What's the difference between sockaddr, sockaddr_in, and sockaddr_in6?

Question

I know that sockaddr_in is for IPv4, and sockaddr_in6 for IPv6. The confusion to me is the difference between sockaddr and sockaddr_in[6].

Some functions accept sockaddr and some functions accept sockaddr_in or sockaddr_in6, so:

• what's the rule?
• And why is there a need for two different structures?

And because the sizeof(sockaddr_in6) > sizeof(sockaddr) == sizeof(sockaddr_in).

• Does that mean we should always use sockaddr_in6 to allocate memory in stack and cast to sockaddr and sockaddr_in if we need to support ipv4 and ipv6?

One example is: we have a socket, and we want to get the string ip address of it (it can be ipv4 or ipv6).

We first call getsockname to get an addr and then call inet_ntop based on the addr.sa_family.

Is there anything wrong with this code snippet?

char ipStr[256];
sockaddr_in6 addr_inv6;
sockaddr* addr = (sockaddr*)&addr_inv6;
sockaddr_in* addr_in = (sockaddr_in*)&addr_inv6;

socklen_t len = sizeof(addr_inv6);
getsockname(_socket, addr, &len);

if (addr->sa_family == AF_INET6) {
    inet_ntop(addr_inv6.sin6_family, &addr_inv6.sin6_addr, ipStr, sizeof(ipStr)); 
    // <<<<<<<<IS THIS LINE VALID, getsockname expected a sockaddr, but we use 
    // it output parameter as sockaddr_in6.
} else {
    inet_ntop(addr_in->sin_family, &addr_in->sin_addr, ipStr, sizeof(ipStr));
}

Answer 1

sockaddr_in and sockaddr_in6 are both structures where first member is a sockaddr structure.

According to the C standard, the address of a structure and its first member are the same, so you can cast the pointer to sockaddr_in(6) in a pointer to sockaddr.

Functions taking sockaddr_in(6) as parameter may modify the sockaddr part, and functions taking sockaddr as parameter just care about that part.

It's a bit like inheritance.