SOCKET keepalive

https://tldp.org/HOWTO/TCP-Keepalive-HOWTO/programming.html#examples

/* --- begin of keepalive test program --- */

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>

int main(void);

int main()
{
int s;
int optval;
socklen_t optlen = sizeof(optval);

/* Create the socket */
if((s = socket(PF_INET, SOCK_STREAM, IPPROTO_TCP)) < 0) {
perror("socket()");
exit(EXIT_FAILURE);
}

/* Check the status for the keepalive option */
if(getsockopt(s, SOL_SOCKET, SO_KEEPALIVE, &optval, &optlen) < 0) {
perror("getsockopt()");
close(s);
exit(EXIT_FAILURE);
}
printf("SO_KEEPALIVE is %s\n", (optval ? "ON" : "OFF"));

/* Set the option active */
optval = 1;
optlen = sizeof(optval);
if(setsockopt(s, SOL_SOCKET, SO_KEEPALIVE, &optval, optlen) < 0) {
perror("setsockopt()");
close(s);
exit(EXIT_FAILURE);
}
printf("SO_KEEPALIVE set on socket\n");

/* Check the status again */
if(getsockopt(s, SOL_SOCKET, SO_KEEPALIVE, &optval, &optlen) < 0) {
perror("getsockopt()");
close(s);
exit(EXIT_FAILURE);
}
printf("SO_KEEPALIVE is %s\n", (optval ? "ON" : "OFF"));

close(s);

exit(EXIT_SUCCESS);
}

/* --- end of keepalive test program --- */