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

[Felipe Franciosi]

Actually, I just noticed Pavel is in the middle of submitting a "replay 
additions" series (currently at v4).

Pavel: is this something you can address as part of that series?

Thanks,
Felipe

> On 21 Sep 2016, at 11:12, Felipe Franciosi <address@hidden> wrote:
> 
> 
>> 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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>