UMOUNT
Section: Linux Programmer's Manual (2)
Updated: 2017-09-15
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NAME
umount, umount2 - unmount filesystem
SYNOPSIS
#include <sys/mount.h>
int umount(const char *target);
int umount2(const char *target, int flags);
DESCRIPTION
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
On success, zero is returned.
On error, -1 is returned, and
errno
is set appropriately.
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.
- 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.
- 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
MNT_DETACH
and
MNT_EXPIRE
are available in glibc since version 2.11.
CONFORMING TO
These functions are Linux-specific and should not be used in
programs intended to be portable.
NOTES
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()
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
mount(2),
mount_namespaces(7),
path_resolution(7),
mount(8),
umount(8)
COLOPHON
This page is part of release 4.13 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/.
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- RETURN VALUE
-
- ERRORS
-
- VERSIONS
-
- CONFORMING TO
-
- NOTES
-
- umount() and shared mount points
-
- Historical details
-
- SEE ALSO
-
- COLOPHON
-
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