sockaddr_storage

As mentioned earlier, sockaddr and sockaddr_in6 have different sizes. Because of this, struct sockaddr cannot be used to allocate storage and then be cast to a sockaddr_in6 pointer. If static allocation of storage for sockaddr_in6 (or even sockaddr_in) structures is needed, struct sockaddr_storage should be used. Here is an example:

struct sockaddr_storage newaddr;

...

msgsock = accept(listen_socket,(struct sockaddr*) &newaddr, &newaddrlen);

In addition to being large enough to accommodate all known protocol-specific socket address structures (including sockaddr_in6), sockaddr_storage is aligned at an appropriate boundary so that protocol-specific socket address data-structure pointers can be cast to it. This enables it to access fields without experiencing alignment problems.

Wildcard Addresses

To allow the protocol implementation to choose the source address for a connection or datagram with IPv4, a constant of INADDR_ANY (the wildcard address) is used as the address in the bind() call.

The IPv6 address type (in6_addr) is a structure. A constant cannot be used in an assignment for this variable, but one can be used to initialize the structure. Thus, we end up with two possible ways to provide the wildcard address.

The global variable, in6addr_any, can be used in an assignment—for example:

sin6.sin6_addr = in6addr_any;

Or the constant, IN6ADDR_ANY_INIT, can be used to initialize the address structure (at declaration time only)—for example:

struct in6_addr anyaddr = IN6ADDR_ANY_INIT;

in6addr_loopback and IN6ADDR_LOOPBACK_INIT

Similarly, the INADDR_LOOPBACK constant is used in IPv4 connect(), send(), and WSASendMsg() calls to communicate with services that reside on the local node. For IPv6 loopback, a global variable (in6addr_loopback) is used for assignment and a constant (IN6ADDR_LOOPBACK_INIT) is used for initialization at declaration time.

Note The IPv4 INADDR_XXX constants were defined in host-byte order. The IPv6 equivalents are defined in network-byte order.