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

...