umount, umount2 - unmount filesystem
int umount(const char *target);
int umount2(const char *target, int flags);
() 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
(), 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 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_EXPIRE unmounts an expired
mount point. This flag cannot be specified with either MNT_FORCE or
- 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
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.
- 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
- 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
- The caller does not have the required privileges.
are available in glibc since version
These functions are Linux-specific and should not be used in programs intended
to be portable.
Shared mount points cause any mount activity on a mount point, including
() operations, to be forwarded to every shared mount point in the
peer group and every slave mount of that peer group. This means that
() 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_DETACH
every mount in the mount namespace to be lazily unmounted.
To ensure umount
() does not propagate in this fashion, the mount point
may be remounted using a mount
() call with a mount_flags
argument that includes both MS_REC
() being called.
The original umount
() function was called as umount(device)
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
was removed, leaving only umount(dir)
devices can be mounted in more than one place, so specifying the device does
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