$NetBSD: NOTES,v 1.3 2006/04/18 11:40:26 salo Exp $ POSIX and init: -------------- POSIX.1 does not define 'init' but it mentions it in a few places. B.2.2.2, p205 line 873: This is part of the extensive 'job control' glossary entry. This specific reference says that 'init' must by default provide protection from job control signals to jobs it starts -- it sets SIGTSTP, SIGTTIN and SIGTTOU to SIG_IGN. B.2.2.2, p206 line 889: Here is a reference to 'vhangup'. It says, 'POSIX.1 does not specify how controlling terminal access is affected by a user logging out (that is, by a controlling process terminating).' vhangup() is recognized as one way to handle the problem. I'm not clear what happens in Reno; I have the impression that when the controlling process terminates, references to the controlling terminal are converted to references to a 'dead' vnode. I don't know whether vhangup() is required. B.2.2.2, p206 line 921: Orphaned process groups bear indirectly on this issue. A session leader's process group is considered to be orphaned; that is, it's immune to job control signals from the terminal. B.2.2.2, p233 line 2055: 'Historically, the implementation-dependent process that inherits children whose parents have terminated without waiting on them is called "init" and has a process ID of 1.' It goes on to note that it used to be the case that 'init' was responsible for sending SIGHUP to the foreground process group of a tty whose controlling process has exited, using vhangup(). It is now the responsibility of the kernel to do this when the controlling process calls _exit(). The kernel is also responsible for sending SIGCONT to stopped process groups that become orphaned. This is like old BSD but entire process groups are signaled instead of individual processes. In general it appears that the kernel now automatically takes care of orphans, relieving 'init' of any responsibility. Specifics are listed on the _exit() page (p50). On setsid(): ----------- It appears that neither getty nor login call setsid(), so init must do this -- seems reasonable. B.4.3.2 p 248 implies that this is the way that 'init' should work; it says that setsid() should be called after forking. Process group leaders cannot call setsid() -- another reason to fork! Of course setsid() causes the current process to become a process group leader, so we can only call setsid() once. Note that the controlling terminal acquires the session leader's process group when opened. Controlling terminals: --------------------- B.7.1.1.3 p276: 'POSIX.1 does not specify a mechanism by which to allocate a controlling terminal. This is normally done by a system utility (such as 'getty') and is considered ... outside the scope of POSIX.1.' It goes on to say that historically the first open() of a tty in a session sets the controlling terminal. P130 has the full details; nothing particularly surprising. The glossary p12 describes a 'controlling process' as the first process in a session that acquires a controlling terminal. Access to the terminal from the session is revoked if the controlling process exits (see p50, in the discussion of process termination). Design notes: ------------ your generic finite state machine we are fascist about which signals we elect to receive, even signals purportedly generated by hardware handle fatal errors gracefully if possible (we reboot if we goof!!) if we get a segmentation fault etc., print a message on the console and spin for a while before rebooting (this at least decreases the amount of paper consumed :-) apply hysteresis to rapidly exiting gettys check wait status of children we reap don't wait for stopped children don't use SIGCHILD, it's too expensive but it may close windows and avoid races, sigh look for EINTR in case we need to change state init is responsible for utmp and wtmp maintenance (ick) maybe now we can consider replacements? maintain them in parallel init only removes utmp and closes out wtmp entries... necessary states and state transitions (gleaned from the man page): 1: single user shell (with password checking?); on exit, go to 2 2: run rc script, on exit 0 check if init.root sysctl != "/", if it differs then fork + chroot into the value of init.root and run /etc/rc inside the chroot: on exit 0, go to 3; on exit N (error), go to 1 (applies also to /etc/rc when init.root == "/") 3: read ttys file: on completion, go to 4. If we did chroot in state 2, we chroot after forking each getty to the same dir (init.root is not re-read) 4: multi-user operation: on SIGTERM, go to 7; on SIGHUP, go to 5; on SIGTSTP, go to 6 5: clean up mode (re-read ttys file, killing off controlling processes on lines that are now 'off', starting them on lines newly 'on') on completion, go to 4 6: boring mode (no new sessions); signals as in 4 7: death: send SIGHUP to all controlling processes, reap for 30 seconds, then go to 1 (warn if not all processes died, i.e. wait blocks) Given the -s flag, we start at state 1; otherwise state 2