umount(2) - Linux man page

Name

umount, umount2 - unmount file system

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) file system mounted on target.

Appropriate privilege (Linux: the CAP_SYS_ADMIN capability) is required to unmount file systems.

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): Force unmount even if busy. This can cause data loss. (Only for NFS mounts.)
MNT_DETACH (since Linux 2.4.11): Perform a lazy unmount: make the mount point unavailable for new accesses, 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 file systems.

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 file-system type independent errors. Each file system type may have its own special errors and its own special behavior. See the kernel source code for details.

EAGAIN: A call to umount2() specifying MNT_EXPIRE successfully marked an unbusy file system 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. Or, umount2() was called with MNT_EXPIRE and either MNT_DETACH or MNT_FORCE.
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 only 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

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).

Referenced By

capabilities(7), clone(2), eject(1), proc(5), syscalls(2)