Thomas Huth <th...@redhat.com> writes: > On 2019-01-09 14:10, Markus Armbruster wrote: >> Thomas Huth <th...@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
That's a compilation failure. "Support is experimental" makes me afraid of run time failures. If we truly want C11, shouldn't we bump minimum required GCC to 4.9?