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?
