I have written a multiclient Server program in C on SuSE Linux Enterprise Server 12.3 (x86_64), I am using one thread per client to receive data.
My problem is:
I am using one terminal to run the server, and using several other terminals to telnet to my server (as client). I have used recv() in the server to receive data from client, I have also applied checks for return value of recv() i.e. Error on -1; Conn. Closed on 0 & Normal operation else. I have not used any flags in recv().
My program works fine if I just close the telnet session (i.e. disconnect client) normally using Ctrl+] and close, but if I forcefully terminate the client using kill <pid> then my server is unable to detect loss of connection.
How to fix that?
Constraint: I do not want to put condition on client side, I want to fix this on server side only.
You can enable SO_KEEPALIVE on the socket in your server.
/* enable keep-alive on the socket */
int one = 1;
setsockopt(sock, SOL_SOCKET, SO_KEEPALIVE, &one, sizeof(one));
By default, when keep-alive is enabled, the connection has to be idle for 2 hours before a keep-alive probe is attempted. You can adjust the keep-alive times to be a little more aggressive by adjusting the TCP_KEEPIDLE parameter:
int idletime = 120; /* in seconds */
setsockopt(sock, IPPROTO_TCP, TCP_KEEPIDLE, &idletime, sizeof(idletime));
When a probe is sent, it expects an acknowledgement from the other end. If there is an acknowledgement, the probe stays silent until the idle timer expires again. The keep-alive probe is retried again, by default every 75 seconds, if no acknowledgement to the probe is received. This can be adjusted with the TCP_KEEPINTVL option. The TCP_KEEPCNT option controls how many successive failures triggers the connection to be dropped. By default, that number is 9.
These options are available on Linux. BSD has similar options, but are named differently.