mount - mount filesystem
int mount(const char *source, const char *target,
const char *filesystemtype, unsigned long mountflags,
const void *data);
() attaches the filesystem specified by source
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
(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.
argument is interpreted by the different filesystems. Typically
it is a string of comma-separated options understood by this filesystem. See
(8) for details of the options available for each filesystem type.
A call to mount
() performs one of a number of general types of operation,
depending on the bits specified in mountflags
. The choice of which
operation to perform is determined by testing the bits set in
, with the tests being conducted in the order listed here:
- Remount an existing mount: mountflags includes
- 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
- Move an existing mount to a new location: mountflags includes
- Create a new mount: mountflags includes none of the above
Each of these operations is detailed later in this page. Further flags may be
specified in mountflags
to modify the behavior of mount
The list below describes the additional flags that can be specified in
. 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 using
- 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 timestamps are
updated only when:
- the inode needs to be updated for some change unrelated to file
- the application employs fsync(2), syncfs(2), or
- an undeleted inode is evicted from memory; or
- more than 24 hours have passed since the inode was written to disk.
- 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. (Mandatory 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 and a kernel configured with the
- Do not update access times for (all types of) files on this
- Do not allow access to devices (special files) on this filesystem.
- 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 capabilities 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
- 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
- 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 same
- 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, some of the above flags are settable on a per-mount
basis, while others apply to the superblock of the mounted filesystem, meaning
that all mounts of the same filesystem share those flags. (Previously, all of
the flags were per-superblock.)
The per-mount-point flags are as follows:
- Since Linux 2.4: MS_NODEV, MS_NOEXEC, and MS_NOSUID
flags are settable on a per-mount-point basis.
- Since Linux 2.6.16: MS_NOATIME and MS_NODIRATIME.
- Since Linux 2.6.20: MS_RELATIME.
The following flags are per-superblock: MS_DIRSYNC
, and MS_SYNCHRONOUS
. The initial
settings of these flags are determined on the first mount of the filesystem,
and will be shared by all subsequent mounts of the same filesystem.
Subsequently, the settings of the flags can be changed via a remount operation
(see below). Such changes will be visible via all mount points associated with
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 superblock. The mounted
filesystem will be writable only if neither the filesystem nor the mountpoint
are flagged as read-only.
An existing mount may be remounted by specifying MS_REMOUNT
. This allows you to change the mountflags
of an existing mount without having to unmount and remount the
should be the same value specified in the initial
arguments are ignored.
arguments should match the values used in
the original mount
() call, except for those parameters that are being
The following mountflags
can be changed: MS_LAZYTIME
(whose effect is to clear the MS_NOATIME
flags), and MS_SYNCHRONOUS
. Attempts to change the
setting of the MS_DIRSYNC
flags during a remount
are silently ignored. Note that changes to per-superblock flags are visible
via all mount points of the associated filesystem (because the per-superblock
flags are shared by all mount points).
Since Linux 3.17, if none of MS_NOATIME
, or MS_STRICTATIME
is specified in
, then the remount operation preserves the existing values of
these flags (rather than defaulting to MS_RELATIME
Since Linux 2.6.26, the MS_REMOUNT
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
MS_REMOUNT | MS_BIND | MS_RDONLY
will make access through this mountpoint read-only, without affecting other
(available since Linux 2.4), then
perform 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
arguments are ignored.
The remaining bits (other than MS_REC
, described below) in the
argument are also ignored. (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 recursive bind mount
operation is performed: all submounts under the source
than unbindable mounts) are also bind mounted at the corresponding location in
includes one of MS_SHARED
, 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 other flags that can be specified while changing the propagation type
(described below) and MS_SILENT
(which is ignored).
, and data
arguments are ignored.
The meanings of the propagation type flags are as follows:
- Make this mount point shared. Mount and unmount events immediately 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 unchanged.
- 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 a
- 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
performed on a directory subtree, any unbindable mounts within the subtree
are automatically pruned (i.e., not replicated) when replicating that
subtree to produce the target subtree.
By default, changing the propagation type affects only the target
point. If the MS_REC
flag is also specified in mountflags
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_namespaces
contains the flag MS_MOVE
(available since Linux
2.4.18), then move a subtree: source
specifies an existing mount point
specifies the new location to which that mount point is to
be relocated. The move is atomic: at no point is the subtree unmounted.
The remaining bits in the mountflags
argument are ignored, as are the
If none of MS_REMOUNT
, or MS_UNBINDABLE
is specified in
, then mount
() performs its default action: creating a
new mount point. source
specifies the source for the new mount point,
specifies the directory at which to create the mount point.
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
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.
- A component of a path was not searchable. (See also
- Mounting a read-only filesystem was attempted without giving the
- The filesystem may be read-only for various reasons, including: it resides
on a read-only optical disk; it is resides on a device with a physical
switch that has been set to mark the device read-only; the filesystem
implementation was compiled with read-only support; or errors were
detected when initially mounting the filesystem, so that it was marked
read-only and can't be remounted as read-write (until the errors are
- Some filesystems instead return the error EROFS on an attempt to
mount a read-only filesystem.
- The block device source is located on a filesystem mounted with the
- 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.
- source cannot be remounted read-only, because it still holds files
open for writing.
- One of the pointer arguments points outside the user address space.
- source had an invalid superblock.
- A remount operation (MS_REMOUNT) was attempted, but source
was not already mounted on target.
- A move operation (MS_MOVE) was attempted, but the mount tree under
source includes unbindable mounts and target is a mount
point that has propagation type MS_SHARED.
- A move operation (MS_MOVE) was attempted, but the parent mount of
source mount has propagation type MS_SHARED.
- A move operation (MS_MOVE) was attempted, but source was not
a mount point, or was '/'.
- mountflags includes more than one of MS_SHARED,
MS_PRIVATE, MS_SLAVE, or MS_UNBINDABLE.
- mountflags includes MS_SHARED, MS_PRIVATE,
MS_SLAVE, or MS_UNBINDABLE and also includes a flag other
than MS_REC or MS_SILENT.
- An attempt was made to bind mount an unbindable mount.
- 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 bound.
- Too many links encountered during pathname resolution.
- A move operation was attempted, and target is a descendant of
- (In case no block device is required:) Table of dummy devices is
- 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
- source is not a block device (and a device was required).
- target, or a prefix of source, is not a directory.
- The major number of the block device source is out of range.
- The caller does not have the required privileges.
- Mounting a read-only filesystem was attempted without giving the
MS_RDONLY flag. See EACCES, above.
The definitions of MS_DIRSYNC
, and MS_UNBINDABLE
were added to glibc headers in
This function is Linux-specific and should not be used in programs intended to
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.
argument may have the magic number 0xC0ED (
) in the top 16 bits. (All of the other flags discussed in
DESCRIPTION occupy the low order 16 bits of mountflags
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
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
Linux 2.4 the set-user-ID and set-group-ID bits are just silently ignored in
Starting with kernel 2.4.19, Linux provides mount namespaces. A mount namespace
is the set of filesystem mounts that are visible to a process. Mount
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
A process can obtain a private mount namespace if: it was created using the
flag, in which case its new namespace is
initialized to be a copy
of the namespace of the process that called
(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 processes
(except child processes that the caller subsequently creates) and vice versa.
For further details on mount namespaces, see mount_namespaces
Each mount point has a parent mount point. The overall parental relationship of
all mount points defines the single directory hierarchy seen by the processes
within a mount namespace.
The parent of a new mount point is defined when the mount point is created. In
the usual case, the parent of a new mount is the mount point of the filesystem
containing the directory or file at which the new mount is attached. In the
case where a new mount is stacked on top of an existing mount, the parent of
the new mount is the previous mount that was stacked at that location.
The parental relationship between mount points can be discovered via the
file (see below).
The Linux-specific /proc/[pid]/mounts
file exposes the list of mount
points in the mount namespace of the process with the specified ID. The
file exposes even more information about mount
points, including the propagation type and mount ID information that makes it
possible to discover the parental relationship between mount points. See
(5) and mount_namespaces
(7) for details of this file.