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