Does this C inheritance implementation contain undefined behavior?

Question

struct parent
{
    char a;
    char b;
};

struct child
{
    struct parent parent;
    int c;
    char d;
};

struct grandchild
{
    struct child child;
    long e;
};

void print_parent_val(struct parent *p)
{
    printf("%d\n", p->a);
}

int main (int argc, char **argv)
{
    struct grandchild g;
    g.child.parent.a = 69;
    print_parent_val((struct parent *)&g);
    return 0;
}

The program compiles (no warnings) and runs fine and prints 69 as expected. I have never seen any code that implements this inheritance technique so I'm very hesitant to see this as "OK".

EDIT: What about converting the grandchild to just a child? Is the mid-generation conversion also possible?

void print_parent_val(struct child *c)
{
    printf("%c\n", c->d);
}

int main (int argc, char **argv)
{
    struct grandchild g;
    g.child.parent.a = 69;
    g.child.d = 'w';
    print_parent_val((struct child *)&g);
    return 0;
}

Answer 1

As I can see, there's no scope of undefined behaviour here.

Quoting C11, chapter §6.7.2.1, Structure and union specifiers, (emphasis mine)

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.

So, with the current snippet (and approach), you should be good to go.