man2/mount.2

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