MOUNT

NAME

SYNOPSIS

#include <sys/mount.h>

int mount(const char *source, const char *target,
          const char *filesystemtype, unsigned long mountflags,
          const void *data);

DESCRIPTION

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. 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.

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_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.

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

ERRORS

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

NOTES

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

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.  

SEE ALSO

COLOPHON