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

       umount, umount2 - unmount filesystem

       #include <sys/mount.h>

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

       umount() and umount2() remove the attachment of the (topmost) 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 following 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 unavailable for new
              accesses, immediately disconnect the filesystem and all
              filesystems mounted below it from each other and from the mount
              table, and actually perform the unmount when the mount ceases to
              be busy.

       MNT_EXPIRE (since Linux 2.6.8)
              Mark the mount as expired.  If a mount is not currently in use,
              then an initial call to umount2() with this flag fails with the
              error EAGAIN, but marks the mount as expired.  The mount remains
              expired as long as it isn't accessed by any process.  A second
              umount2() call specifying MNT_EXPIRE unmounts an expired mount.
              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
              programs that allow unprivileged users to unmount filesystems.

       On success, zero is returned.  On error, -1 is returned, and errno is set
       to indicate the error.

       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 target is locked; see mount_namespaces(7).

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

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

              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.

       MNT_DETACH and MNT_EXPIRE are available in glibc since version 2.11.

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

   umount() and shared mounts
       Shared mounts cause any mount activity on a mount, including umount()
       operations, to be forwarded to every shared mount 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 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 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).

       mount(2), mount_namespaces(7), path_resolution(7), mount(8), umount(8)

       This page is part of release 5.13 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

Linux                              2021-08-27                          UMOUNT(2)