mount()
attaches the file system specified by
source
(which is often a device name, but can also be a directory name
or a dummy) to the directory specified by
target.
Appropriate privilege (Linux: the
CAP_SYS_ADMIN
capability) is required to mount file systems.
Since Linux 2.4 a single file system can be visible at
multiple mount points, and multiple mounts can be stacked
on the same mount point.
Values for the
filesystemtype
argument supported by the kernel are listed in
/proc/filesystems
(like "minix", "ext2", "ext3", "jfs", "xfs", "reiserfs",
"msdos", "proc", "nfs", "iso9660" etc.).
Further types may become available when the appropriate modules
are loaded.
The
mountflags
argument may have the magic number 0xC0ED (MS_MGC_VAL)
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:
MS_BIND (Linux 2.4 onwards)
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
chroot(2)
jails.
The
filesystemtype
and
data
arguments are ignored.
Up until Linux 2.6.26,
mountflags
was also ignored
(the bind mount has the same mount options as
the underlying mount point).
Since Linux 2.6.26, the
MS_RDONLY
flag is honored when making a bind mount.
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
chattr(1).)
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
fcntl(2).)
MS_MOVE
Move a subtree.
source
specifies an existing mount point and
target
specifies the new location.
The move is atomic: at no point is the subtree unmounted.
The
filesystemtype, mountflags, and data
arguments are ignored.
MS_NOATIME
Do not update access times for (all types of) files on this file system.
MS_NODEV
Do not allow access to devices (special files) on this file system.
MS_NODIRATIME
Do not update access times for directories on this file system.
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 file system.
MS_NOSUID
Do not honor set-user-ID and set-group-ID bits when executing
programs from this file system.
MS_RDONLY
Mount file system read-only.
MS_RELATIME (Since Linux 2.6.20)
When a file on this file system is accessed,
only update the file's last access time (atime) 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.
MS_REMOUNT
Remount an existing mount.
This allows you to change the
mountflags
and
data
of an existing mount without having to unmount and remount the file system.
source
and
target
should be the same values specified in the initial
mount()
call;
filesystemtype
is ignored.
The following
mountflags
can be changed:
MS_RDONLY,
MS_SYNCHRONOUS,
MS_MANDLOCK;
before kernel 2.6.16, the following could also be changed:
MS_NOATIME
and
MS_NODIRATIME;
and, additionally, before kernel 2.4.10, the following could also be changed:
MS_NOSUID,
MS_NODEV,
MS_NOEXEC.
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_SYNCHRONOUS
Make writes on this file system synchronous (as though
the
O_SYNC
flag to
open(2)
was specified for all file opens to this file system).
From Linux 2.4 onwards, the
MS_NODEV, MS_NOEXEC, and MS_NOSUID
flags are settable on a per-mount-point basis.
From kernel 2.6.16 onwards,
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.
The
data
argument is interpreted by the different file systems.
Typically it is a string of comma-separated options
understood by this file system.
See
mount(8)
for details of the options available for each filesystem type.
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 kernel source code for details.
EACCES
A component of a path was not searchable.
(See also
path_resolution(7).)
Or, mounting a read-only filesystem was attempted without giving the
MS_RDONLY
flag.
Or, the block device
source
is located on a filesystem mounted with the
MS_NODEV
option.
EBUSY
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
target
because
target
is still busy (it is the working directory of some task,
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.
Or, a remount
(MS_REMOUNT)
was attempted, but
source
was not already mounted on
target.
Or, a move
(MS_MOVE)
was attempted, but
source
was not a mount point, or was aq/aq.
ELOOP
Too many links encountered during pathname resolution.
Or, a move was attempted, while
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.
CONFORMING TO
This function is Linux-specific and should not be used in
programs intended to be portable.
NOTES
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 file system 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 there
was 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()
CLONE_NEWNS
flag,
in which case its new namespace is initialized to be a
copy
of the namespace of the process that called
clone();
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/self
file exposes the list of mount points in the mount
namespace of the process with the specified ID; see
proc(5)
for details.
This page is part of release 3.14 of the Linux
man-pages
project.
A description of the project,
and information about reporting bugs,
can be found at
http://www.kernel.org/doc/man-pages/.
