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

From: Thomas Huth
Subject: Re: [Qemu-devel] [PATCH] configure: Force the C standard to gnu11
Date: Wed, 9 Jan 2019 14:20:02 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.9.1

On 2019-01-09 13:58, Daniel P. Berrangé wrote:
> On Wed, Jan 09, 2019 at 01:52:02PM +0100, Thomas Huth wrote:
>> 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?
> Our code is already cleanly compiling with gnu99 standard - the problem
> is merely that we sometimes introduce regressions due to not enforcing
> that standard level. I don't think the features in gnu11 are compelling
> enough to justify using something that's declared experimental.
What about the duplicated typedef problem? See:


That one occured with Clang, but I think we've had plenty of these in
the past with GCC, too...


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