debconf & daemons

[guido . guenther]

Hi,
The tutorial of debconf reads:
> Your postinst uses debconf and starts a daemon that doesn't close
> all inherited file descriptors (all such daemons are buggy,
> really). This makes debconf hang, because the debconf frontend
> waits for the daemon to close the fd's before continuing.
> Note
> that if you use confmodule.sh, the program probably needs to close
> fd's 0, 1, 2, and _3_.
I know that 0,1,2 are stdout, stdin & stderr. But I wonder what "3" refers to?
Bye,
        Guido