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Re: [Qemu-devel] [PATCH] replay: Fix build with -Werror=unused-result
From: |
Felipe Franciosi |
Subject: |
Re: [Qemu-devel] [PATCH] replay: Fix build with -Werror=unused-result |
Date: |
Wed, 21 Sep 2016 14:26:48 +0000 |
> 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
>
- [Qemu-devel] [PATCH] replay: Fix build with -Werror=unused-result, Felipe Franciosi, 2016/09/21
- Re: [Qemu-devel] [PATCH] replay: Fix build with -Werror=unused-result, Pavel Dovgalyuk, 2016/09/21
- Re: [Qemu-devel] [PATCH] replay: Fix build with -Werror=unused-result, Markus Armbruster, 2016/09/21
- Re: [Qemu-devel] [PATCH] replay: Fix build with -Werror=unused-result, Felipe Franciosi, 2016/09/21
- Re: [Qemu-devel] [PATCH] replay: Fix build with -Werror=unused-result, Pavel Dovgalyuk, 2016/09/21
- Re: [Qemu-devel] [PATCH] replay: Fix build with -Werror=unused-result, Daniel P. Berrange, 2016/09/21
- Re: [Qemu-devel] [PATCH] replay: Fix build with -Werror=unused-result, Felipe Franciosi, 2016/09/21
- Re: [Qemu-devel] [PATCH] replay: Fix build with -Werror=unused-result, Felipe Franciosi, 2016/09/21
- Re: [Qemu-devel] [PATCH] replay: Fix build with -Werror=unused-result, Markus Armbruster, 2016/09/21
- Re: [Qemu-devel] [PATCH] replay: Fix build with -Werror=unused-result, Eric Blake, 2016/09/21
- Re: [Qemu-devel] [PATCH] replay: Fix build with -Werror=unused-result,
Felipe Franciosi <=
- Re: [Qemu-devel] [PATCH] replay: Fix build with -Werror=unused-result, Daniel P. Berrange, 2016/09/21
- Re: [Qemu-devel] [PATCH] replay: Fix build with -Werror=unused-result, Felipe Franciosi, 2016/09/21
- Re: [Qemu-devel] [PATCH] replay: Fix build with -Werror=unused-result, Eric Blake, 2016/09/21
- Re: [Qemu-devel] [PATCH] replay: Fix build with -Werror=unused-result, Markus Armbruster, 2016/09/21
- Re: [Qemu-devel] [PATCH] replay: Fix build with -Werror=unused-result, Eric Blake, 2016/09/21
- Re: [Qemu-devel] [PATCH] replay: Fix build with -Werror=unused-result, Daniel P. Berrange, 2016/09/22
- Re: [Qemu-devel] [PATCH] replay: Fix build with -Werror=unused-result, Markus Armbruster, 2016/09/22
- Re: [Qemu-devel] [PATCH] replay: Fix build with -Werror=unused-result, Eric Blake, 2016/09/22
- Re: [Qemu-devel] [PATCH] replay: Fix build with -Werror=unused-result, Markus Armbruster, 2016/09/23
- Re: [Qemu-devel] [PATCH] replay: Fix build with -Werror=unused-result, Felipe Franciosi, 2016/09/23