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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) 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:

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).
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.
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.
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 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.

A call to umount2() specifying MNT_EXPIRE successfully marked an unbusy filesystem as expired.
target could not be unmounted because it is busy.
target points outside the user address space.
target is not a mount point.
target is locked; see mount_namespaces(7).
umount2() was called with MNT_EXPIRE and either MNT_DETACH or MNT_FORCE.
umount2() was called with an invalid flag value in flags.
A pathname was longer than MAXPATHLEN.
A pathname was empty or had a nonexistent component.
The kernel could not allocate a free page to copy filenames or data into.
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 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.

SEE ALSO

mount(2), mount_namespaces(7), path_resolution(7), mount(8), umount(8)

2024-05-02 Linux man-pages (unreleased)