MOUNT(2) Linux Programmer's Manual MOUNT(2)
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 sys-
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 (e.g., "minix", "ext2", "ext3", "jfs", "xfs", "reiserfs", "msdos",
"proc", "nfs", "iso9660"). Further types may become available when the appropriate mod-
ules 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 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 using chattr(1).)
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 system.
Do not allow access to devices (special files) on this file system.
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 system.
Do not honor set-user-ID and set-group-ID bits when executing programs from this
Mount file system read-only.
MS_RELATIME (Since Linux 2.6.20)
When a file on this file system 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 use-
ful 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 behav-
ior 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 filesys-
temtype 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_NODI-
RATIME; 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_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, 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 set-
table on a per-mount-point basis. The MS_RELATIME flag is also settable on a per-mount-
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 set appropriately.
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.
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 MS_NODEV
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 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 '/'.
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.
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.
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 added
to glibc headers in version 2.12.
This function is Linux-specific and should not be used in programs intended to be porta-
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 names-
pace is the set of file system mounts that are visible to a process. Mount-point names-
paces 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 names-
pace 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 subse-
quently 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), namespaces(7), path_resolution(7), mount(8), umount(8)
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Linux 2012-07-05 MOUNT(2)