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NAME

       mount - mount filesystem

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

       #include <sys/mount.h>

       int mount(const char *source, const char *target,
                 const char *filesystemtype, unsigned long mountflags,
                 const void *_Nullable 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.  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 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 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 above.)

       MS_MANDLOCK
              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.

       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 filesystem.

       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 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 nnp_nosuid_transition.

       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).

       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 properly.

       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() 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.

       The following mountflags can be changed: MS_LAZYTIME, MS_MANDLOCK, MS_NOATIME,  MS_NODEV,  MS_NODIRATIME,
       MS_NOEXEC, MS_NOSUID, MS_RELATIME, MS_RDONLY, MS_STRICTATIME (whose effect is to clear the MS_NOATIME and
       MS_RELATIME flags), and MS_SYNCHRONOUS.  Attempts to change the setting of the MS_DIRSYNC  and  MS_SILENT
       flags  during  a remount are silently ignored.  Note that changes to per-superblock flags are visible via
       all 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:

           MS_REMOUNT | MS_BIND | MS_RDONLY

       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) jails.

       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:

       MS_SHARED
              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.

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

       MS_SLAVE
              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.

       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
              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
       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.   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.

RETURN VALUE

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

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_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 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 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 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.

       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.

STANDARDS

       Linux.

HISTORY

       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.

       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.

NOTES

   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 location.

       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.

SEE ALSO

       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)