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

From: Markus Armbruster
Subject: Re: [Qemu-devel] [PATCH] configure: Force the C standard to gnu11
Date: Wed, 09 Jan 2019 15:20:07 +0100
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/26.1 (gnu/linux)

Thomas Huth <address@hidden> writes:

> On 2019-01-09 14:10, Markus Armbruster wrote:
>> Thomas Huth <address@hidden> writes:
>>> On 2019-01-09 12:44, Daniel P. Berrangé wrote:
>>>> 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
>>>>        ....snip...
>>>>      'gnu11'
>>>>      'gnu1x'
>>>>           GNU dialect of ISO C11.  Support is incomplete and
>>>>           experimental.  The name 'gnu1x' is deprecated."
>>> Ok. Looks like the "Support is incomplete and experimental" sentence has
>>> been removed with GCC 4.9.0 here. So GCC 4.8 is likely pretty close
>>> already. IMHO we could give it a try and enable gnu11 for QEMU with GCC
>>> v4.8, too. If we later find problems, we could still switch back to
>>> gnu99 instead. Other opinions?
>> Switchinh back could be somewhat painful if we already started using C11
>> features.  And if we don't plan to, then what exactly will -std=gnu11
>> buy us?
> With C11, we get safety for the "duplicated typedef" problem that we run
> into regularly again and again, see e.g.:
> https://lists.gnu.org/archive/html/qemu-devel/2018-11/msg05829.html

That's a compilation failure.  "Support is experimental" makes me afraid
of run time failures.

If we truly want C11, shouldn't we bump minimum required GCC to 4.9?

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