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. 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.
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
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_MANDLOCKPermit 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_NOATIMEDo not update access times for (all types of) files on this filesystem.
MS_NODEVDo not allow access to devices (special files) on this filesystem.
MS_NODIRATIMEDo 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_NOEXECDo not allow programs to be executed from this filesystem.
MS_NOSUIDDo not honor set-user-ID and set-group-ID bits or file capabilities when executing programs from this filesystem.
MS_RDONLYMount 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_SYNCHRONOUSMake 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.
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.
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.
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_SHAREDMake 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_PRIVATEMake this mount point private. Mount and unmount events do not propagate into or out of this mount point.
MS_SLAVEIf 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_UNBINDABLEMake 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).
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.
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 to
indicate the error.
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 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.
The block device source is located on a
filesystem mounted with the MS_NODEV option.
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 '/'.
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.
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
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 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, 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.
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).
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).
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.
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)
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/.
|
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> %%%LICENSE_START(VERBATIM) 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 professionally. Formatted or processed versions of this manual, if unaccompanied by the source, must acknowledge the copyright and authors of this work. %%%LICENSE_END 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. |