Launch server service

Once socket is created successfully, we can launch server or client. Let's focus on server firstly.

No matter server uses TCP or UDP, the following code is shared (in local_listen function):

......
ret = setsockopt(s, SOL_SOCKET, SO_REUSEPORT, &x, sizeof(x));
if (ret == -1)
    err(1, NULL);

set_common_sockopts(s, res->ai_family);

if (bind(s, (struct sockaddr *)res->ai_addr,
    res->ai_addrlen) == 0)
    break; 
......

SO_REUSEPORT is a common option when implementing server. About its detailed explanation, you can refer UNIX Network Programming, Volume 1, chapter 7, Socket Options. Personally, I think the SO_REUSEPORT's most important effect is you can bind port successfully after restarting server immediately. Otherwise you will encounter following annoying error:

Address already in use

The set_common_sockopts() will be discussed further. bind() function is used to bind socket and pair of IP address & port.

If the server uses connection-less UDP protocol, then all work is done. But for TCP, there are 2 further steps:

......
if (!uflag && s != -1) {
    if (listen(s, 1) < 0)
        err(1, "listen");
}
......

socklen_t len;
struct sockaddr_storage cliaddr;
......
len = sizeof(cliaddr);
connfd = accept4(s, (struct sockaddr *)&cliaddr,
                &len, SOCK_NONBLOCK);
......

Server waits for the client's connection, and only serves only one connection at a time. Once the connection is established successfully, accept4() will return new socket that is in non-blocking mode. Now the server can use the fresh socket to communicate with client.