Long ago I internalized the point that sizeof() does not return the minimum number of bytes that need to be copied in an aggregate assignment but rather the spacing between elements in an array:
struct foo { ... }; foo a[2];
((char*)&a[0]) + sizeof(foo) equals ((char*)&a[1])
This is a bit of a hole in the language definition since there really is no way to obtain that minimum number.
struct foo { ... some_type last_member; };
offsetof(struct foo, last_member) + sizeof a[0].last_member;
> Hence using sizeof() as the length of a
memcpy() to set a freestanding risks trashing some other object if the compiler has practiced any kind storage packing.
Overwriting padding bytes is guaranteed to be safe.
Segher
