[PATCH] Define alignof using _Alignof when using C11 or newer

[PATCH] Define alignof using _Alignof when using C11 or newer

[Khem Raj]

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

clang 16+ has started to flag this [2]

Fixes build when using -std >= gnu11 and using clang16+

Older compilers gcc < 4.9 or clang < 8 has buggy _Alignof even though it
may support C11, exclude those compilers too

[1] https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2350.htm
[2] https://reviews.llvm.org/D133574

Upstream-Status: Pending
Signed-off-by: Khem Raj <raj.khem@gmail.com>
---
 sha1.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/sha1.c b/sha1.c
index 89b32f4..1e4ad5d 100644
--- a/sha1.c
+++ b/sha1.c
@@ -229,7 +229,17 @@ sha1_process_bytes (const void *buffer, size_t len, struct sha1_ctx *ctx)
   if (len >= 64)
     {
 #if !_STRING_ARCH_unaligned
-# define alignof(type) offsetof (struct { char c; type x; }, x)
+/* 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.  */
+# if (!defined __STDC_VERSION__ || __STDC_VERSION__ < 201112 \
+      || (defined __GNUC__ && __GNUC__ < 4 + (__GNUC_MINOR__ < 9) \
+   && !defined __clang__) \
+      || (defined __clang__ && __clang_major__ < 8))
+#  define alignof(type) offsetof (struct { char c; type x; }, x)
+# else
+#  define alignof(type) _Alignof(type)
+# endif
 # define UNALIGNED_P(p) (((size_t) p) % alignof (sha1_uint32) != 0)
       if (UNALIGNED_P (buffer))
 	while (len > 64)
-- 
2.39.0

Re: [PATCH] Define alignof using _Alignof when using C11 or newer

[Jes Sorensen]

On 1/18/23 03:32, Khem Raj wrote:
> 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
> 
> clang 16+ has started to flag this [2]
> 
> Fixes build when using -std >= gnu11 and using clang16+
> 
> Older compilers gcc < 4.9 or clang < 8 has buggy _Alignof even though it
> may support C11, exclude those compilers too
> 
> [1] https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2350.htm
> [2] https://reviews.llvm.org/D133574
> 
> Upstream-Status: Pending
> Signed-off-by: Khem Raj <raj.khem@gmail.com>
> ---
>  sha1.c | 12 +++++++++++-
>  1 file changed, 11 insertions(+), 1 deletion(-)

Applied,

Thanks,
Jes