Modulo any padding rules, I would be surprised if C didn't store them in the same offsets, the compatible-type specification[1] and equality operator[2] (also see footnote 109) would make implementation much harder if you tried to do it any other way. The way to work around that of course, and to make sure that no matter what, the parent/child objects are compatible to the parent-only level is to do something like:

    // this structure defines a node of a linked list and
    // it only holds the pointers to the next and the previous
    // nodes in the linked list.
    struct list_head {
        struct list_head *next; // pointer to the node next to the current one
        struct list_head *prev; // pointer to the node previous to the current one
    };

    // list_int holds a list_head and an integer data member
    struct list_int {
        struct list_head list;  // common next and prev pointers
        int value;              // specific member as per implementation
    };

    // list_str holds a list_head and a char * data member
    struct list_str {
        struct list_head list;  // common next and prev pointers
        char \* str;             // specific member as per implementation
    };

Often the 'parent' structure would have an 'int objType', which can be switch'd on, to make sure that the receiving function knows what to do. I'm not really seeing any undefined behaviour here.

I'm pretty sure this technique is decades old, btw. I know at one point the linux kernel used it, not sure if it still does..

If you're happy with being C11-compliant, then remove any names for the 'parent' structure in the child structures, making them "anonymous structs" at which point they are [3] considered to be part of the same struct as the parent.

[1]: http://port70.net/~nsz/c/c11/n1570.html#6.2.7p1

[2]: http://port70.net/~nsz/c/c11/n1570.html#6.5.9p6

[3]: http://port70.net/~nsz/c/c11/n1570.html#6.7.2.1p13