MOUNT(2) Linux Programmer's Manual MOUNT(2)
mount - mount filesystem
int mount(const char *source, const char *target,
const char *filesystemtype, unsigned long mountflags,
const void *data);
mount() attaches the filesystem specified by source (which is often a
pathname referring to a device, but can also be the pathname of a
directory or file, or a dummy string) to the location (a directory or
file) specified by the pathname in target.
Appropriate privilege (Linux: the CAP_SYS_ADMIN capability) is required
to mount filesystems.
Values for the filesystemtype argument supported by the kernel are
listed in /proc/filesystems (e.g., "btrfs", "ext4", "jfs", "xfs",
"vfat", "fuse", "tmpfs", "cgroup", "proc", "mqueue", "nfs", "cifs",
"iso9660"). Further types may become available when the appropriate
modules are loaded.
The data argument is interpreted by the different filesystems. Typi-
cally it is a string of comma-separated options understood by this
filesystem. See mount(8) for details of the options available for each
A call to mount() performs one of a number of general types of opera-
tion, depending on the bits specified in mountflags. The choice of
which operation to perform is determined by testing the bits set in
mountflags, with the tests being conducted in the order listed here:
* Remount an existing mount: mountflags includes MS_REMOUNT.
* Create a bind mount: mountflags includes MS_BIND.
* Change the propagation type of an existing mount: mountflags
includes one of MS_SHARED, MS_PRIVATE, MS_SLAVE, or MS_UNBINDABLE.
* Move an existing mount to a new location: mountflags includes
* Create a new mount: mountflags includes none of the above flags.
Each of these operations is detailed later in this page. Further flags
may be specified in mountflags to modify the behavior of mount(), as
Additional mount flags
The list below describes the additional flags that can be specified in
mountflags. Note that some operation types ignore some or all of these
flags, as described later in this page.
MS_DIRSYNC (since Linux 2.5.19)
Make directory changes on this filesystem synchronous. (This
property can be obtained for individual directories or subtrees
MS_LAZYTIME (since Linux 4.0)
Reduce on-disk updates of inode timestamps (atime, mtime, ctime)
by maintaining these changes only in memory. The on-disk time-
stamps are updated only when:
(a) the inode needs to be updated for some change unrelated to
(b) the application employs fsync(2), syncfs(2), or sync(2);
(c) an undeleted inode is evicted from memory; or
(d) more than 24 hours have passed since the inode was written
This mount option significantly reduces writes needed to update
the inode's timestamps, especially mtime and atime. However, in
the event of a system crash, the atime and mtime fields on disk
might be out of date by up to 24 hours.
Examples of workloads where this option could be of significant benefit
include frequent random writes to preallocated files, as well as cases
where the MS_STRICTATIME mount option is also enabled. (The advantage
of combining MS_STRICTATIME and MS_LAZYTIME is that stat(2) will return
the correctly updated atime, but the atime updates will be flushed to
disk only in the cases listed above.)
Permit mandatory locking on files in this filesystem. (Manda-
tory locking must still be enabled on a per-file basis, as
described in fcntl(2).) Since Linux 4.5, this mount option
requires the CAP_SYS_ADMIN capability.
Do not update access times for (all types of) files on this
Do not allow access to devices (special files) on this filesys-
Do not update access times for directories on this filesystem.
This flag provides a subset of the functionality provided by
MS_NOATIME; that is, MS_NOATIME implies MS_NODIRATIME.
Do not allow programs to be executed from this filesystem.
Do not honor set-user-ID and set-group-ID bits or file capabili-
ties when executing programs from this filesystem.
Mount filesystem read-only.
MS_REC (since Linux 2.4.11)
Used in conjunction with MS_BIND to create a recursive bind
mount, and in conjunction with the propagation type flags to
recursively change the propagation type of all of the mounts in
a subtree. See below for further details.
MS_RELATIME (since Linux 2.6.20)
When a file on this filesystem is accessed, update the file's
last access time (atime) only if the current value of atime is
less than or equal to the file's last modification time (mtime)
or last status change time (ctime). This option is useful for
programs, such as mutt(1), that need to know when a file has
been read since it was last modified. Since Linux 2.6.30, the
kernel defaults to the behavior provided by this flag (unless
MS_NOATIME was specified), and the MS_STRICTATIME flag is
required to obtain traditional semantics. In addition, since
Linux 2.6.30, the file's last access time is always updated if
it is more than 1 day old.
MS_SILENT (since Linux 2.6.17)
Suppress the display of certain (printk()) warning messages in
the kernel log. This flag supersedes the misnamed and obsolete
MS_VERBOSE flag (available since Linux 2.4.12), which has the
MS_STRICTATIME (since Linux 2.6.30)
Always update the last access time (atime) when files on this
filesystem are accessed. (This was the default behavior before
Linux 2.6.30.) Specifying this flag overrides the effect of
setting the MS_NOATIME and MS_RELATIME flags.
Make writes on this filesystem synchronous (as though the O_SYNC
flag to open(2) was specified for all file opens to this
From Linux 2.4 onward, the MS_NODEV, MS_NOEXEC, and MS_NOSUID flags are
settable on a per-mount-point basis. From kernel 2.6.16 onward,
MS_NOATIME and MS_NODIRATIME are also settable on a per-mount-point
basis. The MS_RELATIME flag is also settable on a per-mount-point
basis. Since Linux 2.6.16, MS_RDONLY can be set or cleared on a per-
mount-point basis as well as on the underlying filesystem. The mounted
filesystem will be writable only if neither the filesystem nor the
mountpoint are flagged as read-only.
Remounting an existing mount
An existing mount may be remounted by specifying MS_REMOUNT in mount-
flags. This allows you to change the mountflags and data of an exist-
ing mount without having to unmount and remount the filesystem. target
should be the same value specified in the initial mount() call.
The source and filesystemtype arguments are ignored.
The mountflags and data arguments should match the values used in the
original mount() call, except for those parameters that are being
deliberately changed. Another exception is that MS_BIND has a differ-
ent meaning for remount, and it should be included only if explicitly
The following mountflags can be changed: MS_LAZYTIME, MS_MANDLOCK,
MS_NOATIME, MS_NODEV, MS_NODIRATIME, MS_NOEXEC, MS_NOSUID, MS_RELATIME,
MS_RDONLY, and MS_SYNCHRONOUS. Attempts to change the setting of the
MS_DIRSYNC flag during a remount are silently ignored.
Since Linux 3.17, if none of MS_NOATIME, MS_NODIRATIME, MS_RELATIME, or
MS_STRICTATIME is specified in mountflags, then the remount operation
preserves the existing values of these flags (rather than defaulting to
Since Linux 2.6.26, this flag can be used with MS_BIND to modify only
the per-mount-point flags. This is particularly useful for setting or
clearing the "read-only" flag on a mount point without changing the
underlying filesystem. Specifying mountflags as:
MS_REMOUNT | MS_BIND | MS_RDONLY
will make access through this mountpoint read-only, without affecting
other mount points.
Creating a bind mount
If mountflags includes MS_BIND (available since Linux 2.4), then per-
form a bind mount. A bind mount makes a file or a directory subtree
visible at another point within the single directory hierarchy. Bind
mounts may cross filesystem boundaries and span chroot(2) jails.
The filesystemtype and data arguments are ignored.
The remaining bits in the mountflags argument are also ignored, with
the exception of MS_REC. (The bind mount has the same mount options as
the underlying mount point.) However, see the discussion of remounting
above, for a method of making an existing bind mount read-only.
By default, when a directory is bind mounted, only that directory is
mounted; if there are any submounts under the directory tree, they are
not bind mounted. If the MS_REC flag is also specified, then a recur-
sive bind mount operation is performed: all submounts under the source
subtree (other than unbindable mounts) are also bind mounted at the
corresponding location in the target subtree.
Changing the propagation type of an existing mount
If mountflags includes one of MS_SHARED, MS_PRIVATE, MS_SLAVE, or
MS_UNBINDABLE (all available since Linux 2.6.15), then the propagation
type of an existing mount is changed. If more than one of these flags
is specified, an error results.
The only flags that can be used with changing the propagation type are
MS_REC and MS_SILENT.
The source, filesystemtype, and data arguments are ignored.
The meanings of the propagation type flags are as follows:
Make this mount point shared. Mount and unmount events immedi-
ately under this mount point will propagate to the other mount
points that are members of this mount's peer group. Propagation
here means that the same mount or unmount will automatically
occur under all of the other mount points in the peer group.
Conversely, mount and unmount events that take place under peer
mount points will propagate to this mount point.
Make this mount point private. Mount and unmount events do not
propagate into or out of this mount point.
If this is a shared mount point that is a member of a peer group
that contains other members, convert it to a slave mount. If
this is a shared mount point that is a member of a peer group
that contains no other members, convert it to a private mount.
Otherwise, the propagation type of the mount point is left
When a mount point is a slave, mount and unmount events propagate into
this mount point from the (master) shared peer group of which it was
formerly a member. Mount and unmount events under this mount point do
not propagate to any peer.
A mount point can be the slave of another peer group while at the same
time sharing mount and unmount events with a peer group of which it is
Make this mount unbindable. This is like a private mount, and
in addition this mount can't be bind mounted. When a recursive
bind mount (mount() with the MS_BIND and MS_REC flags) is per-
formed on a directory subtree, any bind mounts within the sub-
tree are automatically pruned (i.e., not replicated) when repli-
cating that subtree to produce the target subtree.
By default, changing the propagation type affects only the target mount
point. If the MS_REC flag is also specified in mountflags, then the
propagation type of all mount points under target is also changed.
For further details regarding mount propagation types (including the
default propagation type assigned to new mounts), see mount_names-
Moving a mount
If mountflags contains the flag MS_MOVE (available since Linux 2.4.18),
then move a subtree: source specifies an existing mount point and tar-
get specifies the new location to which that mount point is to be relo-
cated. The move is atomic: at no point is the subtree unmounted.
The remaining bits in the mountflags argument are ignored, as are the
filesystemtype and data arguments.
Creating a new mount point
If none of MS_REMOUNT, MS_BIND, MS_MOVE, MS_SHARED, MS_PRIVATE,
MS_SLAVE, or MS_UNBINDABLE is specified in mountflags, then mount()
performs its default action: creating a new mount point. source speci-
fies the source for the new mount point, and target specifies the
directory at which to create the mount point.
The filesystemtype and data arguments are employed, and further bits
may be specified in mountflags to modify the behavior of the call.
On success, zero is returned. On error, -1 is returned, and errno is
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.
EACCES A component of a path was not searchable. (See also path_reso-
EACCES Mounting a read-only filesystem was attempted without giving the
EACCES The block device source is located on a filesystem mounted with
the MS_NODEV option.
EBUSY An attempt was made to stack a new mount directly on top of an
existing mount point that was created in this mount namespace
with the same source and target.
EBUSY source cannot be remounted read-only, because it still holds
files open for writing.
EFAULT One of the pointer arguments points outside the user address
EINVAL source had an invalid superblock.
EINVAL A remount operation (MS_REMOUNT) was attempted, but source was
not already mounted on target.
EINVAL A move operation (MS_MOVE) was attempted, but source was not a
mount point, or was '/'.
EINVAL mountflags includes more than one of MS_SHARED, MS_PRIVATE,
MS_SLAVE, or MS_UNBINDABLE.
EINVAL mountflags includes MS_SHARED, MS_PRIVATE, MS_SLAVE, or
MS_UNBINDABLE and also includes a flag other than MS_REC or
EINVAL An attempt was made to bind mount an unbindable mount.
EINVAL In an unprivileged mount namespace (i.e., a mount namespace
owned by a user namespace that was created by an unprivileged
user), a bind mount operation (MS_BIND) was attempted without
specifying (MS_REC), which would have revealed the filesystem
tree underneath one of the submounts of the directory being
ELOOP Too many links encountered during pathname resolution.
ELOOP A move operation was attempted, and target is a descendant of
EMFILE (In case no block device is required:) Table of dummy devices is
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
source is not a block device (and a device was required).
target, or a prefix of source, 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.
The definitions of MS_DIRSYNC, MS_MOVE, MS_PRIVATE, MS_REC, MS_RELA-
TIME, MS_SHARED, MS_SLAVE, MS_STRICTATIME and MS_UNBINDABLE were added
to glibc headers in version 2.12.
This function is Linux-specific and should not be used in programs
intended to be portable.
Since Linux 2.4 a single filesystem can be mounted at multiple mount
points, and multiple mounts can be stacked on the same mount point.
The mountflags argument may have the magic number 0xC0ED (MS_MGC_VAL)
in the top 16 bits. (All of the other flags discussed in DESCRIPTION
occupy the low order 16 bits of mountflags.) Specifying MS_MGC_VAL was
required in kernel versions prior to 2.4, but since Linux 2.4 is no
longer required and is ignored if specified.
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.
Starting with kernel 2.4.19, Linux provides per-process mount names-
paces. A mount namespace is the set of filesystem mounts that are vis-
ible to a process. Mount-point namespaces can be (and usually are)
shared between multiple processes, and changes to the namespace (i.e.,
mounts and unmounts) by one process are visible to all other processes
sharing the same namespace. (The pre-2.4.19 Linux situation can be
considered as one in which a single namespace was shared by every
process on the system.)
A child process created by fork(2) shares its parent's mount namespace;
the mount namespace is preserved across an execve(2).
A process can obtain a private mount namespace if: it was created using
the clone(2) CLONE_NEWNS flag, in which case its new namespace is ini-
tialized to be a copy of the namespace of the process that called
clone(2); or it calls unshare(2) with the CLONE_NEWNS flag, which
causes the caller's mount namespace to obtain a private copy of the
namespace that it was previously sharing with other processes, so that
future mounts and unmounts by the caller are invisible to other pro-
cesses (except child processes that the caller subsequently creates)
and vice versa.
The Linux-specific /proc/[pid]/mounts file exposes the list of mount
points in the mount namespace of the process with the specified ID; see
proc(5) for details.
mountpoint(1), umount(2), mount_namespaces(7), path_resolution(7),
findmnt(8), lsblk(8), mount(8), umount(8)
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Linux 2018-02-02 MOUNT(2)