCC since 2011 (v4.6.0)[1], and by clang
> > >>>>> since 2012 (v3.1)[2].
> > >>>>
> > >>>> In configure we mandate either GCC v4.8 or better, or
> > >>>> clang v3.4 or better, or XCode Clang v5.1 or better
> > >>>> (Apple uses a different version numbering setup to upstream).
> > >>>> So you should probably double-check that that xcode clang has
> > >>>> what you want, but it looks like we're good to go otherwise.
> > >>>
> > >>> Can anybody confirm if the following is accurate?
> > >>>
> > >>> https://gist.github.com/yamaya/2924292#file-xcode-clang-vers-L67
> > >>> # Xcode 5.1 (5B130a)
> > >>> Apple LLVM version 5.1 (clang-503.0.38) (based on LLVM 3.4svn)
> > >>> Target: x86_64-apple-darwin13.1.0
> > >>> Thread model: posix
> > >>>
> > >>> If we know we have GCC 4.8+ or clang 3.4+, can we move to C11 and
> > >>> start using -std=gnu11?
> > >>
> > >> You don't have to switch to gnu11, redefintions of typedefs are already
> > >> fine in gnu99, they are a gnu extension there to the c99 standard.
> > >>
> > >> See also:
> > >> https://git.qemu.org/?p=qemu.git;a=commitdiff;h=7be41675f7cb16b
> > >>
> > >> https://www.mail-archive.com/qemu-devel@nongnu.org/msg585581.html
> > > 
> > > They still trigger a warning with gnu99 on clang:
> > > 
> > > $ clang --version
> > > clang version 10.0.0 (Fedora 10.0.0-2.fc32)
> > > Target: x86_64-unknown-linux-gnu
> > > Thread model: posix
> > > InstalledDir: /usr/bin
> > > $ cat test.c
> > > typedef struct A A;
> > > typedef struct A A;
> > > $ clang -std=gnu11 -c test.c
> > > $ clang -std=gnu99 -c test.c
> > > test.c:2:18: warning: redefinition of typedef 'A' is a C11 feature [-Wtypedef-redefinition]
> > > typedef struct A A;
> > 
> > Ah, right, I forgot about that ... so for clang, we silence that warning
> > via CFLAGS in the configure script. See commit e6e90feedb706b1.
> 
> Nice, I hadn't seen that.  This means we don't need C11 for
> supporting redefinition of typedefs.
> 
> Now, do we have other reasons for not moving to C11?  It would be
> nice to make QEMU_GENERIC unnecessary and just use _Generic, for
> example.

When we set std=gnu99 in:


  commit 7be41675f7cb16be7c8d2554add7a63fa43781a8
  Author: Thomas Huth <thuth@redhat.com>
  Date:   Mon Jan 7 11:25:22 2019 +0100

    configure: Force the C standard to gnu99

we chose to not use gnu11, because this standard level is marked as
experimental in GCC 4.8 and thus we felt it wasn't a good idea to
rely on.


Regards,
Daniel
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