ffm man page on DigitalUNIX

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ffm(4)									ffm(4)

       ffm - File-on-File Mounting File System

       Interfaces  documented on this reference page conform to industry stan‐
       dards 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, 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 -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  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)

       Interfaces: streamio(7)

       Files: fstab(4)

       Standards: standards(5) delim off


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