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     NAME
          mount - mount filesystem

     SYNOPSIS
          #include <sys/mount.h>

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

     DESCRIPTION
          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 ker-
          nel 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 filesys-
          tems.  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.

          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

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             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 directo-
               ries 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 mem-
               ory.  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,

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               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 func-
               tionality provided by MS_NOATIME; that is, MS_NOATIME
               implies MS_NODIRATIME.

          MS_NOEXEC
               Do not allow programs to be executed from this filesys-
               tem.

          MS_NOSUID
               Do not honor set-user-ID and set-group-ID bits or file
               capabilities when executing programs from this filesys-
               tem.

          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 fur-
               ther 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.

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          MS_SILENT (since Linux 2.6.17)
               Suppress the display of certain (printk()) warning mes-
               sages in the kernel log.  This flag supersedes the mis-
               named 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 filesys-
          tem superblock.  The mounted filesystem will be writable

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          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 parame-
          ters 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 vis-
          ible 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.

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          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 opera-
          tion 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
          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 propaga-
               tion type of the mount point is left unchanged.

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               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 repli-
               cating 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: creat-
          ing 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 fur-
          ther bits may be specified in mountflags to modify the
          behavior of the call.

     RETURN VALUE
          On success, zero is returned.  On error, -1 is returned, and

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          errno is set appropriately.

     ERRORS
          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.

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          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 par-
               ent 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 aq/aq.

          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 descen-
               dant of source.

          EMFILE

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

     NOTES
          Since Linux 2.4 a single filesystem can be mounted at multi-
          ple 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

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          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 names-
          paces.  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 names-
          pace.  (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 names-
          pace 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 pro-
          cesses, 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

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          that was stacked at that location.

          The parental relationship between mount points can be dis-
          covered 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 pos-
          sible to discover the parental relationship between mount
          points.  See proc(5) and mount_namespaces(7) for details of
          this file.

     SEE ALSO
          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
          This page is part of release 5.10 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/.

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