From Linux Man Pages
mount, umount - mount and unmount filesystems
SYNOPSIS
#include <sys/mount.h>
int mount(const char *source, const char *target, const char *filesys-
temtype, unsigned long mountflags, const void *data);
int umount(const char *target);
int umount2(const char *target, int flags);
DESCRIPTION
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 direc-
tory 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 loaded.
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:
MS_BIND
(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 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 chattr(8).)
MS_MANDLOCK
Permit mandatory locking on files in this file system. (Manda-
tory locking must still be enabled on a per-file basis, as
described in fcntl(2).)
MS_MOVE
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.
MS_NOATIME
Do not update access times for (all types of) files on this file
system.
MS_NODEV
Do not allow access to devices (special files) on this file sys-
tem.
MS_NODIRATIME
Do not update access times for directories on this file system.
MS_NOEXEC
Do not allow programs to be executed from this file system.
MS_NOSUID
Do not honour set-user-ID and set-group-ID bits when executing
programs from this file system.
MS_RDONLY
Mount file system read-only.
MS_REMOUNT
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.
MS_SYNCHRONOUS
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. Typi-
cally 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:
MNT_FORCE (since Linux 2.6.116)
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 cur-
rently 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.
RETURN VALUE
On success, zero is returned. On error, -1 is returned, and errno is
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 kernel source code for details.
EACCES A component of a path was not searchable. (See also path_resolu-
tion(2).) Or, mounting a read-only filesystem was attempted
without giving the MS_RDONLY flag. Or, the block device source
is located on a filesystem mounted with the MS_NODEV option.
EAGAIN A call to umount2() specifying MNT_EXPIRE successfully marked an
unbusy file system as expired.
EBUSY 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 work-
ing directory of some task, the mount point of another device,
has open files, etc.). Or, it could not be unmounted because it
is busy.
EFAULT One of the pointer arguments points outside the user address
space.
EINVAL 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. Or, umount2() was called with MNT_FORCE and
either MNT_DETACH or MNT_FORCE.
ELOOP Too many link encountered during pathname resolution. Or, a
move was attempted, while target is a descendant of source.
EMFILE (In case no block device is required:) Table of dummy devices is
full.
ENAMETOOLONG
A pathname was longer than MAXPATHLEN.
ENODEV Filesystemtype not configured in the kernel.
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.
ENOTBLK
source is not a block device (and a device was required).
ENOTDIR
The second argument, or a prefix of the first argument, is not a
directory.
ENXIO The major number of the block device source is out of range.
EPERM The caller does not have the required privileges.
CONFORMING TO
These functions are Linux-specific and should not be used in programs
intended to be portable.
HISTORY
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.
RELATED
path_resolution(2), mount(8), umount(8)
CATEGORY