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internals: Use Standard C offsetof macro if available

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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>
This commit is contained in:
Simon McVittie 2021-12-10 15:12:59 +00:00
parent 83232393ef
commit a24cf746e2
1 changed files with 5 additions and 0 deletions
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5
dbus/dbus-internals.h
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@ -193,8 +193,13 @@ 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))