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)
- 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_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
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
- 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 into.
- 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.
umount() and shared mount points¶
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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