So_Keepalive - HP Rp3440-4 - 9000 - 0 MB RAM Programmer's Manual

Advanced Topics for Stream Sockets
Socket Options
Network Daemon Server Listening at Port 2000.
When the
network daemon accepts a connection request, the accepted socket will
bind to port 2000 and to the address where the daemon is running (e.g.
192.6.250.100). If you executed netstat an, the output would
resemble:
Active connections (including servers)
Proto Recv-Q Send-Q Local Address Foreign Address (state)
tcp 0 0 192.6.250.100.2000 192.6.250.101.4000 ESTABLISHED
tcp 0 0 *.2000 *.* LISTEN
New Connection Established, Daemon Server Still
Listening.
Here the network daemon has established a connection to
the client (192.6.250.101.4000) with a new server socket. The
original network daemon server continues to listen for more connection
requests.
If the listening network daemon process is killed, attempts to restart the
daemon fail if SO_REUSEADDR is not set. The restart fails because the
daemon attempts to bind to port 2000 and a wildcard IP address (e.g.
*.2000). The wildcard address matches the address of the established
connection (192.6.250.100), so the bind aborts to avoid duplicate
socket naming.
When SO_REUSEADDR is set, bind ignores the wildcard match, so the
network daemon can be restarted. An example usage of this option is:
int optval = 1;
setsockopt (s, SOL_SOCKET, SO_REUSEADDR, &optval,
sizeof(optval));
bind (s, &sin, sizeof(sin));

SO_KEEPALIVE

This option is AF_INET socket-specific.
This option enables the periodic transmission of messages on a connected
socket. This occurs at the transport level and does not require any work
in your application programs.
If the peer socket does not respond to these messages, the connection is
considered broken. The next time one of your processes attempts to use a
connection that is considered broken, the process is notified (with a
SIGPIPE signal if you are trying to send, or an end-of-file condition if you
are trying to receive) that the connection is broken.
66 Chapter 3