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

       mount - mount filesystem

       #include <sys/mount.h>

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

       mount()  attaches  the filesystem specified by source (which is often a
       pathname referring to a device, but can also be the pathname of  a  di-
       rectory  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 in-
          cludes 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  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  en-
              abled.   (The advantage of combining MS_STRICTATIME and MS_LAZY-
              TIME 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.   (Manda-
              tory  locking  must still be enabled on a per-file basis, as de-
              scribed in fcntl(2).)  Since Linux 4.5, this  mount  option  re-
              quires 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 filesys-

              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 capabili-
              ties 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 re-
              cursively  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 re-
              quired 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

       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 set-
          table on a per-mount-point basis.

       *  Since Linux 2.6.16: MS_NOATIME and MS_NODIRATIME.

       *  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.  Subse-
       quently, the settings of the flags can be changed via a remount  opera-
       tion  (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 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 de-
       liberately changed.

       The following mountflags  can  be  changed:  MS_LAZYTIME,  MS_MANDLOCK,
       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 ig-
       nored.   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

       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  read-

       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 other flags that can be specified while changing the propaga-
       tion 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 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 oc-
              cur under all of the other mount points in the peer group.  Con-
              versely, mount and unmount events that  take  place  under  peer
              mount points will propagate to this mount point.

              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  un-

              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 un-
              der 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 recursive
              bind mount (mount() with the MS_BIND and MS_REC flags)  is  per-
              formed  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_name-

   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  di-
       rectory 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.

       On success, zero is returned.  On error, -1 is returned, and  errno  is
       set appropriately.

       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-

       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  de-
              vice 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 re-
              mounted 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 un-
              der  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 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_UN-
              BINDABLE  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

              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.

       The definitions of MS_DIRSYNC, MS_MOVE,  MS_PRIVATE,  MS_REC,  MS_RELA-
       to glibc headers in version 2.12.

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

       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 mul-
       tiple  processes,  and  changes  to the namespace (i.e., mounts and un-
       mounts) 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 sys-

       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.

       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  rela-
       tionship  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 cre-
       ated.   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

       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)

       This page is part of release 5.07 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                             2019-08-02                          MOUNT(2)

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