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     lofs - loopback virtual file system

     #include <sys/param.h>
     #include <sys/mount.h>

     int mount (const char* dir, const char* virtual, int mflag, lofs, NULL, 0);

     The loopback file system device  allows  new,  virtual  file
     systems  to  be  created,  which  provide access to existing
     files using alternate pathnames. Once the virtual file  sys-
     tem is created, other file systems can be mounted within it,
     without affecting the original file  system.  However,  file
     systems  which  are  subsequently  mounted onto the original
     file system are visible to the virtual file  system,  unless
     or  until  the corresponding mount point in the virtual file
     system is covered by a file system mounted there.

     virtual is the mount point for the virtual file system.  dir
     is the pathname of the existing file system. mflag specifies
     the mount options; the  MS_DATA bit in  mflag must  be  set.
     If  the  MS_RDONLY bit in  mflag is not set, accesses to the
     loop back file system are the same  as  for  the  underlying
     file  system.  Otherwise,  all accesses in the loopback file
     system will be read-only. All  other  mount(2)  options  are
     inherited from the underlying file systems.

     A loopback mount of `/' onto /tmp/newroot allows the  entire
     file  system  hierarchy  to  appear as if it were duplicated
     under /tmp/newroot, including any file systems mounted  from
     remote  NFS  servers.  All  files  would  then be accessible
     either from a pathname relative to `/' or  from  a  pathname
     relative to /tmp/newroot until such time as a file system is
     mounted in /tmp/newroot, or any of its subdirectories.

     Loopback mounts of `/' can be performed in conjunction  with
     the  chroot(2)  system  call,  to provide a complete virtual
     file system to a process or family of processes.

     Recursive traversal of loopback mount points is not allowed.
     After   the   loopback   mount  of  /tmp/newroot,  the  file
     /tmp/newroot/tmp/newroot does not contain yet  another  file
     system  hierarchy;  rather,  it appears just as /tmp/newroot
     did before the loopback mount was performed (for example, as
     an empty directory).

     lofs file systems are mounted using:

       mount -F lofs /tmp /mnt

See Also
     lofiadm(1M),  mount(1M),  chroot(2),   mount(2),   sysfs(2),
     vfstab(4), lofi(7D)

     All access to entries in lofs mounted file  systems  map  to
     their  underlying  file  system.  If  a  mount point is made
     available in multiple locations via lofs and is busy in  any
     of  those  locations,  an  attempt to mount a file system at
     that mount point fails unless the overlay flag is specified.
     See mount(1M). Examples of a mount point being busy within a
     lofs mount include having a file system mounted on it or  it
     being a processes' current working directory.

     Because of the potential for confusing  users  and  applica-
     tions,  you should use loopback mounts with care. A loopback
     mount entry in /etc/vfstab must be placed  after  the  mount
     points  of  both  directories  it  depends on.  This is most
     easily accomplished by  making the loopback mount entry  the
     last in  /etc/vfstab.
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