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
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
- 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
- Create a new mount: mountflags includes none of the
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
- 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
- the application employs fsync(2), syncfs(2),
- an undeleted inode is evicted from memory; or
- 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
mount option is also enabled. (The advantage of
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
- Do not update access times for (all types of) files on this
- Do not allow access to devices (special files) on this
- 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
- Do not allow programs to be executed from this
- 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 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 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
From Linux 2.4 onward, the MS_NODEV
flags are settable on a per-mount-point basis. From kernel
2.6.16 onward, MS_NOATIME
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.
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
deliberately changed. Another exception is that MS_BIND
has a different
meaning for remount, and it should be included only if explicitly desired.
The following mountflags
can be changed: MS_LAZYTIME
. 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
, 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, 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
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(2)
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 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 flags that can be used with changing the propagation type are
, 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
- 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
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 member.
- 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 bind 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(7)
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 MS_RDONLY flag.
- The block device source is located on a filesystem
mounted with the MS_NODEV option.
- source is already mounted.
- source cannot be remounted read-only, because it
still holds files open for writing.
- source cannot be mounted on target because
target is still busy (it is the working directory of some thread,
the mount point of another device, has open files, etc.).
- One of the pointer arguments points outside the user
- 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
source was not a mount point, or was '/'.
- mountflags includes more than one of
MS_SHARED, MS_PRIVATE, MS_SLAVE, or
- 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.
- Too many links encountered during pathname resolution.
- A move operation was attempted, and target is a
descendant of source.
- (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
- target, or a prefix of source, is not a
- The major number of the block device source is out
- The caller does not have the required privileges.
The definitions of MS_DIRSYNC
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 per-process mount namespaces. A
mount namespace is the set of filesystem mounts that are visible 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
flag, in which case its new namespace is
initialized to be a copy
of the namespace of the process that called
; 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.
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
This page is part of release 4.14 of the Linux man-pages
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