signal problems on BSD

[Guy Harris]

>  Because I believe that the kernel SIGHUP functionality only works when
>a dialup line or a hard-wired terminal line (i.e. something that init
>deals with, I believe) is the login tty in question.

Well, it's a bit more complicated.  Your original article said:

> While sh automatically sends a HUP signal to all its children when you
> log out (actually, I'm not sure the shell does this actively; it's
> might be a side-effect of the way and process groups et al work in
> BSD) ...

and Maarten replied:

> It's always been the kernel.  Quoting from termio(4) on SunOS 4.0.3c:
> 
>   Modem Disconnect
>      If a modem disconnect is detected, and the  CLOCAL  flag  is
>      not set in the c_cflag field, a SIGHUP signal is sent to all
>      processes in the distinguished process group associated with
>      this  terminal.  ...

The latter has been true since Time Immemorial; however, it only
delivers the SIGHUP if you actually get a "modem disconnect".  Note,
though, that both the 4.3BSD and SunOS 4.x pseudo-tty drivers (and, I
suspect, the S5R4 pseudo-tty mechanism as well) treat the final "close"
of the controller side of a pseudo-tty as being the moral equivalent of
a "modem disconnect", and send a SIGHUP exactly as it's done for Carrier
Detect dropping on a "real" tty line.  So your belief that the kernel
SIGHUP functionality only works on a dialup or hardwired terminal line
is incorrect; it works on pseudo-ttys as well.

However, there is *another* source of SIGHUP.  In BSD, "getty",
"rlogind", and "telnetd" issue a "vhangup" call on the tty they're
using, in order to blow any old processes off that line.  One
consequence of "vhangup" is that, if the tty is still open, a SIGHUP
gets sent to its process group.  In System V, when a "process group
leader" (a login shell, for example) exits, and:

	1) it has a controlling tty

and

	2) that tty's process group is the process group of which that
	   process is a leader,

a SIGHUP is given to that process group.

Of course, if you have a job control shell, neither of the SIGHUPs
mentioned above will reach background processes, as stated by Maarten.