UMOUNT(2)                 (2020-06-09)                  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 (top-
          most) filesystem 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)
               Ask the filesystem to abort pending requests before
               attempting the unmount.  This may allow the unmount to
               complete without waiting for an inaccessible server,
               but could cause data loss.  If, after aborting
               requests, some processes still have active references
               to the filesystem, the unmount will still fail.  As at
               Linux 4.12, MNT_FORCE is supported only on the follow-
               ing filesystems: 9p (since Linux 2.6.16), ceph (since
               Linux 2.6.34), cifs (since Linux 2.6.12), fuse (since
               Linux 2.6.16), lustre (since Linux 3.11), and NFS
               (since Linux 2.1.116).

          MNT_DETACH (since Linux 2.4.11)
               Perform a lazy unmount: make the mount point unavail-
               able for new accesses, immediately disconnect the
               filesystem and all filesystems mounted below it from
               each other and from the mount table, and actually per-
               form 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 currently 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

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               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 programs 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 file-
               names or data into.

          EPERM
               The caller does not have the required privileges.

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     VERSIONS
          MNT_DETACH and MNT_EXPIRE are available in glibc since ver-
          sion 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, including umount() 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() 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() 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() 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), mount_namespaces(7), path_resolution(7), mount(8),
          umount(8)

     COLOPHON
          This page is part of release 5.10 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 https://www.kernel.org/doc/man-pages/.

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