NAME    [Toc]    [Back]

       ffm - File-on-File Mounting File System

STANDARDS    [Toc]    [Back]

       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    [Toc]    [Back]

       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    [Toc]    [Back]

       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    [Toc]    [Back]

       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    [Toc]    [Back]

       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



                                                           ffm(4)