[Qemu-devel] Re: [PATCH] Suppress warning: zero-length gnu_printf format string

[Paolo Bonzini <pbonzini@redhat.com>]

On 10/14/2010 06:38 PM, Blue Swirl wrote:
> On Thu, Oct 14, 2010 at 9:20 AM, Markus Armbruster<armbru@redhat.com>  wrote:
>> Blue Swirl<blauwirbel@gmail.com>  writes:
>>
>>> On Wed, Oct 13, 2010 at 7:19 AM, Markus Armbruster<armbru@redhat.com>  wrote:
>>>> Blue Swirl<blauwirbel@gmail.com>  writes:
>>>>
>>>>> On Mon, Oct 11, 2010 at 12:52 PM, Markus Armbruster<armbru@redhat.com>  wrote:
>>>>>> Warns about this line in check-qjson.c:
>>>>>>     QObject *obj = qobject_from_json("");
>>>>>>
>>>>>> The obvious fix (add -Wno-format-zero-length to gcc_flags) doesn't
>>>>>> work, because -Wall switches it on again.  Fix by putting configured
>>>>>> flags last.
>>>>>
>>>>> This would disable the flag globally. I'd rather disable the flag only
>>>>> for check-qjson.o
>>>>
>>>> Is this warning worth the hassle?  What's the problem with empty format
>>>> strings?
>>>
>>> Your fix solves this specific case, but it also degrades the gcc
>>> checks of the mainstream code (slightly). I think the test suite need
>>> not follow the level of checking that should be applied to mainstream,
>>> or at least the warnings there should not be fatal.
>>
>> "Degrade" implies we miss something that's "wrong" enough to be worth
>> avoiding.  What's wrong with empty format strings?
>
> They generate useless calls to the formatting function, wasting
> performance (slightly). Since there are no calls currently, this is of
> course hypothetical.

It is even more hypothetical when empty-format printfs are optimized 
away by GCC:

$ gcc -x c - -O2 -S -o -
#include <stdio.h>
main() { printf (""); }

	.file	""
	.text
	.p2align 4,,15
.globl main
	.type	main, @function
main:
.LFB11:
	.cfi_startproc
	rep
	ret
	.cfi_endproc

and other attribute-printf-marked functions are probably not noop when 
the format argument is empty.

Paolo