mount(2) — Linux manual page


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.

       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

       *  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

       *  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.)

              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.

              Do not update access times for (all types of) files on this

              Do not allow access to devices (special files) on this

              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.

              Do not allow programs to be executed from this filesystem.

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

              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.

              Make writes on this filesystem synchronous (as though the
              O_SYNC flag to open(2) was specified for all file opens to
              this filesystem).

       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()

       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_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:


       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 per‐
       form 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 mount‐
       flags 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

       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 propaga‐
       tion 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 propa‐
       gation type are MS_REC (described below) and MS_SILENT (which is ig‐

       The source, filesystemtype, and data arguments are ignored.

       The meanings of the propagation type flags are as follows:

              Make this mount point shared.  Mount and unmount events imme‐
              diately 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 au‐
              tomatically 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

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

              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 propa‐
              gate 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.

              Make this mount unbindable.  This is like a private mount, and
              in addition this mount can't be bind mounted.  When a recur‐
              sive 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

       For further details regarding mount propagation types (including the
       default propagation type assigned to new mounts), see

   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

       The remaining bits in the mountflags argument are ignored, as are the
       filesystemtype and data arguments.

   Creating a new mount point
       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 appropriately.

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

       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

       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

       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

       ELOOP  Too many links encountered during pathname resolution.

       ELOOP  A move operation was attempted, and target is a descendant of

       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.

       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,
       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 5.09 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

Linux                            2020-08-13                         MOUNT(2)

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