mount(2) - Linux manual page (original) (raw)


mount(2) System Calls Manual mount(2)

NAME top

   mount - mount filesystem

LIBRARY top

   Standard C library (_libc_, _-lc_)

SYNOPSIS top

   **#include <sys/mount.h>**

   **int mount(const char ***_source_**, const char ***_target_**,**
             **const char ***_filesystemtype_**, unsigned long** _mountflags_**,**
             **const void *_Nullable** _data_**);**

DESCRIPTION top

   **mount**() attaches the filesystem specified by _source_ (which is
   often a pathname referring to a device, but can also be the
   pathname of a directory or file, or a dummy string) to the
   location (a directory or file) specified by the pathname in
   _target_.

   Appropriate privilege (Linux: the **CAP_SYS_ADMIN** capability) is
   required to mount filesystems.

   Values for the _filesystemtype_ argument supported by the kernel are
   listed in _/proc/filesystems_ (e.g., "btrfs", "ext4", "jfs", "xfs",
   "vfat", "fuse", "tmpfs", "cgroup", "proc", "mqueue", "nfs",
   "cifs", "iso9660").  Further types may become available when the
   appropriate modules are loaded.

   The _data_ argument is interpreted by the different filesystems.
   Typically it is a string of comma-separated options understood by
   this filesystem.  See [mount(8)](../man8/mount.8.html) for details of the options
   available for each filesystem type.  This argument may be
   specified as NULL, if there are no options.

   A call to **mount**() performs one of a number of general types of
   operation, depending on the bits specified in _mountflags_.  The
   choice of which operation to perform is determined by testing the
   bits set in _mountflags_, with the tests being conducted in the
   order listed here:

   •  Remount an existing mount: _mountflags_ includes **MS_REMOUNT**.

   •  Create a bind mount: _mountflags_ includes **MS_BIND**.

   •  Change the propagation type of an existing mount: _mountflags_
      includes one of **MS_SHARED**, **MS_PRIVATE**, **MS_SLAVE**, or
      **MS_UNBINDABLE**.

   •  Move an existing mount to a new location: _mountflags_ includes
      **MS_MOVE**.

   •  Create a new mount: _mountflags_ includes none of the above
      flags.

   Each of these operations is detailed later in this page.  Further
   flags may be specified in _mountflags_ to modify the behavior of
   **mount**(), as described below.

Additional mount flags The list below describes the additional flags that can be specified in mountflags. Note that some operation types ignore some or all of these flags, as described later in this page.

   **MS_DIRSYNC** (since Linux 2.5.19)
          Make directory changes on this filesystem synchronous.
          (This property can be obtained for individual directories
          or subtrees using [chattr(1)](../man1/chattr.1.html).)

   **MS_LAZYTIME** (since Linux 4.0)
          Reduce on-disk updates of inode timestamps (atime, mtime,
          ctime) by maintaining these changes only in memory.  The
          on-disk timestamps are updated only when:

          •  the inode needs to be updated for some change unrelated
             to file timestamps;

          •  the application employs [fsync(2)](../man2/fsync.2.html), [syncfs(2)](../man2/syncfs.2.html), or [sync(2)](../man2/sync.2.html);

          •  an undeleted inode is evicted from memory; or

          •  more than 24 hours have passed since the inode was
             written to disk.

          This mount option significantly reduces writes needed to
          update the inode's timestamps, especially mtime and atime.
          However, in the event of a system crash, the atime and
          mtime fields on disk might be out of date by up to 24
          hours.

          Examples of workloads where this option could be of
          significant benefit include frequent random writes to
          preallocated files, as well as cases where the
          **MS_STRICTATIME** mount option is also enabled.  (The
          advantage of combining **MS_STRICTATIME** and **MS_LAZYTIME** is
          that [stat(2)](../man2/stat.2.html) will return the correctly updated atime, but
          the atime updates will be flushed to disk only in the cases
          listed above.)

   **MS_MANDLOCK**
          Permit mandatory locking on files in this filesystem.
          (Mandatory locking must still be enabled on a per-file
          basis, as described in [fcntl(2)](../man2/fcntl.2.html).)  Since Linux 4.5, this
          mount option requires the **CAP_SYS_ADMIN** capability and a
          kernel configured with the **CONFIG_MANDATORY_FILE_LOCKING**
          option.  Mandatory locking has been fully deprecated in
          Linux 5.15, so this flag should be considered deprecated.

   **MS_NOATIME**
          Do not update access times for (all types of) files on this
          filesystem.

   **MS_NODEV**
          Do not allow access to devices (special files) on this
          filesystem.

   **MS_NODIRATIME**
          Do not update access times for directories on this
          filesystem.  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 filesystem.

   **MS_NOSUID**
          Do not honor set-user-ID and set-group-ID bits or file
          capabilities when executing programs from this filesystem.
          In addition, SELinux domain transitions require the
          permission _nosuidtransition_, which in turn needs also the
          policy capability _nnpnosuidtransition_.

   **MS_RDONLY**
          Mount filesystem read-only.

   **MS_REC** (since Linux 2.4.11)
          Used in conjunction with **MS_BIND** to create a recursive bind
          mount, and in conjunction with the propagation type flags
          to recursively change the propagation type of all of the
          mounts in a subtree.  See below for further details.

   **MS_RELATIME** (since Linux 2.6.20)
          When a file on this filesystem 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 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_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 filesystem 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 filesystem synchronous (as though the
          **O_SYNC** flag to [open(2)](../man2/open.2.html) was specified for all file opens to
          this filesystem).

   **MS_NOSYMFOLLOW** (since Linux 5.10)
          Do not follow symbolic links when resolving paths.
          Symbolic links can still be created, and [readlink(1)](../man1/readlink.1.html),
          [readlink(2)](../man2/readlink.2.html), [realpath(1)](../man1/realpath.1.html), and [realpath(3)](../man3/realpath.3.html) all still work
          properly.

   From Linux 2.4 onward, some of the above flags are settable on a
   per-mount basis, while others apply to the superblock of the
   mounted filesystem, meaning that all mounts of the same filesystem
   share those flags.  (Previously, all of the flags were per-
   superblock.)

   The per-mount-point flags are as follows:

   •  Since Linux 2.4: **MS_NODEV**, **MS_NOEXEC**, and **MS_NOSUID** flags are
      settable on a per-mount-point basis.

   •  Additionally, since Linux 2.6.16: **MS_NOATIME** and **MS_NODIRATIME**.

   •  Additionally, since Linux 2.6.20: **MS_RELATIME**.

   The following flags are per-superblock: **MS_DIRSYNC**, **MS_LAZYTIME**,
   **MS_MANDLOCK**, **MS_SILENT**, and **MS_SYNCHRONOUS**.  The initial settings
   of these flags are determined on the first mount of the
   filesystem, and will be shared by all subsequent mounts of the
   same filesystem.  Subsequently, the settings of the flags can be
   changed via a remount operation (see below).  Such changes will be
   visible via all mounts associated with the filesystem.

   Since Linux 2.6.16, **MS_RDONLY** can be set or cleared on a per-
   mount-point basis as well as on the underlying filesystem
   superblock.  The mounted filesystem will be writable only if
   neither the filesystem nor the mountpoint are flagged as read-
   only.

Remounting an existing mount An existing mount may be remounted by specifying MS_REMOUNT in mountflags. This allows you to change the mountflags and data of an existing mount without having to unmount and remount the filesystem. target should be the same value specified in the initial mount() call.

   The _source_ and _filesystemtype_ arguments are ignored.

   The _mountflags_ and _data_ arguments should match the values used in
   the original **mount**() call, except for those parameters that are
   being deliberately changed.

   The following _mountflags_ can be changed: **MS_LAZYTIME**, **MS_MANDLOCK**,
   **MS_NOATIME**, **MS_NODEV**, **MS_NODIRATIME**, **MS_NOEXEC**, **MS_NOSUID**,
   **MS_RELATIME**, **MS_RDONLY**, **MS_STRICTATIME** (whose effect is to clear
   the **MS_NOATIME** and **MS_RELATIME** flags), and **MS_SYNCHRONOUS**.
   Attempts to change the setting of the **MS_DIRSYNC** and **MS_SILENT**
   flags during a remount are silently ignored.  Note that changes to
   per-superblock flags are visible via all mounts of the associated
   filesystem (because the per-superblock flags are shared by all
   mounts).

   Since Linux 3.17, if none of **MS_NOATIME**, **MS_NODIRATIME**,
   **MS_RELATIME**, or **MS_STRICTATIME** is specified in _mountflags_, then
   the remount operation preserves the existing values of these flags
   (rather than defaulting to **MS_RELATIME**).

   Since Linux 2.6.26, the **MS_REMOUNT** flag can be used with **MS_BIND**
   to modify only the per-mount-point flags.  This is particularly
   useful for setting or clearing the "read-only" flag on a mount
   without changing the underlying filesystem.  Specifying _mountflags_
   as:

       MS_REMOUNT | MS_BIND | MS_RDONLY

   will make access through this mountpoint read-only, without
   affecting other mounts.

Creating a bind mount If mountflags includes MS_BIND (available since Linux 2.4), then perform a bind mount. A bind mount makes a file or a directory subtree visible at another point within the single directory hierarchy. Bind mounts may cross filesystem boundaries and span chroot(2) jails.

   The _filesystemtype_ and _data_ arguments are ignored.

   The remaining bits (other than **MS_REC**, described below) in the
   _mountflags_ argument are also ignored.  (The bind mount has the
   same mount options as the underlying mount.)  However, see the
   discussion of remounting above, for a method of making an existing
   bind mount read-only.

   By default, when a directory is bind mounted, only that directory
   is mounted; if there are any submounts under the directory tree,
   they are not bind mounted.  If the **MS_REC** flag is also specified,
   then a recursive bind mount operation is performed: all submounts
   under the _source_ subtree (other than unbindable mounts) are also
   bind mounted at the corresponding location in the _target_ subtree.

Changing the propagation type of an existing mount If mountflags includes one of MS_SHARED, MS_PRIVATE, MS_SLAVE, or MS_UNBINDABLE (all available since Linux 2.6.15), then the propagation type of an existing mount is changed. If more than one of these flags is specified, an error results.

   The only other flags that can be specified while changing the
   propagation type are **MS_REC** (described below) and **MS_SILENT** (which
   is ignored).

   The _source_, _filesystemtype_, and _data_ arguments are ignored.

   The meanings of the propagation type flags are as follows:

   **MS_SHARED**
          Make this mount shared.  Mount and unmount events
          immediately under this mount will propagate to the other
          mounts that are members of this mount's peer group.
          Propagation here means that the same mount or unmount will
          automatically occur under all of the other mounts in the
          peer group.  Conversely, mount and unmount events that take
          place under peer mounts will propagate to this mount.

   **MS_PRIVATE**
          Make this mount private.  Mount and unmount events do not
          propagate into or out of this mount.

   **MS_SLAVE**
          If this is a shared mount that is a member of a peer group
          that contains other members, convert it to a slave mount.
          If this is a shared mount that is a member of a peer group
          that contains no other members, convert it to a private
          mount.  Otherwise, the propagation type of the mount is
          left unchanged.

          When a mount is a slave, mount and unmount events propagate
          into this mount from the (master) shared peer group of
          which it was formerly a member.  Mount and unmount events
          under this mount do not propagate to any peer.

          A mount can be the slave of another peer group while at the
          same time sharing mount and unmount events with a peer
          group of which it is a member.

   **MS_UNBINDABLE**
          Make this mount unbindable.  This is like a private mount,
          and in addition this mount can't be bind mounted.  When a
          recursive bind mount (**mount**() with the **MS_BIND** and **MS_REC**
          flags) is performed on a directory subtree, any unbindable
          mounts within the subtree are automatically pruned (i.e.,
          not replicated) when replicating that subtree to produce
          the target subtree.

   By default, changing the propagation type affects only the _target_
   mount.  If the **MS_REC** flag is also specified in _mountflags_, then
   the propagation type of all mounts under _target_ is also changed.

   For further details regarding mount propagation types (including
   the default propagation type assigned to new mounts), see
   [mount_namespaces(7)](../man7/mount%5Fnamespaces.7.html).

Moving a mount If mountflags contains the flag MS_MOVE (available since Linux 2.4.18), then move a subtree: source specifies an existing mount and target specifies the new location to which that mount is to be relocated. The move is atomic: at no point is the subtree unmounted.

   The remaining bits in the _mountflags_ argument are ignored, as are
   the _filesystemtype_ and _data_ arguments.

Creating a new mount If none of MS_REMOUNT, MS_BIND, MS_MOVE, MS_SHARED, MS_PRIVATE, MS_SLAVE, or MS_UNBINDABLE is specified in mountflags, then mount() performs its default action: creating a new mount. source specifies the source for the new mount, and target specifies the directory at which to create the mount point.

   The _filesystemtype_ and _data_ arguments are employed, and further
   bits may be specified in _mountflags_ to modify the behavior of the
   call.

RETURN VALUE top

   On success, zero is returned.  On error, -1 is returned, and _[errno](../man3/errno.3.html)_
   is set to indicate the error.

ERRORS top

   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 Linux kernel source
   code for details.

   **EACCES** A component of a path was not searchable.  (See also
          [path_resolution(7)](../man7/path%5Fresolution.7.html).)

   **EACCES** Mounting a read-only filesystem was attempted without
          giving the **MS_RDONLY** flag.

          The filesystem may be read-only for various reasons,
          including: it resides on a read-only optical disk; it is
          resides on a device with a physical switch that has been
          set to mark the device read-only; the filesystem
          implementation was compiled with read-only support; or
          errors were detected when initially mounting the
          filesystem, so that it was marked read-only and can't be
          remounted as read-write (until the errors are fixed).

          Some filesystems instead return the error **EROFS** on an
          attempt to mount a read-only filesystem.

   **EACCES** The block device _source_ is located on a filesystem mounted
          with the **MS_NODEV** option.

   **EBUSY** An attempt was made to stack a new mount directly on top of
          an existing mount point that was created in this mount
          namespace with the same _source_ and _target_.

   **EBUSY** _source_ cannot be remounted read-only, because it still
          holds files open for writing.

   **EFAULT** One of the pointer arguments points outside the user
          address space.

   **EINVAL** _source_ had an invalid superblock.

   **EINVAL** A remount operation (**MS_REMOUNT**) was attempted, but _source_
          was not already mounted on _target_.

   **EINVAL** A move operation (**MS_MOVE**) was attempted, but the mount
          tree under _source_ includes unbindable mounts and _target_ is
          a mount that has propagation type **MS_SHARED**.

   **EINVAL** A move operation (**MS_MOVE**) was attempted, but the parent
          mount of _source_ mount has propagation type **MS_SHARED**.

   **EINVAL** A move operation (**MS_MOVE**) was attempted, but _source_ was
          not a mount, or was '/'.

   **EINVAL** A bind operation (**MS_BIND**) was requested where _source_
          referred a mount namespace magic link (i.e., a
          _/proc/_pid_/ns/mnt_ magic link or a bind mount to such a link)
          and the propagation type of the parent mount of _target_ was
          **MS_SHARED**, but propagation of the requested bind mount
          could lead to a circular dependency that might prevent the
          mount namespace from ever being freed.

   **EINVAL** _mountflags_ includes more than one of **MS_SHARED**, **MS_PRIVATE**,
          **MS_SLAVE**, or **MS_UNBINDABLE**.

   **EINVAL** _mountflags_ includes **MS_SHARED**, **MS_PRIVATE**, **MS_SLAVE**, or
          **MS_UNBINDABLE** and also includes a flag other than **MS_REC** or
          **MS_SILENT**.

   **EINVAL** An attempt was made to bind mount an unbindable mount.

   **EINVAL** In an unprivileged mount namespace (i.e., a mount namespace
          owned by a user namespace that was created by an
          unprivileged user), a bind mount operation (**MS_BIND**) was
          attempted without specifying (**MS_REC**), which would have
          revealed the filesystem tree underneath one of the
          submounts of the directory being bound.

   **ELOOP** Too many links encountered during pathname resolution.

   **ELOOP** A move operation was attempted, and _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.

   **EPERM** An attempt was made to modify (**MS_REMOUNT**) the **MS_RDONLY**,
          **MS_NOSUID**, or **MS_NOEXEC** flag, or one of the "atime" flags
          (**MS_NOATIME**, **MS_NODIRATIME**, **MS_RELATIME**) of an existing
          mount, but the mount is locked; see [mount_namespaces(7)](../man7/mount%5Fnamespaces.7.html).

   **EROFS** Mounting a read-only filesystem was attempted without
          giving the **MS_RDONLY** flag.  See **EACCES**, above.

STANDARDS top

   Linux.

HISTORY top

   The definitions of **MS_DIRSYNC**, **MS_MOVE**, **MS_PRIVATE**, **MS_REC**,
   **MS_RELATIME**, **MS_SHARED**, **MS_SLAVE**, **MS_STRICTATIME**, and
   **MS_UNBINDABLE** were added to glibc headers in glibc 2.12.

   Since Linux 2.4 a single filesystem can be mounted at multiple
   mount points, and multiple mounts can be stacked on the same mount
   point.

   The _mountflags_ argument may have the magic number 0xC0ED
   (**MS_MGC_VAL**) in the top 16 bits.  (All of the other flags
   discussed in DESCRIPTION occupy the low order 16 bits of
   _mountflags_.)  Specifying **MS_MGC_VAL** was required before Linux 2.4,
   but since Linux 2.4 is no longer required and is ignored if
   specified.

   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.

NOTES top

Mount namespaces Starting with Linux 2.4.19, Linux provides mount namespaces. A mount namespace is the set of filesystem mounts that are visible to a process. Mount 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)](../man2/fork.2.html) shares its parent's mount
   namespace; the mount namespace is preserved across an [execve(2)](../man2/execve.2.html).

   A process can obtain a private mount namespace if: it was created
   using the [clone(2)](../man2/clone.2.html) **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)](../man2/clone.2.html); or it calls [unshare(2)](../man2/unshare.2.html) 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.

   For further details on mount namespaces, see [mount_namespaces(7)](../man7/mount%5Fnamespaces.7.html).

Parental relationship between mounts Each mount has a parent mount. The overall parental relationship of all mounts defines the single directory hierarchy seen by the processes within a mount namespace.

   The parent of a new mount is defined when the mount is created.
   In the usual case, the parent of a new mount is the mount of the
   filesystem containing the directory or file at which the new mount
   is attached.  In the case where a new mount is stacked on top of
   an existing mount, the parent of the new mount is the previous
   mount that was stacked at that location.

   The parental relationship between mounts can be discovered via the
   _/proc/_pid_/mountinfo_ file (see below).

/proc/pid_/mounts_ and _/proc/_pid_/mountinfo_ The Linux-specific _/proc/_pid_/mounts_ file exposes the list of mounts in the mount namespace of the process with the specified ID. The _/proc/_pid_/mountinfo_ file exposes even more information about mounts, including the propagation type and mount ID information that makes it possible to discover the parental relationship between mounts. See proc(5) and mount_namespaces(7) for details of this file.

SEE ALSO top

   [mountpoint(1)](../man1/mountpoint.1.html), [chroot(2)](../man2/chroot.2.html), [FS_IOC_SETFLAGS(2const)](../man2/FS%5FIOC%5FSETFLAGS.2const.html),
   [mount_setattr(2)](../man2/mount%5Fsetattr.2.html), [pivot_root(2)](../man2/pivot%5Froot.2.html), [umount(2)](../man2/umount.2.html), [mount_namespaces(7)](../man7/mount%5Fnamespaces.7.html),
   [path_resolution(7)](../man7/path%5Fresolution.7.html), [findmnt(8)](../man8/findmnt.8.html), [lsblk(8)](../man8/lsblk.8.html), [mount(8)](../man8/mount.8.html), [umount(8)](../man8/umount.8.html)

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Linux man-pages 6.10 2024-07-23 mount(2)


Pages that refer to this page:access(2), execve(2), fcntl(2), FS_IOC_SETFLAGS(2const), mkdir(2), mknod(2), mount_setattr(2), open(2), pivot_root(2), seccomp_unotify(2), statfs(2), syscalls(2), umount(2), statvfs(3), filesystems(5), proc_pid_mountinfo(5), systemd.exec(5), capabilities(7), cgroups(7), inode(7), mount_namespaces(7), namespaces(7), user_namespaces(7), mount(8)