UNIX for Dummies Questions & Answers

I'm trying to wrap my head around process control in Unix... particularly init and how it fits in to the equation. Init is responsible for creating sessions by spawning instances of getty which calls login effectively creating sessions right? Why then can init belong to a session? If you ps jx 1, you will see init belongs to SID 1. Is SID 1 a pseudo session or just a bogus number added as a formality to the output of ps? I notice that a lot of other low level user processes belong to this special session. Could someone explain to me a little more about it? I've been googling all over the place today for a clearer understanding.

Thanks.

Dex

init, the instance with a pid of 1, is not created via a fork(). The kernel handcrafts it and can do whatever it wants. That said, I don't really see anything terribly odd about sid 1 and I don't understand your confusion.

init was always created with a ppid of 0, even before there was a process 0. Today process 0 tends to be its own parent. There is a lot of weird stuff about non-forked processes. Why did you zero in on sid?

In any event sid 1 is a real sid, but it's not obvious what else it might be. What do you mean by psuedo-session?

Ok... thanks Perderabo. That helps. I was thinking of sessions as something created via the login process and didn't understand how login could create the session that init was in. It sounds like sessions are not as tightly coupled with login as I initially thought. When you say “there are a lot of weird stuff about non-forked processes” do you mean that others exist or that there are other weird things about init? We are talking about process control in a systems programming course I'm taking, so that is why I zeroed in on SID. I figured the value aught to be -1 like TGPID.

Stuff like this depends on the version of unix. But most have a process 0. Read the man page for setsid(). Also read this: http://www.erlenstar.demon.co.uk/unix/faq_2.html#SEC16
and notice that correctly written daemons put themselves into a separate session.

Great link. Steps one and two of creating a daemon finely filled in the gaps and clarified it for me. Thanks again!