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.