path: root/man2/mount.2

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.\" 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@gmx.net>
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.\" Modified 1996-11-04 by Eric S. Raymond <esr@thyrsus.com>
.\" Modified 2001-10-13 by Michael Kerrisk <mtk-manpages@gmx.net>
.\"	Added note on historical behaviour of MS_NOSUID
.\" Modified 2002-05-16 by Michael Kerrisk <mtk-manpages@gmx.net>
.\"	Extensive changes and additions
.\" Modified 2002-05-27 by aeb
.\" Modified 2002-06-11 by Michael Kerrisk <mtk-manpages@gmx.net>
.\"	Enhanced descriptions of MS_MOVE, MS_BIND, and MS_REMOUNT
.\" Modified 2004-06-17 by Michael Kerrisk <mtk-manpages@gmx.net>
.\" 2005-05-18, mtk, Added MNT_EXPIRE, plus a few other tidy-ups.
.\"
.TH MOUNT 2 2004-05-18 "Linux" "Linux Programmer's Manual"
.SH NAME
mount, umount, umount2 \- mount and unmount filesystems
.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 );
.sp
.BI "int umount(const char *" target );
.sp
.BI "int umount2(const char *" target ", int " flags );
.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 .

.BR umount ()
and
.BR umount2 ()
remove the attachment of the (topmost) filesystem mounted on
.IR target .

Appropriate privilege (Linux: the
.B CAP_SYS_ADMIN
capability) is required to mount and unmount 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
.IR filesystemtype
argument supported by the kernel are listed in
.I /proc/filesystems
(like "minix", "ext2", "msdos", "proc", "nfs", "iso9660" etc.).
Further types may become available when the appropriate modules
are loaded.

The
.IR 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 <linux/fs.h> for libc4 and libc5
and in <sys/mount.h> for glibc2) in the low order 16 bits:
.\" FIXME 2.6.15 added flags for "shared sub-tree" functionality:
.\" MS_UNBINDABLE, MS_PRIVATE, MS_SHARED, MS_SLAVE
.\" These need to be documented on this page.
.\" See Documentation/sharedsubtree.txt
.TP
.B MS_BIND
(Linux 2.4 onwards)
.\" since 2.4.0-test9
Perform a bind mount, making a file or a directory subtree visible at
another point within a file system.
Bind mounts may cross file system boundaries and span
.BR chroot (2)
jails.
The
.IR filesystemtype ", " mountflags ", and " data
arguments are ignored.
.\" with the exception of the "hidden" MS_REC mountflags bit
.TP
.BR MS_DIRSYNC " (since Linux 2.5.19)"
Make directory changes on this file system synchronous.
(This property can be obtained for individual directories
or subtrees using
.BR chattr (8).)
.TP
.B MS_MANDLOCK
Permit mandatory locking on files in this file system.
(Mandatory locking must still be enabled on a per-file basis,
as described in
.BR fcntl (2).)
.\" FIXME Say more about MS_MOVE
.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 file system.
.TP
.B MS_NODEV
Do not allow access to devices (special files) on this file system.
.TP
.B MS_NODIRATIME
Do not update access times for directories on this file system.
.TP
.B MS_NOEXEC
Do not allow programs to be executed from this file system.
.\" (Possibly useful for a file system 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 honour set-user-ID and set-group-ID bits when executing
programs from this file system.
.\" (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 file system 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 sub-tree flags.
.TP
.BR MS_RELATIME "(Since Linux 2.6.20)"
When a file on this file system is accessed,
only update the file's last accessed time (atime) if the current value
of atime is less than or equal to the file's last modified (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.
.TP
.B MS_REMOUNT
Remount an existing mount.
This is allows you to change the
.I mountflags
and
.I data
of an existing mount without having to unmount and remount the file system.
.I source
and
.I target
should be the same values specified in the initial
.BR mount ()
call;
.I filesystemtype
is ignored.

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:
.BR MS_NOATIME
and
.BR MS_NODIRATIME ;
and, additionally, before kernel 2.4, the following could also be changed:
.BR MS_NOSUID ,
.BR MS_NODEV ,
.BR MS_NOEXEC .
.TP
.B MS_SYNCHRONOUS
Make writes on this file system synchronous (as though
the
.B O_SYNC
flag to
.BR open (2)
was specified for all file opens to this file system).
.PP
From Linux 2.4 onwards, the
.BR MS_NODEV ", " MS_NOEXEC ", and " MS_NOSUID
flags are settable on a per-mount-point basis.
From kernel 2.6.16 onwards,
.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
.IR data
argument is interpreted by the different file systems.
Typically it is a string of comma-separated options
understood by this file system.
See
.BR mount (8)
for details of the options available for each filesystem type.
.PP
.\" Note: the kernel naming differs from the glibc naming
.\" umount2 is the glibc name for what the kernel now calls umount
.\" and umount is the glibc name for oldumount
Linux 2.1.116 added the
.BR umount2 ()
system call, which, like
.BR umount (),
unmounts a target, but allows additional
.I flags
controlling the behaviour of the operation:
.TP
.BR MNT_FORCE " (since Linux 2.1.116)"
Force unmount even if busy.
This can cause data loss.
(Only for NFS mounts.)
.\" FIXME Can MNT_FORCE result in data loss?  According to
.\" the Solaris manual page it can cause data loss on Solaris.
.\" If the same holds on Linux, then this should be documented.
.TP
.BR MNT_DETACH " (since Linux 2.4.11)"
Perform a lazy unmount: make the mount point unavailable for
new accesses, and actually perform the unmount when the mount point
ceases to be busy.
.TP
.BR MNT_EXPIRE " (since Linux 2.6.8)"
Mark the mount point as expired.
If a mount point is not currently in use, then an initial call to
.BR umount2 ()
with this flag fails with the error
.BR EAGAIN ,
but marks the mount point as expired.
The mount point remains expired as long as it isn't accessed
by any process.
A second
.BR umount2 ()
call specifying
.B MNT_EXPIRE
unmounts an expired mount point.
This flag cannot be specified with either
.B MNT_FORCE
or
.BR MNT_DETACH .
.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 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 EAGAIN
A call to
.BR umount2 ()
specifying
.B MNT_EXPIRE
successfully marked an unbusy file system as expired.
.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 task,
the mount point of another device, has open files, etc.).
Or, it could not be unmounted because it is busy.
.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 '/'.
Or, an unmount was attempted, but
.I target
was not a mount point.
Or,
.BR umount2 ()
was called with
.B MNT_EXPIRE
and either
.B MNT_DETACH
or
.BR MNT_FORCE .
.TP
.B ELOOP
Too many link 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 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
The second argument, or a prefix of the first argument, 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 "CONFORMING TO"
These functions are Linux specific and should not be used in
programs intended to be portable.
.SH NOTES
.SS Linux Notes
The original
.BR umount ()
function was called as \fIumount(device)\fP and would return ENOTBLK
when called with something other than a block device.
In Linux 0.98p4 a call \fIumount(dir)\fP was added, in order to
support anonymous devices.
In Linux 2.3.99-pre7 the call \fIumount(device)\fP was removed,
leaving only \fIumount(dir)\fP (since now devices can be mounted
in more than one place, so specifying the device does not suffice).
.LP
The original MS_SYNC flag was renamed MS_SYNCHRONOUS in 1.1.69
when a different MS_SYNC was added to <mman.h>.
.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.
.SH "SEE ALSO"
.BR path_resolution (7),
.BR mount (8),
.BR umount (8)