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 benefit
87 include frequent random writes to preallocated files, as well as cases
88 where the MS_STRICTATIME mount option is also enabled. (The advantage
89 of combining MS_STRICTATIME and MS_LAZYTIME is that stat(2) will return
90 the correctly updated atime, but the atime updates will be flushed to
91 disk only in the cases listed above.)
92 MS_MANDLOCK
94 Permit mandatory locking on files in this filesystem. (Manda‐
95 tory locking must still be enabled on a per-file basis, as
96 described in fcntl(2).) Since Linux 4.5, this mount option
97 requires the CAP_SYS_ADMIN capability.
98 MS_NOATIME
100 Do not update access times for (all types of) files on this
101 filesystem.
102 MS_NODEV
104 Do not allow access to devices (special files) on this filesys‐
105 tem.
106 MS_NODIRATIME
108 Do not update access times for directories on this filesystem.
109 This flag provides a subset of the functionality provided by
110 MS_NOATIME; that is, MS_NOATIME implies MS_NODIRATIME.
111 MS_NOEXEC
113 Do not allow programs to be executed from this filesystem.
114 MS_NOSUID
116 Do not honor set-user-ID and set-group-ID bits or file capabili‐
117 ties when executing programs from this filesystem.
118 MS_RDONLY
120 Mount filesystem read-only.
121 MS_REC (since Linux 2.4.11)
123 Used in conjunction with MS_BIND to create a recursive bind
124 mount, and in conjunction with the propagation type flags to
125 recursively change the propagation type of all of the mounts in
126 a subtree. See below for further details.
127 MS_RELATIME (since Linux 2.6.20)
129 When a file on this filesystem is accessed, update the file's
130 last access time (atime) only if the current value of atime is
131 less than or equal to the file's last modification time (mtime)
132 or last status change time (ctime). This option is useful for
133 programs, such as mutt(1), that need to know when a file has
134 been read since it was last modified. Since Linux 2.6.30, the
135 kernel defaults to the behavior provided by this flag (unless
136 MS_NOATIME was specified), and the MS_STRICTATIME flag is
137 required to obtain traditional semantics. In addition, since
138 Linux 2.6.30, the file's last access time is always updated if
139 it is more than 1 day old.
140 MS_SILENT (since Linux 2.6.17)
142 Suppress the display of certain (printk()) warning messages in
143 the kernel log. This flag supersedes the misnamed and obsolete
144 MS_VERBOSE flag (available since Linux 2.4.12), which has the
145 same meaning.
146 MS_STRICTATIME (since Linux 2.6.30)
148 Always update the last access time (atime) when files on this
149 filesystem are accessed. (This was the default behavior before
150 Linux 2.6.30.) Specifying this flag overrides the effect of
151 setting the MS_NOATIME and MS_RELATIME flags.
152 MS_SYNCHRONOUS
154 Make writes on this filesystem synchronous (as though the O_SYNC
155 flag to open(2) was specified for all file opens to this
156 filesystem).
157 From Linux 2.4 onward, the MS_NODEV, MS_NOEXEC, and MS_NOSUID flags are
159 settable on a per-mount-point basis. From kernel 2.6.16 onward,
160 MS_NOATIME and MS_NODIRATIME are also settable on a per-mount-point
161 basis. The MS_RELATIME flag is also settable on a per-mount-point
162 basis. Since Linux 2.6.16, MS_RDONLY can be set or cleared on a per-
163 mount-point basis as well as on the underlying filesystem. The mounted
164 filesystem will be writable only if neither the filesystem nor the
165 mountpoint are flagged as read-only.
166 Remounting an existing mount
168 An existing mount may be remounted by specifying MS_REMOUNT in mount‐
169 flags. This allows you to change the mountflags and data of an exist‐
170 ing mount without having to unmount and remount the filesystem. target
171 should be the same value specified in the initial mount() call.
172 The source and filesystemtype arguments are ignored.
174 The mountflags and data arguments should match the values used in the
176 original mount() call, except for those parameters that are being
177 deliberately changed. Another exception is that MS_BIND has a differ‐
178 ent meaning for remount, and it should be included only if explicitly
179 desired.
180 The following mountflags can be changed: MS_LAZYTIME, MS_MANDLOCK,
182 MS_NOATIME, MS_NODEV, MS_NODIRATIME, MS_NOEXEC, MS_NOSUID, MS_RELATIME,
183 MS_RDONLY, and MS_SYNCHRONOUS. Attempts to change the setting of the
184 MS_DIRSYNC flag during a remount are silently ignored.
185 Since Linux 3.17, if none of MS_NOATIME, MS_NODIRATIME, MS_RELATIME, or
187 MS_STRICTATIME is specified in mountflags, then the remount operation
188 preserves the existing values of these flags (rather than defaulting to
189 MS_RELATIME).
190 Since Linux 2.6.26, this flag can be used with MS_BIND to modify only
192 the per-mount-point flags. This is particularly useful for setting or
193 clearing the "read-only" flag on a mount point without changing the
194 underlying filesystem. Specifying mountflags as:
195 MS_REMOUNT | MS_BIND | MS_RDONLY
197 will make access through this mountpoint read-only, without affecting
199 other mount points.
200 Creating a bind mount
202 If mountflags includes MS_BIND (available since Linux 2.4), then per‐
203 form a bind mount. A bind mount makes a file or a directory subtree
204 visible at another point within the single directory hierarchy. Bind
205 mounts may cross filesystem boundaries and span chroot(2) jails.
206 The filesystemtype and data arguments are ignored.
208 The remaining bits in the mountflags argument are also ignored, with
210 the exception of MS_REC. (The bind mount has the same mount options as
211 the underlying mount point.) However, see the discussion of remounting
212 above, for a method of making an existing bind mount read-only.
213 By default, when a directory is bind mounted, only that directory is
215 mounted; if there are any submounts under the directory tree, they are
216 not bind mounted. If the MS_REC flag is also specified, then a recur‐
217 sive bind mount operation is performed: all submounts under the source
218 subtree (other than unbindable mounts) are also bind mounted at the
219 corresponding location in the target subtree.
220 Changing the propagation type of an existing mount
222 If mountflags includes one of MS_SHARED, MS_PRIVATE, MS_SLAVE, or
223 MS_UNBINDABLE (all available since Linux 2.6.15), then the propagation
224 type of an existing mount is changed. If more than one of these flags
225 is specified, an error results.
226 The only flags that can be used with changing the propagation type are
228 MS_REC and MS_SILENT.
229 The source, filesystemtype, and data arguments are ignored.
231 The meanings of the propagation type flags are as follows:
233 MS_SHARED
235 Make this mount point shared. Mount and unmount events immedi‐
236 ately under this mount point will propagate to the other mount
237 points that are members of this mount's peer group. Propagation
238 here means that the same mount or unmount will automatically
239 occur under all of the other mount points in the peer group.
240 Conversely, mount and unmount events that take place under peer
241 mount points will propagate to this mount point.
242 MS_PRIVATE
244 Make this mount point private. Mount and unmount events do not
245 propagate into or out of this mount point.
246 MS_SLAVE
248 If this is a shared mount point that is a member of a peer group
249 that contains other members, convert it to a slave mount. If
250 this is a shared mount point that is a member of a peer group
251 that contains no other members, convert it to a private mount.
252 Otherwise, the propagation type of the mount point is left
253 unchanged.
254 When a mount point is a slave, mount and unmount events propagate into
256 this mount point from the (master) shared peer group of which it was
257 formerly a member. Mount and unmount events under this mount point do
258 not propagate to any peer.
259 A mount point can be the slave of another peer group while at the same
261 time sharing mount and unmount events with a peer group of which it is
262 a member.
263 MS_UNBINDABLE
265 Make this mount unbindable. This is like a private mount, and
266 in addition this mount can't be bind mounted. When a recursive
267 bind mount (mount() with the MS_BIND and MS_REC flags) is per‐
268 formed on a directory subtree, any bind mounts within the sub‐
269 tree are automatically pruned (i.e., not replicated) when repli‐
270 cating that subtree to produce the target subtree.
271 By default, changing the propagation type affects only the target mount
273 point. If the MS_REC flag is also specified in mountflags, then the
274 propagation type of all mount points under target is also changed.
275 For further details regarding mount propagation types (including the
277 default propagation type assigned to new mounts), see mount_names‐
278 paces(7).
279 Moving a mount
281 If mountflags contains the flag MS_MOVE (available since Linux 2.4.18),
282 then move a subtree: source specifies an existing mount point and tar‐
283 get specifies the new location to which that mount point is to be relo‐
284 cated. The move is atomic: at no point is the subtree unmounted.
285 The remaining bits in the mountflags argument are ignored, as are the
287 filesystemtype and data arguments.
288 Creating a new mount point
290 If none of MS_REMOUNT, MS_BIND, MS_MOVE, MS_SHARED, MS_PRIVATE,
291 MS_SLAVE, or MS_UNBINDABLE is specified in mountflags, then mount()
292 performs its default action: creating a new mount point. source speci‐
293 fies the source for the new mount point, and target specifies the
294 directory at which to create the mount point.
295 The filesystemtype and data arguments are employed, and further bits
297 may be specified in mountflags to modify the behavior of the call.
298
300 On success, zero is returned. On error, -1 is returned, and errno is
301 set appropriately.
302
304 The error values given below result from filesystem type independent
305 errors. Each filesystem type may have its own special errors and its
306 own special behavior. See the Linux kernel source code for details.
307 EACCES A component of a path was not searchable. (See also path_reso‐
309 lution(7).)
310 EACCES Mounting a read-only filesystem was attempted without giving the
312 MS_RDONLY flag.
313 EACCES The block device source is located on a filesystem mounted with
315 the MS_NODEV option.
316 EBUSY An attempt was made to stack a new mount directly on top of an
318 existing mount point that was created in this mount namespace
319 with the same source and target.
320 EBUSY source cannot be remounted read-only, because it still holds
322 files open for writing.
323 EFAULT One of the pointer arguments points outside the user address
325 space.
326 EINVAL source had an invalid superblock.
328 EINVAL A remount operation (MS_REMOUNT) was attempted, but source was
330 not already mounted on target.
331 EINVAL A move operation (MS_MOVE) was attempted, but source was not a
333 mount point, or was '/'.
334 EINVAL mountflags includes more than one of MS_SHARED, MS_PRIVATE,
336 MS_SLAVE, or MS_UNBINDABLE.
337 EINVAL mountflags includes MS_SHARED, MS_PRIVATE, MS_SLAVE, or
339 MS_UNBINDABLE and also includes a flag other than MS_REC or
340 MS_SILENT.
341 EINVAL An attempt was made to bind mount an unbindable mount.
343 EINVAL In an unprivileged mount namespace (i.e., a mount namespace
345 owned by a user namespace that was created by an unprivileged
346 user), a bind mount operation (MS_BIND) was attempted without
347 specifying (MS_REC), which would have revealed the filesystem
348 tree underneath one of the submounts of the directory being
349 bound.
350 ELOOP Too many links encountered during pathname resolution.
352 ELOOP A move operation was attempted, and target is a descendant of
354 source.
355 EMFILE (In case no block device is required:) Table of dummy devices is
357 full.
358 ENAMETOOLONG
360 A pathname was longer than MAXPATHLEN.
361 ENODEV filesystemtype not configured in the kernel.
363 ENOENT A pathname was empty or had a nonexistent component.
365 ENOMEM The kernel could not allocate a free page to copy filenames or
367 data into.
368 ENOTBLK
370 source is not a block device (and a device was required).
371 ENOTDIR
373 target, or a prefix of source, is not a directory.
374 ENXIO The major number of the block device source is out of range.
376 EPERM The caller does not have the required privileges.
378
380 The definitions of MS_DIRSYNC, MS_MOVE, MS_PRIVATE, MS_REC, MS_RELA‐
381 TIME, MS_SHARED, MS_SLAVE, MS_STRICTATIME and MS_UNBINDABLE were added
382 to glibc headers in version 2.12.
383
385 This function is Linux-specific and should not be used in programs
386 intended to be portable.
387
389 Since Linux 2.4 a single filesystem can be mounted at multiple mount
390 points, and multiple mounts can be stacked on the same mount point.
391 The mountflags argument may have the magic number 0xC0ED (MS_MGC_VAL)
393 in the top 16 bits. (All of the other flags discussed in DESCRIPTION
394 occupy the low order 16 bits of mountflags.) Specifying MS_MGC_VAL was
395 required in kernel versions prior to 2.4, but since Linux 2.4 is no
396 longer required and is ignored if specified.
397 The original MS_SYNC flag was renamed MS_SYNCHRONOUS in 1.1.69 when a
399 different MS_SYNC was added to <mman.h>.
400 Before Linux 2.4 an attempt to execute a set-user-ID or set-group-ID
402 program on a filesystem mounted with MS_NOSUID would fail with EPERM.
403 Since Linux 2.4 the set-user-ID and set-group-ID bits are just silently
404 ignored in this case.
405 Per-process namespaces
407 Starting with kernel 2.4.19, Linux provides per-process mount names‐
408 paces. A mount namespace is the set of filesystem mounts that are vis‐
409 ible to a process. Mount-point namespaces can be (and usually are)
410 shared between multiple processes, and changes to the namespace (i.e.,
411 mounts and unmounts) by one process are visible to all other processes
412 sharing the same namespace. (The pre-2.4.19 Linux situation can be
413 considered as one in which a single namespace was shared by every
414 process on the system.)
415 A child process created by fork(2) shares its parent's mount namespace;
417 the mount namespace is preserved across an execve(2).
418 A process can obtain a private mount namespace if: it was created using
420 the clone(2) CLONE_NEWNS flag, in which case its new namespace is ini‐
421 tialized to be a copy of the namespace of the process that called
422 clone(2); or it calls unshare(2) with the CLONE_NEWNS flag, which
423 causes the caller's mount namespace to obtain a private copy of the
424 namespace that it was previously sharing with other processes, so that
425 future mounts and unmounts by the caller are invisible to other pro‐
426 cesses (except child processes that the caller subsequently creates)
427 and vice versa.
428 The Linux-specific /proc/[pid]/mounts file exposes the list of mount
430 points in the mount namespace of the process with the specified ID; see
431 proc(5) for details.
432
434 mountpoint(1), umount(2), mount_namespaces(7), path_resolution(7),
435 findmnt(8), lsblk(8), mount(8), umount(8)
436
438 This page is part of release 4.16 of the Linux man-pages project. A
439 description of the project, information about reporting bugs, and the
440 latest version of this page, can be found at
441 https://www.kernel.org/doc/man-pages/.
442Linux 2018-02-02 MOUNT(2)