mount — mount filesystem
||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
Appropriate privilege (Linux: the
CAP_SYS_ADMIN capability) is required to
Values for the
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.
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
one of a number of general types of operation. depending on
the bits specified in
mountflags. The choice of
operation is determined by testing the bits set in
mountflags, with the tests
being conducted in the order listed here:
Remount an existing mount:
Create a bind mount:
Change the propagation type of an existing mount:
includes one of
Move an existing mount to a new location:
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
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:
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 However, in the event of a system crash, the atime and mtime fields on disk might be out of date by up to 24 hours.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
option is also enabled. (The advantage of combining
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
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 filesystem.
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,
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 further
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
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
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
Make writes on this filesystem synchronous (as
to open(2) was
specified for all file opens to this filesystem).
From Linux 2.4 onward, the
MS_NOSUID flags are settable on a
per-mount-point basis. From kernel 2.6.16 onward,
MS_NODIRATIME are also settable on a
per-mount-point basis. The
MS_RELATIME flag is also settable on a
An existing mount may be remounted by specifying
mountflags. This allows you
to change the
data of an existing mount
without having to unmount and remount the filesystem.
target should be
the same value specified in the initial
arguments are ignored.
should match the values used in the original
mount() call, except for those parameters
that are being deliberately changed.
mountflags can be changed:
MS_SYNCHRONOUS. Attempts to change the
setting of the
flag during a remount are silently ignored.
Since Linux 3.17, if none of
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 also be used to make
an existing bind mount read-only by specifying
MS_REMOUNT | MS_BIND | MS_RDONLY
Note that only the
MS_RDONLY setting of the bind mount can
be changed in this manner.
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.
data arguments are
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
includes one of
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
The only flags that can be used with changing the
propagation type are
data arguments are
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. This is the default propagation type for newly created mount points.
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.
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(2) with the
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
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, see mount_namespaces(7).
contains the flag
(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
If none of
MS_UNBINDABLE is specified in
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.
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
errno is set
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 path_resolution(7).)
Mounting a read-only filesystem was attempted
without giving the
The block device
source is located on a
filesystem mounted with the
cannot be remounted read-only, because it still holds
files open for writing.
cannot be mounted on
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 address space.
an invalid superblock.
A remount operation (
MS_REMOUNT) was attempted, but
source was not
already mounted on
A move operation (
MS_MOVE) was attempted, but
source was not
a mount point, or was '/'.
includes more than one of
MS_UNBINDABLE and also includes a
flag other than
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
(In case no block device is required:) Table of dummy devices is full.
A pathname was longer than
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.
not a block device (and a device was required).
target, or a
source, is not a
The major number of the block device
source is out of
The caller does not have the required privileges.
The definitions of
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.
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
required in kernel versions prior to 2.4, but since Linux 2.4
is no longer required and is ignored if specified.
1.1.69 when a different
was added to
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 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 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
/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.
This page is part of release 4.07 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
Copyright (C) 1993 Rickard E. Faith <faithcs.unc.edu>
and Copyright (C) 1994 Andries E. Brouwer <aebcwi.nl>
and Copyright (C) 2002, 2005, 2016 Michael Kerrisk <mtk.manpagesgmail.com>
Permission is granted to make and distribute verbatim copies of this
manual provided the copyright notice and this permission notice are
preserved on all copies.
Permission is granted to copy and distribute modified versions of this
manual under the conditions for verbatim copying, provided that the
entire resulting derived work is distributed under the terms of a
permission notice identical to this one.
Since the Linux kernel and libraries are constantly changing, this
manual page may be incorrect or out-of-date. The author(s) assume no
responsibility for errors or omissions, or for damages resulting from
the use of the information contained herein. The author(s) may not
have taken the same level of care in the production of this manual,
which is licensed free of charge, as they might when working
Formatted or processed versions of this manual, if unaccompanied by
the source, must acknowledge the copyright and authors of this work.
Modified 1996-11-04 by Eric S. Raymond <esrthyrsus.com>
Modified 2001-10-13 by Michael Kerrisk <mtk.manpagesgmail.com>
Added note on historical behavior of MS_NOSUID
Modified 2002-05-16 by Michael Kerrisk <mtk.manpagesgmail.com>
Extensive changes and additions
Modified 2002-05-27 by aeb
Modified 2002-06-11 by Michael Kerrisk <mtk.manpagesgmail.com>
Enhanced descriptions of MS_MOVE, MS_BIND, and MS_REMOUNT
Modified 2004-06-17 by Michael Kerrisk <mtk.manpagesgmail.com>
2005-05-18, mtk, Added MNT_EXPIRE, plus a few other tidy-ups.
2008-10-06, mtk: move umount*() material into separate umount.2 page.
2008-10-06, mtk: Add discussion of namespaces.