diff options
| -rw-r--r-- | include/ruby/internal/stdalign.h | 17 |
1 files changed, 13 insertions, 4 deletions
diff --git a/include/ruby/internal/stdalign.h b/include/ruby/internal/stdalign.h index 2bd8d94197..122f649b37 100644 --- a/include/ruby/internal/stdalign.h +++ b/include/ruby/internal/stdalign.h @@ -112,10 +112,19 @@ struct rbimpl_alignof { # define RBIMPL_ALIGNOF __alignof #else -# /* :BEWARE: It is an undefined behaviour to define a struct/union inside of -# * `offsetof()`! This section is the last resort. If your compiler somehow -# * supports querying alignment of a type please add your own definition of -# * `RBIMPL_ALIGNOF` instead. */ +# /* :BEWARE: This is the last resort. If your compiler somehow supports +# * querying the alignment of a type, you definitely should use that instead. +# * There are 2 known pitfalls for this fallback implementation: +# * +# * Fitst, it is either an undefined behaviour (C) or an explicit error (C++) +# * to define a struct inside of `offsetof`. C compilers tend to accept such +# * things, but AFAIK C++ has no room to allow. +# * +# * Second, there exist T such that `struct { char _; T t; }` is invalid. A +# * known example is when T is a struct with a flexible array member. Such +# * struct cannot be enclosed into another one. +# */ +# /* see: http://www.open-std.org/jtc1/sc22/wg14/www/docs/n2083.htm */ # /* see: http://www.open-std.org/jtc1/sc22/wg14/www/docs/n2350.htm */ # define RBIMPL_ALIGNOF(T) offsetof(struct { char _; T t; }, t) |