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


From: Paolo Bonzini
Subject: [Qemu-devel] Re: [PATCH] Suppress warning: zero-length gnu_printf format string
Date: Thu, 14 Oct 2010 19:52:16 +0200
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On 10/14/2010 06:38 PM, Blue Swirl wrote:
On Thu, Oct 14, 2010 at 9:20 AM, Markus Armbruster<address@hidden>  wrote:
Blue Swirl<address@hidden>  writes:

On Wed, Oct 13, 2010 at 7:19 AM, Markus Armbruster<address@hidden>  wrote:
Blue Swirl<address@hidden>  writes:

On Mon, Oct 11, 2010 at 12:52 PM, Markus Armbruster<address@hidden>  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



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