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

[Felipe Franciosi]

> On 21 Sep 2016, at 11:07, Daniel P. Berrange <address@hidden> wrote:
> 
> On Wed, Sep 21, 2016 at 10:00:23AM +0000, Felipe Franciosi wrote:
>> 
>>> On 21 Sep 2016, at 07:24, Markus Armbruster <address@hidden> wrote:
>>> 
>>> "Pavel Dovgalyuk" <address@hidden> writes:
>>> 
>>>>> From: Felipe Franciosi [mailto:address@hidden
>>>>> If compiling with -Werror=unused-result, replay-internal.c won't build
>>>>> due to a call to fwrite() where the returned value is ignored. A simple
>>>>> cast to (void) is not sufficient on recent GCCs, so this fixes it.
>>>>> 
>>>>> Signed-off-by: Felipe Franciosi <address@hidden>
>>>>> ---
>>>>> replay/replay-internal.c | 2 +-
>>>>> 1 file changed, 1 insertion(+), 1 deletion(-)
>>>>> 
>>>>> diff --git a/replay/replay-internal.c b/replay/replay-internal.c
>>>>> index 5835e8d..6978d76 100644
>>>>> --- a/replay/replay-internal.c
>>>>> +++ b/replay/replay-internal.c
>>>>> @@ -65,7 +65,7 @@ void replay_put_array(const uint8_t *buf, size_t size)
>>>>> {
>>>>>    if (replay_file) {
>>>>>        replay_put_dword(size);
>>>>> -        fwrite(buf, 1, size, replay_file);
>>>>> +        (void)(fwrite(buf, 1, size, replay_file)+1);
>>>>>    }
>>>>> }
>>>> 
>>>> This looks very weird.
>> 
>> Oh I couldn't agree more. I hate this syntax. It is, however, the simplest 
>> way to get around this issue. See:
>> http://stackoverflow.com/questions/11888594/ignoring-return-values-in-c
> 
> 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

I like that better. Do you want a series with a patch to add this macro to 
include/qemu/compiler.h and another making replay_put_array() use it?

Cheers,
Felipe

> 
> 
> /* 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
> 
> GNULIB makes it availavble under LGPLv2.1+
> 
> eg used as:
> 
>   ignore_value(fwrite(buf, 1, size, replay_file));
> 
> Regards,
> Daniel
> -- 
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