umount

UMOUNT(2)                  Linux Programmer's Manual                 UMOUNT(2)

NAME
       umount, umount2 - unmount filesystem

SYNOPSIS
       #include <sys/mount.h>

       int umount(const char *target);

       int umount2(const char *target, int flags);

DESCRIPTION
       umount()  and umount2() remove the attachment of the (topmost) filesys-
       tem mounted on target.

       Appropriate privilege (Linux: the CAP_SYS_ADMIN capability) is required
       to unmount filesystems.

       Linux  2.1.116  added  the umount2() system call, which, like umount(),
       unmounts a target, but allows additional flags controlling the behavior
       of the operation:

       MNT_FORCE (since Linux 2.1.116)
              Force  unmount  even  if busy.  This can cause data loss.  (Only
              for NFS mounts.)

       MNT_DETACH (since Linux 2.4.11)
              Perform a lazy unmount: make the mount point unavailable for new
              accesses, immediately disconnect the filesystem and all filesys-
              tems mounted below it from each other and from the mount  table,
              and  actually perform the unmount when the mount point ceases to
              be busy.

       MNT_EXPIRE (since Linux 2.6.8)
              Mark the mount point as expired.  If a mount point is  not  cur-
              rently  in use, then an initial call to umount2() with this flag
              fails with the error  EAGAIN,  but  marks  the  mount  point  as
              expired.   The  mount  point remains expired as long as it isn't
              accessed by any process.  A  second  umount2()  call  specifying
              MNT_EXPIRE unmounts an expired mount point.  This flag cannot be
              specified with either MNT_FORCE or MNT_DETACH.

       UMOUNT_NOFOLLOW (since Linux 2.6.34)
              Don't dereference target if it is a symbolic  link.   This  flag
              allows  security problems to be avoided in set-user-ID-root pro-
              grams that allow unprivileged users to unmount filesystems.

RETURN VALUE
       On success, zero is returned.  On error, -1 is returned, and  errno  is
       set appropriately.

ERRORS
       The  error  values  given below result from filesystem type independent
       errors.  Each filesystem type may have its own special errors  and  its
       own special behavior.  See the Linux kernel source code for details.

       EAGAIN A call to umount2() specifying MNT_EXPIRE successfully marked an
              unbusy filesystem as expired.

       EBUSY  target could not be unmounted because it is busy.

       EFAULT target points outside the user address space.

       EINVAL target is not a mount point.

       EINVAL umount2() was called with MNT_EXPIRE and  either  MNT_DETACH  or
              MNT_FORCE.

       EINVAL (since Linux 2.6.34)
              umount2() was called with an invalid flag value in flags.

       ENAMETOOLONG
              A pathname was longer than MAXPATHLEN.

       ENOENT A pathname was empty or had a nonexistent component.

       ENOMEM The  kernel  could not allocate a free page to copy filenames or
              data into.

       EPERM  The caller does not have the required privileges.

VERSIONS
       MNT_DETACH and MNT_EXPIRE are available in glibc since version 2.11.

CONFORMING TO
       These functions are Linux-specific and should not be used  in  programs
       intended to be portable.

NOTES
   umount() and shared mount points
       Shared  mount points cause any mount activity on a mount point, includ-
       ing umount(2) operations, to be forwarded to every shared  mount  point
       in the peer group and every slave mount of that peer group.  This means
       that umount(2) of any peer in a set of shared mounts will cause all  of
       its  peers  to  be unmounted and all of their slaves to be unmounted as
       well.

       This propagation of unmount activity can be particularly surprising  on
       systems where every mount point is shared by default.  On such systems,
       recursively bind mounting the root directory of the filesystem  onto  a
       subdirectory   and   then   later  unmounting  that  subdirectory  with
       MNT_DETACH will cause every mount in the mount namespace to  be  lazily
       unmounted.

       To ensure umount(2) does not propagate in this fashion, the mount point
       may be remounted using a mount(2) call with a mount_flags argument that
       includes both MS_REC and MS_PRIVATE prior to umount(2) being called.

   Historical details
       The  original  umount() function was called as umount(device) and would
       return ENOTBLK when called with something other than  a  block  device.
       In  Linux  0.98p4,  a  call  umount(dir) was added, in order to support
       anonymous devices.  In Linux 2.3.99-pre7, the call  umount(device)  was
       removed,  leaving only umount(dir) (since now devices can be mounted in
       more than one place, so specifying the device does not suffice).

SEE ALSO
       mount(2), path_resolution(7), mount(8), umount(8)

COLOPHON
       This page is part of release 4.04 of the Linux  man-pages  project.   A
       description  of  the project, information about reporting bugs, and the
       latest    version    of    this    page,    can     be     found     at
       http://www.kernel.org/doc/man-pages/.

Linux                             2015-03-29                         UMOUNT(2)
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