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ffm(4)								       ffm(4)


  ffm -	File-on-File Mounting File System


  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.


  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.

  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


  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


  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.


  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)