Casting between sockaddr and sockaddr_in6

Question

I've been reading Beej's Guide to Network Programming and in one of his examples, he casts a pointer to struct sockaddr to a struct sockaddr_in6 pointer like shown below.

    void *addr;
    char *ipver;

    // get the pointer to the address itself,
    // different fields in IPv4 and IPv6:
    if (p->ai_family == AF_INET) { // IPv4
        struct sockaddr_in *ipv4 = (struct sockaddr_in *)p->ai_addr;
        addr = &(ipv4->sin_addr);
        ipver = "IPv4";
    } else { // IPv6
        struct sockaddr_in6 *ipv6 = (struct sockaddr_in6 *)p->ai_addr;
        addr = &(ipv6->sin6_addr);
        ipver = "IPv6";
    }

How is this possible, since the sizes of the structs are different?

Answer 1

ai_family and ai_addr are fields of the addrinfo struct, so presumably the code you are quoting had called getaddrinfo() beforehand.

The result of getaddrinfo() is a NULL-terminated linked list of addrinfo structs, where the addrinfo::ai_addr field is a pointer to an allocated memory block that is of sufficient size to hold a socket address of the reported addrinfo::ai_family type. The size of the address is reported in the addrinfo::ai_addrlen field.

For AF_INET, the addrinfo::ai_addr field is pointing at a memory block containing a sockaddr_in struct.

For AF_INET6, the addrinfo::ai_addr field is pointing at a memory block containing a sockaddr_in6 struct.

That is why the type-casts work.

The addrinfo::ai_addr field is declared as struct sockaddr* so it can be passed as-is to the addr parameter of the bind() and connect() functions without type-casting. The addrinfo::ai_addrlen field can be passed as-is to their addrlen parameter.