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

[Markus Armbruster]

"Daniel P. Berrange" <address@hidden> writes:

> On Wed, Sep 21, 2016 at 02:26:48PM +0000, Felipe Franciosi wrote:
>> 
>> > 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.
>
> 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+
>    */

Our tree has a mix of licenses, which is enough of a pain.  Mixing
licenses within *files* is even worse, and might not even be legally
sound.

Relicense the whole file under our preferred license GPLv2+?