mount(2)



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

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 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.  Typi-
       cally 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  opera-
       tion,  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  time-
              stamps 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.  (Manda-
              tory 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.

       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 filesys-
              tem.

       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 capabili-
              ties when executing programs from this filesystem.

       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).

       From Linux 2.4 onward, the MS_NODEV, MS_NOEXEC, and MS_NOSUID flags are
       settable  on  a  per-mount-point  basis.   From  kernel  2.6.16 onward,
       MS_NOATIME and MS_NODIRATIME are also  settable  on  a  per-mount-point
       basis.   The  MS_RELATIME  flag  is  also settable on a per-mount-point
       basis.  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.  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 mount-
       flags.  This allows you to change the mountflags and data of an  exist-
       ing 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.  Another exception is that MS_BIND has a  differ-
       ent  meaning  for remount, and it should be included only if explicitly
       desired.

       The following mountflags  can  be  changed:  MS_LAZYTIME,  MS_MANDLOCK,
       MS_NOATIME, MS_NODEV, MS_NODIRATIME, MS_NOEXEC, MS_NOSUID, MS_RELATIME,
       MS_RDONLY, and MS_SYNCHRONOUS.  Attempts to change the setting  of  the
       MS_DIRSYNC flag during a remount are silently ignored.

       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, this 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  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  in the mountflags argument are also ignored, with
       the exception of MS_REC.  (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  recur-
       sive  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 flags that can be used with changing the propagation type  are
       MS_REC and MS_SILENT.

       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 immedi-
              ately 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  per-
              formed  on  a directory subtree, any bind mounts within the sub-
              tree 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_names-
       paces(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  tar-
       get specifies the new location to which that mount point is to be relo-
       cated.  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 speci-
       fies 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
       On success, zero is returned.  On error, -1 is returned, and  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_reso-
              lution(7).)

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

       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 source was not a
              mount point, or was '/'.

       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.

VERSIONS
       The  definitions  of  MS_DIRSYNC, MS_MOVE, MS_PRIVATE, MS_REC, MS_RELA-
       TIME, 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  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.

   Per-process namespaces
       Starting  with  kernel  2.4.19, Linux provides per-process mount names-
       paces.  A mount namespace is the set of filesystem mounts that are vis-
       ible  to  a  process.   Mount-point 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  ini-
       tialized  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 pro-
       cesses (except child processes that the  caller  subsequently  creates)
       and vice versa.

       The  Linux-specific  /proc/[pid]/mounts  file exposes the list of mount
       points in the mount namespace of the process with the specified ID; see
       proc(5) for details.

SEE ALSO
       mountpoint(1),   umount(2),   mount_namespaces(7),  path_resolution(7),
       findmnt(8), lsblk(8), mount(8), umount(8)

COLOPHON
       This page is part of release 4.16 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                             2018-02-02                          MOUNT(2)

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