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

[Felipe Franciosi]

> On 21 Sep 2016, at 14:55, Eric Blake <address@hidden> 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.

Thanks,
Felipe

> 
> -- 
> Eric Blake   eblake redhat com    +1-919-301-3266
> Libvirt virtualization library http://libvirt.org
>