[Bug c/49863] New: -Wunused-result not behaving correctly

[Bug c/49863] New: -Wunused-result not behaving correctly

[antomicx at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49863

           Summary: -Wunused-result not behaving correctly
           Product: gcc
           Version: 4.5.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: antomicx@gmail.com


With a snippet as simple as the following

# cat main.c

#include <stdlib.h>

int main() {
  system("ls");

  return 0;
}

I'm getting these, different, compilation results:

1) # gcc main.c
Output: empty

2) # gcc -Wunused-result main.c
Output: empty

3) # gcc -W -Wall -Wextra main.c
Output: empty

4) # gcc -W -Wall -Wextra -Wunused-result main.c
Output: empty

5) # gcc -O0 -W -Wall -Wextra -Wunused-result main.c 
Output: empty

6) # gcc -O1 main.c # same thing with -O2 -O3
Output: 
main.c: In function ‘main’:
main.c:4:9: warning: ignoring return value of ‘system’, declared with attribute
warn_unused_result

Why is putting -Ox with x > 0 the only way to get the correct compilation
warning when invoking the 'system' call without parsing its result?

# gcc -v

Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/x86_64-linux-gnu/gcc/x86_64-linux-gnu/4.5.2/lto-wrapper
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu/Linaro
4.5.2-8ubuntu4' --with-bugurl=file:///usr/share/doc/gcc-4.5/README.Bugs
--enable-languages=c,c++,fortran,objc,obj-c++ --prefix=/usr
--program-suffix=-4.5 --enable-shared --enable-multiarch
--with-multiarch-defaults=x86_64-linux-gnu --enable-linker-build-id
--with-system-zlib --libexecdir=/usr/lib/x86_64-linux-gnu
--without-included-gettext --enable-threads=posix
--with-gxx-include-dir=/usr/include/c++/4.5 --libdir=/usr/lib/x86_64-linux-gnu
--enable-nls --with-sysroot=/ --enable-clocale=gnu --enable-libstdcxx-debug
--enable-libstdcxx-time=yes --enable-plugin --enable-gold --enable-ld=default
--with-plugin-ld=ld.gold --enable-objc-gc --disable-werror --with-arch-32=i686
--with-tune=generic --enable-checking=release --build=x86_64-linux-gnu
--host=x86_64-linux-gnu --target=x86_64-linux-gnu
Thread model: posix
gcc version 4.5.2 (Ubuntu/Linaro 4.5.2-8ubuntu4)

[Bug c/49863] -Wunused-result not behaving correctly

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49863

Richard Guenther <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |INVALID

--- Comment #1 from Richard Guenther <rguenth at gcc dot gnu.org> 2011-07-27 10:19:33 UTC ---
Because glibc annotates system with the attribute only if you use
-D_FORTIFY_SOURCE=1 or higher, and then only if optimization is enabled:

/usr/include/features.h:

#if defined _FORTIFY_SOURCE && _FORTIFY_SOURCE > 0 \
    && __GNUC_PREREQ (4, 1) && defined __OPTIMIZE__ && __OPTIMIZE__ > 0
# if _FORTIFY_SOURCE > 1
#  define __USE_FORTIFY_LEVEL 2
# else
#  define __USE_FORTIFY_LEVEL 1
# endif
#else
# define __USE_FORTIFY_LEVEL 0
#endif

/usr/include/sys/cdefs.h:

/* If fortification mode, we warn about unused results of certain
   function calls which can lead to problems.  */
#if __GNUC_PREREQ (3,4)
# define __attribute_warn_unused_result__ \
   __attribute__ ((__warn_unused_result__))
# if __USE_FORTIFY_LEVEL > 0
#  define __wur __attribute_warn_unused_result__
# endif
#else
# define __attribute_warn_unused_result__ /* empty */
#endif
#ifndef __wur
# define __wur /* Ignore */
#endif

...

[Bug c/49863] -Wunused-result not behaving correctly

[antomicx at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49863

--- Comment #2 from antomicx at gmail dot com 2011-07-27 10:29:46 UTC ---
(In reply to comment #1)
> Because glibc annotates system with the attribute only if you use
> -D_FORTIFY_SOURCE=1 or higher, and then only if optimization is enabled:

I got it. Just out of curiosity, is this feature documented somewhere (apart
from source code, obviously ;-) )?

Thanks!

[Bug c/49863] -Wunused-result not behaving correctly

[jim.king at simplivity dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49863

James E. King, III <jim.king at simplivity dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jim.king at simplivity dot
                   |                            |com

--- Comment #3 from James E. King, III <jim.king at simplivity dot com> 2012-05-23 17:15:46 UTC ---
I would like to recommend that this issue be re-opened.  By leaving this issue
alone, debug and optimized builds behave differently.  I would like to have the
same warnings enabled for both debug and optimized builds, and currently the
only way to do that is to disable the unused result code checking for the
release build.  This seems counter-productive.  If I declare
-D_FORTIFY_SOURCE=2 on the command line, the headers should not override me
just because it is not an optimized build.  Is there some other unforeseen
reason for this rule, such as that the compiler actually cannot check for this
warning unless doing optimizations?

[Bug c/49863] -Wunused-result not behaving correctly

[pinskia at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49863

--- Comment #4 from Andrew Pinski <pinskia at gcc dot gnu.org> 2012-05-23 17:20:52 UTC ---
(In reply to comment #3)
> If I declare
> -D_FORTIFY_SOURCE=2 on the command line, the headers should not override me
> just because it is not an optimized build.  Is there some other unforeseen
> reason for this rule, such as that the compiler actually cannot check for this
> warning unless doing optimizations?

You should report the bug in glibc instead since GCC does not have control over
these headers.