C typecasting for structure pointers

Question

Confused in some example with realization of trivial intrusive container in C. I have structures:

struct List {
    struct Link* first;
    struct Link* last;
};

And Link for become the nodes of list:

struct Link {
    struct Link* pre;
    struct Link* suc;
}; 

Some pseudo Node for storing specified values in List:

struct Name {
    struct Link lnk;
    char* n;
};

Function for pushing values in List:

void push_back(struct List* lst, struct Link* p) {
    assert(lst);
    {
        struct Link* last = lst->last;
        if (last) {
            p->pre = last;          
            last->suc = p;                                      
        }
        else {
            lst->first = p;
            p->pre = 0;
        }
        lst->last = p;
        p->suc = 0;
    }
}

And usage of List:

int main() {

    int count = 0;
    struct List names;
    struct Link* curr;
    init(&names);

    push_back(&names, (struct Link*)make_name("Norah"));
    push_back(&names, (struct Link*)make_name("Annemarie"));
    push_back(&names, (struct Link*)make_name("Kris"));

    curr = names.first;
    for (; curr != 0; curr = curr->suc) {
        count++;
        printf("element %d: %s\n", count, ((struct Name*)curr)->n);
    }
    return 0;
}

Question is, how typecast is working in this example? when sizeof(Link)==8 and sizeof(Name)==12 The pointer curr pointing to allocated memory for type Name , this structure not have suc and pre members,just lnk object of type Name (not pointer), and in code no any primary access for this member.

Answer 1

It is relying on the fact that struct Link is the first field of the struct Name . You can pretend the 12 bytes pointed to by a struct Name* is actually an 8 byte struct Link , and disregard the remaining 4 bytes. See the following questions:

• Struct pointer compatibility
• Is casting a struct T* to a struct C* undefined behavior, if T's first field is a C?
• Does accessing the first field of a struct via a C cast violate strict aliasing?

Answer 2

Thanks guys! Im lost from mind that struct Link have two first members which each one 4 bytes. When struct Name is come to push_back() via argument, the function is operate with him like a struct Link .Cuz struct Name have 8 byte member in a head, functions just placing data about suc and pre members in this 8 bytes. I can replace lnk member from Name to any other with data with size 8 bytes.

Have i nice day!