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The Init Daemon

Initial Ram Disk to Rootfs transition

After mounting the root filesystem, either the kernel or the initial ram disk startup process is expected to exec the init program from the root filesystem.

At the current time, there is no support for re-scanning the service files yet, so when init is started, the final configuration in /etc/init.d has to be present. As a result, we currently cannot perform mounting of /etc/ or packing init into the initial ram disk and doing the rootfs transition.

Also, as a result of this, changing the service configuration requires a system reboot to be effective.

This will change in the future.

Service Types and Targets

A service file has a type, specifying whether the service should be run once or restarted when it terminates and a target, specifying when the service should be run.

The target is similar to a runlevel in System V init. The init daemon currently knows about the following targets:

  • boot
  • reboot
  • shutdown

When init is run, it starts all the services for the boot target. From the boot target it can transition to any other target and execute the services for the specified target.

The reboot and shutdown targets cannot transition to any other target and when invoked, cause initd to drop everything else it intended to do.

For the reboot and shutdown targets, respawn type processes are no longer restarted when they terminate and once all services have been executed, the init program performs a hard system reboot or power off.

The init program tries to capture the CTRL+ALT+DELETE key sequence (or its local equivalent) and transitions to the reboot target if pressed.

Service Configuration Rescan

TBD

Control Socket and Signals

The init program catches the following signals:

  • SIGCHLD
  • SIGINT
  • SIGTERM

The SIGCHLD handler implements standard process reaping. If a terminated process belongs to one of the supervised services, the configured action is taken (e.g. restarting it).

When SIGINT is caugth, init transitions to the reboot target. Similarly, SIGTERM causes init to transition to the shutdown target.

For more complex tasks, init creates a control socket that the command line tools included in this package can use. For the time being, the control socket can only tell the init daemon to transition to the reboot or shutdown target.