umount, umount2 - unmount filesystem
Standard C library (
libc,
-lc)
#include <sys/mount.h>
int umount(const char *target);
int umount2(const char *target, int flags);
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.
On success, zero is returned. On error, -1 is returned, and
errno is set
to indicate the error.
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.
MNT_DETACH and
MNT_EXPIRE are available since glibc 2.11.
These functions are Linux-specific and should not be used in programs intended
to be portable.
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.
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).
mount(2),
mount_namespaces(7),
path_resolution(7),
mount(8),
umount(8)