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Re: Moving to C11? (was Re: Redefinition of typedefs (C11 feature))

From: Thomas Huth
Subject: Re: Moving to C11? (was Re: Redefinition of typedefs (C11 feature))
Date: Mon, 14 Sep 2020 07:39:09 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.6.0

On 13/09/2020 04.51, Eduardo Habkost wrote:
> On Sat, Sep 12, 2020 at 08:45:19AM +0200, Thomas Huth wrote:
>> On 11/09/2020 22.06, Eduardo Habkost wrote:
>>> On Fri, Sep 11, 2020 at 08:06:10PM +0100, Peter Maydell wrote:
>>>> On Fri, 11 Sep 2020 at 19:49, Eduardo Habkost <ehabkost@redhat.com> wrote:
>>>>> I'm wondering: do our supported build host platforms all include
>>>>> compilers that are new enough to let us redefine typedefs?
>>>>> The ability to redefine typedefs is a C11 feature which would be
>>>>> very useful for simplifying our QOM boilerplate code.  The
>>>>> feature is supported by GCC since 2011 (v4.6.0)[1], and by clang
>>>>> since 2012 (v3.1)[2].
>>>> In configure we mandate either GCC v4.8 or better, or
>>>> clang v3.4 or better, or XCode Clang v5.1 or better
>>>> (Apple uses a different version numbering setup to upstream).
>>>> So you should probably double-check that that xcode clang has
>>>> what you want, but it looks like we're good to go otherwise.
>>> Can anybody confirm if the following is accurate?
>>> https://gist.github.com/yamaya/2924292#file-xcode-clang-vers-L67
>>> # Xcode 5.1 (5B130a)
>>> Apple LLVM version 5.1 (clang-503.0.38) (based on LLVM 3.4svn)
>>> Target: x86_64-apple-darwin13.1.0
>>> Thread model: posix
>>> If we know we have GCC 4.8+ or clang 3.4+, can we move to C11 and
>>> start using -std=gnu11?
>> You don't have to switch to gnu11, redefintions of typedefs are already
>> fine in gnu99, they are a gnu extension there to the c99 standard.
>> See also:
>> https://git.qemu.org/?p=qemu.git;a=commitdiff;h=7be41675f7cb16b
>> https://www.mail-archive.com/qemu-devel@nongnu.org/msg585581.html
> They still trigger a warning with gnu99 on clang:
> $ clang --version
> clang version 10.0.0 (Fedora 10.0.0-2.fc32)
> Target: x86_64-unknown-linux-gnu
> Thread model: posix
> InstalledDir: /usr/bin
> $ cat test.c
> typedef struct A A;
> typedef struct A A;
> $ clang -std=gnu11 -c test.c
> $ clang -std=gnu99 -c test.c
> test.c:2:18: warning: redefinition of typedef 'A' is a C11 feature 
> [-Wtypedef-redefinition]
> typedef struct A A;

Ah, right, I forgot about that ... so for clang, we silence that warning
via CFLAGS in the configure script. See commit e6e90feedb706b1.


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