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Re: [libvirt] Compiler warnings



At 10/18/2011 09:32 AM, Wen Congyang Write:
> At 10/18/2011 01:04 AM, Eric Blake Write:
>> On 10/17/2011 11:03 AM, Eric Blake wrote:
>>> On 10/17/2011 10:17 AM, Alex Jia wrote:
>>>> Eric,
>>>> It's latest libvirt upstream, current commit id is commit 0a71c79.
>>>>
>>>> # rpm -q glibc
>>>> glibc-2.12-1.42.el6.x86_64
>>>>
>>>
>>>> 430 if (strchr(toescape, *cur))
>>>
>>>>>> CC libvirt_util_la-buf.lo
>>>>>> util/buf.c: In function 'virBufferEscape':
>>>>>> util/buf.c:430: warning: logical '&&' with non-zero constant will
>>>>>> always
>>>>>> evaluate as true [-Wlogical-op]
>>>
>>> Something's screwy. There's no && in that line, unless strchr() is a
>>> macro based on your particular compilation flags. I'm suspecting that
>>> this is a case of -D_FORTIFY_SOURCE going awry. I don't see how this
>>> could possibly be a libvirt issue; more likely, it is an issue with the
>>> choice of compiler flags you are using, coupled with an issue in the
>>> system headers causing the preprocessor to expand strchr() into
>>> something that tickles a spurious gcc warning.
>>
>> Just to make sure, could you show 'make V=1' output, so we know exactly
>> which compiler flags are in effect?
>>
> 
> I can reproduce this problem, here is 'make V=1' output:
> 
> =====================================================
> make[3]: Entering directory `/root/rpmbuild/BUILD/libvirt-0.9.6/src'
> /bin/sh ../libtool  --tag=CC   --mode=compile gcc -std=gnu99 -DHAVE_CONFIG_H -I. -I.. -I../gnulib/lib -I../gnulib/lib -I../include -I../src/util -I../include -DIN_LIBVIRT        -I/usr/include/libxml2   -Wall -W -Wformat-y2k -Wformat-security -Winit-self -Wmissing-include-dirs -Wunused -Wunknown-pragmas -Wstrict-aliasing -Wshadow -Wpointer-arith -Wbad-function-cast -Wcast-align -Wwrite-strings -Wlogical-op -Waggregate-return -Wstrict-prototypes -Wold-style-definition -Wmissing-prototypes -Wmissing-declarations -Wmissing-noreturn -Wmissing-format-attribute -Wredundant-decls -Wnested-externs -Winline -Winvalid-pch -Wvolatile-register-var -Wdisabled-optimization -Wbuiltin-macro-redefined -Wmudflap -Wpacked-bitfield-compat -Wsync-nand -Wattributes -Wcoverage-mismatch -Wmultichar -Wno-missing-field-initializers -Wno-sign-compare -Wframe-larger-than=4096 -Wp,-D_FORTIFY_SOURCE=2 -fstack-protector-all --param=ssp-buffer-size=4 -fexceptions -fasynchronous-unwind-tables -fdiagnostics
-s
> how-option -funit-at-a-time -Werror      -O2 -g -pipe -Wall -Wp,-D_FORTIFY_SOURCE=2 -fexceptions -fstack-protector --param=ssp-buffer-size=4 -m64 -mtune=generic -MT libvirt_util_la-buf.lo -MD -MP -MF .deps/libvirt_util_la-buf.Tpo -c -o libvirt_util_la-buf.lo `test -f 'util/buf.c' || echo './'`util/buf.c
> libtool: compile:  gcc -std=gnu99 -DHAVE_CONFIG_H -I. -I.. -I../gnulib/lib -I../gnulib/lib -I../include -I../src/util -I../include -DIN_LIBVIRT -I/usr/include/libxml2 -Wall -W -Wformat-y2k -Wformat-security -Winit-self -Wmissing-include-dirs -Wunused -Wunknown-pragmas -Wstrict-aliasing -Wshadow -Wpointer-arith -Wbad-function-cast -Wcast-align -Wwrite-strings -Wlogical-op -Waggregate-return -Wstrict-prototypes -Wold-style-definition -Wmissing-prototypes -Wmissing-declarations -Wmissing-noreturn -Wmissing-format-attribute -Wredundant-decls -Wnested-externs -Winline -Winvalid-pch -Wvolatile-register-var -Wdisabled-optimization -Wbuiltin-macro-redefined -Wmudflap -Wpacked-bitfield-compat -Wsync-nand -Wattributes -Wcoverage-mismatch -Wmultichar -Wno-missing-field-initializers -Wno-sign-compare -Wframe-larger-than=4096 -Wp,-D_FORTIFY_SOURCE=2 -fstack-protector-all --param=ssp-buffer-size=4 -fexceptions -fasynchronous-unwind-tables -fdiagnostics-show-option -funit-at-a-time -Werro
r 
> -O2 -g -pipe -Wall -Wp,-D_FORTIFY_SOURCE=2 -fexceptions -fstack-protector --param=ssp-buffer-size=4 -m64 -mtune=generic -MT libvirt_util_la-buf.lo -MD -MP -MF .deps/libvirt_util_la-buf.Tpo -c util/buf.c  -fPIC -DPIC -o .libs/libvirt_util_la-buf.o
> cc1: warnings being treated as errors
> util/buf.c: In function 'virBufferEscape':
> util/buf.c:430: error: logical '&&' with non-zero constant will always evaluate as true [-Wlogical-op]
> make[3]: *** [libvirt_util_la-buf.lo] Error 1
> =====================================================
> 
> If I remove -O2, I can build util/buf.c.

I test strchr in a small program:
=====================================================
# cat test.c
#include <stdio.h>
#include <string.h>

int main() {
    char *s = "abcdefg";
    int chr = (int)'d';
    char *d;

    d = strchr(s, chr);
    if (d)
        printf("%s\n", d);

    return 0;
}
# gcc -std=c99 -Wall -o test test.c -Wlogical-op -Werror
# gcc -std=c99 -O2 -Wall -o test test.c -Wlogical-op -Werror
cc1: warnings being treated as errors
test.c: In function ‘main’:
test.c:9: error: logical ‘&&’ with non-zero constant will always evaluate as true
# gcc -std=c99 -Wall -o test test.c -Wlogical-op -Werror
# gcc -std=c99 -Wall -o test test.c -Werror
=====================================================

The building failed only when '-O2' and '-Wlogical-op' are specified at the same time.

Thanks
Wen Congyang

> 
> I am still investigating this problem.
> 
> Thanks
> Wen Congyang
> 
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