mount - mount filesystem
Standard C library (libc, -lc)
#include <sys/mount.h>
int mount(const char *source, const char *target,
const char *filesystemtype, unsigned long mountflags,
const void *_Nullable 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.
Make directory changes on this filesystem synchronous. (This property can be obtained for individual directories or subtrees using chattr(1).)
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:
the inode needs to be updated for some change unrelated to file timestamps;
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 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.)
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. Mandatory locking has been fully deprecated in Linux 5.15, so this flag should be considered deprecated.
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, MS_NOATIME implies MS_NODIRATIME.
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. In addition, SELinux
domain transitions require the permission nosuid_transition,
which in turn needs also the policy capability
nnp_nosuid_transition.
Mount filesystem read-only.
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.
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.
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.
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.
Make writes on this filesystem synchronous (as though the O_SYNC flag to open(2) was specified for all file opens to this filesystem).
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 mounts 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 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_RDONLYwill make access through this mountpoint read-only, without affecting other mounts.
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.) 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:
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 unmount events that take place under peer mounts will propagate to this mount.
Make this mount private. Mount and unmount events do not propagate into or out of this mount.
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.
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. 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_namespaces(7).
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.
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.
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 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, 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.
An attempt was made to modify (MS_REMOUNT) the MS_RDONLY, MS_NOSUID, or MS_NOEXEC flag, or one of the "atime" flags (MS_NOATIME, MS_NODIRATIME, MS_RELATIME) of an existing mount, but the mount is locked; see mount_namespaces(7).
Mounting a read-only filesystem was attempted without giving the MS_RDONLY flag. See EACCES, above.
Linux.
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 glibc 2.12.
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.
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 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 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/mountinfoThe 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.
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)