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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | STANDARDS | HISTORY | NOTES | SEE ALSO | COLOPHON |
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umount(2) System Calls Manual umount(2)
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.
Linux.
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).
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 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.
mount(2), mount_namespaces(7), path_resolution(7), mount(8),
umount(8)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.9.1.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2024-06-26. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
man-pages@man7.org
Linux man-pages 6.9.1 2024-05-02 umount(2)
Pages that refer to this page: mount(2), syscalls(2), proc_pid_mountinfo(5), capabilities(7), mount_namespaces(7), mount(8), umount(8)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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