man2/mount.2

   1 .\" Copyright (C) 1993 Rickard E. Faith <faith@cs.unc.edu>
   2 .\" and Copyright (C) 1994 Andries E. Brouwer <aeb@cwi.nl>
   3 .\" and Copyright (C) 2002, 2005, 2016 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 .\" 2008-10-06, mtk: move umount*() material into separate umount.2 page.
  38 .\" 2008-10-06, mtk: Add discussion of namespaces.
  39 .\"
  40 .TH MOUNT 2 2016-03-15 "Linux" "Linux Programmer's Manual"
  41 .SH NAME
  42 mount \- mount filesystem
  43 .SH SYNOPSIS
  44 .nf
  45 .B "#include <sys/mount.h>"
  46 .sp
  47 .BI "int mount(const char *" source ", const char *" target ,
  48 .BI "          const char *" filesystemtype ", unsigned long " mountflags ,
  49 .BI "          const void *" data );
  50 .fi
  51 .SH DESCRIPTION
  52 .BR mount ()
  53 attaches the filesystem specified by
  54 .I source
  55 (which is often a pathname referring to a device,
  56 but can also be the pathname of a directory or file,
  57 or a dummy string) to the location (a directory or file)
  58 specified by the pathname in
  59 .IR target .
  60
  61 Appropriate privilege (Linux: the
  62 .B CAP_SYS_ADMIN
  63 capability) is required to mount filesystems.
  64
  65 Values for the
  66 .I filesystemtype
  67 argument supported by the kernel are listed in
  68 .I /proc/filesystems
  69 (e.g., "btrfs", "ext4", "jfs", "xfs", "vfat", "fuse",
  70 "tmpfs", "cgroup", "proc", "mqueue", "nfs", "cifs", "iso9660").
  71 Further types may become available when the appropriate modules
  72 are loaded.
  73
  74 The
  75 .I data
  76 argument is interpreted by the different filesystems.
  77 Typically it is a string of comma-separated options
  78 understood by this filesystem.
  79 See
  80 .BR mount (8)
  81 for details of the options available for each filesystem type.
  82
  83 A call to
  84 .BR mount ()
  85 performs one of a number of general types of operation.
  86 depending on the bits specified in
  87 .IR mountflags .
  88 The choice of operation is determined by testing the bits set in
  89 .IR mountflags ,
  90 with the tests being conducted in the order listed here:
  91 .IP * 3
  92 Remount an existing mount:
  93 .IR mountflags
  94 includes
  95 .BR MS_REMOUNT .
  96 .IP *
  97 Create a bind mount:
  98 .IR mountflags
  99 includes
 100 .BR MS_BIND .
 101 .IP *
 102 Change the propagation type of an existing mount:
 103 .IR mountflags
 104 includes one of
 105 .BR MS_SHARED ,
 106 .BR MS_PRIVATE ,
 107 .BR MS_SLAVE ,
 108 or
 109 .BR MS_UNBINDABLE .
 110 .IP *
 111 Move an existing mount to a new location:
 112 .IR mountflags
 113 includes
 114 .BR MS_MOVE .
 115 .IP *
 116 Create a new mount:
 117 .IR mountflags
 118 includes none of the above flags.
 119 .PP
 120 Each of these operations is detailed later in this page.
 121 Further flags may be specified in
 122 .IR mountflags
 123 to modify the behavior of
 124 .BR mount (),
 125 as described below.
 126 .\"
 127 .SS Additional mount flags
 128 The list below describes the additional flags that can be specified in
 129 .IR mountflags .
 130 Note that some operation types ignore some or all of these flags,
 131 as described later in this page.
 132 .\"
 133 .\" FIXME 2.6.25 Added MS_I_VERSION, which needs to be documented.
 134 .\"
 135 .TP
 136 .BR MS_DIRSYNC " (since Linux 2.5.19)"
 137 Make directory changes on this filesystem synchronous.
 138 (This property can be obtained for individual directories
 139 or subtrees using
 140 .BR chattr (1).)
 141 .TP
 142 .BR MS_LAZYTIME " (since Linux 4.0)"
 143 .\" commit 0ae45f63d4ef8d8eeec49c7d8b44a1775fff13e8
 144 .\" commit fe032c422c5ba562ba9c2d316f55e258e03259c6
 145 .\" commit a26f49926da938f47561f386be56a83dd37a496d
 146 Reduce on-disk updates of inode timestamps (atime, mtime, ctime)
 147 by maintaining these changes only in memory.
 148 The on-disk timestamps are updated only when:
 149 .RS
 150 .IP (a) 5
 151 the inode needs to be updated for some change unrelated to file timestamps;
 152 .IP (b)
 153 the application employs
 154 .BR fsync (2),
 155 .BR syncfs (2),
 156 or
 157 .BR sync (2);
 158 .IP (c)
 159 an undeleted inode is evicted from memory; or
 160 .IP (d)
 161 more than 24 hours have passed since the inode was written to disk.
 162 .RE
 163 .IP
 164 This mount option significantly reduces writes
 165 needed to update the inode's timestamps, especially mtime and atime.
 166 However, in the event of a system crash, the atime and mtime fields
 167 on disk might be out of date by up to 24 hours.
 168
 169 Examples of workloads where this option could be of significant benefit
 170 include frequent random writes to preallocated files,
 171 as well as cases where the
 172 .B MS_STRICTATIME
 173 mount option is also enabled.
 174 (The advantage of combining
 175 .BR MS_STRICTATIME
 176 and
 177 .BR MS_LAZYTIME
 178 is that
 179 .BR stat (2)
 180 will return the correctly updated atime, but the atime updates
 181 will be flushed to disk only in the cases listed above.)
 182 .TP
 183 .B MS_MANDLOCK
 184 Permit mandatory locking on files in this filesystem.
 185 (Mandatory locking must still be enabled on a per-file basis,
 186 as described in
 187 .BR fcntl (2).)
 188 Since Linux 4.5,
 189 .\" commit 95ace75414f312f9a7b93d873f386987b92a5301
 190 this mount option requires the
 191 .B CAP_SYS_ADMIN
 192 capability.
 193 .\" FIXME Describe the MS_MOVE flag in more detail
 194 .TP
 195 .B MS_NOATIME
 196 Do not update access times for (all types of) files on this filesystem.
 197 .TP
 198 .B MS_NODEV
 199 Do not allow access to devices (special files) on this filesystem.
 200 .TP
 201 .B MS_NODIRATIME
 202 Do not update access times for directories on this filesystem.
 203 This flag provides a subset of the functionality provided by
 204 .BR MS_NOATIME ;
 205 that is,
 206 .BR MS_NOATIME
 207 implies
 208 .BR MS_NODIRATIME .
 209 .TP
 210 .B MS_NOEXEC
 211 Do not allow programs to be executed from this filesystem.
 212 .\" (Possibly useful for a filesystem that contains non-Linux executables.
 213 .\" Often used as a security feature, e.g., to make sure that restricted
 214 .\" users cannot execute files uploaded using ftp or so.)
 215 .TP
 216 .B MS_NOSUID
 217 Do not honor set-user-ID and set-group-ID bits or file capabilities
 218 when executing programs from this filesystem.
 219 .\" (This is a security feature to prevent users executing set-user-ID and
 220 .\" set-group-ID programs from removable disk devices.)
 221 .TP
 222 .B MS_RDONLY
 223 Mount filesystem read-only.
 224 .TP
 225 .BR MS_REC " (since Linux 2.4.11)"
 226 Used in conjunction with
 227 .BR MS_BIND
 228 to create a recursive bind mount,
 229 and in conjunction with the propagation type flags to recursively change
 230 the propagation type of all of the mounts in a subtree.
 231 See below for further details.
 232 .TP
 233 .BR MS_RELATIME " (since Linux 2.6.20)"
 234 When a file on this filesystem is accessed,
 235 update the file's last access time (atime) only if the current value
 236 of atime is less than or equal to the file's last modification time (mtime)
 237 or last status change time (ctime).
 238 This option is useful for programs, such as
 239 .BR mutt (1),
 240 that need to know when a file has been read since it was last modified.
 241 Since Linux 2.6.30, the kernel defaults to the behavior provided
 242 by this flag (unless
 243 .BR MS_NOATIME
 244 was specified), and the
 245 .B MS_STRICTATIME
 246 flag is required to obtain traditional semantics.
 247 In addition, since Linux 2.6.30,
 248 the file's last access time is always updated if it
 249 is more than 1 day old.
 250 .\" Matthew Garrett notes in the patch that added this behavior
 251 .\" that this lets utilities such as tmpreaper (which deletes
 252 .\" files based on last access time) work correctly.
 253 .TP
 254 .BR MS_SILENT " (since Linux 2.6.17)"
 255 Suppress the display of certain
 256 .RI ( printk ())
 257 warning messages in the kernel log.
 258 This flag supersedes the misnamed and obsolete
 259 .BR MS_VERBOSE
 260 flag (available since Linux 2.4.12), which has the same meaning.
 261 .TP
 262 .BR MS_STRICTATIME " (since Linux 2.6.30)"
 263 Always update the last access time (atime) when files on this
 264 filesystem are accessed.
 265 (This was the default behavior before Linux 2.6.30.)
 266 Specifying this flag overrides the effect of setting the
 267 .BR MS_NOATIME
 268 and
 269 .BR MS_RELATIME
 270 flags.
 271 .TP
 272 .B MS_SYNCHRONOUS
 273 Make writes on this filesystem synchronous (as though
 274 the
 275 .B O_SYNC
 276 flag to
 277 .BR open (2)
 278 was specified for all file opens to this filesystem).
 279 .PP
 280 From Linux 2.4 onward, the
 281 .BR MS_NODEV ", " MS_NOEXEC ", and " MS_NOSUID
 282 flags are settable on a per-mount-point basis.
 283 From kernel 2.6.16 onward,
 284 .B MS_NOATIME
 285 and
 286 .B MS_NODIRATIME
 287 are also settable on a per-mount-point basis.
 288 The
 289 .B MS_RELATIME
 290 flag is also settable on a per-mount-point basis.
 291 .\"
 292 .SS Remounting an existing mount
 293 An existing mount may be remounted by specifying
 294 .B MS_REMOUNT
 295 in
 296 .IR mountflags .
 297 This allows you to change the
 298 .I mountflags
 299 and
 300 .I data
 301 of an existing mount without having to unmount and remount the filesystem.
 302 .I target
 303 should be the same value specified in the initial
 304 .BR mount ()
 305 call.
 306
 307 The
 308 .I source
 309 and
 310 .I filesystemtype
 311 arguments are ignored.
 312
 313 The
 314 .I mountflags
 315 and
 316 .I data
 317 arguments should match the values used in the original
 318 .BR mount ()
 319 call, except for those parameters that are being deliberately changed.
 320
 321 The following
 322 .I mountflags
 323 can be changed:
 324 .BR MS_MANDLOCK ,
 325 .BR MS_NOATIME ,
 326 .BR MS_NODEV ,
 327 .BR MS_NODIRATIME ,
 328 .BR MS_NOEXEC ,
 329 .BR MS_NOSUID ,
 330 .BR MS_RDONLY ,
 331 and
 332 .BR MS_SYNCHRONOUS .
 333 Attempts to change the setting of the
 334 .\" See the definition of MS_RMT_MASK in include/uapi/linux/fs.h
 335 .BR MS_DIRSYNC
 336 flag during a remount are silently ignored.
 337
 338 Since Linux 3.17,
 339 .\" commit ffbc6f0ead47fa5a1dc9642b0331cb75c20a640e
 340 if none of
 341 .BR MS_NOATIME ,
 342 .BR MS_NODIRATIME ,
 343 .BR MS_RELATIME ,
 344 or
 345 .BR MS_STRICTATIME
 346 is specified in
 347 .IR mountflags ,
 348 then the remount operation preserves the existing values of these flags
 349 (rather than defaulting to
 350 .BR MS_RELATIME ).
 351
 352 Since Linux 2.6.26, this flag can also be used to make
 353 .\" See https://lwn.net/Articles/281157/
 354 an existing bind mount read-only by specifying
 355 .IR mountflags
 356 as:
 357
 358     MS_REMOUNT | MS_BIND | MS_RDONLY
 359
 360 Note that only the
 361 .BR MS_RDONLY
 362 setting of the bind mount can be changed in this manner.
 363 .\"
 364 .SS Creating a bind mount
 365 If
 366 .I mountflags
 367 includes
 368 .BR MS_BIND
 369 (available since Linux 2.4),
 370 .\" since 2.4.0-test9
 371 then perform a bind mount.
 372 A bind mount makes a file or a directory subtree visible at
 373 another point within the single directory hierarchy.
 374 Bind mounts may cross filesystem boundaries and span
 375 .BR chroot (2)
 376 jails.
 377
 378 The
 379 .IR filesystemtype
 380 and
 381 .IR data
 382 arguments are ignored.
 383
 384 The remaining bits in the
 385 .I mountflags
 386 argument are also ignored, with the exception of
 387 .BR MS_REC .
 388 (The bind mount has the same mount options as
 389 the underlying mount point.)
 390 However, see the discussion of remounting above,
 391 for a method of making an existing bind mount read-only.
 392
 393 By default, when a directory is bind mounted,
 394 only that directory is mounted;
 395 if there are any submounts under the directory tree,
 396 they are not bind mounted.
 397 If the
 398 .BR MS_REC
 399 flag is also specified, then a recursive bind mount operation is performed:
 400 all submounts under the
 401 .I source
 402 subtree (other than unbindable mounts)
 403 are also bind mounted at the corresponding location in the
 404 .I target
 405 subtree.
 406 .\"
 407 .SS Changing the propagation type of an existing mount
 408 If
 409 .IR mountflags
 410 includes one of
 411 .BR MS_SHARED ,
 412 .BR MS_PRIVATE ,
 413 .BR MS_SLAVE ,
 414 or
 415 .BR MS_UNBINDABLE
 416 (all available since Linux 2.6.15),
 417 then the propagation type of an existing mount is changed.
 418 If more than one of these flags is specified, an error results.
 419
 420 The only flags that can be used with changing the propagation type are
 421 .BR MS_REC
 422 and
 423 .BR MS_SILENT .
 424
 425 The
 426 .IR source ,
 427 .IR filesystemtype ,
 428 and
 429 .IR data
 430 arguments are ignored.
 431
 432 The meanings of the propagation type flags are as follows:
 433 .TP
 434 .BR MS_SHARED
 435 Make this mount point shared.
 436 Mount and unmount events immediately under this mount point will propagate
 437 to the other mount points that are members of this mount's peer group.
 438 Propagation here means that the same mount or unmount will automatically
 439 occur under all of the other mount points in the peer group.
 440 Conversely, mount and unmount events that take place under
 441 peer mount points will propagate to this mount point.
 442 .TP
 443 .BR MS_PRIVATE
 444 Make this mount point private.
 445 Mount and unmount events do not propagate into or out of this mount point.
 446 This is the default propagation type for newly created mount points.
 447 .TP
 448 .BR MS_SLAVE
 449 If this is a shared mount point that is a member of a peer group
 450 that contains other members, convert it to a slave mount.
 451 If this is a shared mount point that is a member of a peer group
 452 that contains no other members, convert it to a private mount.
 453 Otherwise, the propagation type of the mount point is left unchanged.
 454
 455 When a mount point is a slave,
 456 mount and unmount events propagate into this mount point from
 457 the (master) shared peer group of which it was formerly a member.
 458 Mount and unmount events under this mount point do not propagate to any peer.
 459
 460 A mount point can be the slave of another peer group
 461 while at the same time sharing mount and unmount events
 462 with a peer group of which it is a member.
 463 .TP
 464 .BR MS_UNBINDABLE
 465 Make this mount unbindable.
 466 This is like a private mount,
 467 and in addition this mount can't be bind mounted.
 468 When a recursive bind mount
 469 .RB ( mount (2)
 470 with the
 471 .BR MS_BIND
 472 and
 473 .BR MS_REC
 474 flags) is performed on a directory subtree,
 475 any bind mounts within the subtree are automatically pruned
 476 (i.e., not replicated)
 477 when replicating that subtree to produce the target subtree.
 478 .PP
 479 By default, changing the propagation type affects only the
 480 .I target
 481 mount point.
 482 If the
 483 .B MS_REC
 484 flag is also specified in
 485 .IR mountflags ,
 486 then the propagation type of all mount points under
 487 .IR target
 488 is also changed.
 489 .\"
 490 .SS Moving a mount
 491 If
 492 .I mountflags
 493 contains the flag
 494 .BR MS_MOVE
 495 (available since Linux 2.4.18),
 496 then move a subtree:
 497 .I source
 498 specifies an existing mount point and
 499 .I target
 500 specifies the new location to which that mount point is to be relocated.
 501 The move is atomic: at no point is the subtree unmounted.
 502
 503 The remaining bits in the
 504 .IR mountflags
 505 argument are ignored, as are the
 506 .IR filesystemtype
 507 and
 508 .IR data
 509 arguments.
 510 .\"
 511 .SS Creating a new mount point
 512 If none of
 513 .BR MS_REMOUNT ,
 514 .BR MS_BIND ,
 515 .BR MS_MOVE ,
 516 .BR MS_SHARED ,
 517 .BR MS_PRIVATE ,
 518 .BR MS_SLAVE ,
 519 or
 520 .BR MS_UNBINDABLE
 521 is specified in
 522 .IR mountflags ,
 523 then
 524 .BR mount ()
 525 performs its default action: creating a new mount point.
 526 .IR source
 527 specifies the source for the new mount point, and
 528 .IR target
 529 specifies the directory at which to create the mount point.
 530
 531 The
 532 .I filesystemtype
 533 and
 534 .I data
 535 arguments are employed, and further bits may be specified in
 536 .IR mountflags
 537 to modify the behavior of the call.
 538 .\"
 539 .SH RETURN VALUE
 540 On success, zero is returned.
 541 On error, \-1 is returned, and
 542 .I errno
 543 is set appropriately.
 544 .SH ERRORS
 545 The error values given below result from filesystem type independent
 546 errors.
 547 Each filesystem type may have its own special errors and its
 548 own special behavior.
 549 See the Linux kernel source code for details.
 550 .TP
 551 .B EACCES
 552 A component of a path was not searchable.
 553 (See also
 554 .BR path_resolution (7).)
 555 .TP
 556 .B EACCES
 557 Mounting a read-only filesystem was attempted without giving the
 558 .B MS_RDONLY
 559 flag.
 560 .TP
 561 .B EACCES
 562 The block device
 563 .I source
 564 is located on a filesystem mounted with the
 565 .B MS_NODEV
 566 option.
 567 .\" mtk: Probably: write permission is required for MS_BIND, with
 568 .\" the error EPERM if not present; CAP_DAC_OVERRIDE is required.
 569 .TP
 570 .B EBUSY
 571 .I source
 572 is already mounted.
 573 .TP
 574 .B EBUSY
 575 .I source
 576 cannot be remounted read-only,
 577 because it still holds files open for writing.
 578 .TP
 579 .B EBUSY
 580 .I source
 581 cannot be mounted on
 582 .I target
 583 because
 584 .I target
 585 is still busy (it is the working directory of some thread,
 586 the mount point of another device, has open files, etc.).
 587 .TP
 588 .B EFAULT
 589 One of the pointer arguments points outside the user address space.
 590 .TP
 591 .B EINVAL
 592 .I source
 593 had an invalid superblock.
 594 .TP
 595 .B EINVAL
 596 A remount operation
 597 .RB ( MS_REMOUNT )
 598 was attempted, but
 599 .I source
 600 was not already mounted on
 601 .IR target .
 602 .TP
 603 .B EINVAL
 604 A move operation
 605 .RB ( MS_MOVE )
 606 was attempted, but
 607 .I source
 608 was not a mount point, or was \(aq/\(aq.
 609 .TP
 610 .B EINVAL
 611 .I mountflags
 612 includes more than one of
 613 .BR MS_SHARED ,
 614 .BR MS_PRIVATE ,
 615 .BR MS_SLAVE ,
 616 or
 617 .BR MS_UNBINDABLE .
 618 .TP
 619 .B EINVAL
 620 .I mountflags
 621 includes
 622 .BR MS_SHARED ,
 623 .BR MS_PRIVATE ,
 624 .BR MS_SLAVE ,
 625 or
 626 .BR MS_UNBINDABLE
 627 and also includes a flag other than
 628 .BR MS_REC
 629 or
 630 .BR MS_SILENT .
 631 .TP
 632 .BR EINVAL
 633 An attempt was made to bind mount an unbindable mount.
 634 .TP
 635 .B ELOOP
 636 Too many links encountered during pathname resolution.
 637 .TP
 638 .B ELOOP
 639 A move operation was attempted, and
 640 .I target
 641 is a descendant of
 642 .IR source .
 643 .TP
 644 .B EMFILE
 645 (In case no block device is required:)
 646 Table of dummy devices is full.
 647 .TP
 648 .B ENAMETOOLONG
 649 A pathname was longer than
 650 .BR MAXPATHLEN .
 651 .TP
 652 .B ENODEV
 653 .I filesystemtype
 654 not configured in the kernel.
 655 .TP
 656 .B ENOENT
 657 A pathname was empty or had a nonexistent component.
 658 .TP
 659 .B ENOMEM
 660 The kernel could not allocate a free page to copy filenames or data into.
 661 .TP
 662 .B ENOTBLK
 663 .I source
 664 is not a block device (and a device was required).
 665 .TP
 666 .B ENOTDIR
 667 .IR target ,
 668 or a prefix of
 669 .IR source ,
 670 is not a directory.
 671 .TP
 672 .B ENXIO
 673 The major number of the block device
 674 .I source
 675 is out of range.
 676 .TP
 677 .B EPERM
 678 The caller does not have the required privileges.
 679 .SH VERSIONS
 680 The definitions of
 681 .BR MS_DIRSYNC ,
 682 .BR MS_MOVE ,
 683 .BR MS_PRIVATE ,
 684 .BR MS_REC ,
 685 .BR MS_RELATIME ,
 686 .BR MS_SHARED ,
 687 .BR MS_SLAVED ,
 688 .BR MS_STRICTATIME
 689 and
 690 .BR MS_UNBINDABLE
 691 were added to glibc headers in version 2.12.
 692 .\"
 693 .SH CONFORMING TO
 694 This function is Linux-specific and should not be used in
 695 programs intended to be portable.
 696 .SH NOTES
 697 Since Linux 2.4 a single filesystem can be mounted at
 698 multiple mount points, and multiple mounts can be stacked
 699 on the same mount point.
 700 .\" Multiple mounts on same mount point: since 2.3.99pre7.
 701
 702 The
 703 .I mountflags
 704 argument may have the magic number 0xC0ED (\fBMS_MGC_VAL\fP)
 705 in the top 16 bits.
 706 (All of the other flags discussed in DESCRIPTION
 707 occupy the low order 16 bits of
 708 .IR mountflags .)
 709 Specifying
 710 .BR MS_MGC_VAL
 711 was required in kernel versions prior to 2.4,
 712 but since Linux 2.4 is no longer required and is ignored if specified.
 713
 714 The original
 715 .B MS_SYNC
 716 flag was renamed
 717 .B MS_SYNCHRONOUS
 718 in 1.1.69
 719 when a different
 720 .B MS_SYNC
 721 was added to \fI<mman.h>\fP.
 722 .LP
 723 Before Linux 2.4 an attempt to execute a set-user-ID or set-group-ID program
 724 on a filesystem mounted with
 725 .B MS_NOSUID
 726 would fail with
 727 .BR EPERM .
 728 Since Linux 2.4 the set-user-ID and set-group-ID bits are
 729 just silently ignored in this case.
 730 .\" The change is in patch-2.4.0-prerelease.
 731 .\"
 732
 733 .SS Per-process namespaces
 734 Starting with kernel 2.4.19, Linux provides
 735 per-process mount namespaces.
 736 A mount namespace is the set of filesystem mounts that
 737 are visible to a process.
 738 Mount-point namespaces can be (and usually are)
 739 shared between multiple processes,
 740 and changes to the namespace (i.e., mounts and unmounts) by one process
 741 are visible to all other processes sharing the same namespace.
 742 (The pre-2.4.19 Linux situation can be considered as one in which
 743 a single namespace was shared by every process on the system.)
 744
 745 A child process created by
 746 .BR fork (2)
 747 shares its parent's mount namespace;
 748 the mount namespace is preserved across an
 749 .BR execve (2).
 750
 751 A process can obtain a private mount namespace if:
 752 it was created using the
 753 .BR clone (2)
 754 .BR CLONE_NEWNS
 755 flag,
 756 in which case its new namespace is initialized to be a
 757 .I copy
 758 of the namespace of the process that called
 759 .BR clone (2);
 760 or it calls
 761 .BR unshare (2)
 762 with the
 763 .BR CLONE_NEWNS
 764 flag,
 765 which causes the caller's mount namespace to obtain a private copy
 766 of the namespace that it was previously sharing with other processes,
 767 so that future mounts and unmounts by the caller are invisible
 768 to other processes (except child processes that the caller
 769 subsequently creates) and vice versa.
 770
 771 The Linux-specific
 772 .I /proc/PID/mounts
 773 file exposes the list of mount points in the mount
 774 namespace of the process with the specified ID; see
 775 .BR proc (5)
 776 for details.
 777 .SH SEE ALSO
 778 .BR umount (2),
 779 .BR namespaces (7),
 780 .BR path_resolution (7),
 781 .BR lsblk (8),
 782 .BR findmnt (8),
 783 .BR mount (8),
 784 .BR umount (8)