MOUNT(2) Linux Programmer's Manual MOUNT(2)
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 target.
Only the super-user may mount and unmount filesystems. Since Linux 2.4 a single filesys-
tem 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:
(Linux 2.4 onwards) Perform a bind mount, making a file or a directory subtree vis-
ible 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.
Permit mandatory locking on files in this file system. (Mandatory locking must
still be enabled on a per-file basis, as described in fcntl(2).)
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 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-UID and set-GID bits when executing programs from this file sys-
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 mounts.)
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
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 filesys-
tem type may have its own special errors and its own special behavior. See the kernel
source code for details.
EPERM The user is not the super-user.
ENODEV Filesystemtype not configured in the kernel.
Source is not a block device (and a device was required).
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 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.
EINVAL 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
The second argument, or a prefix of the first argument, is not a directory.
EFAULT One of the pointer arguments points outside the user address space.
ENOMEM The kernel could not allocate a free page to copy filenames or data into.
A pathname was longer than MAXPATHLEN.
ENOENT A pathname was empty or had a nonexistent component.
ELOOP Too many link encountered during pathname resolution. Or, a move was attempted,
while target is a descendant of source.
EACCES A component of a path was not searchable.
Or, mounting a read-only filesystem was attempted without giving the MS_RDONLY
Or, the block device Source is located on a filesystem mounted with the MS_NODEV
ENXIO The major number of the block device source is out of range.
EMFILE (In case no block device is required:) Table of dummy devices is full.
These functions are Linux-specific and should not be used in programs intended to be por-
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.
Linux 2.5 2002-06-11 MOUNT(2)