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ffm(4)


NAME

  ffm - File-on-File Mounting File System


STANDARDS

  Interfaces documented on this reference page conform to industry standards
  as follows:

  fattach():  XSH4.2

  fdetach():  XSH4.2

  Refer to standards(5) for more information about industry standards and
  their associated tags.


DESCRIPTION

  The File-on-File Mounting (FFM) file system allows regular files, character
  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 invisible.

  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 accessed.

  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 separate file
  descriptors.


EXAMPLES

  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 -t ffm 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 -t ffm -o clone a_file b_file


RESTRICTIONS

  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 configure
  the kernel option FFM_FS into the kernel.  See System Administration for
  information about configuring the kernel.


RELATED INFORMATION

  Commands: fdetach(8), mount(8)

  Functions: fattach(3), fdetach(3), isastream(3), chmod(2), mount(2)

  Interfaces: streamio(7)

  Files: fstab(4)

  Standards: standards(5)

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