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@gmx.net>
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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@gmx.net>
  29 .\"     Added note on historical behavior of MS_NOSUID
  30 .\" Modified 2002-05-16 by Michael Kerrisk <mtk-manpages@gmx.net>
  31 .\"     Extensive changes and additions
  32 .\" Modified 2002-05-27 by aeb
  33 .\" Modified 2002-06-11 by Michael Kerrisk <mtk-manpages@gmx.net>
  34 .\"     Enhanced descriptions of MS_MOVE, MS_BIND, and MS_REMOUNT
  35 .\" Modified 2004-06-17 by Michael Kerrisk <mtk-manpages@gmx.net>
  36 .\" 2005-05-18, mtk, Added MNT_EXPIRE, plus a few other tidy-ups.
  37 .\"
  38 .TH MOUNT 2 2004-05-18 "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 .IR 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 .IR 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 .B MS_BIND
  97 (Linux 2.4 onwards)
  98 .\" since 2.4.0-test9
  99 Perform a bind mount, making a file or a directory subtree visible at
 100 another point within a file system.
 101 Bind mounts may cross file system boundaries and span
 102 .BR chroot (2)
 103 jails.
 104 The
 105 .IR filesystemtype ", " mountflags ", and " data
 106 arguments are ignored.
 107 .\" with the exception of the "hidden" MS_REC mountflags bit
 108 .TP
 109 .BR MS_DIRSYNC " (since Linux 2.5.19)"
 110 Make directory changes on this file system synchronous.
 111 (This property can be obtained for individual directories
 112 or subtrees using
 113 .BR chattr (8).)
 114 .TP
 115 .B MS_MANDLOCK
 116 Permit mandatory locking on files in this file system.
 117 (Mandatory locking must still be enabled on a per-file basis,
 118 as described in
 119 .BR fcntl (2).)
 120 .\" FIXME Say more about MS_MOVE
 121 .TP
 122 .B MS_MOVE
 123 Move a subtree.
 124 .I source
 125 specifies an existing mount point and
 126 .I target
 127 specifies the new location.
 128 The move is atomic: at no point is the subtree unmounted.
 129 The
 130 .IR filesystemtype ", " mountflags ", and " data
 131 arguments are ignored.
 132 .TP
 133 .B MS_NOATIME
 134 Do not update access times for (all types of) files on this file system.
 135 .TP
 136 .B MS_NODEV
 137 Do not allow access to devices (special files) on this file system.
 138 .TP
 139 .B MS_NODIRATIME
 140 Do not update access times for directories on this file system.
 141 .TP
 142 .B MS_NOEXEC
 143 Do not allow programs to be executed from this file system.
 144 .\" (Possibly useful for a file system that contains non-Linux executables.
 145 .\" Often used as a security feature, e.g., to make sure that restricted
 146 .\" users cannot execute files uploaded using ftp or so.)
 147 .TP
 148 .B MS_NOSUID
 149 Do not honor set-user-ID and set-group-ID bits when executing
 150 programs from this file system.
 151 .\" (This is a security feature to prevent users executing set-user-ID and
 152 .\" set-group-ID programs from removable disk devices.)
 153 .TP
 154 .B MS_RDONLY
 155 Mount file system read-only.
 156 .\"
 157 .\" FIXME Document MS_REC, available since 2.4.11.
 158 .\" This flag has meaning in conjunction with MS_BIND and
 159 .\" also with the shared sub-tree flags.
 160 .TP
 161 .BR MS_RELATIME "(Since Linux 2.6.20)"
 162 When a file on this file system is accessed,
 163 only update the file's last accessed time (atime) if the current value
 164 of atime is less than or equal to the file's last modified (mtime)
 165 or last status change time (ctime).
 166 This option is useful for programs, such as
 167 .BR mutt (1),
 168 that need to know when a file has been read since it was last modified.
 169 .TP
 170 .B MS_REMOUNT
 171 Remount an existing mount.
 172 This is allows you to change the
 173 .I mountflags
 174 and
 175 .I data
 176 of an existing mount without having to unmount and remount the file system.
 177 .I source
 178 and
 179 .I target
 180 should be the same values specified in the initial
 181 .BR mount ()
 182 call;
 183 .I filesystemtype
 184 is ignored.
 185
 186 The following
 187 .I mountflags
 188 can be changed:
 189 .BR MS_RDONLY ,
 190 .BR MS_SYNCHRONOUS ,
 191 .BR MS_MANDLOCK ;
 192 before kernel 2.6.16, the following could also be changed:
 193 .BR MS_NOATIME
 194 and
 195 .BR MS_NODIRATIME ;
 196 and, additionally, before kernel 2.4, the following could also be changed:
 197 .BR MS_NOSUID ,
 198 .BR MS_NODEV ,
 199 .BR MS_NOEXEC .
 200 .TP
 201 .B MS_SYNCHRONOUS
 202 Make writes on this file system synchronous (as though
 203 the
 204 .B O_SYNC
 205 flag to
 206 .BR open (2)
 207 was specified for all file opens to this file system).
 208 .PP
 209 From Linux 2.4 onwards, the
 210 .BR MS_NODEV ", " MS_NOEXEC ", and " MS_NOSUID
 211 flags are settable on a per-mount-point basis.
 212 From kernel 2.6.16 onwards,
 213 .B MS_NOATIME
 214 and
 215 .B MS_NODIRATIME
 216 are also settable on a per-mount-point basis.
 217 The
 218 .B MS_RELATIME
 219 flag is also settable on a per-mount-point basis.
 220 .PP
 221 The
 222 .IR data
 223 argument is interpreted by the different file systems.
 224 Typically it is a string of comma-separated options
 225 understood by this file system.
 226 See
 227 .BR mount (8)
 228 for details of the options available for each filesystem type.
 229 .PP
 230 .\" Note: the kernel naming differs from the glibc naming
 231 .\" umount2 is the glibc name for what the kernel now calls umount
 232 .\" and umount is the glibc name for oldumount
 233 Linux 2.1.116 added the
 234 .BR umount2 ()
 235 system call, which, like
 236 .BR umount (),
 237 unmounts a target, but allows additional
 238 .I flags
 239 controlling the behavior of the operation:
 240 .TP
 241 .BR MNT_FORCE " (since Linux 2.1.116)"
 242 Force unmount even if busy.
 243 This can cause data loss.
 244 (Only for NFS mounts.)
 245 .\" FIXME Can MNT_FORCE result in data loss?  According to
 246 .\" the Solaris manual page it can cause data loss on Solaris.
 247 .\" If the same holds on Linux, then this should be documented.
 248 .TP
 249 .BR MNT_DETACH " (since Linux 2.4.11)"
 250 Perform a lazy unmount: make the mount point unavailable for
 251 new accesses, and actually perform the unmount when the mount point
 252 ceases to be busy.
 253 .TP
 254 .BR MNT_EXPIRE " (since Linux 2.6.8)"
 255 Mark the mount point as expired.
 256 If a mount point is not currently in use, then an initial call to
 257 .BR umount2 ()
 258 with this flag fails with the error
 259 .BR EAGAIN ,
 260 but marks the mount point as expired.
 261 The mount point remains expired as long as it isn't accessed
 262 by any process.
 263 A second
 264 .BR umount2 ()
 265 call specifying
 266 .B MNT_EXPIRE
 267 unmounts an expired mount point.
 268 This flag cannot be specified with either
 269 .B MNT_FORCE
 270 or
 271 .BR MNT_DETACH .
 272 .SH "RETURN VALUE"
 273 On success, zero is returned.
 274 On error, \-1 is returned, and
 275 .I errno
 276 is set appropriately.
 277 .SH ERRORS
 278 The error values given below result from filesystem type independent
 279 errors.
 280 Each filesystem type may have its own special errors and its
 281 own special behavior.
 282 See the kernel source code for details.
 283 .TP
 284 .B EACCES
 285 A component of a path was not searchable.
 286 (See also
 287 .BR path_resolution (7).)
 288 Or, mounting a read-only filesystem was attempted without giving the
 289 .B MS_RDONLY
 290 flag.
 291 Or, the block device
 292 .I source
 293 is located on a filesystem mounted with the
 294 .B MS_NODEV
 295 option.
 296 .\" mtk: Probably: write permission is required for MS_BIND, with
 297 .\" the error EPERM if not present; CAP_DAC_OVERRIDE is required.
 298 .TP
 299 .B EAGAIN
 300 A call to
 301 .BR umount2 ()
 302 specifying
 303 .B MNT_EXPIRE
 304 successfully marked an unbusy file system as expired.
 305 .TP
 306 .B EBUSY
 307 .I source
 308 is already mounted.
 309 Or, it cannot be remounted read-only,
 310 because it still holds files open for writing.
 311 Or, it cannot be mounted on
 312 .I target
 313 because
 314 .I target
 315 is still busy (it is the working directory of some task,
 316 the mount point of another device, has open files, etc.).
 317 Or, it could not be unmounted because it is busy.
 318 .TP
 319 .B EFAULT
 320 One of the pointer arguments points outside the user address space.
 321 .TP
 322 .B EINVAL
 323 .I source
 324 had an invalid superblock.
 325 Or, a remount
 326 .RB ( MS_REMOUNT )
 327 was attempted, but
 328 .I source
 329 was not already mounted on
 330 .IR target .
 331 Or, a move
 332 .RB ( MS_MOVE )
 333 was attempted, but
 334 .I source
 335 was not a mount point, or was '/'.
 336 Or, an unmount was attempted, but
 337 .I target
 338 was not a mount point.
 339 Or,
 340 .BR umount2 ()
 341 was called with
 342 .B MNT_EXPIRE
 343 and either
 344 .B MNT_DETACH
 345 or
 346 .BR MNT_FORCE .
 347 .TP
 348 .B ELOOP
 349 Too many link encountered during pathname resolution.
 350 Or, a move was attempted, while
 351 .I target
 352 is a descendant of
 353 .IR source .
 354 .TP
 355 .B EMFILE
 356 (In case no block device is required:)
 357 Table of dummy devices is full.
 358 .TP
 359 .B ENAMETOOLONG
 360 A pathname was longer than
 361 .BR MAXPATHLEN .
 362 .TP
 363 .B ENODEV
 364 .I filesystemtype
 365 not configured in the kernel.
 366 .TP
 367 .B ENOENT
 368 A pathname was empty or had a nonexistent component.
 369 .TP
 370 .B ENOMEM
 371 The kernel could not allocate a free page to copy filenames or data into.
 372 .TP
 373 .B ENOTBLK
 374 .I source
 375 is not a block device (and a device was required).
 376 .TP
 377 .B ENOTDIR
 378 The second argument, or a prefix of the first argument, is not
 379 a directory.
 380 .TP
 381 .B ENXIO
 382 The major number of the block device
 383 .I source
 384 is out of range.
 385 .TP
 386 .B EPERM
 387 The caller does not have the required privileges.
 388 .SH "CONFORMING TO"
 389 These functions are Linux specific and should not be used in
 390 programs intended to be portable.
 391 .SH NOTES
 392 .SS Linux Notes
 393 The original
 394 .BR umount ()
 395 function was called as \fIumount(device)\fP and would return
 396 .B ENOTBLK
 397 when called with something other than a block device.
 398 In Linux 0.98p4 a call \fIumount(dir)\fP was added, in order to
 399 support anonymous devices.
 400 In Linux 2.3.99-pre7 the call \fIumount(device)\fP was removed,
 401 leaving only \fIumount(dir)\fP (since now devices can be mounted
 402 in more than one place, so specifying the device does not suffice).
 403 .LP
 404 The original
 405 .BR MS_SYNC
 406 flag was renamed
 407 .BR MS_SYNCHRONOUS
 408 in 1.1.69
 409 when a different
 410 .BR MS_SYNC
 411 was added to \fI<mman.h>\fP.
 412 .LP
 413 Before Linux 2.4 an attempt to execute a set-user-ID or set-group-ID program
 414 on a filesystem mounted with
 415 .B MS_NOSUID
 416 would fail with
 417 .BR EPERM .
 418 Since Linux 2.4 the set-user-ID and set-group-ID bits are
 419 just silently ignored in this case.
 420 .\" The change is in patch-2.4.0-prerelease.
 421 .SH "SEE ALSO"
 422 .BR path_resolution (7),
 423 .BR mount (8),
 424 .BR umount (8)