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NAME
mount - mount filesystem
SYNOPSIS
#include <sys/mount.h>
int mount(const char *source, const char *target,
const char *filesystemtype, unsigned long mountflags,
const void *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 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 fsync(2), syncfs(2), or 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.
RETURN VALUE
On success, zero is returned. On error, -1 is returned, and errno is set appropriately.
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_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.
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.
VERSIONS
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.
CONFORMING TO
This function is Linux-specific and should not be used in programs intended 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 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.
SEE ALSO
mountpoint(1), umount(2), mount_namespaces(7), path_resolution(7), findmnt(8), lsblk(8), mount(8), umount(8)
COLOPHON
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