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mount - mount file system
int mount(const char *source, const char *target,
const char *filesystemtype, unsigned long mountflags,
const void *data);
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 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). 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
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).)
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 file
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 MS_NODIRATIME.
Do not allow programs to be executed from this file system.
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. 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
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
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
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.
On success, zero is returned. On error, -1 is returned, and errno is
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 kernel source code for details.
EACCES 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
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 thread, the mount point of another
device, has open files, etc.).
EFAULT One of the pointer arguments points outside the user address
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 '/'.
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
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
source is not a block device (and a device was required).
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.
The definitions of MS_DIRSYNC, MS_MOVE, MS_REC, MS_RELATIME, 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 be portable.
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 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 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.
umount(2), path_resolution(7), mount(8), umount(8)
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