What Is The Purpose Of A sockaddr_in In This Program? - c

Question

For one of my classes I have to program a server and a client, and we were given some sample code to work with. Here is the snippet I am confused about:
main()
{
int sock, sock_current, cc, fromlen, tolen; /*sd is the socket */
int addrlen;
struct sockaddr_in sin;
struct sockaddr_in pin;
/* get an internet domain socket */
if ((sock = socket(AF_INET, SOCK_STREAM, 0)) == -1) {
perror("socket");
exit(1);
}
/* complete the socket structure */
memset(&sin, 0, sizeof(sin));
sin.sin_family = AF_INET;
sin.sin_addr.s_addr = INADDR_ANY;
sin.sin_port = htons(PORT);
/* bind the socket to the port number */
if (bind(sock, (struct sockaddr *) &sin, sizeof(sin)) == -1) {
perror("bind");
exit(1);
}
I think that the purpose of the sockaddr_in struct sin is to store a local IP address to associate with sock when it gets bound. Am I correct on that? If I am correct, how does this snippet of code accomplish that? I don't get it:
/* complete the socket structure */
memset(&sin, 0, sizeof(sin));
sin.sin_family = AF_INET;
sin.sin_addr.s_addr = INADDR_ANY;
sin.sin_port = htons(PORT);

Answer 1

The bind function assigns a local protocol address to a socket. The purpose of sin here is to tell bind which local address to assign.
memset(&sin, 0, sizeof(sin));
sin.sin_family = AF_INET; //it's an IPv4 address
sin.sin_addr.s_addr = INADDR_ANY; //wildcard IP address
sin.sin_port = htons(PORT); //bind to this port number

Answer 2

In addition to what Yu said (which is all correct), if you don't call bind before calling listen() (which is likely right after the snippet you posted), it will listen on a random TCP port (because you opened an IPv4 TCP socket when you called socket()).
This is actually useful sometimes -- I have an app that broadcasts the port it runs on over the LAN for clients to find it, so it doesn't matter what port it's on.
Also, you can bind before you call connect(), if you want, to force what port or interface the outgoing connection will be made on, but this is also uncommon.