struct sockaddr.sa_data appears to be too small

Question

I am doing some interposition in Mac OS X (essentially intercepting C calls) and I noticed that the ping application tries to call the sendto function with an addrlen value of 16. In sys/socket.h I can see clearly that the sa_data array only holds up to 14 bytes:

/*
 * [XSI] Structure used by kernel to store most addresses.
 */
struct sockaddr {
    __uint8_t   sa_len;     /* total length */
    sa_family_t sa_family;  /* [XSI] address family */
    char        sa_data[14];    /* [XSI] addr value (actually larger) */
};

It freaks me out that the comment says "actually larger" but there's not much I can do about that.

Anyway, the man page shows the signature for the sendto function looks like this:

ssize_t
sendto(int socket,
  const void *buffer,
  size_t length,
  int flags,
  const struct sockaddr *dest_addr,
  socklen_t dest_len);

And then the man page specifically calls out what to do if the length value is too long for a message to be delivered atomically. It ignores the situation where dest_len is larger than what dest_addr.sa_data can hold.

If I try to copy 16 bytes of data into another dest_addr.sa_data from the caller it fails and the application crashes, as it should. Am I misunderstanding how this field is used? Why does the comment in the header say "actually larger" but then assigns a fixed size to the array?

Answer 1

You never actually are supposed to use struct sockaddr. It's like void in void *, a stand-in for the actual sockaddr_foo structures.