A Socket Address is a host.port pair (communication is between host.port pairs -- one on the server, the other on the client). We know how to determine host numbers and service numbers so we're well on our way to filling out a structure were we specify those numbers. The structure is sockaddr_in, which has the address family is AF_INET as in this fragment:

int tcpopen(host,service)

char *service, *host;

{ int unit;

struct sockaddr_in sin;

struct servent *sp;

struct hostent *hp;

etc...

if ((sp=getservbyname(service,"tcp"))

== NULL) then error...

if ((hp=gethostbyname(host)) == NULL)

then error...

bzero((char *)&sin, sizeof(sin));

sin.sin_family=AF_INET;

bcopy(hp->h_addr,(char *)&sin.sin_addr,

hp->h_length);

sin.sin_port=sp->s_port;

etc...

The code fragment is filling in the IP address type AF_INET, port number and IP address in the Socket Address structure -- the address of the remote host.port where we want to connect to find a service.

There's a generic Socket Address structure, a sockaddr, used for communication in arbitrary domains. It has an address family field and an address (or data) field:

/* from: /usr/include/sys/socket.h */

struct sockaddr {

u_short sa_family; /*address family */

char sa_data[14];/*max 14 byte addr*/

};

The sockaddr_in structure is for

Internet Socket Addresses

address family AF_INET). An instance

of the generic socket address.

/* from: /usr/include/netinet/in.h */

struct sockaddr_in {

short sin_family; /* AF_INET */

u_short sin_port; /* service port */

struct in_addr sin_addr; /*host number */

char sin_zero[8]; /* not used */

};

The family defines the interpretation of the data. In other domains addressing will be different -- services in the UNIX domain are names (eg. /dev/printer). In the sockaddr_in structure we've got fields to specify a port and a host IP number (and 8 octets that aren't used at all!). That structure specifies one end of an IPC connection. Creating that structure and filling in the right numbers has been pretty easy so far.