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

[Eduardo Habkost]

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;
                 ^
test.c:1:18: note: previous definition is here
typedef struct A A;
                 ^
1 warning generated.
$ 

-- 
Eduardo