https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109956
--- Comment #13 from Pascal Cuoq <pascal_cuoq at hotmail dot com> --- @Martin I completely agree with comment 12, however about the last paragraph, I would like to point out that for purposes of memcpy'ing to or from such a struct with initialized FAM, it is enough to recommend that programmers use the simple formula “offsetof(struct foo, t) + n * sizeof(char)” (or “offsetof(struct foo, t[n])”. The part that is not copied is the part that they did not intend to use when they chose the initializer of the FAM, and that they cannot portably use because of the padding that may or may not exist for a different target architecture. So since: First, GCC currently does not always reserve enough room to allow “memcpy(…, …, sizeof(struct foo) + n * sizeof(char))”, and second, using the time-proven formula as argument of malloc technically does not always allocate enough space to make it valid to access p->t[n-1] according to the strict interpretation of the words “it behaves as if that member were replaced with the longest array (with the same element type) that would not make the structure larger than the object being accessed”, we might as well start recommending that C programmers use “offsetof(struct foo, t) + n * sizeof(char)” as argument of memcpy, and either clarify the meaning of the words “it behaves as if…” in the C standard or prepare for a very unpleasant discussion when we have to tell them the formula they have to use as argument of malloc.
