What is the purpose of the sa_data field in a sockaddr? - c

Question

int bind(int sockfd, const struct sockaddr *addr, socklen_t addrlen);
The actual structure passed for the addr argument will depend on the address family. The sockaddr structure is defined as something like:
struct sockaddr {
sa_family_t sa_family;
char sa_data[14];
}
So for an IPv4 address (AF_INET), the actual struct that will be passed is this:
/* Source http://linux.die.net/man/7/ip */
struct sockaddr_in {
sa_family_t sin_family; /* address family: AF_INET */
in_port_t sin_port; /* port in network byte order */
struct in_addr sin_addr; /* internet address */
};
/* Internet address. */
struct in_addr {
uint32_t s_addr; /* address in network byte order */
};
Does the bind code read the sockaddr.sa_family value and depending on the value it finds, it will then cast the sockaddr struct into the appropriate struct such as sockaddr_in?
Why is the sa_data set to 14 characters? If I understand correct, the sa_data field is just a field that will have large enough memory space to fit all address family types? Presumably the original designers anticipated that 14 characters would be wide enough to fit all future types.

Answer 1

According to the glibc manual:
The length 14 of sa_data is essentially arbitrary.
And the FreeBSD developers handbook mentions the following:
Please note the vagueness with which the sa_data field is declared,
just as an array of 14 bytes, with the comment hinting there can be
more than 14 of them.
This vagueness is quite deliberate. Sockets is a very powerful
interface. While most people perhaps think of it as nothing more than
the Internet interface—and most applications probably use it for that
nowadays—sockets can be used for just about any kind of interprocess
communications, of which the Internet (or, more precisely, IP) is only
one.
Yes, the sa_family field is used to recognize how to treat the struct passed (which is cast to struct sockaddr* in a call to bind). You can read more about how it works also in a FreeBSD developers handbook.
And actually there are "polymorphic" (sub)types of sockaddr, in which sa_data contains more than 16 bytes, for example:
struct sockaddr_un {
sa_family_t sun_family; /* AF_UNIX */
char sun_path[108]; /* pathname */
};

Answer 2

The sockaddr struct is used as a tagged union. By reading the sa_family field it can be cast to a struct of the proper form.
The 14 bytes is arbitrary. It's big enough to hold IPv4 addresses, but not big enough to hold IPv6 addresses. There is also a sockaddr_storage struct which is big enough for both. Reading the Microsoft docs on SOCKADDR_STORAGE, it comes in at 128 bytes, so much larger than needed for IPv6. Checking some Linux headers, it seems to be at least that large there as well.
For reference, the IPv6 struct is:
struct sockaddr_in6 {
u_int16_t sin6_family; // address family, AF_INET6
u_int16_t sin6_port; // port number, Network Byte Order
u_int32_t sin6_flowinfo; // IPv6 flow information
struct in6_addr sin6_addr; // IPv6 address
u_int32_t sin6_scope_id; // Scope ID
};
struct in6_addr {
unsigned char s6_addr[16]; // IPv6 address
};
As you can see, the 16 byte s6_addr field is already bigger than the 14 byte sa_data field on it's own. Total size after the sa_family field is 26 bytes.