Re: [Qemu-devel] [PATCH] replay: Fix build with -Werror=unused-result

["Daniel P. Berrange" <berrange@redhat.com>]

On Wed, Sep 21, 2016 at 02:26:48PM +0000, Felipe Franciosi wrote:
> 
> > On 21 Sep 2016, at 14:55, Eric Blake <eblake@redhat.com> wrote:
> > 
> > On 09/21/2016 07:31 AM, Markus Armbruster wrote:
> >>> 
> >>> If we want to ignore return value reliably, lets just pull in the
> >>> ignore_value macro from gnulib which is known to work across GCC
> >>> versions
> >>> 
> >>> 
> >>> /* Normally casting an expression to void discards its value, but GCC
> >>>   versions 3.4 and newer have __attribute__ ((__warn_unused_result__))
> >>>   which may cause unwanted diagnostics in that case.  Use __typeof__
> >>>   and __extension__ to work around the problem, if the workaround is
> >>>   known to be needed.  */
> >>> #if 3 < __GNUC__ + (4 <= __GNUC_MINOR__)
> >>> # define ignore_value(x) \
> >>>    (__extension__ ({ __typeof__ (x) __x = (x); (void) __x; }))
> >>> #else
> >>> # define ignore_value(x) ((void) (x))
> >>> #endif
> >> 
> >> Casting a value to void is the traditional and obvious way to say "yes,
> >> I mean to ignore this value".  Now compilers start to reply "no, you
> >> don't".  We can invent new (and less obvious) ways to say "yes, I do",
> >> and compilers can then learn them so they can again reply "no, you
> >> don't".  Why have compilers started to behave like two-year-olds?
> > 
> > gcc has been doing the "__warn_unused_value__ means cast-to-void is
> > insufficient" complaint for years (since at least 2008, per the gnulib
> > history).  But the gnulib workaround has also been effectively silencing
> > it for years (it was actually my work in 2011, commit 939dedd, which
> > came up with the form listed above).  The other nice thing about
> > "ignore_value(wur_function())" is that you are avoiding a cast in your
> > local code, and the burden of shutting up the annoying compiler is
> > hidden behind a macro that can easily be changed to affect all clients
> > of the macro, should gcc regress yet again and we need some other
> > formula to shut it up.
> > 
> > And yes, the gnulib mailing list has threads complaining about gcc's
> > behavior back when the macro had to be invented, and again when glibc
> > added wur markings to functions that can legitimately be ignored
> > (fread() is one of them; because there are valid programming paradigms
> > where you check ferror() later on rather than having to check every
> > intermediate fread(), at the expense of less-specific error messages).
> 
> What's the best way to bring gnulib's ignore-value.h into Qemu? I'd think we could just add to include/qemu/compiler.h something like:
> 
> ----------------------8<----------------------
> #if QEMU_GNUC_PREREQ(3, 4)
> /* From gnulib's ignore-value.h by Jim Meyering, Eric Blake and Padraig Brady */
> # define ignore_value(x) \,
>          (__extension__ ({ __typeof__ (x) __x = (x); (void) __x; }))
> #else
> # define ignore_value(x) ((void) (x))
> #endif
> ----------------------8<----------------------
> 
> But I'm not sure if that suffices to meet GPL's requirements.

The compiler.h file has no license header, just a comment
saying "public domain", which is obviously not the case
if you add this macro.

Given that you'll need to explicitly mention the license terms
for ignore_value. eg with a comment line like

  /* The ignore_value() macro is taken from GNULIB ignore-value.h,
   * licensed under the terms of the LGPLv2+
   */

Regards,
Daniel
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