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

[Thomas Huth <thuth@redhat.com>]

On 2019-01-09 14:10, Markus Armbruster wrote:
> Thomas Huth <thuth@redhat.com> 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

 Thomas