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MOUNT(2) Linux Programmer's Manual MOUNT(2)
mount - mount filesystem
#include <sys/mount.h>
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.
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.
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 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.
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).
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
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. 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, 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 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 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 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 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:
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 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
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.
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 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.
EACCES The block device source is located on a filesystem mounted
with the MS_NODEV option.
EBUSY source is already mounted.
EBUSY source cannot be remounted read-only, because it still holds
files open for writing.
EBUSY 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.).
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 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
MS_SILENT.
EINVAL An attempt was made to bind mount an unbindable mount.
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.
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.
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.
Per-process namespaces
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 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.
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)
This page is part of release 4.12 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/.
Linux 2017-07-13 MOUNT(2)
Pages that refer to this page: access(2), execve(2), fcntl(2), ioctl_iflags(2), mkdir(2), mknod(2), open(2), statfs(2), syscalls(2), umount(2), statvfs(3), systemd.exec(5), capabilities(7), cgroups(7), inode(7), mount_namespaces(7), namespaces(7), user_namespaces(7), mount(8)