Wandering Thoughts archives

Init's (historical) roles

Historically (by which I mean since at least V6 Unix), init aka PID 1 has had three and then four roles:

1. It inherits orphan processes, ie processes that have had their regular parent exit. Doing this almost certainly simplified a bunch of V7 kernel code because it meant that every process has a parent process.

2. Starting up the user level of Unix on boot. Originally this was done by running a monolithic shell script, as can still be sort of seen in OpenBSD. System V init modularized and generalized it into the multi-file form.

3. Starting, managing, and restarting the getty processes for the console and (other) serial lines. System V init generalized this so that init started and restarted whatever you told it to via entries in /etc/inittab.

4. Shutting down the user level of the system and rebooting. This role first appeared in System V init, using the modularity that it had introduced for booting. Modern BSDs also give init responsibility for rebooting (and it will run a shell script as part of this), but as late as 4.4 BSD reboot(8) did almost all of the work itself and there was no concept of running a shell script to bring services down in any orderly way; reboot(8) just killed everything in sight.

(Really. You can read the 4.4 BSD reboot(8) source if you want, it's not long. The violence starts at the 'kill(-1, SIGTERM)'.)

Putting the three (and then four) roles on the shoulders of a single process is likely due to both conservation of resources in early Unixes (given that they ran in very limited environments they likely didn't want to take up memory with extra programs) and simple least complexity and effort. Once you had init as the inheritor of orphan processes you might as well make it do all the other roles since it was already there. Why throw in additional programs without a good need? It probably helped that even in V7 the other two roles were pretty simple and minimal, per eg the V7 /etc/rc.

As a historical note, it was BSD Unix that decided that init was so crucial that the system should be rebooted if it ever exited. V7 Unix will probably get into an odd state if init ever exits but as far as I can tell from the kernel source PID 1 is not treated specially as far as exiting goes; as a practical matter V7 Unix just assumes it will never happen. Even what happens if /etc/init can't be executed on boot is not strictly a kernel thing in V7.

(In the initial environment of BSD, this decision was probably doubly correct. Even if you never have to deal with any orphaned processes or the kernel cleaned them up itself (let's wave our hands aggressively here), losing init means that getty processes will not be restarted on serial lines when people log out, which over time makes it impossible for anyone to log in. Of course in the modern era of networked machines this is no longer such an issue and you probably care a lot more about sshd than about gettys.)

Some modern init systems have split some or most of these roles out from PID 1. Solaris, for example, moved everything except the first role to separate processes (the SMF stuff runs in svc.startd et al and getty processes are handled through ttymon and sac).