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Re: [Qemu-devel] [PATCH] configure: Force the C standard to gnu11

From: Daniel P . Berrangé
Subject: Re: [Qemu-devel] [PATCH] configure: Force the C standard to gnu11
Date: Wed, 9 Jan 2019 11:44:59 +0000
User-agent: Mutt/1.10.1 (2018-07-13)

On Wed, Jan 09, 2019 at 12:25:43PM +0100, Thomas Huth wrote:
> On 2019-01-09 11:58, Daniel P. Berrangé wrote:
> > On Mon, Jan 07, 2019 at 11:45:26AM +0100, Thomas Huth wrote:
> >> Different versions of GCC and Clang use different versions of the C 
> >> standard.
> >> This repeatedly caused problems already, e.g. with duplicated typedefs:
> >>
> >>  https://lists.gnu.org/archive/html/qemu-devel/2018-11/msg05829.html
> >>
> >> or with for-loop variable initializers:
> >>
> >>  https://lists.gnu.org/archive/html/qemu-devel/2019-01/msg00237.html
> >>
> >> To avoid these problems, we should enforce the C language version to the
> >> same level for all compilers. Since our minimum compiler versions are
> >> GCC v4.8 and Clang v3.4 now, and both basically support "gnu11" already,
> >> this seems to be a good choice.
> > 
> > In 4.x   gnu11 is marked as experimental. I'm not really comfortable
> > using experimental features - even if its warning free there's a risk
> > it would silently mis-compile something.
> > 
> > gnu99 is ok with 4.x - it is merely "incomplete".
> gnu11 has the big advantage that it also fixes the problem with
> duplicated typedefs that are reported by older versions of Clang.
> Are you sure about the experimental character in 4.x? I just looked at
> https://gcc.gnu.org/onlinedocs/gcc-4.8.5/gcc/Standards.html and it says:
> "A fourth version of the C standard, known as C11, was published in 2011
> as ISO/IEC 9899:2011. GCC has limited incomplete support for parts of
> this standard, enabled with -std=c11 or -std=iso9899:2011."
> It does not say anything about "experimental" there. The word
> "experimental" is only used for the C++ support, but we hardly have C++
> code in QEMU -- if you worry about that, I could simply drop the
> "-std=gnu++11" part from my patch?

I was looking at the "info gcc" docs on RHEL7, gcc-4.8.5-16.el7_4.1.x86_64:

  "3.4 Options Controlling C Dialect


          ISO C11, the 2011 revision of the ISO C standard.  Support is
          incomplete and experimental.  The name 'c1x' is deprecated.


          GNU dialect of ISO C11.  Support is incomplete and
          experimental.  The name 'gnu1x' is deprecated."

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