umount2(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 filesystems.

RETURN VALUE         top

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

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

       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 2.11.

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 5.09 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                            2020-06-09                        UMOUNT(2)

Pages that refer to this page: syscalls(2)