struct addrinfo is returned by
getaddrinfo(), and contains, on success, a linked list of such
structs for a specified hostname and/or service.
ai_addr member isn’t actually a
struct sockaddr, because that
struct is merely a generic one that contains common members for all the others, and is used in order to determine what type of struct you actually have. Depending upon what you pass to
getaddrinfo(), and what that function found out,
ai_addr might actually be a pointer to
struct sockaddr_in, or
struct sockaddr_in6, or whatever else, depending upon what is appropriate for that particular address entry. This is one good reason why they’re kept “separate”, because that member might point to one of a bunch of different types of
structs, which it couldn’t do if you tried to hardcode all the members into
struct addrinfo, because those different
structs have different members.
This is probably the easiest way to get this information if you have a hostname, but it’s not the only way. For an IPv4 connection, you can just populate a
struct sockaddr_in structure yourself, if you want to and you have the data to do so, and avoid going through the rigamarole of calling
getaddrinfo(), which you might have to wait for if it needs to go out into the internet to collect the information for you. You don’t have to use
struct addrinfo at all.