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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 13:52:02 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.9.1

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?


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