Mandatory locking (cont)

Requires that the filesystem is mounted with the 'mand' option:
      # mount -oremount,mand /var
      $ mount | grep /var
      /dev/hda3 on /var type ext3 (rw,mand)

Requires that the file to be locked has setgid bit set and doesn't have the group execute bit set:
      $ ls -l file
      /dev/hda3 on /var type ext3 (rw,mand)
      -rw-r-Sr--  2 reegen reegen  17282 Oct 13 12:17 file

For more info about locking, see the man pages, and /usr/src/linux/Documentation/{locks,mandatory}.txt