umount(2) — Linux manual page


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

NAME         top

       umount, umount2 - unmount filesystem

SYNOPSIS         top

       #include <sys/mount.h>

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

DESCRIPTION         top

       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 point 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 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 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

RETURN VALUE         top

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

ERRORS         top

       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.

              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         top

       MNT_DETACH and MNT_EXPIRE are available in glibc since version

CONFORMING TO         top

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

NOTES         top

   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         top

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

COLOPHON         top

       This page is part of release 5.11 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                          2021-03-22                      UMOUNT(2)

Pages that refer to this page: mount(2)syscalls(2)proc(5)capabilities(7)mount_namespaces(7)mount(8)umount(8)