Re: [PATCH] disas: Fix build with glib2.0 >=2.67.3

[Peter Maydell]

On Tue, 23 Feb 2021 at 15:03, Christian Ehrhardt
<christian.ehrhardt@canonical.com> wrote:
>
> glib2.0 introduced a change in 2.67.3 and later which triggers an
> issue [1] for anyone including it's headers in a "extern C" context
> which a few files in disas/* do. An example of such an include chain
> and error look like:
>
> ../../disas/arm-a64.cc
> In file included from /usr/include/glib-2.0/glib/gmacros.h:241,
>                  from 
> /usr/lib/x86_64-linux-gnu/glib-2.0/include/glibconfig.h:9,
>                  from /usr/include/glib-2.0/glib/gtypes.h:32,
>                  from /usr/include/glib-2.0/glib/galloca.h:32,
>                  from /usr/include/glib-2.0/glib.h:30,
>                  from /<<BUILDDIR>>/qemu-5.2+dfsg/include/glib-compat.h:32,
>                  from /<<BUILDDIR>>/qemu-5.2+dfsg/include/qemu/osdep.h:126,
>                  from ../../disas/arm-a64.cc:21:
> /usr/include/c++/10/type_traits:56:3: error: template with C linkage
>    56 |   template<typename _Tp, _Tp __v>
>       |   ^~~~~~~~
> ../../disas/arm-a64.cc:20:1: note: ‘extern "C"’ linkage started here
>    20 | extern "C" {
>       | ^~~~~~~~~~
>
> To fix that move the include of osdep.h out of that section. It was added
> already as C++ fixup by e78490c44: "disas/arm-a64.cc: Include osdep.h first".
>
> [1]: https://gitlab.gnome.org/GNOME/glib/-/issues/2331

I'm not convinced by this as a fix, though I'm happy to be corrected
by somebody with a fuller understanding of C++. glib.h may be supposed
to work as a C++ header, but osdep.h as a whole is definitely a C header,
so I think it ought to be inside 'extern C'; and it has to be
the first header included; and it happens to want to include glib.h.

Fixing glib.h seems like it would be nicer, assuming they haven't
already shipped this breakage. Failing that, does it work to do:

/*
 * glib.h expects to be included as a C++ header if we're
 * building a C++ file, but osdep.h and thus glib-compat.h are
 * C headers and should be inside an "extern C" block.
 */
#ifdef __cplusplus
extern "C++" {
#include <glib.h>
#if defined(G_OS_UNIX)
#include <glib-unix.h>
#endif
}

in include/glib-compat.h ?

thanks
-- PMM