man7/inotify.7

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  27 .TH INOTIFY 7 2019-03-06 "Linux" "Linux Programmer's Manual"
  28 .SH NAME
  29 inotify \- monitoring filesystem events
  30 .SH DESCRIPTION
  31 The
  32 .I inotify
  33 API provides a mechanism for monitoring filesystem events.
  34 Inotify can be used to monitor individual files,
  35 or to monitor directories.
  36 When a directory is monitored, inotify will return events
  37 for the directory itself, and for files inside the directory.
  38 .PP
  39 The following system calls are used with this API:
  40 .IP * 3
  41 .BR inotify_init (2)
  42 creates an inotify instance and returns a file descriptor
  43 referring to the inotify instance.
  44 The more recent
  45 .BR inotify_init1 (2)
  46 is like
  47 .BR inotify_init (2),
  48 but has a
  49 .IR flags
  50 argument that provides access to some extra functionality.
  51 .IP *
  52 .BR inotify_add_watch (2)
  53 manipulates the "watch list" associated with an inotify instance.
  54 Each item ("watch") in the watch list specifies the pathname of
  55 a file or directory,
  56 along with some set of events that the kernel should monitor for the
  57 file referred to by that pathname.
  58 .BR inotify_add_watch (2)
  59 either creates a new watch item, or modifies an existing watch.
  60 Each watch has a unique "watch descriptor", an integer
  61 returned by
  62 .BR inotify_add_watch (2)
  63 when the watch is created.
  64 .IP *
  65 When events occur for monitored files and directories,
  66 those events are made available to the application as structured data that
  67 can be read from the inotify file descriptor using
  68 .BR read (2)
  69 (see below).
  70 .IP *
  71 .BR inotify_rm_watch (2)
  72 removes an item from an inotify watch list.
  73 .IP *
  74 When all file descriptors referring to an inotify
  75 instance have been closed (using
  76 .BR close (2)),
  77 the underlying object and its resources are
  78 freed for reuse by the kernel;
  79 all associated watches are automatically freed.
  80 .PP
  81 With careful programming,
  82 an application can use inotify to efficiently monitor and cache
  83 the state of a set of filesystem objects.
  84 However, robust applications should allow for the fact that bugs
  85 in the monitoring logic or races of the kind described below
  86 may leave the cache inconsistent with the filesystem state.
  87 It is probably wise to do some consistency checking,
  88 and rebuild the cache when inconsistencies are detected.
  89 .SS Reading events from an inotify file descriptor
  90 To determine what events have occurred, an application
  91 .BR read (2)s
  92 from the inotify file descriptor.
  93 If no events have so far occurred, then,
  94 assuming a blocking file descriptor,
  95 .BR read (2)
  96 will block until at least one event occurs
  97 (unless interrupted by a signal,
  98 in which case the call fails with the error
  99 .BR EINTR ;
 100 see
 101 .BR signal (7)).
 102 .PP
 103 Each successful
 104 .BR read (2)
 105 returns a buffer containing one or more of the following structures:
 106 .PP
 107 .in +4n
 108 .EX
 109 struct inotify_event {
 110     int      wd;       /* Watch descriptor */
 111 .\" FIXME . The type of the 'wd' field should probably be "int32_t".
 112 .\" I submitted a patch to fix this.  See the LKML thread
 113 .\" "[patch] Fix type errors in inotify interfaces", 18 Nov 2008
 114 .\" Glibc bug filed: http://sources.redhat.com/bugzilla/show_bug.cgi?id=7040
 115     uint32_t mask;     /* Mask describing event */
 116     uint32_t cookie;   /* Unique cookie associating related
 117                           events (for rename(2)) */
 118     uint32_t len;      /* Size of \fIname\fP field */
 119     char     name[];   /* Optional null-terminated name */
 120 };
 121 .EE
 122 .in
 123 .PP
 124 .I wd
 125 identifies the watch for which this event occurs.
 126 It is one of the watch descriptors returned by a previous call to
 127 .BR inotify_add_watch (2).
 128 .PP
 129 .I mask
 130 contains bits that describe the event that occurred (see below).
 131 .PP
 132 .I cookie
 133 is a unique integer that connects related events.
 134 Currently, this is used only for rename events, and
 135 allows the resulting pair of
 136 .B IN_MOVED_FROM
 137 and
 138 .B IN_MOVED_TO
 139 events to be connected by the application.
 140 For all other event types,
 141 .I cookie
 142 is set to 0.
 143 .PP
 144 The
 145 .I name
 146 field is present only when an event is returned
 147 for a file inside a watched directory;
 148 it identifies the filename within to the watched directory.
 149 This filename is null-terminated,
 150 and may include further null bytes (\(aq\e0\(aq) to align subsequent reads to a
 151 suitable address boundary.
 152 .PP
 153 The
 154 .I len
 155 field counts all of the bytes in
 156 .IR name ,
 157 including the null bytes;
 158 the length of each
 159 .I inotify_event
 160 structure is thus
 161 .IR "sizeof(struct inotify_event)+len" .
 162 .PP
 163 The behavior when the buffer given to
 164 .BR read (2)
 165 is too small to return information about the next event depends
 166 on the kernel version: in kernels before 2.6.21,
 167 .BR read (2)
 168 returns 0; since kernel 2.6.21,
 169 .BR read (2)
 170 fails with the error
 171 .BR EINVAL .
 172 Specifying a buffer of size
 173 .PP
 174     sizeof(struct inotify_event) + NAME_MAX + 1
 175 .PP
 176 will be sufficient to read at least one event.
 177 .SS inotify events
 178 The
 179 .BR inotify_add_watch (2)
 180 .I mask
 181 argument and the
 182 .I mask
 183 field of the
 184 .I inotify_event
 185 structure returned when
 186 .BR read (2)ing
 187 an inotify file descriptor are both bit masks identifying
 188 inotify events.
 189 The following bits can be specified in
 190 .I mask
 191 when calling
 192 .BR inotify_add_watch (2)
 193 and may be returned in the
 194 .I mask
 195 field returned by
 196 .BR read (2):
 197 .RS 4
 198 .TP
 199 .BR IN_ACCESS " (+)"
 200 File was accessed (e.g.,
 201 .BR read (2),
 202 .BR execve (2)).
 203 .TP
 204 .BR IN_ATTRIB " (*)"
 205 Metadata changed\(emfor example, permissions (e.g.,
 206 .BR chmod (2)),
 207 timestamps (e.g.,
 208 .BR utimensat (2)),
 209 extended attributes
 210 .RB ( setxattr (2)),
 211 link count (since Linux 2.6.25; e.g.,
 212 .\" FIXME .
 213 .\" Events do not occur for link count changes on a file inside a monitored
 214 .\" directory. This differs from other metadata changes for files inside
 215 .\" a monitored directory.
 216 for the target of
 217 .BR link (2)
 218 and for
 219 .BR unlink (2)),
 220 and user/group ID (e.g.,
 221 .BR chown (2)).
 222 .TP
 223 .BR IN_CLOSE_WRITE " (+)"
 224 File opened for writing was closed.
 225 .TP
 226 .BR IN_CLOSE_NOWRITE " (*)"
 227 File or directory not opened for writing was closed.
 228 .TP
 229 .BR IN_CREATE " (+)"
 230 File/directory created in watched directory (e.g.,
 231 .BR open (2)
 232 .BR O_CREAT ,
 233 .BR mkdir (2),
 234 .BR link (2),
 235 .BR symlink (2),
 236 .BR bind (2)
 237 on a UNIX domain socket).
 238 .TP
 239 .BR IN_DELETE " (+)"
 240 File/directory deleted from watched directory.
 241 .TP
 242 .B IN_DELETE_SELF
 243 Watched file/directory was itself deleted.
 244 (This event also occurs if an object is moved to another filesystem,
 245 since
 246 .BR mv (1)
 247 in effect copies the file to the other filesystem and
 248 then deletes it from the original filesystem.)
 249 In addition, an
 250 .B IN_IGNORED
 251 event will subsequently be generated for the watch descriptor.
 252 .TP
 253 .BR IN_MODIFY " (+)"
 254 File was modified (e.g.,
 255 .BR write (2),
 256 .BR truncate (2)).
 257 .TP
 258 .B IN_MOVE_SELF
 259 Watched file/directory was itself moved.
 260 .TP
 261 .BR IN_MOVED_FROM " (+)"
 262 Generated for the directory containing the old filename
 263 when a file is renamed.
 264 .TP
 265 .BR IN_MOVED_TO " (+)"
 266 Generated for the directory containing the new filename
 267 when a file is renamed.
 268 .TP
 269 .BR IN_OPEN " (*)"
 270 File or directory was opened.
 271 .RE
 272 .PP
 273 Inotify monitoring is inode-based: when monitoring a file
 274 (but not when monitoring the directory containing a file),
 275 an event can be generated for activity on any link to the file
 276 (in the same or a different directory).
 277 .PP
 278 When monitoring a directory:
 279 .IP * 3
 280 the events marked above with an asterisk (*) can occur both
 281 for the directory itself and for objects inside the directory; and
 282 .IP *
 283 the events marked with a plus sign (+) occur only for objects
 284 inside the directory (not for the directory itself).
 285 .PP
 286 .IR Note :
 287 when monitoring a directory,
 288 events are not generated for the files inside the directory
 289 when the events are performed via a pathname (i.e., a link)
 290 that lies outside the monitored directory.
 291 .PP
 292 When events are generated for objects inside a watched directory, the
 293 .I name
 294 field in the returned
 295 .I inotify_event
 296 structure identifies the name of the file within the directory.
 297 .PP
 298 The
 299 .B IN_ALL_EVENTS
 300 macro is defined as a bit mask of all of the above events.
 301 This macro can be used as the
 302 .I mask
 303 argument when calling
 304 .BR inotify_add_watch (2).
 305 .PP
 306 Two additional convenience macros are defined:
 307 .RS 4
 308 .TP
 309 .BR IN_MOVE
 310 Equates to
 311 .BR "IN_MOVED_FROM | IN_MOVED_TO" .
 312 .TP
 313 .BR IN_CLOSE
 314 Equates to
 315 .BR "IN_CLOSE_WRITE | IN_CLOSE_NOWRITE" .
 316 .RE
 317 .PP
 318 The following further bits can be specified in
 319 .I mask
 320 when calling
 321 .BR inotify_add_watch (2):
 322 .RS 4
 323 .TP
 324 .BR IN_DONT_FOLLOW " (since Linux 2.6.15)"
 325 Don't dereference
 326 .I pathname
 327 if it is a symbolic link.
 328 .TP
 329 .BR IN_EXCL_UNLINK " (since Linux 2.6.36)"
 330 .\" commit 8c1934c8d70b22ca8333b216aec6c7d09fdbd6a6
 331 By default, when watching events on the children of a directory,
 332 events are generated for children even after they have been unlinked
 333 from the directory.
 334 This can result in large numbers of uninteresting events for
 335 some applications (e.g., if watching
 336 .IR /tmp ,
 337 in which many applications create temporary files whose
 338 names are immediately unlinked).
 339 Specifying
 340 .B IN_EXCL_UNLINK
 341 changes the default behavior,
 342 so that events are not generated for children after
 343 they have been unlinked from the watched directory.
 344 .TP
 345 .B IN_MASK_ADD
 346 If a watch instance already exists for the filesystem object corresponding to
 347 .IR pathname ,
 348 add (OR) the events in
 349 .I mask
 350 to the watch mask (instead of replacing the mask);
 351 the error
 352 .B EINVAL
 353 results if
 354 .B IN_MASK_CREATE
 355 is also specified.
 356 .TP
 357 .B IN_ONESHOT
 358 Monitor the filesystem object corresponding to
 359 .I pathname
 360 for one event, then remove from
 361 watch list.
 362 .TP
 363 .BR IN_ONLYDIR " (since Linux 2.6.15)"
 364 Watch
 365 .I pathname
 366 only if it is a directory;
 367 the error
 368 .B ENOTDIR
 369 results if
 370 .I pathname
 371 is not a directory.
 372 Using this flag provides an application with a race-free way of
 373 ensuring that the monitored object is a directory.
 374 .TP
 375 .BR IN_MASK_CREATE " (since Linux 4.18)"
 376 Watch
 377 .I pathname
 378 only if it does not already have a watch associated with it;
 379 the error
 380 .B EEXIST
 381 results if
 382 .I pathname
 383 is already being watched.
 384 .IP
 385 Using this flag provides an application with a way of ensuring
 386 that new watches do not modify existing ones.
 387 This is useful because multiple paths may refer to the same inode,
 388 and multiple calls to
 389 .BR inotify_add_watch (2)
 390 without this flag may clobber existing watch masks.
 391 .RE
 392 .PP
 393 The following bits may be set in the
 394 .I mask
 395 field returned by
 396 .BR read (2):
 397 .RS 4
 398 .TP
 399 .B IN_IGNORED
 400 Watch was removed explicitly
 401 .RB ( inotify_rm_watch (2))
 402 or automatically (file was deleted, or filesystem was unmounted).
 403 See also BUGS.
 404 .TP
 405 .B IN_ISDIR
 406 Subject of this event is a directory.
 407 .TP
 408 .B IN_Q_OVERFLOW
 409 Event queue overflowed
 410 .RI ( wd
 411 is \-1 for this event).
 412 .TP
 413 .B IN_UNMOUNT
 414 Filesystem containing watched object was unmounted.
 415 In addition, an
 416 .B IN_IGNORED
 417 event will subsequently be generated for the watch descriptor.
 418 .RE
 419 .SS Examples
 420 Suppose an application is watching the directory
 421 .I dir
 422 and the file
 423 .IR dir/myfile
 424 for all events.
 425 The examples below show some events that will be generated
 426 for these two objects.
 427 .RS 4
 428 .TP
 429 fd = open("dir/myfile", O_RDWR);
 430 Generates
 431 .B IN_OPEN
 432 events for both
 433 .I dir
 434 and
 435 .IR dir/myfile .
 436 .TP
 437 read(fd, buf, count);
 438 Generates
 439 .B IN_ACCESS
 440 events for both
 441 .I dir
 442 and
 443 .IR dir/myfile .
 444 .TP
 445 write(fd, buf, count);
 446 Generates
 447 .B IN_MODIFY
 448 events for both
 449 .I dir
 450 and
 451 .IR dir/myfile .
 452 .TP
 453 fchmod(fd, mode);
 454 Generates
 455 .B IN_ATTRIB
 456 events for both
 457 .I dir
 458 and
 459 .IR dir/myfile .
 460 .TP
 461 close(fd);
 462 Generates
 463 .B IN_CLOSE_WRITE
 464 events for both
 465 .I dir
 466 and
 467 .IR dir/myfile .
 468 .RE
 469 .PP
 470 Suppose an application is watching the directories
 471 .I dir1
 472 and
 473 .IR dir2 ,
 474 and the file
 475 .IR dir1/myfile .
 476 The following examples show some events that may be generated.
 477 .RS 4
 478 .TP
 479 link("dir1/myfile", "dir2/new");
 480 Generates an
 481 .B IN_ATTRIB
 482 event for
 483 .IR myfile
 484 and an
 485 .B IN_CREATE
 486 event for
 487 .IR dir2 .
 488 .TP
 489 rename("dir1/myfile", "dir2/myfile");
 490 Generates an
 491 .B IN_MOVED_FROM
 492 event for
 493 .IR dir1 ,
 494 an
 495 .B IN_MOVED_TO
 496 event for
 497 .IR dir2 ,
 498 and an
 499 .B IN_MOVE_SELF
 500 event for
 501 .IR myfile .
 502 The
 503 .B IN_MOVED_FROM
 504 and
 505 .B IN_MOVED_TO
 506 events will have the same
 507 .I cookie
 508 value.
 509 .RE
 510 .PP
 511 Suppose that
 512 .IR dir1/xx
 513 and
 514 .IR dir2/yy
 515 are (the only) links to the same file, and an application is watching
 516 .IR dir1 ,
 517 .IR dir2 ,
 518 .IR dir1/xx ,
 519 and
 520 .IR dir2/yy .
 521 Executing the following calls in the order given below will generate
 522 the following events:
 523 .RS 4
 524 .TP
 525 unlink("dir2/yy");
 526 Generates an
 527 .BR IN_ATTRIB
 528 event for
 529 .IR xx
 530 (because its link count changes)
 531 and an
 532 .B IN_DELETE
 533 event for
 534 .IR dir2 .
 535 .TP
 536 unlink("dir1/xx");
 537 Generates
 538 .BR IN_ATTRIB ,
 539 .BR IN_DELETE_SELF ,
 540 and
 541 .BR IN_IGNORED
 542 events for
 543 .IR xx ,
 544 and an
 545 .BR IN_DELETE
 546 event for
 547 .IR dir1 .
 548 .RE
 549 .PP
 550 Suppose an application is watching the directory
 551 .IR dir
 552 and (the empty) directory
 553 .IR dir/subdir .
 554 The following examples show some events that may be generated.
 555 .RS 4
 556 .TP
 557 mkdir("dir/new", mode);
 558 Generates an
 559 .B "IN_CREATE | IN_ISDIR"
 560 event for
 561 .IR dir .
 562 .TP
 563 rmdir("dir/subdir");
 564 Generates
 565 .B IN_DELETE_SELF
 566 and
 567 .B IN_IGNORED
 568 events for
 569 .IR subdir ,
 570 and an
 571 .B "IN_DELETE | IN_ISDIR"
 572 event for
 573 .IR dir .
 574 .RE
 575 .SS /proc interfaces
 576 The following interfaces can be used to limit the amount of
 577 kernel memory consumed by inotify:
 578 .TP
 579 .I /proc/sys/fs/inotify/max_queued_events
 580 The value in this file is used when an application calls
 581 .BR inotify_init (2)
 582 to set an upper limit on the number of events that can be
 583 queued to the corresponding inotify instance.
 584 Events in excess of this limit are dropped, but an
 585 .B IN_Q_OVERFLOW
 586 event is always generated.
 587 .TP
 588 .I /proc/sys/fs/inotify/max_user_instances
 589 This specifies an upper limit on the number of inotify instances
 590 that can be created per real user ID.
 591 .TP
 592 .I /proc/sys/fs/inotify/max_user_watches
 593 This specifies an upper limit on the number of watches
 594 that can be created per real user ID.
 595 .SH VERSIONS
 596 Inotify was merged into the 2.6.13 Linux kernel.
 597 The required library interfaces were added to glibc in version 2.4.
 598 .RB ( IN_DONT_FOLLOW ,
 599 .BR IN_MASK_ADD ,
 600 and
 601 .B IN_ONLYDIR
 602 were added in glibc version 2.5.)
 603 .SH CONFORMING TO
 604 The inotify API is Linux-specific.
 605 .SH NOTES
 606 Inotify file descriptors can be monitored using
 607 .BR select (2),
 608 .BR poll (2),
 609 and
 610 .BR epoll (7).
 611 When an event is available, the file descriptor indicates as readable.
 612 .PP
 613 Since Linux 2.6.25,
 614 signal-driven I/O notification is available for inotify file descriptors;
 615 see the discussion of
 616 .B F_SETFL
 617 (for setting the
 618 .B O_ASYNC
 619 flag),
 620 .BR F_SETOWN ,
 621 and
 622 .B F_SETSIG
 623 in
 624 .BR fcntl (2).
 625 The
 626 .I siginfo_t
 627 structure (described in
 628 .BR sigaction (2))
 629 that is passed to the signal handler has the following fields set:
 630 .IR si_fd
 631 is set to the inotify file descriptor number;
 632 .IR si_signo
 633 is set to the signal number;
 634 .IR si_code
 635 is set to
 636 .BR POLL_IN ;
 637 and
 638 .B POLLIN
 639 is set in
 640 .IR si_band .
 641 .PP
 642 If successive output inotify events produced on the
 643 inotify file descriptor are identical (same
 644 .IR wd ,
 645 .IR mask ,
 646 .IR cookie ,
 647 and
 648 .IR name ),
 649 then they are coalesced into a single event if the
 650 older event has not yet been read (but see BUGS).
 651 This reduces the amount of kernel memory required for the event queue,
 652 but also means that an application can't use inotify to reliably count
 653 file events.
 654 .PP
 655 The events returned by reading from an inotify file descriptor
 656 form an ordered queue.
 657 Thus, for example, it is guaranteed that when renaming from
 658 one directory to another, events will be produced in the
 659 correct order on the inotify file descriptor.
 660 .PP
 661 The set of watch descriptors that is being monitored via
 662 an inotify file descriptor can be viewed via the entry for
 663 the inotify file descriptor in the process's
 664 .IR /proc/[pid]/fdinfo
 665 directory.
 666 See
 667 .BR proc (5)
 668 for further details.
 669 The
 670 .B FIONREAD
 671 .BR ioctl (2)
 672 returns the number of bytes available to read from an
 673 inotify file descriptor.
 674 .SS Limitations and caveats
 675 The inotify API provides no information about the user or process that
 676 triggered the inotify event.
 677 In particular, there is no easy
 678 way for a process that is monitoring events via inotify
 679 to distinguish events that it triggers
 680 itself from those that are triggered by other processes.
 681 .PP
 682 Inotify reports only events that a user-space program triggers through
 683 the filesystem API.
 684 As a result, it does not catch remote events that occur
 685 on network filesystems.
 686 (Applications must fall back to polling the filesystem
 687 to catch such events.)
 688 Furthermore, various pseudo-filesystems such as
 689 .IR /proc ,
 690 .IR /sys ,
 691 and
 692 .IR /dev/pts
 693 are not monitorable with inotify.
 694 .PP
 695 The inotify API does not report file accesses and modifications that
 696 may occur because of
 697 .BR mmap (2),
 698 .BR msync (2),
 699 and
 700 .BR munmap (2).
 701 .PP
 702 The inotify API identifies affected files by filename.
 703 However, by the time an application processes an inotify event,
 704 the filename may already have been deleted or renamed.
 705 .PP
 706 The inotify API identifies events via watch descriptors.
 707 It is the application's responsibility to cache a mapping
 708 (if one is needed) between watch descriptors and pathnames.
 709 Be aware that directory renamings may affect multiple cached pathnames.
 710 .PP
 711 Inotify monitoring of directories is not recursive:
 712 to monitor subdirectories under a directory,
 713 additional watches must be created.
 714 This can take a significant amount time for large directory trees.
 715 .PP
 716 If monitoring an entire directory subtree,
 717 and a new subdirectory is created in that tree or an existing directory
 718 is renamed into that tree,
 719 be aware that by the time you create a watch for the new subdirectory,
 720 new files (and subdirectories) may already exist inside the subdirectory.
 721 Therefore, you may want to scan the contents of the subdirectory
 722 immediately after adding the watch (and, if desired,
 723 recursively add watches for any subdirectories that it contains).
 724 .PP
 725 Note that the event queue can overflow.
 726 In this case, events are lost.
 727 Robust applications should handle the possibility of
 728 lost events gracefully.
 729 For example, it may be necessary to rebuild part or all of
 730 the application cache.
 731 (One simple, but possibly expensive,
 732 approach is to close the inotify file descriptor, empty the cache,
 733 create a new inotify file descriptor,
 734 and then re-create watches and cache entries
 735 for the objects to be monitored.)
 736 .PP
 737 If a filesystem is mounted on top of a monitored directory,
 738 no event is generated, and no events are generated
 739 for objects immediately under the new mount point.
 740 If the filesystem is subsequently unmounted,
 741 events will subsequently be generated for the directory and
 742 the objects it contains.
 743 .\"
 744 .SS Dealing with rename() events
 745 As noted above, the
 746 .B IN_MOVED_FROM
 747 and
 748 .B IN_MOVED_TO
 749 event pair that is generated by
 750 .BR rename (2)
 751 can be matched up via their shared cookie value.
 752 However, the task of matching has some challenges.
 753 .PP
 754 These two events are usually consecutive in the event stream available
 755 when reading from the inotify file descriptor.
 756 However, this is not guaranteed.
 757 If multiple processes are triggering events for monitored objects,
 758 then (on rare occasions) an arbitrary number of
 759 other events may appear between the
 760 .B IN_MOVED_FROM
 761 and
 762 .B IN_MOVED_TO
 763 events.
 764 Furthermore, it is not guaranteed that the event pair is atomically
 765 inserted into the queue: there may be a brief interval where the
 766 .B IN_MOVED_FROM
 767 has appeared, but the
 768 .B IN_MOVED_TO
 769 has not.
 770 .PP
 771 Matching up the
 772 .B IN_MOVED_FROM
 773 and
 774 .B IN_MOVED_TO
 775 event pair generated by
 776 .BR rename (2)
 777 is thus inherently racy.
 778 (Don't forget that if an object is renamed outside of a monitored directory,
 779 there may not even be an
 780 .BR IN_MOVED_TO
 781 event.)
 782 Heuristic approaches (e.g., assume the events are always consecutive)
 783 can be used to ensure a match in most cases,
 784 but will inevitably miss some cases,
 785 causing the application to perceive the
 786 .B IN_MOVED_FROM
 787 and
 788 .B IN_MOVED_TO
 789 events as being unrelated.
 790 If watch descriptors are destroyed and re-created as a result,
 791 then those watch descriptors will be inconsistent with
 792 the watch descriptors in any pending events.
 793 (Re-creating the inotify file descriptor and rebuilding the cache may
 794 be useful to deal with this scenario.)
 795 .PP
 796 Applications should also allow for the possibility that the
 797 .B IN_MOVED_FROM
 798 event was the last event that could fit in the buffer
 799 returned by the current call to
 800 .BR read (2),
 801 and the accompanying
 802 .B IN_MOVED_TO
 803 event might be fetched only on the next
 804 .BR read (2),
 805 which should be done with a (small) timeout to allow for the fact that
 806 insertion of the
 807 .BR IN_MOVED_FROM - IN_MOVED_TO
 808 event pair is not atomic,
 809 and also the possibility that there may not be any
 810 .B IN_MOVED_TO
 811 event.
 812 .SH BUGS
 813 Before Linux 3.19,
 814 .BR fallocate (2)
 815 did not create any inotify events.
 816 Since Linux 3.19,
 817 .\" commit 820c12d5d6c0890bc93dd63893924a13041fdc35
 818 calls to
 819 .BR fallocate (2)
 820 generate
 821 .B IN_MODIFY
 822 events.
 823 .PP
 824 .\" FIXME . kernel commit 611da04f7a31b2208e838be55a42c7a1310ae321
 825 .\" implies that unmount events were buggy 2.6.11 to 2.6.36
 826 .\"
 827 In kernels before 2.6.16, the
 828 .B IN_ONESHOT
 829 .I mask
 830 flag does not work.
 831 .PP
 832 As originally designed and implemented, the
 833 .B IN_ONESHOT
 834 flag did not cause an
 835 .B IN_IGNORED
 836 event to be generated when the watch was dropped after one event.
 837 However, as an unintended effect of other changes,
 838 since Linux 2.6.36, an
 839 .B IN_IGNORED
 840 event is generated in this case.
 841 .PP
 842 Before kernel 2.6.25,
 843 .\" commit 1c17d18e3775485bf1e0ce79575eb637a94494a2
 844 the kernel code that was intended to coalesce successive identical events
 845 (i.e., the two most recent events could potentially be coalesced
 846 if the older had not yet been read)
 847 instead checked if the most recent event could be coalesced with the
 848 .I oldest
 849 unread event.
 850 .PP
 851 When a watch descriptor is removed by calling
 852 .BR inotify_rm_watch (2)
 853 (or because a watch file is deleted or the filesystem
 854 that contains it is unmounted),
 855 any pending unread events for that watch descriptor remain available to read.
 856 As watch descriptors are subsequently allocated with
 857 .BR inotify_add_watch (2),
 858 the kernel cycles through the range of possible watch descriptors (0 to
 859 .BR INT_MAX )
 860 incrementally.
 861 When allocating a free watch descriptor, no check is made to see whether that
 862 watch descriptor number has any pending unread events in the inotify queue.
 863 Thus, it can happen that a watch descriptor is reallocated even
 864 when pending unread events exist for a previous incarnation of
 865 that watch descriptor number, with the result that the application
 866 might then read those events and interpret them as belonging to
 867 the file associated with the newly recycled watch descriptor.
 868 In practice, the likelihood of hitting this bug may be extremely low,
 869 since it requires that an application cycle through
 870 .B INT_MAX
 871 watch descriptors,
 872 release a watch descriptor while leaving unread events for that
 873 watch descriptor in the queue,
 874 and then recycle that watch descriptor.
 875 For this reason, and because there have been no reports
 876 of the bug occurring in real-world applications,
 877 as of Linux 3.15,
 878 .\" FIXME . https://bugzilla.kernel.org/show_bug.cgi?id=77111
 879 no kernel changes have yet been made to eliminate this possible bug.
 880 .SH EXAMPLE
 881 The following program demonstrates the usage of the inotify API.
 882 It marks the directories passed as a command-line arguments
 883 and waits for events of type
 884 .BR IN_OPEN ,
 885 .BR IN_CLOSE_NOWRITE
 886 and
 887 .BR IN_CLOSE_WRITE .
 888 .PP
 889 The following output was recorded while editing the file
 890 .I /home/user/temp/foo
 891 and listing directory
 892 .IR /tmp .
 893 Before the file and the directory were opened,
 894 .B IN_OPEN
 895 events occurred.
 896 After the file was closed, an
 897 .B IN_CLOSE_WRITE
 898 event occurred.
 899 After the directory was closed, an
 900 .B IN_CLOSE_NOWRITE
 901 event occurred.
 902 Execution of the program ended when the user pressed the ENTER key.
 903 .SS Example output
 904 .in +4n
 905 .EX
 906 $ \fB./a.out /tmp /home/user/temp\fP
 907 Press enter key to terminate.
 908 Listening for events.
 909 IN_OPEN: /home/user/temp/foo [file]
 910 IN_CLOSE_WRITE: /home/user/temp/foo [file]
 911 IN_OPEN: /tmp/ [directory]
 912 IN_CLOSE_NOWRITE: /tmp/ [directory]
 913
 914 Listening for events stopped.
 915 .EE
 916 .in
 917 .SS Program source
 918 \&
 919 .EX
 920 #include <errno.h>
 921 #include <poll.h>
 922 #include <stdio.h>
 923 #include <stdlib.h>
 924 #include <sys/inotify.h>
 925 #include <unistd.h>
 926
 927 /* Read all available inotify events from the file descriptor 'fd'.
 928    wd is the table of watch descriptors for the directories in argv.
 929    argc is the length of wd and argv.
 930    argv is the list of watched directories.
 931    Entry 0 of wd and argv is unused. */
 932
 933 static void
 934 handle_events(int fd, int *wd, int argc, char* argv[])
 935 {
 936     /* Some systems cannot read integer variables if they are not
 937        properly aligned. On other systems, incorrect alignment may
 938        decrease performance. Hence, the buffer used for reading from
 939        the inotify file descriptor should have the same alignment as
 940        struct inotify_event. */
 941
 942     char buf[4096]
 943         __attribute__ ((aligned(__alignof__(struct inotify_event))));
 944     const struct inotify_event *event;
 945     int i;
 946     ssize_t len;
 947     char *ptr;
 948
 949     /* Loop while events can be read from inotify file descriptor. */
 950
 951     for (;;) {
 952
 953         /* Read some events. */
 954
 955         len = read(fd, buf, sizeof buf);
 956         if (len == \-1 && errno != EAGAIN) {
 957             perror("read");
 958             exit(EXIT_FAILURE);
 959         }
 960
 961         /* If the nonblocking read() found no events to read, then
 962            it returns \-1 with errno set to EAGAIN. In that case,
 963            we exit the loop. */
 964
 965         if (len <= 0)
 966             break;
 967
 968         /* Loop over all events in the buffer */
 969
 970         for (ptr = buf; ptr < buf + len;
 971                 ptr += sizeof(struct inotify_event) + event\->len) {
 972
 973             event = (const struct inotify_event *) ptr;
 974
 975             /* Print event type */
 976
 977             if (event\->mask & IN_OPEN)
 978                 printf("IN_OPEN: ");
 979             if (event\->mask & IN_CLOSE_NOWRITE)
 980                 printf("IN_CLOSE_NOWRITE: ");
 981             if (event\->mask & IN_CLOSE_WRITE)
 982                 printf("IN_CLOSE_WRITE: ");
 983
 984             /* Print the name of the watched directory */
 985
 986             for (i = 1; i < argc; ++i) {
 987                 if (wd[i] == event\->wd) {
 988                     printf("%s/", argv[i]);
 989                     break;
 990                 }
 991             }
 992
 993             /* Print the name of the file */
 994
 995             if (event\->len)
 996                 printf("%s", event\->name);
 997
 998             /* Print type of filesystem object */
 999
1000             if (event\->mask & IN_ISDIR)
1001                 printf(" [directory]\en");
1002             else
1003                 printf(" [file]\en");
1004         }
1005     }
1006 }
1007
1008 int
1009 main(int argc, char* argv[])
1010 {
1011     char buf;
1012     int fd, i, poll_num;
1013     int *wd;
1014     nfds_t nfds;
1015     struct pollfd fds[2];
1016
1017     if (argc < 2) {
1018         printf("Usage: %s PATH [PATH ...]\en", argv[0]);
1019         exit(EXIT_FAILURE);
1020     }
1021
1022     printf("Press ENTER key to terminate.\en");
1023
1024     /* Create the file descriptor for accessing the inotify API */
1025
1026     fd = inotify_init1(IN_NONBLOCK);
1027     if (fd == \-1) {
1028         perror("inotify_init1");
1029         exit(EXIT_FAILURE);
1030     }
1031
1032     /* Allocate memory for watch descriptors */
1033
1034     wd = calloc(argc, sizeof(int));
1035     if (wd == NULL) {
1036         perror("calloc");
1037         exit(EXIT_FAILURE);
1038     }
1039
1040     /* Mark directories for events
1041        \- file was opened
1042        \- file was closed */
1043
1044     for (i = 1; i < argc; i++) {
1045         wd[i] = inotify_add_watch(fd, argv[i],
1046                                   IN_OPEN | IN_CLOSE);
1047         if (wd[i] == \-1) {
1048             fprintf(stderr, "Cannot watch '%s'\en", argv[i]);
1049             perror("inotify_add_watch");
1050             exit(EXIT_FAILURE);
1051         }
1052     }
1053
1054     /* Prepare for polling */
1055
1056     nfds = 2;
1057
1058     /* Console input */
1059
1060     fds[0].fd = STDIN_FILENO;
1061     fds[0].events = POLLIN;
1062
1063     /* Inotify input */
1064
1065     fds[1].fd = fd;
1066     fds[1].events = POLLIN;
1067
1068     /* Wait for events and/or terminal input */
1069
1070     printf("Listening for events.\en");
1071     while (1) {
1072         poll_num = poll(fds, nfds, \-1);
1073         if (poll_num == \-1) {
1074             if (errno == EINTR)
1075                 continue;
1076             perror("poll");
1077             exit(EXIT_FAILURE);
1078         }
1079
1080         if (poll_num > 0) {
1081
1082             if (fds[0].revents & POLLIN) {
1083
1084                 /* Console input is available. Empty stdin and quit */
1085
1086                 while (read(STDIN_FILENO, &buf, 1) > 0 && buf != '\en')
1087                     continue;
1088                 break;
1089             }
1090
1091             if (fds[1].revents & POLLIN) {
1092
1093                 /* Inotify events are available */
1094
1095                 handle_events(fd, wd, argc, argv);
1096             }
1097         }
1098     }
1099
1100     printf("Listening for events stopped.\en");
1101
1102     /* Close inotify file descriptor */
1103
1104     close(fd);
1105
1106     free(wd);
1107     exit(EXIT_SUCCESS);
1108 }
1109 .EE
1110 .SH SEE ALSO
1111 .BR inotifywait (1),
1112 .BR inotifywatch (1),
1113 .BR inotify_add_watch (2),
1114 .BR inotify_init (2),
1115 .BR inotify_init1 (2),
1116 .BR inotify_rm_watch (2),
1117 .BR read (2),
1118 .BR stat (2),
1119 .BR fanotify (7)
1120 .PP
1121 .IR Documentation/filesystems/inotify.txt
1122 in the Linux kernel source tree