Readonly files

["Steven Fullmer" <Steven_Fullmer tni com>]

Using chmod, chown and chgrp, a user can get permissions to modify files.  Is there a way to 'lock' files in use as well?  I know of a couple programs that create an entry into a .lock file where, when someone opens the same file, it is opened read-only.  Is this a feature unique to this program, or is it a RH standard, so no two people can modify certain files?  If there is, is there a command that is sent to lock and/or unlock the files/directories in question?

Steven Fullmer