The fuser utility shall write to stdout the process IDs of processes that
have one or more named files open. For block and character special
devices, all processes using files on that device are listed.  A file is
considered open by a process if it was explicitly opened, is the working
directory, root directory, jail root directory, active executable text,
kernel trace file for that process or controlling tty of the process.

The fuser(1) utility is defined in IEEE Std 1003.1-2004 standard, but
doesn't included in standard FreeBSD distribution.

WWW: http://mbsd.msk.ru/stas/fuser.html