Restarting a linux daemon

Question

I have Linux daemon that I have written in C++ that should restart itself when given a "restart"-command from a user over the network through its console. Is this possible? I use a /etc/init.d script. How can I program it to restart itself? Should I launch a new process with a very long delay (one minute) that then fires the shell script again ? Problem is that the daemon may take a very long time to close down and it could take even more than a minute in a worst-case scenario.

Answer 1

There are basically three ways for an application to restart itself:

1. When the application is told to restart, it does proper clean-up, releases all resources it has allocated, and then re-initializes like it was started from scratch.

2. Fork a new process, where the new child process execs itself and the parent process exits normally.

3. The daemon is actually just a wrapper application, much like an init-script. It forks a new process which runs the actual application, while the parent process just waits for it to exit. If the child process (and the real application) returns with a special exit-code, it means that it should be restarted so the forks/execs all over again.

Note that points 2 and 3 are basically the same.

Answer 2

Break down the restart as two steps, stop and start. if your program takes time to stop, it should be handled in the stop function, I can't comment on specifics since I don't know your usecase, but I'd imagine monitoring the process to check if it's terminated will be a graceful way to stop

Answer 3

Do whatever shut-down/clean-up you need to do, then call this:

execl( argv[0], argv, reinterpret_cast< char* >( 0 ) );

Just like fork() and exec(), but skipping the fork. The exec will replace the current process with a new copy of itself. cf. http://linux.die.net/man/3/exec

Answer 4

Your init script should just kill your daemon and start it again. Don't try to restart your daemon FROM your daemon.