casting between sockaddr and sockaddr_in

Question

I came across a socket programming tutorial in which it is quoted

\"a pointer to a struct sockaddr_in can be cast to a pointer to a struct sockad

Answer 1

The different sizes don't matter. Just like you can pass strings of different lengths to the various string-handling functions, you can pass struct sockaddr of different lengths to the various socket-handling functions.

The size of the struct sockaddr is interpreted by the called function per the contents of the sa_family member of the structure. Note also that all functions that take a struct sockaddr * address also take a socklen_t argument that holds the size of the structure being passed.

For example, the struct sockaddr_un structure is 110 bytes:

   struct sockaddr_un {
       sa_family_t sun_family;               /* AF_UNIX */
       char        sun_path[108];            /* pathname */
   };

The called function such as bind() or getpeername() have declarations similar to

int getpeerame(int sockfd, struct sockaddr *addr, socklen_t *addrlen); 

for the very reason that the size(s) of various socket structures vary.

Note that the first member of every struct sockaddr_??? is the sa_family. Thus it's always in the same place.