1MOUNT(2) Linux Programmer's Manual MOUNT(2)
2
6 mount - mount filesystem
7
9 #include <sys/mount.h>
10 int mount(const char *source, const char *target,
12 const char *filesystemtype, unsigned long mountflags,
13 const void *data);
14
16 mount() attaches the filesystem specified by source (which is often a
17 pathname referring to a device, but can also be the pathname of a
18 directory or file, or a dummy string) to the location (a directory or
19 file) specified by the pathname in target.
20 Appropriate privilege (Linux: the CAP_SYS_ADMIN capability) is required
22 to mount filesystems.
23 Values for the filesystemtype argument supported by the kernel are
25 listed in /proc/filesystems (e.g., "btrfs", "ext4", "jfs", "xfs",
26 "vfat", "fuse", "tmpfs", "cgroup", "proc", "mqueue", "nfs", "cifs",
27 "iso9660"). Further types may become available when the appropriate
28 modules are loaded.
29 The data argument is interpreted by the different filesystems. Typi‐
31 cally it is a string of comma-separated options understood by this
32 filesystem. See mount(8) for details of the options available for each
33 filesystem type.
34 A call to mount() performs one of a number of general types of opera‐
36 tion, depending on the bits specified in mountflags. The choice of
37 which operation to perform is determined by testing the bits set in
38 mountflags, with the tests being conducted in the order listed here:
39 * Remount an existing mount: mountflags includes MS_REMOUNT.
41 * Create a bind mount: mountflags includes MS_BIND.
43 * Change the propagation type of an existing mount: mountflags
45 includes one of MS_SHARED, MS_PRIVATE, MS_SLAVE, or MS_UNBINDABLE.
46 * Move an existing mount to a new location: mountflags includes
48 MS_MOVE.
49 * Create a new mount: mountflags includes none of the above flags.
51 Each of these operations is detailed later in this page. Further flags
53 may be specified in mountflags to modify the behavior of mount(), as
54 described below.
55 Additional mount flags
57 The list below describes the additional flags that can be specified in
58 mountflags. Note that some operation types ignore some or all of these
59 flags, as described later in this page.
60 MS_DIRSYNC (since Linux 2.5.19)
62 Make directory changes on this filesystem synchronous. (This
63 property can be obtained for individual directories or subtrees
64 using chattr(1).)
65 MS_LAZYTIME (since Linux 4.0)
67 Reduce on-disk updates of inode timestamps (atime, mtime, ctime)
68 by maintaining these changes only in memory. The on-disk time‐
69 stamps are updated only when:
70 (a) the inode needs to be updated for some change unrelated to
72 file timestamps;
73 (b) the application employs fsync(2), syncfs(2), or sync(2);
75 (c) an undeleted inode is evicted from memory; or
77 (d) more than 24 hours have passed since the inode was written
79 to disk.
80 This mount option significantly reduces writes needed to update
82 the inode's timestamps, especially mtime and atime. However, in
83 the event of a system crash, the atime and mtime fields on disk
84 might be out of date by up to 24 hours.
85 Examples of workloads where this option could be of significant
87 benefit include frequent random writes to preallocated files, as
88 well as cases where the MS_STRICTATIME mount option is also
89 enabled. (The advantage of combining MS_STRICTATIME and
90 MS_LAZYTIME is that stat(2) will return the correctly updated
91 atime, but the atime updates will be flushed to disk only in the
92 cases listed above.)
93 MS_MANDLOCK
95 Permit mandatory locking on files in this filesystem. (Manda‐
96 tory locking must still be enabled on a per-file basis, as
97 described in fcntl(2).) Since Linux 4.5, this mount option
98 requires the CAP_SYS_ADMIN capability and a kernel configured
99 with the CONFIG_MANDATORY_FILE_LOCKING option.
100 MS_NOATIME
102 Do not update access times for (all types of) files on this
103 filesystem.
104 MS_NODEV
106 Do not allow access to devices (special files) on this filesys‐
107 tem.
108 MS_NODIRATIME
110 Do not update access times for directories on this filesystem.
111 This flag provides a subset of the functionality provided by
112 MS_NOATIME; that is, MS_NOATIME implies MS_NODIRATIME.
113 MS_NOEXEC
115 Do not allow programs to be executed from this filesystem.
116 MS_NOSUID
118 Do not honor set-user-ID and set-group-ID bits or file capabili‐
119 ties when executing programs from this filesystem.
120 MS_RDONLY
122 Mount filesystem read-only.
123 MS_REC (since Linux 2.4.11)
125 Used in conjunction with MS_BIND to create a recursive bind
126 mount, and in conjunction with the propagation type flags to
127 recursively change the propagation type of all of the mounts in
128 a subtree. See below for further details.
129 MS_RELATIME (since Linux 2.6.20)
131 When a file on this filesystem is accessed, update the file's
132 last access time (atime) only if the current value of atime is
133 less than or equal to the file's last modification time (mtime)
134 or last status change time (ctime). This option is useful for
135 programs, such as mutt(1), that need to know when a file has
136 been read since it was last modified. Since Linux 2.6.30, the
137 kernel defaults to the behavior provided by this flag (unless
138 MS_NOATIME was specified), and the MS_STRICTATIME flag is
139 required to obtain traditional semantics. In addition, since
140 Linux 2.6.30, the file's last access time is always updated if
141 it is more than 1 day old.
142 MS_SILENT (since Linux 2.6.17)
144 Suppress the display of certain (printk()) warning messages in
145 the kernel log. This flag supersedes the misnamed and obsolete
146 MS_VERBOSE flag (available since Linux 2.4.12), which has the
147 same meaning.
148 MS_STRICTATIME (since Linux 2.6.30)
150 Always update the last access time (atime) when files on this
151 filesystem are accessed. (This was the default behavior before
152 Linux 2.6.30.) Specifying this flag overrides the effect of
153 setting the MS_NOATIME and MS_RELATIME flags.
154 MS_SYNCHRONOUS
156 Make writes on this filesystem synchronous (as though the O_SYNC
157 flag to open(2) was specified for all file opens to this
158 filesystem).
159 From Linux 2.4 onward, some of the above flags are settable on a per-
161 mount basis, while others apply to the superblock of the mounted
162 filesystem, meaning that all mounts of the same filesystem share those
163 flags. (Previously, all of the flags were per-superblock.)
164 The per-mount-point flags are as follows:
166 * Since Linux 2.4: MS_NODEV, MS_NOEXEC, and MS_NOSUID flags are set‐
168 table on a per-mount-point basis.
169 * Since Linux 2.6.16: MS_NOATIME and MS_NODIRATIME.
171 * Since Linux 2.6.20: MS_RELATIME.
173 The following flags are per-superblock: MS_DIRSYNC, MS_LAZYTIME,
175 MS_MANDLOCK, MS_SILENT, and MS_SYNCHRONOUS. The initial settings of
176 these flags are determined on the first mount of the filesystem, and
177 will be shared by all subsequent mounts of the same filesystem. Subse‐
178 quently, the settings of the flags can be changed via a remount opera‐
179 tion (see below). Such changes will be visible via all mount points
180 associated with the filesystem.
181 Since Linux 2.6.16, MS_RDONLY can be set or cleared on a per-mount-
183 point basis as well as on the underlying filesystem superblock. The
184 mounted filesystem will be writable only if neither the filesystem nor
185 the mountpoint are flagged as read-only.
186 Remounting an existing mount
188 An existing mount may be remounted by specifying MS_REMOUNT in mount‐
189 flags. This allows you to change the mountflags and data of an exist‐
190 ing mount without having to unmount and remount the filesystem. target
191 should be the same value specified in the initial mount() call.
192 The source and filesystemtype arguments are ignored.
194 The mountflags and data arguments should match the values used in the
196 original mount() call, except for those parameters that are being
197 deliberately changed.
198 The following mountflags can be changed: MS_LAZYTIME, MS_MANDLOCK,
200 MS_NOATIME, MS_NODEV, MS_NODIRATIME, MS_NOEXEC, MS_NOSUID, MS_RELATIME,
201 MS_RDONLY, MS_STRICTATIME (whose effect is to clear the MS_NOATIME and
202 MS_RELATIME flags), and MS_SYNCHRONOUS. Attempts to change the setting
203 of the MS_DIRSYNC and MS_SILENT flags during a remount are silently
204 ignored. Note that changes to per-superblock flags are visible via all
205 mount points of the associated filesystem (because the per-superblock
206 flags are shared by all mount points).
207 Since Linux 3.17, if none of MS_NOATIME, MS_NODIRATIME, MS_RELATIME, or
209 MS_STRICTATIME is specified in mountflags, then the remount operation
210 preserves the existing values of these flags (rather than defaulting to
211 MS_RELATIME).
212 Since Linux 2.6.26, the MS_REMOUNT flag can be used with MS_BIND to
214 modify only the per-mount-point flags. This is particularly useful for
215 setting or clearing the "read-only" flag on a mount point without
216 changing the underlying filesystem. Specifying mountflags as:
217 MS_REMOUNT | MS_BIND | MS_RDONLY
219 will make access through this mountpoint read-only, without affecting
221 other mount points.
222 Creating a bind mount
224 If mountflags includes MS_BIND (available since Linux 2.4), then per‐
225 form a bind mount. A bind mount makes a file or a directory subtree
226 visible at another point within the single directory hierarchy. Bind
227 mounts may cross filesystem boundaries and span chroot(2) jails.
228 The filesystemtype and data arguments are ignored.
230 The remaining bits (other than MS_REC, described below) in the mount‐
232 flags argument are also ignored. (The bind mount has the same mount
233 options as the underlying mount point.) However, see the discussion of
234 remounting above, for a method of making an existing bind mount read-
235 only.
236 By default, when a directory is bind mounted, only that directory is
238 mounted; if there are any submounts under the directory tree, they are
239 not bind mounted. If the MS_REC flag is also specified, then a recur‐
240 sive bind mount operation is performed: all submounts under the source
241 subtree (other than unbindable mounts) are also bind mounted at the
242 corresponding location in the target subtree.
243 Changing the propagation type of an existing mount
245 If mountflags includes one of MS_SHARED, MS_PRIVATE, MS_SLAVE, or
246 MS_UNBINDABLE (all available since Linux 2.6.15), then the propagation
247 type of an existing mount is changed. If more than one of these flags
248 is specified, an error results.
249 The only other flags that can be specified while changing the propaga‐
251 tion type are MS_REC (described below) and MS_SILENT (which is
252 ignored).
253 The source, filesystemtype, and data arguments are ignored.
255 The meanings of the propagation type flags are as follows:
257 MS_SHARED
259 Make this mount point shared. Mount and unmount events immedi‐
260 ately under this mount point will propagate to the other mount
261 points that are members of this mount's peer group. Propagation
262 here means that the same mount or unmount will automatically
263 occur under all of the other mount points in the peer group.
264 Conversely, mount and unmount events that take place under peer
265 mount points will propagate to this mount point.
266 MS_PRIVATE
268 Make this mount point private. Mount and unmount events do not
269 propagate into or out of this mount point.
270 MS_SLAVE
272 If this is a shared mount point that is a member of a peer group
273 that contains other members, convert it to a slave mount. If
274 this is a shared mount point that is a member of a peer group
275 that contains no other members, convert it to a private mount.
276 Otherwise, the propagation type of the mount point is left
277 unchanged.
278 When a mount point is a slave, mount and unmount events propa‐
280 gate into this mount point from the (master) shared peer group
281 of which it was formerly a member. Mount and unmount events
282 under this mount point do not propagate to any peer.
283 A mount point can be the slave of another peer group while at
285 the same time sharing mount and unmount events with a peer group
286 of which it is a member.
287 MS_UNBINDABLE
289 Make this mount unbindable. This is like a private mount, and
290 in addition this mount can't be bind mounted. When a recursive
291 bind mount (mount() with the MS_BIND and MS_REC flags) is per‐
292 formed on a directory subtree, any unbindable mounts within the
293 subtree are automatically pruned (i.e., not replicated) when
294 replicating that subtree to produce the target subtree.
295 By default, changing the propagation type affects only the target mount
297 point. If the MS_REC flag is also specified in mountflags, then the
298 propagation type of all mount points under target is also changed.
299 For further details regarding mount propagation types (including the
301 default propagation type assigned to new mounts), see mount_names‐
302 paces(7).
303 Moving a mount
305 If mountflags contains the flag MS_MOVE (available since Linux 2.4.18),
306 then move a subtree: source specifies an existing mount point and tar‐
307 get specifies the new location to which that mount point is to be relo‐
308 cated. The move is atomic: at no point is the subtree unmounted.
309 The remaining bits in the mountflags argument are ignored, as are the
311 filesystemtype and data arguments.
312 Creating a new mount point
314 If none of MS_REMOUNT, MS_BIND, MS_MOVE, MS_SHARED, MS_PRIVATE,
315 MS_SLAVE, or MS_UNBINDABLE is specified in mountflags, then mount()
316 performs its default action: creating a new mount point. source speci‐
317 fies the source for the new mount point, and target specifies the
318 directory at which to create the mount point.
319 The filesystemtype and data arguments are employed, and further bits
321 may be specified in mountflags to modify the behavior of the call.
322
324 On success, zero is returned. On error, -1 is returned, and errno is
325 set appropriately.
326
328 The error values given below result from filesystem type independent
329 errors. Each filesystem type may have its own special errors and its
330 own special behavior. See the Linux kernel source code for details.
331 EACCES A component of a path was not searchable. (See also path_reso‐
333 lution(7).)
334 EACCES Mounting a read-only filesystem was attempted without giving the
336 MS_RDONLY flag.
337 The filesystem may be read-only for various reasons, including:
339 it resides on a read-only optical disk; it is resides on a
340 device with a physical switch that has been set to mark the
341 device read-only; the filesystem implementation was compiled
342 with read-only support; or errors were detected when initially
343 mounting the filesystem, so that it was marked read-only and
344 can't be remounted as read-write (until the errors are fixed).
345 Some filesystems instead return the error EROFS on an attempt to
347 mount a read-only filesystem.
348 EACCES The block device source is located on a filesystem mounted with
350 the MS_NODEV option.
351 EBUSY An attempt was made to stack a new mount directly on top of an
353 existing mount point that was created in this mount namespace
354 with the same source and target.
355 EBUSY source cannot be remounted read-only, because it still holds
357 files open for writing.
358 EFAULT One of the pointer arguments points outside the user address
360 space.
361 EINVAL source had an invalid superblock.
363 EINVAL A remount operation (MS_REMOUNT) was attempted, but source was
365 not already mounted on target.
366 EINVAL A move operation (MS_MOVE) was attempted, but the mount tree
368 under source includes unbindable mounts and target is a mount
369 point that has propagation type MS_SHARED.
370 EINVAL A move operation (MS_MOVE) was attempted, but the parent mount
372 of source mount has propagation type MS_SHARED.
373 EINVAL A move operation (MS_MOVE) was attempted, but source was not a
375 mount point, or was '/'.
376 EINVAL mountflags includes more than one of MS_SHARED, MS_PRIVATE,
378 MS_SLAVE, or MS_UNBINDABLE.
379 EINVAL mountflags includes MS_SHARED, MS_PRIVATE, MS_SLAVE, or
381 MS_UNBINDABLE and also includes a flag other than MS_REC or
382 MS_SILENT.
383 EINVAL An attempt was made to bind mount an unbindable mount.
385 EINVAL In an unprivileged mount namespace (i.e., a mount namespace
387 owned by a user namespace that was created by an unprivileged
388 user), a bind mount operation (MS_BIND) was attempted without
389 specifying (MS_REC), which would have revealed the filesystem
390 tree underneath one of the submounts of the directory being
391 bound.
392 ELOOP Too many links encountered during pathname resolution.
394 ELOOP A move operation was attempted, and target is a descendant of
396 source.
397 EMFILE (In case no block device is required:) Table of dummy devices is
399 full.
400 ENAMETOOLONG
402 A pathname was longer than MAXPATHLEN.
403 ENODEV filesystemtype not configured in the kernel.
405 ENOENT A pathname was empty or had a nonexistent component.
407 ENOMEM The kernel could not allocate a free page to copy filenames or
409 data into.
410 ENOTBLK
412 source is not a block device (and a device was required).
413 ENOTDIR
415 target, or a prefix of source, is not a directory.
416 ENXIO The major number of the block device source is out of range.
418 EPERM The caller does not have the required privileges.
420 EROFS Mounting a read-only filesystem was attempted without giving the
422 MS_RDONLY flag. See EACCES, above.
423
425 The definitions of MS_DIRSYNC, MS_MOVE, MS_PRIVATE, MS_REC, MS_RELA‐
426 TIME, MS_SHARED, MS_SLAVE, MS_STRICTATIME, and MS_UNBINDABLE were added
427 to glibc headers in version 2.12.
428
430 This function is Linux-specific and should not be used in programs
431 intended to be portable.
432
434 Since Linux 2.4 a single filesystem can be mounted at multiple mount
435 points, and multiple mounts can be stacked on the same mount point.
436 The mountflags argument may have the magic number 0xC0ED (MS_MGC_VAL)
438 in the top 16 bits. (All of the other flags discussed in DESCRIPTION
439 occupy the low order 16 bits of mountflags.) Specifying MS_MGC_VAL was
440 required in kernel versions prior to 2.4, but since Linux 2.4 is no
441 longer required and is ignored if specified.
442 The original MS_SYNC flag was renamed MS_SYNCHRONOUS in 1.1.69 when a
444 different MS_SYNC was added to <mman.h>.
445 Before Linux 2.4 an attempt to execute a set-user-ID or set-group-ID
447 program on a filesystem mounted with MS_NOSUID would fail with EPERM.
448 Since Linux 2.4 the set-user-ID and set-group-ID bits are just silently
449 ignored in this case.
450 Mount namespaces
452 Starting with kernel 2.4.19, Linux provides mount namespaces. A mount
453 namespace is the set of filesystem mounts that are visible to a
454 process. Mount namespaces can be (and usually are) shared between mul‐
455 tiple processes, and changes to the namespace (i.e., mounts and
456 unmounts) by one process are visible to all other processes sharing the
457 same namespace. (The pre-2.4.19 Linux situation can be considered as
458 one in which a single namespace was shared by every process on the sys‐
459 tem.)
460 A child process created by fork(2) shares its parent's mount namespace;
462 the mount namespace is preserved across an execve(2).
463 A process can obtain a private mount namespace if: it was created using
465 the clone(2) CLONE_NEWNS flag, in which case its new namespace is ini‐
466 tialized to be a copy of the namespace of the process that called
467 clone(2); or it calls unshare(2) with the CLONE_NEWNS flag, which
468 causes the caller's mount namespace to obtain a private copy of the
469 namespace that it was previously sharing with other processes, so that
470 future mounts and unmounts by the caller are invisible to other pro‐
471 cesses (except child processes that the caller subsequently creates)
472 and vice versa.
473 For further details on mount namespaces, see mount_namespaces(7).
475 Parental relationship between mount points
477 Each mount point has a parent mount point. The overall parental rela‐
478 tionship of all mount points defines the single directory hierarchy
479 seen by the processes within a mount namespace.
480 The parent of a new mount point is defined when the mount point is cre‐
482 ated. In the usual case, the parent of a new mount is the mount point
483 of the filesystem containing the directory or file at which the new
484 mount is attached. In the case where a new mount is stacked on top of
485 an existing mount, the parent of the new mount is the previous mount
486 that was stacked at that location.
487 The parental relationship between mount points can be discovered via
489 the /proc/[pid]/mountinfo file (see below).
490 /proc/[pid]/mounts and /proc/[pid]/mountinfo
492 The Linux-specific /proc/[pid]/mounts file exposes the list of mount
493 points in the mount namespace of the process with the specified ID.
494 The /proc/[pid]/mountinfo file exposes even more information about
495 mount points, including the propagation type and mount ID information
496 that makes it possible to discover the parental relationship between
497 mount points. See proc(5) and mount_namespaces(7) for details of this
498 file.
499
501 mountpoint(1), chroot(2), ioctl_iflags(2), pivot_root(2), umount(2),
502 mount_namespaces(7), path_resolution(7), findmnt(8), lsblk(8),
503 mount(8), umount(8)
504
506 This page is part of release 5.04 of the Linux man-pages project. A
507 description of the project, information about reporting bugs, and the
508 latest version of this page, can be found at
509 https://www.kernel.org/doc/man-pages/.
510Linux 2019-08-02 MOUNT(2)