man7/inotify.7

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  27 .TH INOTIFY 7 2014-07-08 "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
  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 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
 103 Each successful
 104 .BR read (2)
 105 returns a buffer containing one or more of the following structures:
 106 .in +4n
 107 .nf
 108
 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 .fi
 122 .in
 123
 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
 129 .I mask
 130 contains bits that describe the event that occurred (see below).
 131
 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
 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 file pathname relative to the watched directory.
 149 This pathname is null-terminated,
 150 and may include further null bytes (\(aq\\0\(aq) to align subsequent reads to a
 151 suitable address boundary.
 152
 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
 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
 174     sizeof(struct inotify_event) + NAME_MAX + 1
 175
 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 for the target of
 213 .BR link (2)
 214 and for
 215 .BR unlink (2)),
 216 and user/group ID (e.g.,
 217 .BR chown (2)).
 218 .TP
 219 .BR IN_CLOSE_WRITE " (*)"
 220 File opened for writing was closed.
 221 .TP
 222 .BR IN_CLOSE_NOWRITE " (*)"
 223 File not opened for writing was closed.
 224 .TP
 225 .BR IN_CREATE " (+)"
 226 File/directory created in watched directory (e.g.,
 227 .BR open (2)
 228 .BR O_CREAT ,
 229 .BR mkdir (2),
 230 .BR link (2),
 231 .BR symlink (2),
 232 .BR bind (2)
 233 on a UNIX domain socket).
 234 .TP
 235 .BR IN_DELETE " (+)"
 236 File/directory deleted from watched directory.
 237 .TP
 238 .B IN_DELETE_SELF
 239 Watched file/directory was itself deleted.
 240 (This event also occurs if an object is moved to another filesystem,
 241 since
 242 .BR mv (1)
 243 in effect copies the file to the other filesystem and
 244 then deletes it from the original filesystem.)
 245 In addition, an
 246 .B IN_IGNORED
 247 event will subsequently be generated for the watch descriptor.
 248 .TP
 249 .BR IN_MODIFY " (*)"
 250 File was modified (e.g.,
 251 .BR write (2),
 252 .BR truncate (2)).
 253 .TP
 254 .B IN_MOVE_SELF
 255 Watched file/directory was itself moved.
 256 .TP
 257 .BR IN_MOVED_FROM " (+)"
 258 Generated for the directory containing the old filename
 259 when a file is renamed.
 260 .TP
 261 .BR IN_MOVED_TO " (+)"
 262 Generated for the directory containing the new filename
 263 when a file is renamed.
 264 .TP
 265 .BR IN_OPEN " (*)"
 266 File was opened.
 267 .RE
 268 .PP
 269 When monitoring a directory:
 270 .IP * 3
 271 the events marked above with an asterisk (*) can occur both
 272 for the directory itself and for objects inside the directory; and
 273 .IP *
 274 the events marked with a plus sign (+) occur only for objects
 275 inside the directory (not for the directory itself).
 276 .PP
 277 When events are generated for objects inside a watched directory, the
 278 .I name
 279 field in the returned
 280 .I inotify_event
 281 structure identifies the name of the file within the directory.
 282 .PP
 283 The
 284 .B IN_ALL_EVENTS
 285 macro is defined as a bit mask of all of the above events.
 286 This macro can be used as the
 287 .I mask
 288 argument when calling
 289 .BR inotify_add_watch (2).
 290
 291 Two additional convenience macros are defined:
 292 .RS 4
 293 .TP
 294 .BR IN_MOVE
 295 Equates to
 296 .BR "IN_MOVED_FROM | IN_MOVED_TO" .
 297 .TP
 298 .BR IN_CLOSE
 299 Equates to
 300 .BR "IN_CLOSE_WRITE | IN_CLOSE_NOWRITE" .
 301 .RE
 302 .PP
 303 The following further bits can be specified in
 304 .I mask
 305 when calling
 306 .BR inotify_add_watch (2):
 307 .RS 4
 308 .TP
 309 .BR IN_DONT_FOLLOW " (since Linux 2.6.15)"
 310 Don't dereference
 311 .I pathname
 312 if it is a symbolic link.
 313 .TP
 314 .BR IN_EXCL_UNLINK " (since Linux 2.6.36)"
 315 .\" commit 8c1934c8d70b22ca8333b216aec6c7d09fdbd6a6
 316 By default, when watching events on the children of a directory,
 317 events are generated for children even after they have been unlinked
 318 from the directory.
 319 This can result in large numbers of uninteresting events for
 320 some applications (e.g., if watching
 321 .IR /tmp ,
 322 in which many applications create temporary files whose
 323 names are immediately unlinked).
 324 Specifying
 325 .B IN_EXCL_UNLINK
 326 changes the default behavior,
 327 so that events are not generated for children after
 328 they have been unlinked from the watched directory.
 329 .TP
 330 .B IN_MASK_ADD
 331 If a watch instance already exists for the filesystem object corresponding to
 332 .IR pathname ,
 333 add (OR) the events in
 334 .I mask
 335 to the watch mask (instead of replacing the mask).
 336 .TP
 337 .B IN_ONESHOT
 338 Monitor the filesystem object corresponding to
 339 .I pathname
 340 for one event, then remove from
 341 watch list.
 342 .TP
 343 .BR IN_ONLYDIR " (since Linux 2.6.15)"
 344 Only watch
 345 .I pathname
 346 if it is a directory.
 347 Using this flag provides an application with a race-free way of
 348 ensuring that the monitored objeect is a directory.
 349 .RE
 350 .PP
 351 The following bits may be set in the
 352 .I mask
 353 field returned by
 354 .BR read (2):
 355 .RS 4
 356 .TP
 357 .B IN_IGNORED
 358 Watch was removed explicitly
 359 .RB ( inotify_rm_watch (2))
 360 or automatically (file was deleted, or filesystem was unmounted).
 361 See also BUGS.
 362 .TP
 363 .B IN_ISDIR
 364 Subject of this event is a directory.
 365 .TP
 366 .B IN_Q_OVERFLOW
 367 Event queue overflowed
 368 .RI ( wd
 369 is \-1 for this event).
 370 .TP
 371 .B IN_UNMOUNT
 372 Filesystem containing watched object was unmounted.
 373 In addition, an
 374 .B IN_IGNORED
 375 event will subsequently be generated for the watch descriptor.
 376 .RE
 377 .SS Examples
 378 Suppose an application is watching the directory
 379 .I dir
 380 and the file
 381 .IR dir/myfile
 382 for all events.
 383 The examples below show some events that will be generated
 384 for these two objects.
 385 .RS 4
 386 .TP
 387 fd = open("dir/myfile", O_RDWR);
 388 Generates
 389 .B IN_OPEN
 390 events for both
 391 .I dir
 392 and
 393 .IR dir/myfile .
 394 .TP
 395 read(fd, buf, count);
 396 Generates
 397 .B IN_ACCESS
 398 events for both
 399 .I dir
 400 and
 401 .IR dir/myfile .
 402 .TP
 403 write(fd, buf, count);
 404 Generates
 405 .B IN_MODIFY
 406 events for both
 407 .I dir
 408 and
 409 .IR dir/myfile .
 410 .TP
 411 fchmod(fd, mode);
 412 Generates
 413 .B IN_ATTRIB
 414 events for both
 415 .I dir
 416 and
 417 .IR dir/myfile .
 418 .TP
 419 close(fd);
 420 Generates
 421 .B IN_CLOSE_WRITE
 422 events for both
 423 .I dir
 424 and
 425 .IR dir/myfile .
 426 .RE
 427 .PP
 428 Suppose an application is watching the directories
 429 .I dir1
 430 and
 431 .IR dir2 ,
 432 and the file
 433 .IR dir1/myfile .
 434 The following examples show some events that may be generated.
 435 .RS 4
 436 .TP
 437 link("dir1/myfile", "dir2/new");
 438 Generates an
 439 .B IN_ATTRIB
 440 event for
 441 .IR myfile
 442 and an
 443 .B IN_CREATE
 444 event for
 445 .IR dir2 .
 446 .TP
 447 rename("dir1/myfile", "dir2/myfile");
 448 Generates an
 449 .B IN_MOVED_FROM
 450 event for
 451 .IR dir1 ,
 452 an
 453 .B IN_MOVED_TO
 454 event for
 455 .IR dir2 ,
 456 and an
 457 .B IN_MOVE_SELF
 458 event for
 459 .IR myfile .
 460 The
 461 .B IN_MOVED_FROM
 462 and
 463 .B IN_MOVED_TO
 464 events will have the same
 465 .I cookie
 466 value.
 467 .RE
 468 .PP
 469 Suppose that
 470 .IR dir1/xx
 471 and
 472 .IR dir2/yy
 473 are (the only) links to the same file, and an application is watching
 474 .IR dir1 ,
 475 .IR dir2 ,
 476 .IR dir1/xx ,
 477 and
 478 .IR dir2/yy .
 479 Executing the following calls in the order given below will generate
 480 the following events:
 481 .RS 4
 482 .TP
 483 unlink("dir2/yy");
 484 Generates an
 485 .BR IN_ATTRIB
 486 event for
 487 .IR xx
 488 (because its link count changes)
 489 and an
 490 .B IN_DELETE
 491 event for
 492 .IR dir2 .
 493 .TP
 494 unlink("dir1/xx");
 495 Generates
 496 .BR IN_ATTRIB ,
 497 .BR IN_DELETE_SELF ,
 498 and
 499 .BR IN_IGNORED
 500 events for
 501 .IR xx ,
 502 and an
 503 .BR IN_DELETE
 504 event for
 505 .IR dir1 .
 506 .RE
 507 .PP
 508 Suppose an application is watching the directory
 509 .IR dir
 510 and (the empty) directory
 511 .IR dir/subdir .
 512 The following examples show some events that may be generated.
 513 .RS 4
 514 .TP
 515 mkdir("dir/new", mode);
 516 Generates an
 517 .B "IN_CREATE | IN_ISDIR"
 518 event for
 519 .IR dir .
 520 .TP
 521 rmdir("dir/subdir");
 522 Generates
 523 .B IN_DELETE_SELF
 524 and
 525 .B IN_IGNORED
 526 events for
 527 .IR subdir ,
 528 and an
 529 .B "IN_DELETE | IN_ISDIR"
 530 event for
 531 .IR dir .
 532 .RE
 533 .SS /proc interfaces
 534 The following interfaces can be used to limit the amount of
 535 kernel memory consumed by inotify:
 536 .TP
 537 .I /proc/sys/fs/inotify/max_queued_events
 538 The value in this file is used when an application calls
 539 .BR inotify_init (2)
 540 to set an upper limit on the number of events that can be
 541 queued to the corresponding inotify instance.
 542 Events in excess of this limit are dropped, but an
 543 .B IN_Q_OVERFLOW
 544 event is always generated.
 545 .TP
 546 .I /proc/sys/fs/inotify/max_user_instances
 547 This specifies an upper limit on the number of inotify instances
 548 that can be created per real user ID.
 549 .TP
 550 .I /proc/sys/fs/inotify/max_user_watches
 551 This specifies an upper limit on the number of watches
 552 that can be created per real user ID.
 553 .SH VERSIONS
 554 Inotify was merged into the 2.6.13 Linux kernel.
 555 The required library interfaces were added to glibc in version 2.4.
 556 .RB ( IN_DONT_FOLLOW ,
 557 .BR IN_MASK_ADD ,
 558 and
 559 .B IN_ONLYDIR
 560 were added in glibc version 2.5.)
 561 .SH CONFORMING TO
 562 The inotify API is Linux-specific.
 563 .SH NOTES
 564 Inotify file descriptors can be monitored using
 565 .BR select (2),
 566 .BR poll (2),
 567 and
 568 .BR epoll (7).
 569 When an event is available, the file descriptor indicates as readable.
 570
 571 Since Linux 2.6.25,
 572 signal-driven I/O notification is available for inotify file descriptors;
 573 see the discussion of
 574 .B F_SETFL
 575 (for setting the
 576 .B O_ASYNC
 577 flag),
 578 .BR F_SETOWN ,
 579 and
 580 .B F_SETSIG
 581 in
 582 .BR fcntl (2).
 583 The
 584 .I siginfo_t
 585 structure (described in
 586 .BR sigaction (2))
 587 that is passed to the signal handler has the following fields set:
 588 .IR si_fd
 589 is set to the inotify file descriptor number;
 590 .IR si_signo
 591 is set to the signal number;
 592 .IR si_code
 593 is set to
 594 .BR POLL_IN ;
 595 and
 596 .B POLLIN
 597 is set in
 598 .IR si_band .
 599
 600 If successive output inotify events produced on the
 601 inotify file descriptor are identical (same
 602 .IR wd ,
 603 .IR mask ,
 604 .IR cookie ,
 605 and
 606 .IR name ),
 607 then they are coalesced into a single event if the
 608 older event has not yet been read (but see BUGS).
 609 This reduces the amount of kernel memory required for the event queue,
 610 but also means that an application can't use inotify to reliably count
 611 file events.
 612
 613 The events returned by reading from an inotify file descriptor
 614 form an ordered queue.
 615 Thus, for example, it is guaranteed that when renaming from
 616 one directory to another, events will be produced in the
 617 correct order on the inotify file descriptor.
 618
 619 The
 620 .B FIONREAD
 621 .BR ioctl (2)
 622 returns the number of bytes available to read from an
 623 inotify file descriptor.
 624 .SS Limitations and caveats
 625 The inotify API provides no information about the user or process that
 626 triggered the inotify event.
 627 In particular, there is no easy
 628 way for a process that is monitoring events via inotify
 629 to distinguish events that it triggers
 630 itself from those that are triggered by other processes.
 631
 632 Inotify reports only events that a user-space program triggers through
 633 the filesystem API.
 634 As a result, it does not catch remote events that occur
 635 on network filesystems.
 636 (Applications must fall back to polling the filesystem
 637 to catch such events.)
 638 Furthermore, various pseudo-filesystems such as
 639 .IR /proc ,
 640 .IR /sys ,
 641 and
 642 .IR /dev/pts
 643 are not monitorable with inotify.
 644
 645 The inotify API does not report file accesses and modifications that
 646 may occur because of
 647 .BR mmap (2),
 648 .BR msync (2),
 649 and
 650 .BR munmap (2).
 651
 652 The inotify API identifies affected files by filename.
 653 However, by the time an application processes an inotify event,
 654 the filename may already have been deleted or renamed.
 655
 656 The inotify API identifies events via watch descriptors.
 657 It is the application's responsibility to cache a mapping
 658 (if one is needed) between watch descriptors and pathnames.
 659 Be aware that directory renamings may affect multiple cached pathnames.
 660
 661 Inotify monitoring of directories is not recursive:
 662 to monitor subdirectories under a directory,
 663 additional watches must be created.
 664 This can take a significant amount time for large directory trees.
 665
 666 If monitoring an entire directory subtree,
 667 and a new subdirectory is created in that tree or an existing directory
 668 is renamed into that tree,
 669 be aware that by the time you create a watch for the new subdirectory,
 670 new files (and subdirectories) may already exist inside the subdirectory.
 671 Therefore, you may want to scan the contents of the subdirectory
 672 immediately after adding the watch (and, if desired,
 673 recursively add watches for any subdirectories that it contains).
 674
 675 Note that the event queue can overflow.
 676 In this case, events are lost.
 677 Robust applications should handle the possibility of
 678 lost events gracefully.
 679 For example, it may be necessary to rebuild part or all of
 680 the application cache.
 681 (One simple, but possibly expensive,
 682 approach is to close the inotify file descriptor, empty the cache,
 683 create a new inotify file descriptor,
 684 and then re-create watches and cache entries
 685 for the objects to be monitored.)
 686 .SS Dealing with rename() events
 687 As noted above, the
 688 .B IN_MOVED_FROM
 689 and
 690 .B IN_MOVED_TO
 691 event pair that is generated by
 692 .BR rename (2)
 693 can be matched up via their shared cookie value.
 694 However, the task of matching has some challenges.
 695
 696 These two events are usually consecutive in the event stream available
 697 when reading from the inotify file descriptor.
 698 However, this is not guaranteed.
 699 If multiple processes are triggering events for monitored objects,
 700 then (on rare occasions) an arbitrary number of
 701 other events may appear between the
 702 .B IN_MOVED_FROM
 703 and
 704 .B IN_MOVED_TO
 705 events.
 706 Furthermore, it is not guaranteed that the event pair is atomically
 707 inserted into the queue: there may be a brief interval where the
 708 .B IN_MOVED_FROM
 709 has appeared, but the
 710 .B IN_MOVED_TO
 711 has not.
 712
 713 Matching up the
 714 .B IN_MOVED_FROM
 715 and
 716 .B IN_MOVED_TO
 717 event pair generated by
 718 .BR rename (2)
 719 is thus inherently racy.
 720 (Don't forget that if an object is renamed outside of a monitored directory,
 721 there may not even be an
 722 .BR IN_MOVED_TO
 723 event.)
 724 Heuristic approaches (e.g., assume the events are always consecutive)
 725 can be used to ensure a match in most cases,
 726 but will inevitably miss some cases,
 727 causing the application to perceive the
 728 .B IN_MOVED_FROM
 729 and
 730 .B IN_MOVED_TO
 731 events as being unrelated.
 732 If watch descriptors are destroyed and re-created as a result,
 733 then those watch descriptors will be inconsistent with
 734 the watch descriptors in any pending events.
 735 (Re-creating the inotify file descriptor and rebuilding the cache may
 736 be useful to deal with this scenario.)
 737
 738 Applications should also allow for the possibility that the
 739 .B IN_MOVED_FROM
 740 event was the last event that could fit in the buffer
 741 returned by the current call to
 742 .BR read (2),
 743 and the accompanying
 744 .B IN_MOVED_TO
 745 event might be fetched only on the next
 746 .BR read (2),
 747 which should be done with a (small) timeout to allow for the fact that
 748 insertion of the
 749 .BR IN_MOVED_FROM - IN_MOVED_TO
 750 event pair is not atomic,
 751 and also the possibility that there may not be any
 752 .B IN_MOVED_TO
 753 event.
 754 .SH BUGS
 755 .\" FIXME . kernel commit 611da04f7a31b2208e838be55a42c7a1310ae321
 756 .\" implies that unmount events were buggy 2.6.11 to 2.6.36
 757 .\"
 758 In kernels before 2.6.16, the
 759 .B IN_ONESHOT
 760 .I mask
 761 flag does not work.
 762
 763 As originally designed and implemented, the
 764 .B IN_ONESHOT
 765 flag did not cause an
 766 .B IN_IGNORED
 767 event to be generated when the watch was dropped after one event.
 768 However, as an unintended effect of other changes,
 769 since Linux 2.6.36, an
 770 .B IN_IGNORED
 771 event is generated in this case.
 772
 773 Before kernel 2.6.25,
 774 .\" commit 1c17d18e3775485bf1e0ce79575eb637a94494a2
 775 the kernel code that was intended to coalesce successive identical events
 776 (i.e., the two most recent events could potentially be coalesced
 777 if the older had not yet been read)
 778 instead checked if the most recent event could be coalesced with the
 779 .I oldest
 780 unread event.
 781
 782 When a watch descriptor is removed by calling
 783 .BR inotify_rm_watch (2)
 784 (or because a watch file is deleted or the filesystem
 785 that contains it is unmounted),
 786 any pending unread events for that watch descriptor remain available to read.
 787 As watch descriptors are subsequently allocated with
 788 .BR inotify_add_watch (2),
 789 the kernel cycles through the range of possible watch descriptors (0 to
 790 .BR INT_MAX )
 791 incrementally.
 792 When allocating a free watch descriptor, no check is made to see whether that
 793 watch descriptor number has any pending unread events in the inotify queue.
 794 Thus, it can happen that a watch descriptor is reallocated even
 795 when pending unread events exist for a previous incarnation of
 796 that watch descriptor number, with the result that the application
 797 might then read those events and interpret them as belonging to
 798 the file associated with the newly recycled watch descriptor.
 799 In practice, the likelihood of hitting this bug may be extremely low,
 800 since it requires that an application cycle through
 801 .B INT_MAX
 802 watch descriptors,
 803 release a watch descriptor while leaving unread events for that
 804 watch descriptor in the queue in the queue,
 805 and then recycle that watch descriptor.
 806 For this reason, and because there have been no reports
 807 of the bug occurring in real-world applications,
 808 as of Linux 3.15,
 809 .\" FIXME . https://bugzilla.kernel.org/show_bug.cgi?id=77111
 810 no kernel changes have yet been made to eliminate this possible bug.
 811 .SH EXAMPLE
 812 The following program demonstrates the usage of the inotify API.
 813 It marks the directories passed as a command-line arguments
 814 and waits for events of type
 815 .BR IN_OPEN ,
 816 .BR IN_CLOSE_NOWRITE
 817 and
 818 .BR IN_CLOSE_WRITE .
 819 .PP
 820 The following output was recorded while editing the file
 821 .I /home/user/temp/foo
 822 and listing directory
 823 .IR /tmp .
 824 Before the file and the directory were opened,
 825 .B IN_OPEN
 826 events occurred.
 827 After the file was closed, an
 828 .B IN_CLOSE_WRITE
 829 event occurred.
 830 After the directory was closed, an
 831 .B IN_CLOSE_NOWRITE
 832 event occurred.
 833 Execution of the program ended when the user pressed the ENTER key.
 834 .SS Example output
 835 .in +4n
 836 .nf
 837 $ \fB./a.out /tmp /home/user/temp\fP
 838 Press enter key to terminate.
 839 Listening for events.
 840 IN_OPEN: /home/user/temp/foo [file]
 841 IN_CLOSE_WRITE: /home/user/temp/foo [file]
 842 IN_OPEN: /tmp/ [directory]
 843 IN_CLOSE_NOWRITE: /tmp/ [directory]
 844
 845 Listening for events stopped.
 846 .fi
 847 .in
 848 .SS Program source
 849 .nf
 850 #include <errno.h>
 851 #include <poll.h>
 852 #include <stdio.h>
 853 #include <stdlib.h>
 854 #include <sys/inotify.h>
 855 #include <unistd.h>
 856
 857 /* Read all available inotify events from the file descriptor 'fd'.
 858    wd is the table of watch descriptors for the directories in argv.
 859    argc is the length of wd and argv.
 860    argv is the list of watched directories.
 861    Entry 0 of wd and argv is unused. */
 862
 863 static void
 864 handle_events(int fd, int *wd, int argc, char* argv[])
 865 {
 866     /* Some systems cannot read integer variables if they are not
 867        properly aligned. On other systems, incorrect alignment may
 868        decrease performance. Hence, the buffer used for reading from
 869        the inotify file descriptor should have the same alignment as
 870        struct inotify_event. */
 871
 872     char buf[4096]
 873         __attribute__ ((aligned(__alignof__(struct inotify_event))));
 874     const struct inotify_event *event;
 875     int i;
 876     ssize_t len;
 877     char *ptr;
 878
 879     /* Loop while events can be read from inotify file descriptor. */
 880
 881     for (;;) {
 882
 883         /* Read some events. */
 884
 885         len = read(fd, buf, sizeof buf);
 886         if (len == \-1 && errno != EAGAIN) {
 887             perror("read");
 888             exit(EXIT_FAILURE);
 889         }
 890
 891         /* If the nonblocking read() found no events to read, then
 892            it returns \-1 with errno set to EAGAIN. In that case,
 893            we exit the loop. */
 894
 895         if (len <= 0)
 896             break;
 897
 898         /* Loop over all events in the buffer */
 899
 900         for (ptr = buf; ptr < buf + len;
 901                 ptr += sizeof(struct inotify_event) + event\->len) {
 902
 903             event = (const struct inotify_event *) ptr;
 904
 905             /* Print event type */
 906
 907             if (event\->mask & IN_OPEN)
 908                 printf("IN_OPEN: ");
 909             if (event\->mask & IN_CLOSE_NOWRITE)
 910                 printf("IN_CLOSE_NOWRITE: ");
 911             if (event\->mask & IN_CLOSE_WRITE)
 912                 printf("IN_CLOSE_WRITE: ");
 913
 914             /* Print the name of the watched directory */
 915
 916             for (i = 1; i < argc; ++i) {
 917                 if (wd[i] == event\->wd) {
 918                     printf("%s/", argv[i]);
 919                     break;
 920                 }
 921             }
 922
 923             /* Print the name of the file */
 924
 925             if (event\->len)
 926                 printf("%s", event\->name);
 927
 928             /* Print type of filesystem object */
 929
 930             if (event\->mask & IN_ISDIR)
 931                 printf(" [directory]\\n");
 932             else
 933                 printf(" [file]\\n");
 934         }
 935     }
 936 }
 937
 938 int
 939 main(int argc, char* argv[])
 940 {
 941     char buf;
 942     int fd, i, poll_num;
 943     int *wd;
 944     nfds_t nfds;
 945     struct pollfd fds[2];
 946
 947     if (argc < 2) {
 948         printf("Usage: %s PATH [PATH ...]\\n", argv[0]);
 949         exit(EXIT_FAILURE);
 950     }
 951
 952     printf("Press ENTER key to terminate.\\n");
 953
 954     /* Create the file descriptor for accessing the inotify API */
 955
 956     fd = inotify_init1(IN_NONBLOCK);
 957     if (fd == \-1) {
 958         perror("inotify_init1");
 959         exit(EXIT_FAILURE);
 960     }
 961
 962     /* Allocate memory for watch descriptors */
 963
 964     wd = calloc(argc, sizeof(int));
 965     if (wd == NULL) {
 966         perror("calloc");
 967         exit(EXIT_FAILURE);
 968     }
 969
 970     /* Mark directories for events
 971        \- file was opened
 972        \- file was closed */
 973
 974     for (i = 1; i < argc; i++) {
 975         wd[i] = inotify_add_watch(fd, argv[i],
 976                                   IN_OPEN | IN_CLOSE);
 977         if (wd[i] == \-1) {
 978             fprintf(stderr, "Cannot watch '%s'\\n", argv[i]);
 979             perror("inotify_add_watch");
 980             exit(EXIT_FAILURE);
 981         }
 982     }
 983
 984     /* Prepare for polling */
 985
 986     nfds = 2;
 987
 988     /* Console input */
 989
 990     fds[0].fd = STDIN_FILENO;
 991     fds[0].events = POLLIN;
 992
 993     /* Inotify input */
 994
 995     fds[1].fd = fd;
 996     fds[1].events = POLLIN;
 997
 998     /* Wait for events and/or terminal input */
 999
1000     printf("Listening for events.\\n");
1001     while (1) {
1002         poll_num = poll(fds, nfds, \-1);
1003         if (poll_num == \-1) {
1004             if (errno == EINTR)
1005                 continue;
1006             perror("poll");
1007             exit(EXIT_FAILURE);
1008         }
1009
1010         if (poll_num > 0) {
1011
1012             if (fds[0].revents & POLLIN) {
1013
1014                 /* Console input is available. Empty stdin and quit */
1015
1016                 while (read(STDIN_FILENO, &buf, 1) > 0 && buf != '\\n')
1017                     continue;
1018                 break;
1019             }
1020
1021             if (fds[1].revents & POLLIN) {
1022
1023                 /* Inotify events are available */
1024
1025                 handle_events(fd, wd, argc, argv);
1026             }
1027         }
1028     }
1029
1030     printf("Listening for events stopped.\\n");
1031
1032     /* Close inotify file descriptor */
1033
1034     close(fd);
1035
1036     free(wd);
1037     exit(EXIT_SUCCESS);
1038 }
1039 .fi
1040 .SH SEE ALSO
1041 .BR inotifywait (1),
1042 .BR inotifywatch (1),
1043 .BR inotify_add_watch (2),
1044 .BR inotify_init (2),
1045 .BR inotify_init1 (2),
1046 .BR inotify_rm_watch (2),
1047 .BR read (2),
1048 .BR stat (2),
1049 .BR fanotify (7)
1050
1051 .IR Documentation/filesystems/inotify.txt
1052 in the Linux kernel source tree