The fist member of a struct is guaranteed not to have padding before it according to the C spec, so for this use case this is always true. Implementations are f

The fist member of a struct is guaranteed not to have padding before it according to the C spec, so for this use case this is always true. Implementations are free to have padding in other places, but pretty much all implementations adhere to a ABI, and that forces the padding to be well defined.