The standard actually specifies (or specified?) that the flexible array member must come after all the members, including the padding. But, from my reading, that wasn't their intent, and is certainly not how compilers implement it. The link in my original post is an acknowledgement from the committee about this and that the standard needs to be updated.
I think the main reason the OP got away with it is because in his structure, "sizeof(struct sdshdr)" is equal to the offset of "buf" in the struct. This is not necessarily true.
I think the main reason the OP got away with it is because in his structure, "sizeof(struct sdshdr)" is equal to the offset of "buf" in the struct. This is not necessarily true.
In particular, see the latest C standard draft (http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1570.pdf), section 6.7.2.1 (Structure and union specifiers), paragraph 18 and 20-21. An example in the standard uses this code:
and says that "but it is possible that"