This is merely a little reminder for myself:
for pid in `ps -C <process-name> -o pid=`; do ls -l "/proc/$pid/fd"; done
On Linux, this returns a list of file handles being held open by all instances of <process-name>.
Update (2019-06-27): Martin Schuster suggested an even nicer (and regarding the output seemingly a more complete) approach to me by email:
lsof -c /^<process-name>$/ -a -d ^mem