coreutils-6.9 and Tru64 UNIX 5.1 FFM mounts

[Albert Chin]

Tru64 UNIX allows you to mount one file on top of another:
  The File-on-File Mounting (FFM) file system allows regular files, character
  device special files, or block device special files to be mounted on regu-
  lar files or directories.

# touch /mnt/foo
# mount -t ffm /dev/mem /mnt/foo
# df -k /mnt/foo 
Filesystem  1024-blocks        Used   Available Capacity  Mounted on
/dev/mem        6290784     2070204     4189840    34%    /mnt/foo

Using GNU df from coreutils-5.9:
# gdf -k /mnt/foo
gdf: `/mnt/foo': Invalid argument
gdf: no file systems processed

-- 
albert chin (address@hidden)