WG14 N2350 made very clear that it is an UB having type definitions within "offsetof" [1]. This patch enhances the implementation of macro alignof_slot to use builtin "_Alignof" to avoid undefined behavior on when using std=c11 or newer. Make exceptions for gcc < 4.9 and clang < 8 where there is bug [3] in _Alignof implementation
clang 16+ has started to flag this [2] Fixes build when using -std >= gnu11 and using clang16+ [1] https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2350.htm [2] https://reviews.llvm.org/D133574 [3] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=52023 Signed-off-by: Khem Raj <[email protected]> --- v1->v2: - Exclude gcc < 4.9 and clang < 8 even though they may have C11 support ChangeLog | 5 +++++ lib/alignof.h | 10 +++++++++- 2 files changed, 14 insertions(+), 1 deletion(-) diff --git a/ChangeLog b/ChangeLog index fb467a3c14..33c0a0bb8a 100644 --- a/ChangeLog +++ b/ChangeLog @@ -1,3 +1,8 @@ +2023-01-15 Khem Raj <[email protected]> + + * lib/alignof.h (alignof_slot): Use _Alignof when using C11 or newer + standard barring GCC < 4.9 and clang < 8. + 2023-01-14 Bruno Haible <[email protected]> error, verror tests: Fix link error when the package uses libintl. diff --git a/lib/alignof.h b/lib/alignof.h index 505ad97aa4..962c8f31f4 100644 --- a/lib/alignof.h +++ b/lib/alignof.h @@ -28,8 +28,16 @@ #if defined __cplusplus template <class type> struct alignof_helper { char __slot1; type __slot2; }; # define alignof_slot(type) offsetof (alignof_helper<type>, __slot2) -#else +/* GCC releases before GCC 4.9 had a bug in _Alignof. See GCC bug 52023 + <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=52023>. + clang versions < 8.0.0 have the same bug. */ +#elif (!defined __STDC_VERSION__ || __STDC_VERSION__ < 201112 \ + || (defined __GNUC__ && __GNUC__ < 4 + (__GNUC_MINOR__ < 9) \ + && !defined __clang__) \ + || (defined __clang__ && __clang_major__ < 8)) # define alignof_slot(type) offsetof (struct { char __slot1; type __slot2; }, __slot2) +#else +# define alignof_slot(type) _Alignof(type) #endif /* alignof_type (TYPE) -- 2.39.0
