mount(2) System Calls Manual mount(2)
NAME
mount - mount filesystem
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#include <sys/mount.h>
int mount(const char *source, const char *target,
const char *filesystemtype, unsigned long mountflags,
const void *_Nullable 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 di-
rectory 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. Typi-
cally 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 opera-
tion, 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 in-
cludes 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 time-
stamps are updated only when:
• the inode needs to be updated for some change unrelated to
file timestamps;
• the application employs fsync(2), syncfs(2), or sync(2);
• an undeleted inode is evicted from memory; or
• 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 en-
abled. (The advantage of combining MS_STRICTATIME and MS_LAZY-
TIME 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. (Manda-
tory locking must still be enabled on a per-file basis, as de-
scribed in fcntl(2).) Since Linux 4.5, this mount option re-
quires the CAP_SYS_ADMIN capability and a kernel configured with
the CONFIG_MANDATORY_FILE_LOCKING option. Mandatory locking has
been fully deprecated in Linux 5.15, so this flag should be con-
sidered deprecated.
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 filesys-
tem.
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 capabili-
ties when executing programs from this filesystem. In addition,
SELinux domain transitions require the permission nosuid_transi-
tion, which in turn needs also the policy capability nnp_no-
suid_transition.
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 re-
cursively 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 re-
quired 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), real-
path(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 set-
table 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. Subse-
quently, the settings of the flags can be changed via a remount opera-
tion (see below). Such changes will be visible via all mounts associ-
ated 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 mount-
flags. This allows you to change the mountflags and data of an exist-
ing 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 de-
liberately 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 ig-
nored. Note that changes to per-superblock flags are visible via all
mounts of the associated filesystem (because the per-superblock flags
are shared by all mounts).
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 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 mounts.
Creating a bind mount
If mountflags includes MS_BIND (available since Linux 2.4), then per-
form 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 mount-
flags argument are also ignored. (The bind mount has the same mount
options as the underlying mount.) However, see the discussion of re-
mounting 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 recur-
sive 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 propaga-
tion type are MS_REC (described below) and MS_SILENT (which is ig-
nored).
The source, filesystemtype, and data arguments are ignored.
The meanings of the propagation type flags are as follows:
MS_SHARED
Make this mount shared. Mount and unmount events immediately
under this mount will propagate to the other mounts 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 mounts in the peer group. Conversely, mount and un-
mount events that take place under peer mounts will propagate to
this mount.
MS_PRIVATE
Make this mount private. Mount and unmount events do not propa-
gate into or out of this mount.
MS_SLAVE
If this is a shared mount that is a member of a peer group that
contains other members, convert it to a slave mount. If this is
a shared mount 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 is left unchanged.
When a mount is a slave, mount and unmount events propagate into
this mount from the (master) shared peer group of which it was
formerly a member. Mount and unmount events under this mount do
not propagate to any peer.
A mount 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 per-
formed 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. If the MS_REC flag is also specified in mountflags, then the
propagation type of all mounts under target is also changed.
For further details regarding mount propagation types (including the
default propagation type assigned to new mounts), see mount_name-
spaces(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 and target
specifies the new location to which that mount 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
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. source specifies
the source for the new mount, 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_reso-
lution(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 de-
vice 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 re-
mounted 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 un-
der source includes unbindable mounts and target is a mount 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, 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_UN-
BINDABLE 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.
EPERM An attempt was made to modify (MS_REMOUNT) the MS_RDONLY, MS_NO-
SUID, or MS_NOEXEC flag, or one of the "atime" flags (MS_NOAT-
IME, MS_NODIRATIME, MS_RELATIME) of an existing mount, but the
mount is locked; see mount_namespaces(7).
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_RELA-
TIME, MS_SHARED, MS_SLAVE, MS_STRICTATIME, and MS_UNBINDABLE were added
to glibc headers in glibc 2.12.
STANDARDS
This function is Linux-specific and should not be used in programs in-
tended 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 before Linux 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 Linux 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 mul-
tiple processes, and changes to the namespace (i.e., mounts and un-
mounts) 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 sys-
tem.)
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 ini-
tialized 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 pro-
cesses (except child processes that the caller subsequently creates)
and vice versa.
For further details on mount namespaces, see mount_namespaces(7).
Parental relationship between mounts
Each mount has a parent mount. The overall parental relationship of
all mounts defines the single directory hierarchy seen by the processes
within a mount namespace.
The parent of a new mount is defined when the mount is created. In the
usual case, the parent of a new mount is the mount 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 mounts 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 mounts
in the mount namespace of the process with the specified ID. The
/proc/[pid]/mountinfo file exposes even more information about mounts,
including the propagation type and mount ID information that makes it
possible to discover the parental relationship between mounts. See
proc(5) and mount_namespaces(7) for details of this file.
SEE ALSO
mountpoint(1), chroot(2), ioctl_iflags(2), mount_setattr(2),
pivot_root(2), umount(2), mount_namespaces(7), path_resolution(7),
findmnt(8), lsblk(8), mount(8), umount(8)
Linux man-pages 6.03 2023-02-05 mount(2)
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