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 behaviour 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 2.6.12" "Linux Programmer's Manual"
  39 .SH NAME
  40 mount, umount \- 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 <linux/fs.h> for libc4 and libc5
  90 and in <sys/mount.h> 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 honour 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.  This is 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 behaviour of the operation:
 239 .TP
 240 .BR MNT_FORCE " (since Linux 2.1.116)"
 241 Force unmount even if busy.  This can cause data loss.
 242 (Only for NFS mounts.)
 243 .\" FIXME Can MNT_FORCE result in data loss?  According to
 244 .\" the Solaris manual page it can cause data loss on Solaris.
 245 .\" If the same holds on Linux, then this should be documented.
 246 .TP
 247 .BR MNT_DETACH " (since Linux 2.4.11)"
 248 Perform a lazy unmount: make the mount point unavailable for
 249 new accesses, and actually perform the unmount when the mount point
 250 ceases to be busy.
 251 .TP
 252 .BR MNT_EXPIRE " (since Linux 2.6.8)"
 253 Mark the mount point as expired.
 254 If a mount point is not currently in use, then an initial call to
 255 .BR umount2 ()
 256 with this flag fails with the error
 257 .BR EAGAIN ,
 258 but marks the mount point as expired.
 259 The mount point remains expired as long as it isn't accessed
 260 by any process.
 261 A second
 262 .BR umount2 ()
 263 call specifying
 264 .B MNT_EXPIRE
 265 unmounts an expired mount point.
 266 This flag cannot be specified with either
 267 .B MNT_FORCE
 268 or
 269 .BR MNT_DETACH .
 270 .SH "RETURN VALUE"
 271 On success, zero is returned.  On error, \-1 is returned, and
 272 .I errno
 273 is set appropriately.
 274 .SH ERRORS
 275 The error values given below result from filesystem type independent
 276 errors. Each filesystem type may have its own special errors and its
 277 own special behavior.  See the kernel source code for details.
 278
 279 .TP
 280 .B EACCES
 281 A component of a path was not searchable. (See also
 282 .BR path_resolution (2).)
 283 Or, mounting a read-only filesystem was attempted without giving the
 284 .B MS_RDONLY
 285 flag.
 286 Or, the block device
 287 .I source
 288 is located on a filesystem mounted with the
 289 .B MS_NODEV
 290 option.
 291 .\" mtk: Probably: write permission is required for MS_BIND, with
 292 .\" the error EPERM if not present; CAP_DAC_OVERRIDE is required.
 293 .TP
 294 .B EAGAIN
 295 A call to
 296 .BR umount2 ()
 297 specifying
 298 .B MNT_EXPIRE
 299 successfully marked an unbusy file system as expired.
 300 .TP
 301 .B EBUSY
 302 .I source
 303 is already mounted. Or, it cannot be remounted read-only,
 304 because it still holds files open for writing.
 305 Or, it cannot be mounted on
 306 .I target
 307 because
 308 .I target
 309 is still busy (it is the working directory of some task,
 310 the mount point of another device, has open files, etc.).
 311 Or, it could not be unmounted because it is busy.
 312 .TP
 313 .B EFAULT
 314 One of the pointer arguments points outside the user address space.
 315 .TP
 316 .B EINVAL
 317 .I source
 318 had an invalid superblock.
 319 Or, a remount
 320 .RB ( MS_REMOUNT )
 321 was attempted, but
 322 .I source
 323 was not already mounted on
 324 .IR target .
 325 Or, a move
 326 .RB ( MS_MOVE )
 327 was attempted, but
 328 .I source
 329 was not a mount point, or was '/'.
 330 Or, an unmount was attempted, but
 331 .I target
 332 was not a mount point.
 333 Or,
 334 .BR umount2 ()
 335 was called with
 336 .B MNT_EXPIRE
 337 and either
 338 .B MNT_DETACH
 339 or
 340 .BR MNT_FORCE .
 341 .TP
 342 .B ELOOP
 343 Too many link encountered during pathname resolution.
 344 Or, a move was attempted, while
 345 .I target
 346 is a descendant of
 347 .IR source .
 348 .TP
 349 .B EMFILE
 350 (In case no block device is required:)
 351 Table of dummy devices is full.
 352 .TP
 353 .B ENAMETOOLONG
 354 A pathname was longer than MAXPATHLEN.
 355 .TP
 356 .B ENODEV
 357 .I filesystemtype
 358 not configured in the kernel.
 359 .TP
 360 .B ENOENT
 361 A pathname was empty or had a nonexistent component.
 362 .TP
 363 .B ENOMEM
 364 The kernel could not allocate a free page to copy filenames or data into.
 365 .TP
 366 .B ENOTBLK
 367 .I source
 368 is not a block device (and a device was required).
 369 .TP
 370 .B ENOTDIR
 371 The second argument, or a prefix of the first argument, is not
 372 a directory.
 373 .TP
 374 .B ENXIO
 375 The major number of the block device
 376 .I source
 377 is out of range.
 378 .TP
 379 .B EPERM
 380 The caller does not have the required privileges.
 381 .SH "CONFORMING TO"
 382 These functions are Linux specific and should not be used in
 383 programs intended to be portable.
 384 .SH HISTORY
 385 The original
 386 .BR umount ()
 387 function was called as \fIumount(device)\fP and would return ENOTBLK
 388 when called with something other than a block device.
 389 In Linux 0.98p4 a call \fIumount(dir)\fP was added, in order to
 390 support anonymous devices.
 391 In Linux 2.3.99-pre7 the call \fIumount(device)\fP was removed,
 392 leaving only \fIumount(dir)\fP (since now devices can be mounted
 393 in more than one place, so specifying the device does not suffice).
 394 .LP
 395 The original MS_SYNC flag was renamed MS_SYNCHRONOUS in 1.1.69
 396 when a different MS_SYNC was added to <mman.h>.
 397 .LP
 398 Before Linux 2.4 an attempt to execute a set-user-ID or set-group-ID program
 399 on a filesystem mounted with
 400 .B MS_NOSUID
 401 would fail with
 402 .BR EPERM .
 403 Since Linux 2.4 the set-user-ID and set-group-ID bits are
 404 just silently ignored in this case.
 405 .\" The change is in patch-2.4.0-prerelease.
 406 .SH "SEE ALSO"
 407 .BR path_resolution (2),
 408 .BR mount (8),
 409 .BR umount (8)