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.\" 2008-10-06, mtk: Created this as a new page by splitting

.\"     umount/umount2 material out of mount.2

.\"

.TH UMOUNT 2 2017-09-15 "Linux" "Linux Programmer's Manual"

.SH NAME

umount, umount2 \- unmount filesystem

.SH SYNOPSIS

.nf

.B "#include <sys/mount.h>"

.PP

.BI "int umount(const char *" target );

.PP

.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

.PP

Appropriate privilege (Linux: the

.B CAP_SYS_ADMIN

capability) is required to unmount filesystems.

.PP

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,

.BR 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 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.

.TP

.BR 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

.BR umount2 ()

with this flag fails with the error

.BR 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

.BR umount2 ()

call specifying

.B MNT_EXPIRE

unmounts an expired mount point.

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 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 appropriately.

.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

.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 VERSIONS

.BR MNT_DETACH

and

.BR MNT_EXPIRE

.\" http://sourceware.org/bugzilla/show_bug.cgi?id=10092

are available in glibc since version 2.11.

.SH CONFORMING TO

These functions are Linux-specific and should not be used in

programs intended to be portable.

.SH NOTES

.SS umount() and shared mount points

Shared mount points cause any mount activity on a mount point, including

.BR 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

.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.

.PP

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

.BR MNT_DETACH

will cause every mount in the mount namespace to be lazily unmounted.

.PP

To ensure

.BR umount ()

does not propagate in this fashion,

the mount point may be remounted using a

.BR mount ()

call with a

.I mount_flags

argument that includes both

.BR MS_REC

and

.BR MS_PRIVATE

prior to

.BR umount ()

being called.

.SS Historical details

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 SEE ALSO

.BR mount (2),

.BR mount_namespaces (7),

.BR path_resolution (7),

.BR mount (8),

.BR umount (8)

.SH COLOPHON

This page is part of release 4.15 of the Linux

.I 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/.