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