man2/mount.2

.\" Copyright (C) 1993 Rickard E. Faith <faith@cs.unc.edu>
.\" and Copyright (C) 1994 Andries E. Brouwer <aeb@cwi.nl>
.\" and Copyright (C) 2002, 2005 Michael Kerrisk <mtk.manpages@gmail.com>
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.\" Modified 1996-11-04 by Eric S. Raymond <esr@thyrsus.com>
.\" Modified 2001-10-13 by Michael Kerrisk <mtk.manpages@gmail.com>
.\"	Added note on historical behavior of MS_NOSUID
.\" Modified 2002-05-16 by Michael Kerrisk <mtk.manpages@gmail.com>
.\"	Extensive changes and additions
.\" Modified 2002-05-27 by aeb
.\" Modified 2002-06-11 by Michael Kerrisk <mtk.manpages@gmail.com>
.\"	Enhanced descriptions of MS_MOVE, MS_BIND, and MS_REMOUNT
.\" Modified 2004-06-17 by Michael Kerrisk <mtk.manpages@gmail.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.
.\"
.TH MOUNT 2 2016-03-15 "Linux" "Linux Programmer's Manual"
.SH NAME
mount \- mount filesystem
.SH SYNOPSIS
.nf
.B "#include <sys/mount.h>"
.sp
.BI "int mount(const char *" source ", const char *" target ,
.BI "          const char *" filesystemtype ", unsigned long " mountflags ,
.BI "          const void *" data );
.fi
.SH DESCRIPTION
.BR mount ()
attaches the filesystem specified by
.I source
(which is often a device name, but can also be a directory name
or a dummy) to the directory specified by
.IR target .

Appropriate privilege (Linux: the
.B CAP_SYS_ADMIN
capability) is required to mount filesystems.

Since Linux 2.4 a single filesystem can be visible at
multiple mount points, and multiple mounts can be stacked
on the same mount point.
.\" Multiple mounts on same mount point: since 2.3.99pre7.

Values for the
.I filesystemtype
argument supported by the kernel are listed in
.I /proc/filesystems
(e.g., "minix", "ext2", "ext3", "jfs", "xfs", "reiserfs",
"msdos", "proc", "nfs", "iso9660").
Further types may become available when the appropriate modules
are loaded.

The
.I mountflags
argument may have the magic number 0xC0ED (\fBMS_MGC_VAL\fP)
in the top 16 bits (this was required in kernel versions prior to 2.4, but
is no longer required and ignored if specified),
and various mount flags
.\" (as defined in \fI<linux/fs.h>\fP for libc4 and libc5
.\" and in \fI<sys/mount.h>\fP for glibc2)
in the low order 16 bits:
.\" FIXME 2.6.15 added flags for "shared subtree" functionality:
.\"
.\"	MS_PRIVATE
.\"		All mounts are private by default. Previously shared mounts
.\"		can be re-marked PRIVATE.
.\"	MS_SHARED
.\"		Mount points that are marked SHARED propagate mount events
.\"		to one another after being cloned.
.\"			mount --make-rshared ==> MS_SHARED | MS_REC
.\"	MS_SLAVE
.\"		A previously shared mount point can be marked SLAVE, meaning
.\"		it receives propagated events, but does not propagate events.
.\"	MS_UNBINDABLE
.\"		mounts cannot be bound into other places, and will not be
.\"		propagated into new subtrees	
.\"
.\" These settings are visible in /proc/$$/mountinfo
.\"
.\" These need to be documented on this page.
.\" See:
.\"
.\" * Documentation/filesystems/sharedsubtree.txt
.\"
.\" * http://lwn.net/Articles/159077/
.\"
.\" * https://www.kernel.org/doc/ols/2006/ols2006v2-pages-209-222.pdf
.\"       Shared-Subtree Concept, Implementation, and Applications in Linux
.\"       Al Viro viro@ftp.linux.org.uk
.\"       Ram Pai linuxram@us.ibm.com
.\"
.\" * http://www.ibm.com/developerworks/linux/library/l-mount-namespaces/index.html
.\"       Applying mount namespaces
.\"
.\"       Uncover practical applications for advanced Linux mounts features
.\"       Serge E. Hallyn (sergeh@us.ibm.com), Software Engineer, IBM
.\"       Ram Pai (linuxram@us.ibm.com), Software Engineer, IBM
.\"       Date:  17 Sep 2007
.\"
.\" FIXME 2.6.25 Added MS_I_VERSION, which needs to be documented.
.\"
.TP
.BR MS_BIND " (Linux 2.4 onward)"
.\" since 2.4.0-test9
Perform a bind mount, making a file or a directory subtree visible at
another point within a filesystem.
Bind mounts may cross filesystem boundaries and span
.BR chroot (2)
jails.
The
.IR filesystemtype
and
.IR data
arguments are ignored.
Up until Linux 2.6.26,
.I mountflags
was also ignored
.\" with the exception of the "hidden" MS_REC mountflags bit
(the bind mount has the same mount options as
the underlying mount point).
.TP
.BR 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
.BR chattr (1).)
.TP
.BR MS_LAZYTIME " (since Linux 4.0)"
.\" commit 0ae45f63d4ef8d8eeec49c7d8b44a1775fff13e8
.\" commit fe032c422c5ba562ba9c2d316f55e258e03259c6
.\" commit a26f49926da938f47561f386be56a83dd37a496d
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:
.RS
.IP (a) 5
the inode needs to be updated for some change unrelated to file timestamps;
.IP (b)
the application employs
.BR fsync (2),
.BR syncfs (2),
or
.BR sync (2);
.IP (c)
an undeleted inode is evicted from memory; or
.IP (d)
more than 24 hours have passed since the inode was written to disk.
.RE
.IP
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
.B MS_STRICTATIME
mount option is also enabled.
(The advantage of combining
.BR MS_STRICTATIME
and
.BR MS_LAZYTIME
is that
.BR stat (2)
will return the correctly updated atime, but the atime updates
will be flushed to disk only in the cases listed above.)
.TP
.B MS_MANDLOCK
Permit mandatory locking on files in this filesystem.
(Mandatory locking must still be enabled on a per-file basis,
as described in
.BR fcntl (2).)
Since Linux 4.5,
.\" commit 95ace75414f312f9a7b93d873f386987b92a5301
this mount option requires the
.B CAP_SYS_ADMIN
capability.
.\" FIXME Describe the MS_MOVE flag in more detail
.TP
.B MS_MOVE
Move a subtree.
.I source
specifies an existing mount point and
.I target
specifies the new location.
The move is atomic: at no point is the subtree unmounted.
The
.IR filesystemtype ", " mountflags ", and " data
arguments are ignored.
.TP
.B MS_NOATIME
Do not update access times for (all types of) files on this filesystem.
.TP
.B MS_NODEV
Do not allow access to devices (special files) on this filesystem.
.TP
.B MS_NODIRATIME
Do not update access times for directories on this filesystem.
This flag provides a subset of the functionality provided by
.BR MS_NOATIME ;
that is,
.BR MS_NOATIME
implies
.BR MS_NODIRATIME .
.TP
.B MS_NOEXEC
Do not allow programs to be executed from this filesystem.
.\" (Possibly useful for a filesystem that contains non-Linux executables.
.\" Often used as a security feature, e.g., to make sure that restricted
.\" users cannot execute files uploaded using ftp or so.)
.TP
.B MS_NOSUID
Do not honor set-user-ID and set-group-ID bits when executing
programs from this filesystem.
.\" (This is a security feature to prevent users executing set-user-ID and
.\" set-group-ID programs from removable disk devices.)
.TP
.B MS_RDONLY
Mount filesystem read-only.
.\"
.\" FIXME Document MS_REC, available since 2.4.11.
.\" This flag has meaning in conjunction with MS_BIND and
.\" also with the shared subtree flags.
.TP
.BR 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
.BR 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
.BR MS_NOATIME
was specified), and the
.B 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.
.\" Matthew Garrett notes in the patch that added this behavior
.\" that this lets utilities such as tmpreaper (which deletes
.\" files based on last access time) work correctly.
.TP
.B MS_REMOUNT
Remount an existing mount.
This allows you to change the
.I mountflags
and
.I data
of an existing mount without having to unmount and remount the filesystem.
.I target
should be the same value specified in the initial
.BR mount ()
call;
.I source
and
.I filesystemtype
are ignored.
The
.I mountflags
and
.I data
arguments should match the values used in the original
.BR mount ()
call, except for those parameters that are being deliberately changed.

The following
.I mountflags
can be changed:
.BR MS_RDONLY ,
.BR MS_SYNCHRONOUS ,
.BR MS_MANDLOCK ;
before kernel 2.6.16, the following could also be changed:
.B MS_NOATIME
and
.BR MS_NODIRATIME ;
and, additionally, before kernel 2.4.10, the following could also be changed:
.BR MS_NOSUID ,
.BR MS_NODEV ,
.BR MS_NOEXEC .
.TP
.BR MS_SILENT " (since Linux 2.6.17)"
Suppress the display of certain
.RI ( printk ())
warning messages in the kernel log.
This flag supersedes the misnamed and obsolete
.BR MS_VERBOSE
flag (available since Linux 2.4.12), which has the same meaning.
.TP
.BR 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
.BR MS_NOATIME
and
.BR MS_RELATIME
flags.
.TP
.B MS_SYNCHRONOUS
Make writes on this filesystem synchronous (as though
the
.B O_SYNC
flag to
.BR open (2)
was specified for all file opens to this filesystem).
.PP
From Linux 2.4 onward, the
.BR MS_NODEV ", " MS_NOEXEC ", and " MS_NOSUID
flags are settable on a per-mount-point basis.
From kernel 2.6.16 onward,
.B MS_NOATIME
and
.B MS_NODIRATIME
are also settable on a per-mount-point basis.
The
.B MS_RELATIME
flag is also settable on a per-mount-point basis.
.PP
The
.I data
argument is interpreted by the different filesystems.
Typically it is a string of comma-separated options
understood by this filesystem.
See
.BR mount (8)
for details of the options available for each filesystem type.
.SH RETURN VALUE
On success, zero is returned.
On error, \-1 is returned, and
.I errno
is set appropriately.
.SH 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.
.TP
.B EACCES
A component of a path was not searchable.
(See also
.BR path_resolution (7).)
Or, mounting a read-only filesystem was attempted without giving the
.B MS_RDONLY
flag.
Or, the block device
.I source
is located on a filesystem mounted with the
.B MS_NODEV
option.
.\" mtk: Probably: write permission is required for MS_BIND, with
.\" the error EPERM if not present; CAP_DAC_OVERRIDE is required.
.TP
.B EBUSY
.I source
is already mounted.
Or, it cannot be remounted read-only,
because it still holds files open for writing.
Or, it cannot be mounted on
.I target
because
.I target
is still busy (it is the working directory of some thread,
the mount point of another device, has open files, etc.).
.TP
.B EFAULT
One of the pointer arguments points outside the user address space.
.TP
.B EINVAL
.I source
had an invalid superblock.
Or, a remount
.RB ( MS_REMOUNT )
was attempted, but
.I source
was not already mounted on
.IR target .
Or, a move
.RB ( MS_MOVE )
was attempted, but
.I source
was not a mount point, or was \(aq/\(aq.
.TP
.B ELOOP
Too many links encountered during pathname resolution.
Or, a move was attempted, while
.I target
is a descendant of
.IR source .
.TP
.B EMFILE
(In case no block device is required:)
Table of dummy devices is full.
.TP
.B ENAMETOOLONG
A pathname was longer than
.BR MAXPATHLEN .
.TP
.B ENODEV
.I filesystemtype
not configured in the kernel.
.TP
.B ENOENT
A pathname was empty or had a nonexistent component.
.TP
.B ENOMEM
The kernel could not allocate a free page to copy filenames or data into.
.TP
.B ENOTBLK
.I source
is not a block device (and a device was required).
.TP
.B ENOTDIR
.IR target ,
or a prefix of
.IR source ,
is not a directory.
.TP
.B ENXIO
The major number of the block device
.I source
is out of range.
.TP
.B EPERM
The caller does not have the required privileges.
.SH VERSIONS
The definitions of
.BR MS_DIRSYNC ,
.BR MS_MOVE ,
.BR MS_REC ,
.BR MS_RELATIME ,
and
.BR MS_STRICTATIME
were added to glibc headers in version 2.12.
.\" FIXME . Definitions of the so-far-undocumented MS_UNBINDABLE, MS_PRIVATE,
.\" MS_SHARED, and MS_SLAVE were (also) only added to glibc headers in 2.12.
.SH CONFORMING TO
This function is Linux-specific and should not be used in
programs intended to be portable.
.SH NOTES
The original
.B MS_SYNC
flag was renamed
.B MS_SYNCHRONOUS
in 1.1.69
when a different
.B MS_SYNC
was added to \fI<mman.h>\fP.
.LP
Before Linux 2.4 an attempt to execute a set-user-ID or set-group-ID program
on a filesystem mounted with
.B MS_NOSUID
would fail with
.BR EPERM .
Since Linux 2.4 the set-user-ID and set-group-ID bits are
just silently ignored in this case.
.\" The change is in patch-2.4.0-prerelease.
.SS 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
.BR fork (2)
shares its parent's mount namespace;
the mount namespace is preserved across an
.BR execve (2).

A process can obtain a private mount namespace if:
it was created using the
.BR clone (2)
.BR CLONE_NEWNS
flag,
in which case its new namespace is initialized to be a
.I copy
of the namespace of the process that called
.BR clone (2);
or it calls
.BR unshare (2)
with the
.BR 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
.I /proc/PID/mounts
file exposes the list of mount points in the mount
namespace of the process with the specified ID; see
.BR proc (5)
for details.
.SH SEE ALSO
.BR umount (2),
.BR namespaces (7),
.BR path_resolution (7),
.BR lsblk (8),
.BR findmnt (8),
.BR mount (8),
.BR umount (8)