man2/mount.2

   1 .\" Hey Emacs! This file is -*- nroff -*- source.
   2 .\"
   3 .\" Copyright (C) 1993 Rickard E. Faith <faith@cs.unc.edu>
   4 .\" and Copyright (C) 1994 Andries E. Brouwer <aeb@cwi.nl>
   5 .\" and Copyright (C) 2002, 2005 Michael Kerrisk <mtk.manpages@gmail.com>
   6 .\"
   7 .\" Permission is granted to make and distribute verbatim copies of this
   8 .\" manual provided the copyright notice and this permission notice are
   9 .\" preserved on all copies.
  10 .\"
  11 .\" Permission is granted to copy and distribute modified versions of this
  12 .\" manual under the conditions for verbatim copying, provided that the
  13 .\" entire resulting derived work is distributed under the terms of a
  14 .\" permission notice identical to this one.
  15 .\"
  16 .\" Since the Linux kernel and libraries are constantly changing, this
  17 .\" manual page may be incorrect or out-of-date.  The author(s) assume no
  18 .\" responsibility for errors or omissions, or for damages resulting from
  19 .\" the use of the information contained herein.  The author(s) may not
  20 .\" have taken the same level of care in the production of this manual,
  21 .\" which is licensed free of charge, as they might when working
  22 .\" professionally.
  23 .\"
  24 .\" Formatted or processed versions of this manual, if unaccompanied by
  25 .\" the source, must acknowledge the copyright and authors of this work.
  26 .\"
  27 .\" Modified 1996-11-04 by Eric S. Raymond <esr@thyrsus.com>
  28 .\" Modified 2001-10-13 by Michael Kerrisk <mtk.manpages@gmail.com>
  29 .\"     Added note on historical behavior of MS_NOSUID
  30 .\" Modified 2002-05-16 by Michael Kerrisk <mtk.manpages@gmail.com>
  31 .\"     Extensive changes and additions
  32 .\" Modified 2002-05-27 by aeb
  33 .\" Modified 2002-06-11 by Michael Kerrisk <mtk.manpages@gmail.com>
  34 .\"     Enhanced descriptions of MS_MOVE, MS_BIND, and MS_REMOUNT
  35 .\" Modified 2004-06-17 by Michael Kerrisk <mtk.manpages@gmail.com>
  36 .\" 2005-05-18, mtk, Added MNT_EXPIRE, plus a few other tidy-ups.
  37 .\"
  38 .TH MOUNT 2 2007-12-17 "Linux" "Linux Programmer's Manual"
  39 .SH NAME
  40 mount, umount, umount2 \- mount and unmount file systems
  41 .SH SYNOPSIS
  42 .nf
  43 .B "#include <sys/mount.h>"
  44 .sp
  45 .BI "int mount(const char *" source ", const char *" target ,
  46 .BI "          const char *" filesystemtype ", unsigned long " mountflags ,
  47 .BI "          const void *" data );
  48 .sp
  49 .BI "int umount(const char *" target );
  50 .sp
  51 .BI "int umount2(const char *" target ", int " flags );
  52 .fi
  53 .SH DESCRIPTION
  54 .BR mount ()
  55 attaches the file system specified by
  56 .I source
  57 (which is often a device name, but can also be a directory name
  58 or a dummy) to the directory specified by
  59 .IR target .
  60
  61 .BR umount ()
  62 and
  63 .BR umount2 ()
  64 remove the attachment of the (topmost) file system mounted on
  65 .IR target .
  66
  67 Appropriate privilege (Linux: the
  68 .B CAP_SYS_ADMIN
  69 capability) is required to mount and unmount file systems.
  70
  71 Since Linux 2.4 a single file system can be visible at
  72 multiple mount points, and multiple mounts can be stacked
  73 on the same mount point.
  74 .\" Multiple mounts on same mount point: since 2.3.99pre7.
  75
  76 Values for the
  77 .I filesystemtype
  78 argument supported by the kernel are listed in
  79 .I /proc/filesystems
  80 (like "minix", "ext2", "msdos", "proc", "nfs", "iso9660" etc.).
  81 Further types may become available when the appropriate modules
  82 are loaded.
  83
  84 The
  85 .I mountflags
  86 argument may have the magic number 0xC0ED (\fBMS_MGC_VAL\fP)
  87 in the top 16 bits (this was required in kernel versions prior to 2.4, but
  88 is no longer required and ignored if specified),
  89 and various mount flags (as defined in \fI<linux/fs.h>\fP for libc4 and libc5
  90 and in \fI<sys/mount.h>\fP for glibc2) in the low order 16 bits:
  91 .\" FIXME 2.6.15 added flags for "shared sub-tree" functionality:
  92 .\" MS_UNBINDABLE, MS_PRIVATE, MS_SHARED, MS_SLAVE
  93 .\" These need to be documented on this page.
  94 .\" See:
  95 .\" Documentation/filesystems/sharedsubtree.txt
  96 .\"
  97 .\" http://lwn.net/Articles/159077/
  98 .\"
  99 .\" http://myweb.sudhaa.com:2022/~ram/sharedsubtree/paper/sharedsubtree.1.pdf
 100 .\" Shared-Subtree Concept, Implementation, and Applications in Linux
 101 .\" Al Viro viro@ftp.linux.org.uk
 102 .\" Ram Pai linuxram@us.ibm.com
 103 .\"
 104 .\" http://foss.in/2005/slides/sharedsubtree1.pdf
 105 .\" Shared Subtree Concept and Implementation in the Linux Kernel
 106 .\" Ram Pai
 107 .\"
 108 .TP
 109 .BR MS_BIND " (Linux 2.4 onwards)"
 110 .\" since 2.4.0-test9
 111 Perform a bind mount, making a file or a directory subtree visible at
 112 another point within a file system.
 113 Bind mounts may cross file system boundaries and span
 114 .BR chroot (2)
 115 jails.
 116 The
 117 .IR filesystemtype ", " mountflags ", and " data
 118 arguments are ignored.
 119 .\" with the exception of the "hidden" MS_REC mountflags bit
 120 .TP
 121 .BR MS_DIRSYNC " (since Linux 2.5.19)"
 122 Make directory changes on this file system synchronous.
 123 (This property can be obtained for individual directories
 124 or subtrees using
 125 .BR chattr (1).)
 126 .TP
 127 .B MS_MANDLOCK
 128 Permit mandatory locking on files in this file system.
 129 (Mandatory locking must still be enabled on a per-file basis,
 130 as described in
 131 .BR fcntl (2).)
 132 .\" FIXME Say more about MS_MOVE
 133 .TP
 134 .B MS_MOVE
 135 Move a subtree.
 136 .I source
 137 specifies an existing mount point and
 138 .I target
 139 specifies the new location.
 140 The move is atomic: at no point is the subtree unmounted.
 141 The
 142 .IR filesystemtype ", " mountflags ", and " data
 143 arguments are ignored.
 144 .TP
 145 .B MS_NOATIME
 146 Do not update access times for (all types of) files on this file system.
 147 .TP
 148 .B MS_NODEV
 149 Do not allow access to devices (special files) on this file system.
 150 .TP
 151 .B MS_NODIRATIME
 152 Do not update access times for directories on this file system.
 153 This flag provides a subset of the functionality provided by
 154 .BR MS_NOATIME ;
 155 that is,
 156 .BR MS_NOATIME
 157 implies
 158 .BR MS_NODIRATIME .
 159 .TP
 160 .B MS_NOEXEC
 161 Do not allow programs to be executed from this file system.
 162 .\" (Possibly useful for a file system that contains non-Linux executables.
 163 .\" Often used as a security feature, e.g., to make sure that restricted
 164 .\" users cannot execute files uploaded using ftp or so.)
 165 .TP
 166 .B MS_NOSUID
 167 Do not honor set-user-ID and set-group-ID bits when executing
 168 programs from this file system.
 169 .\" (This is a security feature to prevent users executing set-user-ID and
 170 .\" set-group-ID programs from removable disk devices.)
 171 .TP
 172 .B MS_RDONLY
 173 Mount file system read-only.
 174 .\"
 175 .\" FIXME Document MS_REC, available since 2.4.11.
 176 .\" This flag has meaning in conjunction with MS_BIND and
 177 .\" also with the shared sub-tree flags.
 178 .TP
 179 .BR MS_RELATIME " (Since Linux 2.6.20)"
 180 When a file on this file system is accessed,
 181 only update the file's last access time (atime) if the current value
 182 of atime is less than or equal to the file's last modification (mtime)
 183 or last status change time (ctime).
 184 This option is useful for programs, such as
 185 .BR mutt (1),
 186 that need to know when a file has been read since it was last modified.
 187 .TP
 188 .B MS_REMOUNT
 189 Remount an existing mount.
 190 This allows you to change the
 191 .I mountflags
 192 and
 193 .I data
 194 of an existing mount without having to unmount and remount the file system.
 195 .I source
 196 and
 197 .I target
 198 should be the same values specified in the initial
 199 .BR mount ()
 200 call;
 201 .I filesystemtype
 202 is ignored.
 203
 204 The following
 205 .I mountflags
 206 can be changed:
 207 .BR MS_RDONLY ,
 208 .BR MS_SYNCHRONOUS ,
 209 .BR MS_MANDLOCK ;
 210 before kernel 2.6.16, the following could also be changed:
 211 .B MS_NOATIME
 212 and
 213 .BR MS_NODIRATIME ;
 214 and, additionally, before kernel 2.4.10, the following could also be changed:
 215 .BR MS_NOSUID ,
 216 .BR MS_NODEV ,
 217 .BR MS_NOEXEC .
 218 .TP
 219 .B MS_SYNCHRONOUS
 220 Make writes on this file system synchronous (as though
 221 the
 222 .B O_SYNC
 223 flag to
 224 .BR open (2)
 225 was specified for all file opens to this file system).
 226 .PP
 227 From Linux 2.4 onwards, the
 228 .BR MS_NODEV ", " MS_NOEXEC ", and " MS_NOSUID
 229 flags are settable on a per-mount-point basis.
 230 From kernel 2.6.16 onwards,
 231 .B MS_NOATIME
 232 and
 233 .B MS_NODIRATIME
 234 are also settable on a per-mount-point basis.
 235 The
 236 .B MS_RELATIME
 237 flag is also settable on a per-mount-point basis.
 238 .PP
 239 The
 240 .I data
 241 argument is interpreted by the different file systems.
 242 Typically it is a string of comma-separated options
 243 understood by this file system.
 244 See
 245 .BR mount (8)
 246 for details of the options available for each filesystem type.
 247 .PP
 248 .\" Note: the kernel naming differs from the glibc naming
 249 .\" umount2 is the glibc name for what the kernel now calls umount
 250 .\" and umount is the glibc name for oldumount
 251 Linux 2.1.116 added the
 252 .BR umount2 ()
 253 system call, which, like
 254 .BR umount (),
 255 unmounts a target, but allows additional
 256 .I flags
 257 controlling the behavior of the operation:
 258 .TP
 259 .BR MNT_FORCE " (since Linux 2.1.116)"
 260 Force unmount even if busy.
 261 This can cause data loss.
 262 (Only for NFS mounts.)
 263 .\" FIXME Can MNT_FORCE result in data loss?  According to
 264 .\" the Solaris manual page it can cause data loss on Solaris.
 265 .\" If the same holds on Linux, then this should be documented.
 266 .TP
 267 .BR MNT_DETACH " (since Linux 2.4.11)"
 268 Perform a lazy unmount: make the mount point unavailable for
 269 new accesses, and actually perform the unmount when the mount point
 270 ceases to be busy.
 271 .TP
 272 .BR MNT_EXPIRE " (since Linux 2.6.8)"
 273 Mark the mount point as expired.
 274 If a mount point is not currently in use, then an initial call to
 275 .BR umount2 ()
 276 with this flag fails with the error
 277 .BR EAGAIN ,
 278 but marks the mount point as expired.
 279 The mount point remains expired as long as it isn't accessed
 280 by any process.
 281 A second
 282 .BR umount2 ()
 283 call specifying
 284 .B MNT_EXPIRE
 285 unmounts an expired mount point.
 286 This flag cannot be specified with either
 287 .B MNT_FORCE
 288 or
 289 .BR MNT_DETACH .
 290 .SH "RETURN VALUE"
 291 On success, zero is returned.
 292 On error, \-1 is returned, and
 293 .I errno
 294 is set appropriately.
 295 .SH ERRORS
 296 The error values given below result from filesystem type independent
 297 errors.
 298 Each filesystem type may have its own special errors and its
 299 own special behavior.
 300 See the kernel source code for details.
 301 .TP
 302 .B EACCES
 303 A component of a path was not searchable.
 304 (See also
 305 .BR path_resolution (7).)
 306 Or, mounting a read-only filesystem was attempted without giving the
 307 .B MS_RDONLY
 308 flag.
 309 Or, the block device
 310 .I source
 311 is located on a filesystem mounted with the
 312 .B MS_NODEV
 313 option.
 314 .\" mtk: Probably: write permission is required for MS_BIND, with
 315 .\" the error EPERM if not present; CAP_DAC_OVERRIDE is required.
 316 .TP
 317 .B EAGAIN
 318 A call to
 319 .BR umount2 ()
 320 specifying
 321 .B MNT_EXPIRE
 322 successfully marked an unbusy file system as expired.
 323 .TP
 324 .B EBUSY
 325 .I source
 326 is already mounted.
 327 Or, it cannot be remounted read-only,
 328 because it still holds files open for writing.
 329 Or, it cannot be mounted on
 330 .I target
 331 because
 332 .I target
 333 is still busy (it is the working directory of some task,
 334 the mount point of another device, has open files, etc.).
 335 Or, it could not be unmounted because it is busy.
 336 .TP
 337 .B EFAULT
 338 One of the pointer arguments points outside the user address space.
 339 .TP
 340 .B EINVAL
 341 .I source
 342 had an invalid superblock.
 343 Or, a remount
 344 .RB ( MS_REMOUNT )
 345 was attempted, but
 346 .I source
 347 was not already mounted on
 348 .IR target .
 349 Or, a move
 350 .RB ( MS_MOVE )
 351 was attempted, but
 352 .I source
 353 was not a mount point, or was '/'.
 354 Or, an unmount was attempted, but
 355 .I target
 356 was not a mount point.
 357 Or,
 358 .BR umount2 ()
 359 was called with
 360 .B MNT_EXPIRE
 361 and either
 362 .B MNT_DETACH
 363 or
 364 .BR MNT_FORCE .
 365 .TP
 366 .B ELOOP
 367 Too many link encountered during pathname resolution.
 368 Or, a move was attempted, while
 369 .I target
 370 is a descendant of
 371 .IR source .
 372 .TP
 373 .B EMFILE
 374 (In case no block device is required:)
 375 Table of dummy devices is full.
 376 .TP
 377 .B ENAMETOOLONG
 378 A pathname was longer than
 379 .BR MAXPATHLEN .
 380 .TP
 381 .B ENODEV
 382 .I filesystemtype
 383 not configured in the kernel.
 384 .TP
 385 .B ENOENT
 386 A pathname was empty or had a nonexistent component.
 387 .TP
 388 .B ENOMEM
 389 The kernel could not allocate a free page to copy filenames or data into.
 390 .TP
 391 .B ENOTBLK
 392 .I source
 393 is not a block device (and a device was required).
 394 .TP
 395 .B ENOTDIR
 396 The second argument, or a prefix of the first argument, is not
 397 a directory.
 398 .TP
 399 .B ENXIO
 400 The major number of the block device
 401 .I source
 402 is out of range.
 403 .TP
 404 .B EPERM
 405 The caller does not have the required privileges.
 406 .SH "CONFORMING TO"
 407 These functions are Linux-specific and should not be used in
 408 programs intended to be portable.
 409 .SH NOTES
 410 .SS Linux Notes
 411 The original
 412 .BR umount ()
 413 function was called as \fIumount(device)\fP and would return
 414 .B ENOTBLK
 415 when called with something other than a block device.
 416 In Linux 0.98p4 a call \fIumount(dir)\fP was added, in order to
 417 support anonymous devices.
 418 In Linux 2.3.99-pre7 the call \fIumount(device)\fP was removed,
 419 leaving only \fIumount(dir)\fP (since now devices can be mounted
 420 in more than one place, so specifying the device does not suffice).
 421 .LP
 422 The original
 423 .B MS_SYNC
 424 flag was renamed
 425 .B MS_SYNCHRONOUS
 426 in 1.1.69
 427 when a different
 428 .B MS_SYNC
 429 was added to \fI<mman.h>\fP.
 430 .LP
 431 Before Linux 2.4 an attempt to execute a set-user-ID or set-group-ID program
 432 on a filesystem mounted with
 433 .B MS_NOSUID
 434 would fail with
 435 .BR EPERM .
 436 Since Linux 2.4 the set-user-ID and set-group-ID bits are
 437 just silently ignored in this case.
 438 .\" The change is in patch-2.4.0-prerelease.
 439 .SH "SEE ALSO"
 440 .BR path_resolution (7),
 441 .BR mount (8),
 442 .BR umount (8)