Kudos for your presentation.
I think your implementation should work fine, because C guarantees that the address of a struct is the address of its initial member. Put aside the statements C makes about alignment of struct-members, this guarantee should mean that, as long as your implementation always puts Link as first member, this should not cause alignment issues.
from here: C99 ยง6.7.2.1:
13 Within a structure object, the non-bit-field members and the units
in which bit-fields reside have addresses that increase in the order
in which they are declared. A pointer to a structure object, suitably
converted, points to its initial member (or if that member is a
bit-field, then to the unit in which it resides), and vice versa.
There may be unnamed padding within a structure object, but not at its
beginning
This should be what you meant to say about Base * and Derived *, although no such thing exists in pure C. They are just structs which happen to have the same memory layout.
However I think it is a bit brittle to implement it like this, because Node and Link directly depend on each other. If you were to change the structure of Node, your code would become invalid. At the moment I don’t see the point in having an extra struct Link, apart from you being able to just write a new Node for a new type reusing Link.
There is actually a linked list implementation that immediately came to mind when I saw your post and works in a way very similar to the way you intend to use your list: the kernel list
It uses the same List-element (list_head):
struct list_head {
struct list_head *next, *prev;
};
It contains this function macro:
#define list_for_each_entry(pos, head, member) \
for (pos = list_first_entry(head, typeof(*pos), member); \
&pos->member != (head); \
pos = list_next_entry(pos, member))
If you look at the way the macro is implemented, you will see that it offers iteration over the entries of a list without knowing anything about the layout of the entries the list is contained in. Assuming I interpret your intention right, I think this is the way you would want it to be.