mount - mount file system
int mount(const char *source, const char *target,
const char *filesystemtype, unsigned long mountflags,
const void *data);
() attaches the file system specified by source
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
(e.g., "minix", "ext2",
"ext3", "jfs", "xfs", "reiserfs",
"msdos", "proc", "nfs", "iso9660").
Further types may become available when the appropriate modules are loaded.
argument may have the magic number 0xC0ED (
) 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 in the low order 16 bits:
- MS_BIND (Linux 2.4 onward)
- 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).
- 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
- Permit mandatory locking on files in this file system.
(Mandatory locking must still be enabled on a per-file basis, as described
- 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.
- Do not update access times for (all types of) files on this
- Do not allow access to devices (special files) on this file
- 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
- Do not allow programs to be executed from this file
- Do not honor set-user-ID and set-group-ID bits when
executing programs from this file system.
- 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. 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.
- 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. target should be the same
value specified in the initial mount() call; source and
filesystemtype are 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_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
- MS_STRICTATIME (Since Linux 2.6.30)
- Always update the last access time (atime) when files on
this file system 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 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 onward, the MS_NODEV
flags are settable on a per-mount-point basis. From kernel
2.6.16 onward, MS_NOATIME
are also settable on
a per-mount-point basis. The MS_RELATIME
flag is also settable on a
argument is interpreted by the different file systems. Typically
it is a string of comma-separated options understood by this file system. See
for details of the options available for each filesystem type.
On success, zero is returned. On error, -1 is returned, and errno
The error values given below result from filesystem type independent errors.
Each file-system 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).) Or, mounting a read-only file system was
attempted without giving the MS_RDONLY flag. Or, the block device
source is located on a file system mounted with the MS_NODEV
- 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 thread, the mount point of another
device, has open files, etc.).
- One of the pointer arguments points outside the user
- 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 '/'.
- Too many links encountered during pathname resolution. Or,
a move was attempted, while target is a descendant of
- (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
- target, or a prefix of source, is not a
- The major number of the block device source is out
- The caller does not have the required privileges.
The definitions of MS_DIRSYNC
, and MS_STRICTATIME
were only added to glibc headers
in version 2.12.
This function is Linux-specific and should not be used in programs intended to
The original MS_SYNC
flag was renamed MS_SYNCHRONOUS
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 file system 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 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 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
flag, in which case its new namespace is
initialized to be a copy
of the namespace of the process that called
; 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
This page is part of release 3.44 of the Linux man-pages
description of the project, and information about reporting bugs, can be found