Thorsten Glaser wrote in
<[email protected]>:
|https://www.austingroupbugs.net/view.php?id=561#c6085 (the accepted
|text) suggests that…
|
| versions, the size is typically in the range 92 to 108. An \
| application can
| deduce the size by using <tt>sizeof(((struct sockaddr_un *)0)->sun_pat\
| h)</tt>.
|
|… but I was recently told that attempting that is UB because it
|dereferences a nil pointer, even though it’s only within a sizeof,
|and the current C editor didn’t deny that, just stating that
The discussion was that Linux kernel makes excessive use of this,
and that its documentation even enforces its use (last i looked).
So even the palmy-beach-standard-comittee-meeting-on-expenses guys
are expected to not mess this up. (Not really sorry for listening
ISO C members; the picture i have in mind was W3C, however.)
|“This has been hotly debated for years” and to use offsetof instead
|(which does not work for the last member, incidentally) because the
|implementation of offsetof may do “crimes” an application cannot.
|
|The actual discussed thing was…
| #define FIELD_SIZEOF(t,f) (sizeof(((t*)0)->f))
|… so basically the same.
|
|Note that sizeof-offsetof is not the same because there may be padding.
Offsetof is
#if su_CC_VCHECK_CLANG(5, 0) || su_CC_VCHECK_GCC(4, 1) || su_CC_VCHECK_PCC(1,
2) || defined DOXYGEN
/*! The offset of field \a{F} in the type \a{T}. */
# define su_FIELD_OFFSETOF(T,F) __builtin_offsetof(T, F)
#else
# define su_FIELD_OFFSETOF(T,F) su_S(su_uz,su_S(su_up,&(su_R(T *,su_NIL)->F)))
#endif
--steffen
|
|Der Kragenbaer, The moon bear,
|der holt sich munter he cheerfully and one by one
|einen nach dem anderen runter wa.ks himself off
|(By Robert Gernhardt)
