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       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 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

       Values  for  the  filesystemtype  argument  supported  by  the  kernel   are   listed   in
       /proc/filesystems (e.g., "btrfs", "ext4", "jfs", "xfs", "vfat", "fuse", "tmpfs", "cgroup",
       "proc", "mqueue", "nfs", "cifs", "iso9660").  Further types may become available when  the
       appropriate modules are loaded.

       The  data  argument is interpreted by the different filesystems.  Typically it is a string
       of comma-separated options understood by this filesystem.  See mount(8) for details of the
       options  available  for  each filesystem type.  This argument may be specified as NULL, if
       there are no options.

       A call to mount() performs one of a number of general types of operation, depending on the
       bits  specified  in mountflags.  The choice of which operation to perform is determined by
       testing the bits set in mountflags, with the tests being conducted  in  the  order  listed

       *  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,

       *  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

       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

              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 filesystem.

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

              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

              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.  In addition, SELinux domain transitions require the
              permission  nosuid_transition,  which  in  turn  needs  also  the policy capability

              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).

       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

       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 mounts 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_NOATIME,  MS_NODEV,
       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  mounts  of  the  associated
       filesystem (because the per-superblock flags are shared by all mounts).

       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 without changing the underlying filesystem.  Specifying mountflags as:


       will make access through this mountpoint read-only, without affecting other mounts.

   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)

       The filesystemtype and data arguments are ignored.

       The remaining bits (other than MS_REC, described below) in  the  mountflags  argument  are
       also  ignored.   (The  bind  mount  has  the  same mount options as the underlying mount.)
       However, see the discussion of remounting above, for a method of making an  existing  bind
       mount read-only.

       By default, when a directory is bind mounted, only that directory is mounted; if there are
       any submounts under the directory tree, they are not bind mounted.  If the MS_REC flag  is
       also  specified,  then  a recursive bind mount operation is performed: all submounts under
       the  source  subtree  (other  than  unbindable  mounts)  are  also  bind  mounted  at  the
       corresponding location in the target subtree.

   Changing the propagation type of an existing mount
       If  mountflags  includes  one  of  MS_SHARED,  MS_PRIVATE, MS_SLAVE, or MS_UNBINDABLE (all
       available since Linux 2.6.15), then the propagation type of an existing mount is  changed.
       If more than one of these flags is specified, an error results.

       The  only other flags that can be specified while changing the propagation type are MS_REC
       (described below) and MS_SILENT (which is ignored).

       The source, filesystemtype, and data arguments are ignored.

       The meanings of the propagation type flags are as follows:

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

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

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

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

              A mount 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 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.  If the MS_REC
       flag is also specified in mountflags, then the propagation type of all mounts 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 and target specifies the new location to which
       that mount 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
       is specified in mountflags, then mount() performs  its  default  action:  creating  a  new
       mount.   source specifies the source for the new mount, 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.


       On  success, zero is returned.  On error, -1 is returned, and errno is set to indicate the


       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

       EACCES The block device source is located  on  a  filesystem  mounted  with  the  MS_NODEV

       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

       EFAULT One of the pointer arguments points outside the user address space.

       EINVAL source had an invalid superblock.

       EINVAL A  remount operation (MS_REMOUNT) was attempted, but source was not already mounted
              on target.

       EINVAL A move operation (MS_MOVE) was attempted, but the mount tree under source  includes
              unbindable mounts and target is a mount 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, 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

       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.

              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.

       EPERM  An  attempt  was made to modify (MS_REMOUNT) the MS_RDONLY, MS_NOSUID, or MS_NOEXEC
              flag, or one of the "atime" flags (MS_NOATIME, MS_NODIRATIME,  MS_RELATIME)  of  an
              existing mount, but the mount is locked; see mount_namespaces(7).

       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_RELATIME,  MS_SHARED,
       MS_SLAVE, MS_STRICTATIME, and MS_UNBINDABLE were added to glibc headers in version 2.12.


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


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

       The parent of a new mount is defined when the mount is created.  In the  usual  case,  the
       parent  of  a new mount is the mount 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

       The parental relationship between mounts 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  mounts in the mount
       namespace of the process with the specified ID.  The  /proc/[pid]/mountinfo  file  exposes
       even  more  information  about  mounts,  including  the  propagation  type  and  mount  ID
       information that makes it possible to discover the parental relationship  between  mounts.
       See proc(5) and mount_namespaces(7) for details of this file.


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


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