mount attaches the filesystem specified by source (which is often
a device name, but can also be a directory name or a dummy) to the directory
specified by target.
umount and umount2 remove the attachment of the (topmost)
filesystem mounted on target.
Appropriate privilege (Linux: the CAP_SYS_ADMIN capability) is required
to mount and unmount filesystems.
Since Linux 2.4 a single filesystem can be visible at multiple mount points, and
multiple mounts can be stacked on the same mount point.
Values for the filesystemtype argument supported by the kernel are listed
in /proc/filesystems (like "minix", "ext2",
"msdos", "proc", "nfs", "iso9660"
etc.). Further types may become available when the appropriate modules are
The mountflags argument may have the magic number 0xC0ED (
MS_MGC_VAL) in the top 16 bits (this was required in kernel versions
prior to 2.4, but is no longer required and ignored if specified), and various
mount flags (as defined in <linux/fs.h> for libc4 and libc5 and in
<sys/mount.h> for glibc2) in the low order 16 bits:
(Linux 2.4 onwards) Perform a bind mount, making a file or
a directory subtree visible at another point within a file system. Bind
mounts may cross file system boundaries and span chroot(2) jails.
The filesystemtype, mountflags, and data arguments
(Since Linux 2.5.19.) Make directory changes on this file
system synchronous. (This property can be obtained for individual
directories or subtrees using chattr(8).)
Permit mandatory locking on files in this file system.
(Mandatory locking must still be enabled on a per-file basis, as described
Move a subtree. source specifies an existing mount
point and target specifies the new location. The move is atomic: at
no point is the subtree unmounted. The filesystemtype,
mountflags, and data arguments are ignored.
Do not update access times for (all types of) files on this
Do not allow access to devices (special files) on this file
Do not update access times for directories on this file
Do not allow programs to be executed from this file
Do not honour set-UID and set-GID bits when executing
programs from this file system.
Mount file system read-only.
Remount an existing mount. This is allows you to change the
mountflags and data of an existing mount without having to
unmount and remount the file system. source and target
should be the same values specified in the initial mount() call;
filesystemtype is ignored.
Make writes on this file system synchronous (as though the
O_SYNC flag to open(2) was specified for all file opens to
this file system).
From Linux 2.4 onwards, the MS_NODEV, MS_NOEXEC, and
MS_NOSUID flags are settable on a per-mount point basis.
The data argument is interpreted by the different file systems. Typically
it is a string of comma-separated options understood by this file system. See
mount(8) for details of the options available for each filesystem type.
Linux 2.1.116 added the umount2() system call, which, like
umount(), unmounts a target, but allows additional flags
controlling the behaviour of the operation:
Force unmount even if busy. (Since 2.1.116. Only for NFS
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. (Since 2.4.11.)
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 kernel source code for details.
A component of a path was not searchable. (See also
Or, mounting a read-only filesystem was attempted without giving the
Or, the block device source is located on a filesystem mounted with
the MS_NODEV option.
source is already mounted. Or, it cannot be
remounted read-only, because it still holds files open for writing. Or, it
cannot be mounted on target because target is still busy (it
is the working directory of some task, the mount point of another device,
has open files, etc.). Or, it could not be unmounted because it is
One of the pointer arguments points outside the user
source had an invalid superblock. Or, a remount was
attempted, while source was not already mounted on target.
Or, a move was attempted, while source was not a mount point, or
was '/'. Or, an umount was attempted, while target was not a mount
Too many link encountered during pathname resolution. Or, a
move was attempted, while target is a descendant of
(In case no block device is required:) Table of dummy
devices is full.
A pathname was longer than MAXPATHLEN.
Filesystemtype not configured in the kernel.
A pathname was empty or had a nonexistent component.
The kernel could not allocate a free page to copy filenames
or data into.
source is not a block device (and a device was
The second argument, or a prefix of the first argument, is
not a directory.
The major number of the block device source is out
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).
The original MS_SYNC flag was renamed MS_SYNCHRONOUS in 1.1.69 when a different
MS_SYNC was added to <mman.h>.
Before Linux 2.4 an attempt to execute a set-UID or set-GID program on a
filesystem mounted with MS_NOSUID would fail with EPERM. Since
Linux 2.4 the set-UID and set-GID bits are just silently ignored in this case.