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       mount - mount filesystem


       Standard C library (libc, -lc)


       #include <sys/mount.h>

       int mount(const char *source, const char *target,
                 const char *filesystemtype, unsigned long mountflags,
                 const void *_Nullable 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:

              •  the inode needs to be updated for some change unrelated to file timestamps;

              •  the application employs fsync(2), syncfs(2), or sync(2);

              •  an undeleted inode is evicted from memory; or

              •  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.  Mandatory locking has been fully
              deprecated in Linux 5.15, so this flag should be considered deprecated.

              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 glibc 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 before Linux 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 Linux 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_setattr(2),  pivot_root(2),  umount(2),
       mount_namespaces(7), path_resolution(7), findmnt(8), lsblk(8), mount(8), umount(8)