<defunct> processes (was Re: Trouble killing processes in SysV/AT)

[Guy Harris]

> My process was there, but so were about 40 processes marked STAT == Z,
> COMMAND == <defunct>.  Trying to kill -9 these failed,

Just as shooting a corpse won't do anything interesting, either.

"<defunct>" means "defunct."  Those processes have already been killed;
however, a dead process remains around in "zombie" form (hence the "Z") until
the parent process does a "wait()" or variant thereof to pick up and dispose of
the corpse.

> I got lucky in comparison to Mr. Paul - at least these went away when the
> parent exited.

When the parent exits, "init" gets custody of the body and disposes of it
rather quickly.

> As an interesting aside, I was running TT == 0 (/dev/tty0), but the
> controlling TT of these defunct processes was drifting all over hell's
> half acre: TT == co, then TT == h1, then TT == dx - all for the same process!

Long-standing 4BSD "ps" bug.  The controlling tty information comes from the U
area of the process; however, zombies don't have U areas.  However, "ps"
doesn't understand the connection between these two facts; it still tries to
extract the controlling tty from its U-area buffer.  This means it picks up the
controlling tty of the last process whose U area it read.

The correct fix is to change the code at the beginning of "save()" from:

	if (mproc->p_stat != SZOMB && getu() == 0)
		return;
	ttyp = gettty();

to something like:

	if (mproc->p_stat != SZOMB) {
		if (getu() == 0)
			return;
		ttyp = gettty();
	} else
		ttyp = "?";	/* zombies are not attached to terminals */