Many traditional daemons do this, as part of their "daemonization" sequence which is meant to avoid inheriting anything from a user's environment, as traditionally (before IPC-oriented init systems such as systemd or Upstart became popular) the administrator would just directly run an /etc/rc.d script from an interactive session, and sometimes would just directly start the daemon binary – which is then supposed to fork/double-fork, re-open stdin/out/err, shake off any controlling ttys, write its own pidfile, and so on.
For example, an old /etc/rc.d/inetd might have been as simple as this:
case $1 in
start)
echo -n "Starting inetd..."
/sbin/inetd
echo "done."
[...]
But there is no cross-platform method for determining which file descriptors are open. Some Unixes have a syscall such as closefrom(), others allow the process to iterate over /proc/self/fd/[0-9], but some don't have anything better than iterating from 3 all the way to the current RLIMIT_NOFILE value and blindly trying to every possible FD.
The latter is what xinetd does, with the assumption that its NOFILE limit will be somewhere around 1k to 4k at most. (Which was generally the case 20-30 years ago when its startup code was written – it hasn't been changed much since then.)
