Question

I have to write a SNMP module which monitor a certain server application that I have written too. The problem is that I have to know if this application is running and I should be able to kill it whenever I can.

I know the port where the application is listening (reading the application configuration file) and I can try to bind this port to a socket in order to know if it is (or isn’t) being used by my application or another which is enough for my module. Here is the code:

int
get_server_status()
{
        struct sockaddr_in local;
        int port,sockfd;

        if (parse_config_file(&port,NULL,CONFIG_FILE_PATH) == -1)
                return -1; //Error

        if ((sockfd = socket ( AF_INET, SOCK_STREAM, 0 )) < 0)
                return -1; //Error

        local.sin_family = AF_INET;
        local.sin_port = htons ( port );
        local.sin_addr.s_addr = inet_addr ("127.0.0.1");

        if (bind(sockfd, (struct sockaddr *) &local, sizeof (local) ) < 0 ){
                if(errno == EADDRINUSE)
                        return 1; //Port being used
                else
                        return -1; //Error
        }else{
                close(sockfd);
                return 0;         //Port not being used

        }
}

The problem comes when I need to kill the application, I don’t know neither its PID nor how to get it. I can get is using netstat -tln <port>but I don’t know how to do it programmatically. Any ideas??

Answer 1

Like netstat, you should read /proc/net/tcp.

Interpreting it:

1. The second field, titled local_address, is the IP and port. 00000000:0050 would be HTTP (the port number is in hex).
2. The 4th field, titled st, is the state. A is TCP_LISTEN.
3. The 10th field, titled inode is the inode number (decimal this time).
4. For each process, /proc/pid/fd/ contains an entry for each open file descriptor. ls -l for socket descriptors shows that it’s a link to socket:[nnnnnn]. The number nnnnnn should match the inode number from /proc/net/tcp.

This makes finding the process quite tiresome, but possible.
Finding the right line in /proc/net/tcp isn’t difficult, and then you can get the inode number.
Finding the process requires you to scan all processes, looking for one which refers this inode number. I know no better way.