Calling "start" to start program and "stop" to close current instance in C

Question

I wrote a simple server in C and would like to have the same functionality calling it as other C daemons (such as calling it with ./ftpd start and closing that instance with ./ftpd stop). Obviously the problem I'm having is that I do not know how to grab the current instance of the running program. I can parse the options just fine (using getopt / optarg) but at the moment, ./my-program stop just starts a new instance vs. calling ./my-program start which starts it up fine.

The reason I want to do this is because another program will be signaling my server to stop, so a call like ./my-program stop is very simple, which can then stop the server loops and close all the open fd's. Thanks!

Is it possible to do this in the C program itself or is this usually handled externally?

Answer 1

You will always run a separate process when running it with 'stop' argument. You need to find the running daemon and sending it a signal to stop it. When your deamon is started (option 'start'), you might want to store its pid somewhere so that when it is started with 'stop' option, it can retrieve the pid of the daemon and send it a signal to stop it. You can also parse the process table to find your daemon, but that will be more complex.

Answer 2

In most Linux distributions that I've come across, daemons are started and stopped via init scripts rather than direct invocation.

For example, on my Ubuntu box I can start and stop wicd by means of:

/etc/init.d/wicd start /etc/init.d/wicd stop

... which I think is what you want to do.

As some other people have noted, to stop a running process you will need to stash its PID somewhere, and then use that PID to send it a signal later on. You could implement this yourself in a script, but most Linux distros provide helper applications to take care of that for you e.g. start-stop-daemon(8) on Debian-derived platforms (Redhat distros have something similar, but I can't remember the application name off the top of my head).

So if it were me, I would write a script to handle the starting and stopping of your C daemon, and I would base that script on one of the others in /etc/init.d as ar mentioned above.

Answer 3

Have a look at the killall command. The start within a script often just looks to see if an instance of the program already exists, and if it doesn't it starts it. The stop often just sends a signal to the service using the killall command.

Similar things that you may be interested in are editing or replacing the configuration files for a daemon and then sending the daemon the SIGHUP signal telling it that it should reread its configuration.

Another related topic is sending a daemon a particular message by calling the daemon's executable with some particular options. This is done this way because the interface that the messages are sent in can change and if it is the same program doing the sending and receiving it is hard to get out of sync. The method of sending messages can vary as well (Unix Domain Sockets, named pipe, editing a configuration file and sending SIGHUP, just about anything you could imagine). One other benefit of this is that the same input verification that the program uses when it runs as a daemon can be used before it sends the message, so if there is an error that the daemon would choke on the instance of the program that you are calling from the command line could go ahead and tell you on the terminal and refuse to pass this on to the daemon.

To find the PID of a running service either use a pid file (daemon creates a file in a certain location with its PID in it -- just like old lock files, or do it the same way that killall does -- look through /proc (which is platform specific).