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

[Markus Armbruster]

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?