umount(2) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | CONFORMING TO | NOTES | SEE ALSO | COLOPHON

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 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.

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 target is locked; see mount_namespaces(7).

       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         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 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).

SEE ALSO         top

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

COLOPHON         top

       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
       https://www.kernel.org/doc/man-pages/.

Linux                          2021-08-27                      UMOUNT(2)

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