Internal Program Version: lsof  4.99.4

Lsof lists information about files opened by processes. An open file
may be a regular file, a directory, a block special file, a character
special file, an executing text reference, a library, a stream, or a
network file (Internet socket, NFS file, or UNIX domain socket.)  A
specific  file or all the files in a file system may be selected by
path.

http://people.freebsd.org/~abe