MOUNT
Section: Linux Programmer's Manual (2)
Updated: 2021-03-22
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NAME
mount - mount filesystem
SYNOPSIS
#include <sys/mount.h>
int mount(const char *source, const char *target,
const char *filesystemtype, unsigned long mountflags,
const void *data);
DESCRIPTION
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.
Typically it is a string of comma-separated options
understood by this filesystem.
See
mount(8)
for details of the options available for each filesystem type.
This argument may be specified as NULL, if there are no options.
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
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
MS_MOVE.
- *
-
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 described below.
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 using
chattr(1).)
- 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:
-
- (a)
-
the inode needs to be updated for some change unrelated to file timestamps;
- (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 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.)
- MS_MANDLOCK
-
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
CONFIG_MANDATORY_FILE_LOCKING
option.
- MS_NOATIME
-
Do not update access times for (all types of) files on this filesystem.
- MS_NODEV
-
Do not allow access to devices (special files) on this filesystem.
- MS_NODIRATIME
-
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.
- MS_NOEXEC
-
Do not allow programs to be executed from this filesystem.
- MS_NOSUID
-
Do not honor set-user-ID and set-group-ID bits or file capabilities
when executing programs from this filesystem.
- MS_RDONLY
-
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 meaning.
- 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.
- MS_SYNCHRONOUS
-
Make writes on this filesystem synchronous (as though
the
O_SYNC
flag to
open(2)
was specified for all file opens to this filesystem).
- MS_NOSYMFOLLOW (since Linux 5.10)
-
Do not follow symbolic links when resolving paths.
Symbolic links can still be created,
and
readlink(1),
readlink(2),
realpath(1),
and
realpath(3)
all still work properly.
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.
- *
-
Additionally, since Linux 2.6.16:
MS_NOATIME
and
MS_NODIRATIME.
- *
-
Additionally, since Linux 2.6.20:
MS_RELATIME.
The following flags are per-superblock:
MS_DIRSYNC,
MS_LAZYTIME,
MS_MANDLOCK,
MS_SILENT,
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 the filesystem.
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.
Remounting an existing mount
An existing mount may be remounted by specifying
MS_REMOUNT
in
mountflags.
This allows you to change the
mountflags
and
data
of an existing 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.
The following
mountflags
can be changed:
MS_LAZYTIME,
MS_MANDLOCK,
MS_NOATIME,
MS_NODEV,
MS_NODIRATIME,
MS_NOEXEC,
MS_NOSUID,
MS_RELATIME,
MS_RDONLY,
MS_STRICTATIME
(whose effect is to clear the
MS_NOATIME
and
MS_RELATIME
flags),
and
MS_SYNCHRONOUS.
Attempts to change the setting of the
MS_DIRSYNC
and
MS_SILENT
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,
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
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
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 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)
jails.
The
filesystemtype
and
data
arguments are ignored.
The remaining bits (other than
MS_REC,
described below) in the
mountflags
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
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 other flags that can be specified while changing
the propagation type are
MS_REC
(described below) and
MS_SILENT
(which is ignored).
The
source,
filesystemtype,
and
data
arguments are ignored.
The meanings of the propagation type flags are as follows:
- MS_SHARED
-
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.
- MS_PRIVATE
-
Make this mount point private.
Mount and unmount events do not propagate into or out of this mount point.
- MS_SLAVE
-
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 member.
- MS_UNBINDABLE
-
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
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_namespaces(7).
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
target
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
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
specifies 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.
RETURN VALUE
On success, zero is returned.
On error, -1 is returned, and
errno
is set to indicate the error.
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 Linux kernel source code for details.
- EACCES
-
A component of a path was not searchable.
(See also
path_resolution(7).)
- EACCES
-
Mounting a read-only filesystem was attempted without giving the
MS_RDONLY
flag.
-
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 fixed).
-
Some filesystems instead return the error
EROFS
on an attempt to mount a read-only filesystem.
- 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 space.
- 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 the mount tree under
source
includes unbindable mounts and
target
is a mount point that has propagation type
MS_SHARED.
- EINVAL
-
A move operation
(MS_MOVE)
was attempted, but the parent mount of
source
mount has propagation type
MS_SHARED.
- EINVAL
-
A move operation
(MS_MOVE)
was attempted, but
source
was not a mount point, or was '/'.
- EINVAL
-
A bind operation
(MS_BIND)
was requested where
source
referred a mount namespace magic link (i.e., a
/proc/[pid]/ns/mnt
magic link or a bind mount to such a link)
and the propagation type of the parent mount of
target
was
MS_SHARED,
but propagation of the requested bind mount could lead to a circular
dependency that might prevent the mount namespace from ever being freed.
- 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
MS_SILENT.
- 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 bound.
- ELOOP
-
Too many links encountered during pathname resolution.
- ELOOP
-
A move operation was attempted, and
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
-
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.
- EROFS
-
Mounting a read-only filesystem was attempted without giving the
MS_RDONLY
flag.
See
EACCES,
above.
VERSIONS
The definitions of
MS_DIRSYNC,
MS_MOVE,
MS_PRIVATE,
MS_REC,
MS_RELATIME,
MS_SHARED,
MS_SLAVE,
MS_STRICTATIME,
and
MS_UNBINDABLE
were added to glibc headers in version 2.12.
CONFORMING TO
This function is Linux-specific and should not be used in
programs intended to be portable.
NOTES
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.
Mount namespaces
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(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 initialized 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 processes (except child processes that the caller
subsequently creates) and vice versa.
For further details on mount namespaces, see
mount_namespaces(7).
Parental relationship between mount points
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
/proc/[pid]/mountinfo
file (see below).
/proc/[pid]/mounts and /proc/[pid]/mountinfo
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
/proc/[pid]/mountinfo
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
proc(5)
and
mount_namespaces(7)
for details of this file.
SEE ALSO
mountpoint(1),
chroot(2),
ioctl_iflags(2),
pivot_root(2),
umount(2),
mount_namespaces(7),
path_resolution(7),
findmnt(8),
lsblk(8),
mount(8),
umount(8)
COLOPHON
This page is part of release 5.11 of the Linux
man-pages
project.
A description of the project,
information about reporting bugs,
and the latest version of this page,
can be found at
https://www.kernel.org/doc/man-pages/.
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- Additional mount flags
-
- Remounting an existing mount
-
- Creating a bind mount
-
- Changing the propagation type of an existing mount
-
- Moving a mount
-
- Creating a new mount point
-
- RETURN VALUE
-
- ERRORS
-
- VERSIONS
-
- CONFORMING TO
-
- NOTES
-
- Mount namespaces
-
- Parental relationship between mount points
-
- /proc/[pid]/mounts and /proc/[pid]/mountinfo
-
- SEE ALSO
-
- COLOPHON
-
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Time: 06:22:43 GMT, May 09, 2021