Re: [Qemu-devel] [PATCH] configure: mark qemu-ga VSS includes as system headers

[Michael Roth <mdroth@linux.vnet.ibm.com>]

Quoting Thomas Huth (2016-06-29 03:27:26)
> On 29.06.2016 01:43, Michael Roth wrote:
> > As of e4650c81, we do w32 builds with -Werror enabled. Unfortunately
> > for cases where we enable VSS support in qemu-ga, we still have
> > warnings generated by VSS includes that ship as part of the Microsoft
> > VSS SDK.
> > 
> > We can selectively address a number of these warnings using
> > 
> >   #pragma GCC diagnostic ignored ...
> > 
> > but at least one of these:
> > 
> >   warning: ‘typedef’ was ignored in this declaration
> > 
> > resulting from declarations of the form:
> > 
> >   typedef struct Blah { ... };
> > 
> > does not provide a specific command-line/pragma option to disable
> > warnings of the sort.
> > 
> > To allow VSS builds to succeed, the next-best option is disabling
> > these warnings on a per-file basis. pragmas like #pragma GCC
> > system_header can be used to declare subsequent includes/declarations
> > as being exempt from normal warnings, but this must be done within
> > a header file.
> > 
> > Since we don't control the VSS SDK, we'd need to rely on a
> > intermediate header include to accomplish this, and
> > since different objects in the VSS link target rely on different
> > headers from the VSS SDK, this would become somewhat of a rat's nest
> > (though not totally unmanageable).
> > 
> > The next step up in granularity is just marking the entire VSS
> > SDK include path as system headers via -isystem. This is a bit more
> > heavy-handed, but since this SDK hasn't changed since 2005, there's
> > likely little to be gained from selectively disabling warnings
> > anyway, so we implement that approach here.
> > 
> > This fixes the -Werror failures in both the configure test and the
> > qga build due to shared reliance on $vss_win32_include. For the
> > same reason, this also enforces a new dependency on -isystem support
> > in the C/C++ compiler when building QGA with VSS enabled.
> 
> Did we ever support any non-GCC-based compiler for building QGA? I don't
> think so, but in the worst case, we could later add a check whether the
> compiler supports that parameter, too...

Not really, but I think it's possible to bootstrap clang using mingw,
which is probably a reasonable option to keep open. That was the main
reason I wanted to point out the new dependency, but it turns out clang
supports -isystem anyway so a check/fallback in configure (like we
have for #pragma GCC diagnostic) is probably not not needed atm.

The other possibility is a native MSVC compile, but I doubt that works
as is (heavy usage of unistd.h, for instance), and I'm not aware of any
good reason to make it work.

> 
> Anyway, I think your patch is a nice and clean way to deal with the
> error messages from these headers, so:
> 
> Reviewed-by: Thomas Huth <thuth@redhat.com>

Thanks!