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

NAME         top

       mount - mount filesystem

SYNOPSIS         top

       #include <sys/mount.h>

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

DESCRIPTION         top

       mount() attaches the filesystem specified by source (which is often a
       device name, but can also be a directory name or a dummy) to the
       directory specified by target.

       Appropriate privilege (Linux: the CAP_SYS_ADMIN capability) is
       required to mount filesystems.

       Since Linux 2.4 a single filesystem can be visible at multiple mount
       points, and multiple mounts can be stacked on the same mount point.

       Values for the filesystemtype argument supported by the kernel are
       listed in /proc/filesystems (e.g., "minix", "ext2", "ext3", "jfs",
       "xfs", "reiserfs", "msdos", "proc", "nfs", "iso9660").  Further types
       may become available when the appropriate modules are loaded.

       The mountflags argument may have the magic number 0xC0ED (MS_MGC_VAL)
       in the top 16 bits (this was required in kernel versions prior to
       2.4, but is no longer required and ignored if specified), and various
       mount flags in the low order 16 bits:

       MS_BIND (Linux 2.4 onward)
              Perform a bind mount, making a file or a directory subtree
              visible at another point within a filesystem.  Bind mounts may
              cross filesystem boundaries and span chroot(2) jails.  The
              filesystemtype and data arguments are ignored.  Up until Linux
              2.6.26, mountflags was also ignored (the bind mount has the
              same mount options as the underlying mount point).

       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 sync(2);

              (c)  an undeleted inode is evicted from memory; or

              (d)  more than 24 hours have passed since the inode was
                   written to disk.

              This mount option significantly reduces writes needed to
              update the inode's timestamps, especially mtime and atime.
              However, in the event of a system crash, the atime and mtime
              fields on disk might be out of date by up to 24 hours.

              Examples of workloads where this option could be of
              significant benefit include frequent random writes to
              preallocated files, as well as cases where the MS_STRICTATIME
              mount option is also enabled.  (The advantage of combining
              MS_STRICTATIME and MS_LAZYTIME is that stat(2) will return the
              correctly updated atime, but the atime updates will be flushed
              to disk only in the cases listed above.)

              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.

              Move a subtree.  source specifies an existing mount point and
              target specifies the new location.  The move is atomic: at no
              point is the subtree unmounted.  The filesystemtype,
              mountflags, and data arguments are ignored.

              Do not update access times for (all types of) files on this

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

              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 when executing
              programs from this filesystem.

              Mount filesystem read-only.

       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.

              Remount an existing mount.  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; source and
              filesystemtype 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

              The following mountflags can be changed: MS_RDONLY,
              MS_SYNCHRONOUS, MS_MANDLOCK; before kernel 2.6.16, the
              following could also be changed: MS_NOATIME and MS_NODIRATIME;
              and, additionally, before kernel 2.4.10, the following could
              also be changed: MS_NOSUID, MS_NODEV, MS_NOEXEC.

       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).

       From Linux 2.4 onward, the MS_NODEV, MS_NOEXEC, and MS_NOSUID flags
       are settable on a per-mount-point basis.  From kernel 2.6.16 onward,
       MS_NOATIME and MS_NODIRATIME are also settable on a per-mount-point
       basis.  The MS_RELATIME flag is also settable on a per-mount-point

       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.

RETURN VALUE         top

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

ERRORS         top

       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).)  Or, mounting a read-only filesystem was
              attempted without giving the MS_RDONLY flag.  Or, the block
              device source is located on a filesystem mounted with the
              MS_NODEV option.

       EBUSY  source is already mounted.  Or, it cannot be remounted read-
              only, because it still holds files open for writing.  Or, it
              cannot be mounted on target because target is still busy (it
              is the working directory of some thread, the mount point of
              another device, has open files, etc.).

       EFAULT One of the pointer arguments points outside the user address

       EINVAL source had an invalid superblock.  Or, a remount (MS_REMOUNT)
              was attempted, but source was not already mounted on target.
              Or, a move (MS_MOVE) was attempted, but source was not a mount
              point, or was '/'.

       ELOOP  Too many links encountered during pathname resolution.  Or, a
              move was attempted, while 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.

VERSIONS         top

       The definitions of MS_DIRSYNC, MS_MOVE, MS_REC, MS_RELATIME, and
       MS_STRICTATIME were added to glibc headers in version 2.12.

CONFORMING TO         top

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

NOTES         top

       The original MS_SYNC flag was renamed MS_SYNCHRONOUS in 1.1.69 when a
       different MS_SYNC was added to <mman.h>.

       Before Linux 2.4 an attempt to execute a set-user-ID or set-group-ID
       program on a filesystem mounted with MS_NOSUID would fail with EPERM.
       Since Linux 2.4 the set-user-ID and set-group-ID bits are just
       silently ignored in this case.

   Per-process namespaces
       Starting with kernel 2.4.19, Linux provides per-process mount
       namespaces.  A mount namespace is the set of filesystem mounts that
       are visible to a process.  Mount-point namespaces can be (and usually
       are) shared between multiple processes, and changes to the namespace
       (i.e., mounts and unmounts) by one process are visible to all other
       processes sharing the same namespace.  (The pre-2.4.19 Linux
       situation can be considered as one in which a single namespace was
       shared by every process on the system.)

       A child process created by fork(2) shares its parent's mount
       namespace; the mount namespace is preserved across an execve(2).

       A process can obtain a private mount namespace if: it was created
       using the clone(2) CLONE_NEWNS flag, in which case its new namespace
       is 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.

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

SEE ALSO         top

       umount(2), namespaces(7), path_resolution(7), lsblk(8), findmnt(8),
       mount(8), umount(8)

COLOPHON         top

       This page is part of release 4.06 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                            2016-03-15                         MOUNT(2)