umount(2) System Calls Manual umount(2)
NAME
umount, umount2 - unmount filesystem
LIBRARY
Standard C library (libc, -lc)
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) filesys-
tem 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 ac-
cesses, immediately disconnect the filesystem and all filesys-
tems 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_DE-
TACH.
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 pro-
grams that allow unprivileged users to unmount filesystems.
RETURN VALUE
On success, zero is returned. On error, -1 is returned, and errno is
set to indicate the error.
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 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.
STANDARDS
Linux.
HISTORY
MNT_DETACH and MNT_EXPIRE are available since glibc 2.11.
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).
NOTES
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 un-
mounted 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, re-
cursively bind mounting the root directory of the filesystem onto a
subdirectory and then later unmounting that subdirectory with MNT_DE-
TACH will cause every mount in the mount namespace to be lazily un-
mounted.
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 in-
cludes both MS_REC and MS_PRIVATE prior to umount() being called.
SEE ALSO
mount(2), mount_namespaces(7), path_resolution(7), mount(8), umount(8)
Linux man-pages 6.7 2023-10-31 umount(2)
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