Proper way to detach from control terminal?

[Guy Harris]

>I would like to know if there is a "proper" way to detach a process from
>its   control   terminal  (once  it  has  fork'ed  and  the  parent  has
>terminated).

Well, it depends on which flavor of UNIX you're using....

>	1.  setpgrp(2)
>
>                This  is  the  method  described in the System V section
>                termio(7), and does seem to work.

One would hope it would, since that's the call you're supposed to use
for that in S5.

>		 The calling  sequence I  am  using  is  simply 
>                setpgrp() for  System  V  and setpgrp(0,getpid())
>                for BSD based  code.   But,  on  BSD systems,  the
>                control terminal is still reported when a ps(1) is
>                done.

That's because "setpgrp" *isn't* the call you're supposed to use on BSD
systems.  "setpgrp" has the side effect of detaching you from your
controlling terminal on S5, but it doesn't have that side effect on BSD.
(The two were invented independently - at least in part - and for
different purposes; the BSD one is used for job control, and processes
are supposed to keep their controlling tty no matter whether they're in
the foreground or in the background.)

>	2.  ioctl(...TIOCSPGRP...)
>
>		Is using ioctl to set the process group any different
>		than using the setpgrp() system call?

Yes, the system call sets the process group of a *process*, while the
"ioctl" (not available currently on vanilla S5) sets the process group
of a *terminal*.  Basically, if you type something like your interrupt
character (RUBOUT, ^C, whatever), the terminal driver sends a signal to
the process group to which it belongs, which means it's delivered to all
processes that are currently in that process group.

>	3.  ioctl(...TIOCNOTTY...)
>
>		I have seen this used in the recently posted plp
>		software, but cannot find any documentation on this
>		setting other than "void tty association" in the
>		header file <sys/ioctl.h> on a BSD system.

That's what "void tty association" means - "void" a process's
"association" with its controling "tty", i.e.  detach itself from that
tty.  ("Void where prohibited by law", on the other hand, is a command
telling you to urinate, say, on the White House lawn. :-))  The
4.3-tahoe TTY(4) manual page says (I think, earlier 4.3BSD manuals and
maybe the 4.2BSD manual said something similar):

	A process can remove the association it has with its controlling
	terminal by opening the file "/dev/tty" and issuing an
	"ioctl(f, TIOCNOTTY, 0);"

	This is often desirable in server processes.

>	4.  closing all terminal files
>
>                I  don't know if I am reading things wrong, but it seems
>                that some programs simple close  stdin/stdout/stderr  on
>                startup  and  open  something  else as stdin.  Does this
>                really do anything?

It doesn't detach you from your controlling terminal, that's for
sure....

>                Should I do this  in  concert  with any/all of the
>                above options?

Yes, you might want to do this if your program is, say, a daemon process
that's trying to run "cleanly" regardless of whether it's started from
"/etc/rc" or from a user's terminal; that way, it won't "hang on" to the
terminal (i.e., it won't hold it open, and won't issue messages to it).

Dave Lennert (formerly of HP, now of Sequent - right, Dave?) wrote a
fairly detailed paper on "how to write a daemon", going into details
about these sorts of things on various UNIX flavors.  I think it
appeared in one of the UNIX magazines within the past few months.