mount(2) — Linux manual page

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MOUNT(2)                Linux Programmer's Manual               MOUNT(2)

NAME         top

       mount - mount filesystem

SYNOPSIS         top

       #include <sys/mount.h>

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

       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:

              (a) the inode needs to be updated for some change
                  unrelated to file timestamps;

              (b) the application employs fsync(2), syncfs(2), or
                  sync(2);

              (c) an undeleted inode is evicted from memory; or

              (d) 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) 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).)  Since Linux 4.5, this
              mount option requires the CAP_SYS_ADMIN capability and a
              kernel configured with the CONFIG_MANDATORY_FILE_LOCKING
              option.

       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 nosuid_transition, which in turn needs also the
              policy capability nnp_nosuid_transition.

       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) 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),
              readlink(2), realpath(1), and realpath(3) 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 mount points 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 mount
       points of the associated filesystem (because the per-superblock
       flags are shared by all mount points).

       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
       point 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 mount points.

   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 point.)  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 point shared.  Mount and unmount events
              immediately under this mount point will propagate to the
              other mount points 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
              mount points in the peer group.  Conversely, mount and
              unmount events that take place under peer mount points
              will propagate to this mount point.

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

       MS_SLAVE
              If this is a shared mount point that is a member of a peer
              group that contains other members, convert it to a slave
              mount.  If this is a shared mount point 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 point is left unchanged.

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

              A mount point 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 point.  If the MS_REC flag is also specified in mountflags,
       then the propagation type of all mount points 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).

   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
       point and target specifies the new location to which that mount
       point 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 point
       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 point.
       source specifies the source for the new mount point, 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 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).)

       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 point 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 point, 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.

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

VERSIONS         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 version 2.12.

CONFORMING TO         top

       This function is Linux-specific and should not be used in
       programs intended to be portable.

NOTES         top

       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 in kernel
       versions prior to 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.

   Mount namespaces
       Starting with kernel 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) 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.

       For further details on mount namespaces, see mount_namespaces(7).

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

       The parent of a new mount point is defined when the mount point
       is created.  In the usual case, the parent of a new mount is the
       mount point 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 mount points 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
       mount points in the mount namespace of the process with the
       specified ID.  The /proc/[pid]/mountinfo file exposes even more
       information about mount points, including the propagation type
       and mount ID information that makes it possible to discover the
       parental relationship between mount points.  See proc(5) and
       mount_namespaces(7) for details of this file.

SEE ALSO         top

       mountpoint(1), chroot(2), ioctl_iflags(2), pivot_root(2),
       umount(2), mount_namespaces(7), path_resolution(7), findmnt(8),
       lsblk(8), mount(8), umount(8)

COLOPHON         top

       This page is part of release 5.12 of the Linux man-pages project.
       A description of the project, information about reporting bugs,
       and the latest version of this page, can be found at
       https://www.kernel.org/doc/man-pages/.

Linux                          2021-03-22                       MOUNT(2)

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