.\" Copyright (C) 1993 Rickard E. Faith <faith@cs.unc.edu>
.\" and Copyright (C) 1994 Andries E. Brouwer <aeb@cwi.nl>
.\" and Copyright (C) 2002, 2005 Michael Kerrisk <mtk.manpages@gmail.com>
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.\" 2008-10-06, mtk: Created this as a new page by splitting
.\"     umount/umount2 material out of mount.2
.\"
.TH umount 2 2024-05-02 "Linux man-pages (unreleased)"
.SH NAME
umount, umount2 \- unmount filesystem
.SH LIBRARY
Standard C library
.RI ( libc ", " \-lc )
.SH SYNOPSIS
.nf
.B "#include <sys/mount.h>"
.P
.BI "int umount(const char *" target );
.BI "int umount2(const char *" target ", int " flags );
.fi
.SH DESCRIPTION
.BR umount ()
and
.BR umount2 ()
remove the attachment of the (topmost) filesystem mounted on
.IR target .
.\" Note: the kernel naming differs from the glibc naming
.\" umount2 is the glibc name for what the kernel now calls umount
.\" and umount is the glibc name for oldumount
.P
Appropriate privilege (Linux: the
.B CAP_SYS_ADMIN
capability) is required to unmount filesystems.
.P
Linux 2.1.116 added the
.BR umount2 ()
system call, which, like
.BR umount (),
unmounts a target, but allows additional
.I flags
controlling the behavior of the operation:
.TP
.BR 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,
.B 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).
.TP
.BR 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.
.TP
.BR 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
.BR umount2 ()
with this flag fails with the error
.BR EAGAIN ,
but marks the mount as expired.
The mount remains expired as long as it isn't accessed
by any process.
A second
.BR umount2 ()
call specifying
.B MNT_EXPIRE
unmounts an expired mount.
This flag cannot be specified with either
.B MNT_FORCE
or
.BR MNT_DETACH .
.TP
.BR UMOUNT_NOFOLLOW " (since Linux 2.6.34)"
.\" Later added to Linux 2.6.33-stable
Don't dereference
.I target
if it is a symbolic link.
This flag allows security problems to be avoided in set-user-ID-\fIroot\fP
programs that allow unprivileged users to unmount filesystems.
.SH RETURN VALUE
On success, zero is returned.
On error, \-1 is returned, and
.I errno
is set to indicate the error.
.SH 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.
.TP
.B EAGAIN
A call to
.BR umount2 ()
specifying
.B MNT_EXPIRE
successfully marked an unbusy filesystem as expired.
.TP
.B EBUSY
.I target
could not be unmounted because it is busy.
.TP
.B EFAULT
.I target
points outside the user address space.
.TP
.B EINVAL
.I target
is not a mount point.
.TP
.B EINVAL
.I target
is locked; see
.BR mount_namespaces (7).
.TP
.B EINVAL
.BR umount2 ()
was called with
.B MNT_EXPIRE
and either
.B MNT_DETACH
or
.BR MNT_FORCE .
.TP
.BR EINVAL " (since Linux 2.6.34)"
.BR umount2 ()
was called with an invalid flag value in
.IR flags .
.TP
.B ENAMETOOLONG
A pathname was longer than
.BR MAXPATHLEN .
.TP
.B ENOENT
A pathname was empty or had a nonexistent component.
.TP
.B ENOMEM
The kernel could not allocate a free page to copy filenames or data into.
.TP
.B EPERM
The caller does not have the required privileges.
.SH STANDARDS
Linux.
.SH HISTORY
.B MNT_DETACH
and
.B MNT_EXPIRE
.\" http://sourceware.org/bugzilla/show_bug.cgi?id=10092
are available since glibc 2.11.
.P
The original
.BR umount ()
function was called as \fIumount(device)\fP and would return
.B ENOTBLK
when called with something other than a block device.
In Linux 0.98p4, a call \fIumount(dir)\fP was added, in order to
support anonymous devices.
In Linux 2.3.99-pre7, the call \fIumount(device)\fP was removed,
leaving only \fIumount(dir)\fP (since now devices can be mounted
in more than one place, so specifying the device does not suffice).
.SH NOTES
.SS umount() and shared mounts
Shared mounts cause any mount activity on a mount, including
.BR umount ()
operations, to be forwarded to every shared mount in the
peer group and every slave mount of that peer group.
This means that
.BR 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.
.P
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
.B MNT_DETACH
will cause every mount in the mount namespace to be lazily unmounted.
.P
To ensure
.BR umount ()
does not propagate in this fashion,
the mount may be remounted using a
.BR mount (2)
call with a
.I mount_flags
argument that includes both
.B MS_REC
and
.B MS_PRIVATE
prior to
.BR umount ()
being called.
.SH SEE ALSO
.BR mount (2),
.BR mount_namespaces (7),
.BR path_resolution (7),
.BR mount (8),
.BR umount (8)