Manpage of UMOUNT

UMOUNT

Section: Linux Programmer's Manual (2)
Updated: 2015-03-29
Index
 

NAME

umount, umount2 - unmount filesystem  

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_ADMINcapability) is required to unmount filesystems.

Linux 2.1.116 added the umount2() system call, which, like umount(), unmounts a target, but allows additional flagscontrolling 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, 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 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_EXPIREunmounts an expired mount point. This flag cannot be specified with either MNT_FORCEor MNT_DETACH.
UMOUNT_NOFOLLOW (since Linux 2.6.34)
Don't dereference targetif it is a symbolic link. This flag allows security problems to be avoided in set-user-ID-rootprograms that allow unprivileged users to unmount filesystems.
 

RETURN VALUE

On success, zero is returned. On error, -1 is returned, and errnois set appropriately.  

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_EXPIREsuccessfully marked an unbusy filesystem as expired.
EBUSY
targetcould not be unmounted because it is busy.
EFAULT
targetpoints outside the user address space.
EINVAL
targetis not a mount point.
EINVAL
umount2() was called with MNT_EXPIREand either MNT_DETACHor 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.
 

VERSIONS

MNT_DETACHand MNT_EXPIREare 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

 

umount() and shared mount points

Shared mount points cause any mount activity on a mount point, including umount(2) operations, to be forwarded to every shared mount point in the peer group and every slave mount of that peer group. This means that umount(2) 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 point 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_DETACHwill cause every mount in the mount namespace to be lazily unmounted.

To ensure umount(2) does not propagate in this fashion, the mount point may be remounted using a mount(2) call with a mount_flagsargument that includes both MS_RECand MS_PRIVATEprior to umount(2) being called.  

Historical details

The original umount() function was called as umount(device) and would return ENOTBLKwhen 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).  

SEE ALSO

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


 

Index

NAME
SYNOPSIS
DESCRIPTION
RETURN VALUE
ERRORS
VERSIONS
CONFORMING TO
NOTES
umount() and shared mount points
Historical details
SEE ALSO

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Time: 22:27:43 GMT, June 20, 2016