Re: [Qemu-devel] [PATCH] Fix gcc-4.6 compiler error

[Zhi Yong Wu]

On Mon, Aug 1, 2011 at 8:30 AM, Anthony Liguori <address@hidden> wrote:
> On 07/29/2011 07:18 PM, Peter Maydell wrote:
>>
>> On 29 July 2011 20:30, Stefan Weil<address@hidden>  wrote:
>>>
>>> Commit 3d3b8303c6f83b9b245bc774af530a6403cc4ce6
>>> breaks builds with gcc-4.6:
>>>
>>> hw/fw_cfg.c: In function ‘probe_splashfile’:
>>> hw/fw_cfg.c:66:9: error: variable ‘fop_ret’ set but not used
>>> [-Werror=unused-but-set-variable]
>>> hw/fw_cfg.c: In function ‘fw_cfg_bootsplash’:
>>> hw/fw_cfg.c:130:9: error: variable ‘fop_ret’ set but not used
>>> [-Werror=unused-but-set-variable]
>>>
>>> Remove fop_ret. Testing the result of fread() is normally
>>> a good idea, but I don't think it is needed here.
>>
>>> @@ -86,7 +85,7 @@ static FILE *probe_splashfile(char *filename, int
>>> *file_sizep, int *file_typep)
>>>     }
>>>     /* check magic ID */
>>>     fseek(fp, 0L, SEEK_SET);
>>> -    fop_ret = fread(buf, 1, 2, fp);
>>> +    (void)fread(buf, 1, 2, fp);
>>
>> Usually this kind of thing is added in order to stop gcc complaining about
>> you ignoring the return value of a function which has been marked (by
>> libc)
>> as 'don't-ignore-return-value'. In such cases a "(void)" is not sufficient
>> to suppress the "return value ignored" warning.
>>
>> At least some of these cases really should be checking fread return
>> values;
>> I see we also don't check fseek() return values either in all places. So
>> the easiest fix is just to check all the fread() calls.
>>
>> Alternative suggestion: it would be easier to just slurp the whole file
>> into memory (which is what we do once we've figured out it's an image)
>> and then check the magic numbers in the memory buffer, which removes
>> a lot of these unchecked function calls altogether. Since we're linking
>> against glib anyway it looks like g_file_get_contents() would do 95%
>> of the work for us. [disclaimer: I haven't used that API myself but it
>> looks the right shape...g_malloc vs qemu_malloc issues, maybe?] Failing
>> that, fopen/fstat/fread/fclose/check magic numbers.
>
> As long as it's not mixed, it shouldn't be a problem.
>
> I think using g_file_get_contents would make sense here.
I have also met this problem on fedora 15 today. Currently i disable
werror option to build successfully. How to completely this problem
regardless of gcc>=4.6?

>
> Regards,
>
> Anthony Liguori
>
>>
>> -- PMM
>>
>>
>
>
>



-- 
Regards,

Zhi Yong Wu