umount() - Unix, Linux System Call
mount, umount - mount and unmount filesystems
int mount(const char *source, const char *target,
const char *filesystemtype, unsigned long mountflags,
const void *data);
int umount(const char *target);
int umount2(const char *target, int flags);
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
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
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
filesystemtype, mountflags, and data arguments are ignored.
MS_DIRSYNC (since Linux 2.5.19) ||
Make directory changes on this file system synchronous.
(This property can be obtained for individual directories
or subtrees using
Permit mandatory locking on files in this file system.
(Mandatory locking must still be enabled on a per-file basis,
as described in
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.
filesystemtype, mountflags, and data arguments are ignored.
Do not update access times for (all types of) files on this file system.
Do not allow access to devices (special files) on this file system.
Do not update access times for directories on this file system.
Do not allow programs to be executed from this file system.
Do not honour set-user-ID and set-group-ID bits when executing
programs from this file system.
Mount file system read-only.
Remount an existing mount. This is allows you to change the
data of an existing mount without having to unmount and remount the file system.
target should be the same values specified in the initial
filesystemtype is ignored.
mountflags can be changed:
MS_RDONLY, MS_SYNCHRONOUS, MS_MANDLOCK; before kernel 2.6.16, the following could also be changed:
MS_NODIRATIME; and, additionally, before kernel 2.4, the following could also be changed:
MS_NOSUID, MS_NODEV, MS_NOEXEC.
Make writes on this file system synchronous (as though
O_SYNC flag to
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.
From kernel 2.6.16 onwards,
MS_NODIRATIME are also settable on a per-mount-point basis.
data argument is interpreted by the different file systems.
Typically it is a string of comma-separated options
understood by this file system.
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:
MNT_FORCE (since Linux 2.1.116) ||
Force unmount even if busy.
(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 isnt accessed
by any process.
umount2() call specifying
MNT_EXPIRE unmounts an expired mount point.
This flag cannot be specified with either
On success, zero is returned. On error, -1 is returned, and errno is set appropriately.
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
A call to
MNT_EXPIRE successfully marked an unbusy file system as expired.
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 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 busy.
One of the pointer arguments points outside the user address space.
source had an invalid superblock.
Or, a remount
(MS_REMOUNT) was attempted, but
source was not already mounted on
target. Or, a move
(MS_MOVE) was attempted, but
source was not a mount point, or was /.
Or, an unmount was attempted, but
target was not a mount point.
umount2() was called with
MNT_EXPIRE and either
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 required).
The second argument, or a prefix of the first argument, is not
The major number of the block device
source is out of range.
The caller does not have the required privileges.
These functions are Linux-specific and should not be used in
programs intended to be portable.
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-user-ID or set-group-ID program
on a filesystem mounted with MS_NOSUID would fail with
EPERM. Since Linux 2.4 the set-user-ID and set-group-ID bits are
just silently ignored in this case.