internals: Use Standard C offsetof macro if available

clang 13 fails to compile our current implementation with:

.../dbus/dbus-message.c:2070:3: error: variable length array folded to constant array as an extension [-Werror,-Wgnu-folding-constant]
  _DBUS_STATIC_ASSERT (_DBUS_ALIGNOF (DBusMessageRealIter) <=
  ^
.../dbus/dbus-internals.h:460:25: note: expanded from macro '_DBUS_STATIC_ASSERT'
  typedef struct { char _assertion[(expr) ? 1 : -1]; } \

This appears to be because the "traditional" definition of
offsetof(), which we're hard-coding here, does not qualify as a constant
expression under C rules due to its use of pointer casts.

Modern compilers like gcc and clang have a built-in implementation
of offsetof that *is* a constant expression.

Signed-off-by: Simon McVittie <smcv@collabora.com>
(cherry picked from commit a24cf746e2a8191070efd1300a15d37284aaf2fa)

diff --git a/dbus/dbus-internals.h b/dbus/dbus-internals.h
index e7649e1..281d6af 100644 (file)
--- a/dbus/dbus-internals.h
+++ b/dbus/dbus-internals.h
@@ -192,8 +192,13 @@ void _dbus_real_assert_not_reached (const char *explanation,
 
 #define _DBUS_ZERO(object) (memset (&(object), '\0', sizeof ((object))))
 
+#ifdef offsetof
+#define _DBUS_STRUCT_OFFSET(struct_type, member) \
+    (offsetof (struct_type, member))
+#else
 #define _DBUS_STRUCT_OFFSET(struct_type, member)       \
     ((intptr_t) ((unsigned char*) &((struct_type*) 0)->member))
+#endif
 
 #define _DBUS_ALIGNOF(type) \
     (_DBUS_STRUCT_OFFSET (struct { char _1; type _2; }, _2))