ffm(4)ffm(4)NAMEffm - File-on-File Mounting File System
Interfaces documented on this reference page conform to industry stan‐
dards as follows:
Refer to standards(5) for more information about industry standards and
their associated tags.
The File-on-File Mounting (FFM) file system allows regular files, char‐
acter device special files, or block device special files to be mounted
on regular files or directories.
The ffm file system is used with the System V Release 4-compatible
library functions fattach(3) and detach(3) to enable a user process to
have one file descriptor pointing to the data associated with a named
file and a named STREAM. When one name is active, the other name is
For example, a user application mounts a file descriptor from a file
named a_file on a file that is named b_file. The file descriptor of
file a_file is accessible by two names, a_file and b_file. However,
when the user application attempts to open either file, only the file
descriptor for a_file is returned: the file descriptor for b_file is
invisible while a_file is mounted over it.
The fattach(3) function mounts a file over another; the fdetach(3)
function removes the association so the underlying file can be
The user process can also mount a regular file over a regular file in
order for it to be a clone of the underlying file. [Do not confuse
this clone with an AdvFS clone fileset.] In this case, the clone file
is a character device special file that is associated with a device
driver that handles such files. As a result, a user can specify one
clone entry and then open this device multiple times. Each time the
device is opened, a new vnode is obtained but exactly the same device
behavoir is also obtained: the behavior is cloned.
That mount occurs if the -o clone option is used in the mount command
or as an element of a ffm line in the /etc/fstab file. In this case,
there are two files with identical contents, separate names, and sepa‐
rate file descriptors.
The following example shows an ffm mount of a_file on b_file. If the
df command were executed, its display would show a_file in the file
system column and b_file in the Mounted on column:
# mount -tffm a_file b_file
The following example shows an ffm mount of a_file on b_file, with the
mount -o clone option specifying that a_file is a clone of b_file.
# mount -tffm-o clone a_file b_file
The user process must be the root user or must be the owner of the
files and must have write permissions for the files.
[Tru64 UNIX] Before you can use the ffm file system, you must config‐
ure the kernel option FFM_FS into the kernel. See System Administra‐
tion for information about configuring the kernel.
Commands: fdetach(8), mount(8)
Functions: fattach(3), fdetach(3), isastream(3), chmod(2), mount(2)
Standards: standards(5) delim off