man2/mount.2

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