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

[Thomas Huth]

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?

 Thomas