man2/mount.2

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   3 .\" Copyright (C) 1993 Rickard E. Faith <faith@cs.unc.edu>
   4 .\" Copyright (C) 1994 Andries E. Brouwer <aeb@cwi.nl>
   5 .\" 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 .B MS_REMOUNT
 162 Remount an existing mount.  This is allows you to change the
 163 .I mountflags
 164 and
 165 .I data
 166 of an existing mount without having to unmount and remount the file system.
 167 .I source
 168 and
 169 .I target
 170 should be the same values specified in the initial
 171 .BR mount ()
 172 call;
 173 .I filesystemtype
 174 is ignored.
 175 .TP
 176 .B MS_SYNCHRONOUS
 177 Make writes on this file system synchronous (as though
 178 the
 179 .B O_SYNC
 180 flag to
 181 .BR open (2)
 182 was specified for all file opens to this file system).
 183 .PP
 184 From Linux 2.4 onwards, the
 185 .BR MS_NODEV ", " MS_NOEXEC ", and " MS_NOSUID
 186 flags are settable on a per-mount-point basis.
 187 Form kernel 2.6.16 onwards,
 188 .B MS_NOATIME
 189 and
 190 .B MS_NODIRATIME
 191 are also settable on a per-mount-point basis.
 192 .\" FIXME Since Linux 2.6.16, MS_NODIRATIME and MS_NOATIME are
 193 .\" also settable on a per-mount basis
 194 .PP
 195 The
 196 .IR data
 197 argument is interpreted by the different file systems.
 198 Typically it is a string of comma-separated options
 199 understood by this file system.
 200 See
 201 .BR mount (8)
 202 for details of the options available for each filesystem type.
 203 .PP
 204 .\" Note: the kernel naming differs from the glibc naming
 205 .\" umount2 is the glibc name for what the kernel now calls umount
 206 .\" and umount is the glibc name for oldumount
 207 Linux 2.1.116 added the
 208 .BR umount2 ()
 209 system call, which, like
 210 .BR umount (),
 211 unmounts a target, but allows additional
 212 .I flags
 213 controlling the behaviour of the operation:
 214 .TP
 215 .BR MNT_FORCE " (since Linux 2.1.116)"
 216 Force unmount even if busy.
 217 (Only for NFS mounts.)
 218 .\" FIXME Can MNT_FORCE result in data loss?  According to
 219 .\" the Solaris manual page it can cause data loss on Solaris.
 220 .\" If the same holds on Linux, then this should be documented.
 221 .TP
 222 .BR MNT_DETACH " (since Linux 2.4.11)"
 223 Perform a lazy unmount: make the mount point unavailable for
 224 new accesses, and actually perform the unmount when the mount point
 225 ceases to be busy.
 226 .TP
 227 .BR MNT_EXPIRE " (since Linux 2.6.8)"
 228 Mark the mount point as expired.
 229 If a mount point is not currently in use, then an initial call to
 230 .BR umount2 ()
 231 with this flag fails with the error
 232 .BR EAGAIN ,
 233 but marks the mount point as expired.
 234 The mount point remains expired as long as it isn't accessed
 235 by any process.
 236 A second
 237 .BR umount2 ()
 238 call specifying
 239 .B MNT_EXPIRE
 240 unmounts an expired mount point.
 241 This flag cannot be specified with either
 242 .B MNT_FORCE
 243 or
 244 .BR MNT_DETACH .
 245 .SH "RETURN VALUE"
 246 On success, zero is returned.  On error, \-1 is returned, and
 247 .I errno
 248 is set appropriately.
 249 .SH ERRORS
 250 The error values given below result from filesystem type independent
 251 errors. Each filesystem type may have its own special errors and its
 252 own special behavior.  See the kernel source code for details.
 253
 254 .TP
 255 .B EACCES
 256 A component of a path was not searchable. (See also
 257 .BR path_resolution (2).)
 258 Or, mounting a read-only filesystem was attempted without giving the
 259 .B MS_RDONLY
 260 flag.
 261 Or, the block device
 262 .I source
 263 is located on a filesystem mounted with the
 264 .B MS_NODEV
 265 option.
 266 .\" mtk: Probably: write permission is required for MS_BIND, with
 267 .\" the error EPERM if not present; CAP_DAC_OVERRIDE is required.
 268 .TP
 269 .B EAGAIN
 270 A call to
 271 .BR umount2 ()
 272 specifying
 273 .B MNT_EXPIRE
 274 successfully marked an unbusy file system as expired.
 275 .TP
 276 .B EBUSY
 277 .I source
 278 is already mounted. Or, it cannot be remounted read-only,
 279 because it still holds files open for writing.
 280 Or, it cannot be mounted on
 281 .I target
 282 because
 283 .I target
 284 is still busy (it is the working directory of some task,
 285 the mount point of another device, has open files, etc.).
 286 Or, it could not be unmounted because it is busy.
 287 .TP
 288 .B EFAULT
 289 One of the pointer arguments points outside the user address space.
 290 .TP
 291 .B EINVAL
 292 .I source
 293 had an invalid superblock.
 294 Or, a remount
 295 .RB ( MS_REMOUNT )
 296 was attempted, but
 297 .I source
 298 was not already mounted on
 299 .IR target .
 300 Or, a move
 301 .RB ( MS_MOVE )
 302 was attempted, but
 303 .I source
 304 was not a mount point, or was '/'.
 305 Or, an unmount was attempted, but
 306 .I target
 307 was not a mount point.
 308 Or,
 309 .BR umount2 ()
 310 was called with
 311 .B MNT_FORCE
 312 and either
 313 .B MNT_DETACH
 314 or
 315 .BR MNT_FORCE .
 316 .TP
 317 .B ELOOP
 318 Too many link encountered during pathname resolution.
 319 Or, a move was attempted, while
 320 .I target
 321 is a descendant of
 322 .IR source .
 323 .TP
 324 .B EMFILE
 325 (In case no block device is required:)
 326 Table of dummy devices is full.
 327 .TP
 328 .B ENAMETOOLONG
 329 A pathname was longer than MAXPATHLEN.
 330 .TP
 331 .B ENODEV
 332 .I Filesystemtype
 333 not configured in the kernel.
 334 .TP
 335 .B ENOENT
 336 A pathname was empty or had a nonexistent component.
 337 .TP
 338 .B ENOMEM
 339 The kernel could not allocate a free page to copy filenames or data into.
 340 .TP
 341 .B ENOTBLK
 342 .I source
 343 is not a block device (and a device was required).
 344 .TP
 345 .B ENOTDIR
 346 The second argument, or a prefix of the first argument, is not
 347 a directory.
 348 .TP
 349 .B ENXIO
 350 The major number of the block device
 351 .I source
 352 is out of range.
 353 .TP
 354 .B EPERM
 355 The caller does not have the required privileges.
 356 .SH "CONFORMING TO"
 357 These functions are Linux-specific and should not be used in
 358 programs intended to be portable.
 359 .SH HISTORY
 360 The original
 361 .BR umount ()
 362 function was called as \fIumount(device)\fP and would return ENOTBLK
 363 when called with something other than a block device.
 364 In Linux 0.98p4 a call \fIumount(dir)\fP was added, in order to
 365 support anonymous devices.
 366 In Linux 2.3.99-pre7 the call \fIumount(device)\fP was removed,
 367 leaving only \fIumount(dir)\fP (since now devices can be mounted
 368 in more than one place, so specifying the device does not suffice).
 369 .LP
 370 The original MS_SYNC flag was renamed MS_SYNCHRONOUS in 1.1.69
 371 when a different MS_SYNC was added to <mman.h>.
 372 .LP
 373 Before Linux 2.4 an attempt to execute a set-user-ID or set-group-ID program
 374 on a filesystem mounted with
 375 .B MS_NOSUID
 376 would fail with
 377 .BR EPERM .
 378 Since Linux 2.4 the set-user-ID and set-group-ID bits are
 379 just silently ignored in this case.
 380 .\" The change is in patch-2.4.0-prerelease.
 381 .SH "SEE ALSO"
 382 .BR path_resolution (2),
 383 .BR mount (8),
 384 .BR umount (8)