man7/inotify.7

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  27 .TH INOTIFY 7 2014-05-23 "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 the events marked with an asterisk (*) above can occur for
 271 files in the directory, in which case the
 272 .I name
 273 field in the returned
 274 .I inotify_event
 275 structure identifies the name of the file within the directory.
 276 .PP
 277 The
 278 .B IN_ALL_EVENTS
 279 macro is defined as a bit mask of all of the above events.
 280 This macro can be used as the
 281 .I mask
 282 argument when calling
 283 .BR inotify_add_watch (2).
 284
 285 Two additional convenience macros are defined:
 286 .RS 4
 287 .TP
 288 .BR IN_MOVE
 289 Equates to
 290 .BR "IN_MOVED_FROM | IN_MOVED_TO" .
 291 .TP
 292 .BR IN_CLOSE
 293 Equates to
 294 .BR "IN_CLOSE_WRITE | IN_CLOSE_NOWRITE" .
 295 .RE
 296 .PP
 297 The following further bits can be specified in
 298 .I mask
 299 when calling
 300 .BR inotify_add_watch (2):
 301 .RS 4
 302 .TP
 303 .BR IN_DONT_FOLLOW " (since Linux 2.6.15)"
 304 Don't dereference
 305 .I pathname
 306 if it is a symbolic link.
 307 .TP
 308 .BR IN_EXCL_UNLINK " (since Linux 2.6.36)"
 309 .\" commit 8c1934c8d70b22ca8333b216aec6c7d09fdbd6a6
 310 By default, when watching events on the children of a directory,
 311 events are generated for children even after they have been unlinked
 312 from the directory.
 313 This can result in large numbers of uninteresting events for
 314 some applications (e.g., if watching
 315 .IR /tmp ,
 316 in which many applications create temporary files whose
 317 names are immediately unlinked).
 318 Specifying
 319 .B IN_EXCL_UNLINK
 320 changes the default behavior,
 321 so that events are not generated for children after
 322 they have been unlinked from the watched directory.
 323 .TP
 324 .B IN_MASK_ADD
 325 If a watch instance already exists for the filesystem object corresponding to
 326 .IR pathname ,
 327 add (OR) the events in
 328 .I mask
 329 to the watch mask (instead of replacing the mask).
 330 .TP
 331 .B IN_ONESHOT
 332 Monitor the filesystem object corresponding to
 333 .I pathname
 334 for one event, then remove from
 335 watch list.
 336 .TP
 337 .BR IN_ONLYDIR " (since Linux 2.6.15)"
 338 Only watch
 339 .I pathname
 340 if it is a directory.
 341 .RE
 342 .PP
 343 The following bits may be set in the
 344 .I mask
 345 field returned by
 346 .BR read (2):
 347 .RS 4
 348 .TP
 349 .B IN_IGNORED
 350 Watch was removed explicitly
 351 .RB ( inotify_rm_watch (2))
 352 or automatically (file was deleted, or filesystem was unmounted).
 353 See also BUGS.
 354 .TP
 355 .B IN_ISDIR
 356 Subject of this event is a directory.
 357 .TP
 358 .B IN_Q_OVERFLOW
 359 Event queue overflowed
 360 .RI ( wd
 361 is \-1 for this event).
 362 .TP
 363 .B IN_UNMOUNT
 364 Filesystem containing watched object was unmounted.
 365 In addition, an
 366 .B IN_IGNORED
 367 event will subsequently be generated for the watch descriptor.
 368 .RE
 369 .SS Examples
 370 Suppose an application is watching the directory
 371 .I dir
 372 and the file
 373 .IR dir/myfile
 374 for all events.
 375 The examples below show some events that will be generated
 376 for these two objects.
 377 .RS 4
 378 .TP
 379 fd = open("dir/myfile", O_RDWR);
 380 Generates
 381 .B IN_OPEN
 382 events for both
 383 .I dir
 384 and
 385 .IR dir/myfile .
 386 .TP
 387 read(fd, buf, count);
 388 Generates
 389 .B IN_ACCESS
 390 events for both
 391 .I dir
 392 and
 393 .IR dir/myfile .
 394 .TP
 395 write(fd, buf, count);
 396 Generates
 397 .B IN_MODIFY
 398 events for both
 399 .I dir
 400 and
 401 .IR dir/myfile .
 402 .TP
 403 fchmod(fd, mode);
 404 Generates
 405 .B IN_ATTRIB
 406 events for both
 407 .I dir
 408 and
 409 .IR dir/myfile .
 410 .TP
 411 close(fd);
 412 Generates
 413 .B IN_CLOSE_WRITE
 414 events for both
 415 .I dir
 416 and
 417 .IR dir/myfile .
 418 .RE
 419 .PP
 420 Suppose an application is watching the directories
 421 .I dir1
 422 and
 423 .IR dir2 ,
 424 and the file
 425 .IR dir1/myfile .
 426 The following examples show some events that may be generated.
 427 .RS 4
 428 .TP
 429 link("dir1/myfile", "dir2/new");
 430 Generates an
 431 .B IN_ATTRIB
 432 event for
 433 .IR myfile
 434 and an
 435 .B IN_CREATE
 436 event for
 437 .IR dir2 .
 438 .TP
 439 rename("dir1/myfile", "dir2/myfile");
 440 Generates an
 441 .B IN_MOVED_FROM
 442 event for
 443 .IR dir1 ,
 444 an
 445 .B IN_MOVED_TO
 446 event for
 447 .IR dir2 ,
 448 and an
 449 .B IN_MOVE_SELF
 450 event for
 451 .IR myfile .
 452 The
 453 .B IN_MOVED_FROM
 454 and
 455 .B IN_MOVED_TO
 456 events will have the same
 457 .I cookie
 458 value.
 459 .RE
 460 .PP
 461 Suppose that
 462 .IR dir1/xx
 463 and
 464 .IR dir2/yy
 465 are (the only) links to the same file, and an application is watching
 466 .IR dir1 ,
 467 .IR dir2 ,
 468 .IR dir1/xx ,
 469 and
 470 .IR dir2/yy .
 471 Executing the following calls in the order given below will generate
 472 the following events:
 473 .RS 4
 474 .TP
 475 unlink("dir2/yy");
 476 Generates an
 477 .BR IN_ATTRIB
 478 event for
 479 .IR xx
 480 (because its link count changes)
 481 and an
 482 .B IN_DELETE
 483 event for
 484 .IR dir2 .
 485 .TP
 486 unlink("dir1/xx");
 487 Generates
 488 .BR IN_ATTRIB ,
 489 .BR IN_DELETE_SELF ,
 490 and
 491 .BR IN_IGNORED
 492 events for
 493 .IR xx ,
 494 and an
 495 .BR IN_DELETE
 496 event for
 497 .IR dir1 .
 498 .RE
 499 .PP
 500 Suppose an application is watching the directory
 501 .IR dir
 502 and (the empty) directory
 503 .IR dir/subdir .
 504 The following examples show some events that may be generated.
 505 .RS 4
 506 .TP
 507 mkdir("dir/new", mode);
 508 Generates an
 509 .B "IN_CREATE | IN_ISDIR"
 510 event for
 511 .IR dir .
 512 .TP
 513 rmdir("dir/subdir");
 514 Generates
 515 .B IN_DELETE_SELF
 516 and
 517 .B IN_IGNORED
 518 events for
 519 .IR subdir ,
 520 and an
 521 .B "IN_DELETE | IN_ISDIR"
 522 event for
 523 .IR dir .
 524 .RE
 525 .SS /proc interfaces
 526 The following interfaces can be used to limit the amount of
 527 kernel memory consumed by inotify:
 528 .TP
 529 .I /proc/sys/fs/inotify/max_queued_events
 530 The value in this file is used when an application calls
 531 .BR inotify_init (2)
 532 to set an upper limit on the number of events that can be
 533 queued to the corresponding inotify instance.
 534 Events in excess of this limit are dropped, but an
 535 .B IN_Q_OVERFLOW
 536 event is always generated.
 537 .TP
 538 .I /proc/sys/fs/inotify/max_user_instances
 539 This specifies an upper limit on the number of inotify instances
 540 that can be created per real user ID.
 541 .TP
 542 .I /proc/sys/fs/inotify/max_user_watches
 543 This specifies an upper limit on the number of watches
 544 that can be created per real user ID.
 545 .SH VERSIONS
 546 Inotify was merged into the 2.6.13 Linux kernel.
 547 The required library interfaces were added to glibc in version 2.4.
 548 .RB ( IN_DONT_FOLLOW ,
 549 .BR IN_MASK_ADD ,
 550 and
 551 .B IN_ONLYDIR
 552 were added in glibc version 2.5.)
 553 .SH CONFORMING TO
 554 The inotify API is Linux-specific.
 555 .SH NOTES
 556 Inotify file descriptors can be monitored using
 557 .BR select (2),
 558 .BR poll (2),
 559 and
 560 .BR epoll (7).
 561 When an event is available, the file descriptor indicates as readable.
 562
 563 Since Linux 2.6.25,
 564 signal-driven I/O notification is available for inotify file descriptors;
 565 see the discussion of
 566 .B F_SETFL
 567 (for setting the
 568 .B O_ASYNC
 569 flag),
 570 .BR F_SETOWN ,
 571 and
 572 .B F_SETSIG
 573 in
 574 .BR fcntl (2).
 575 The
 576 .I siginfo_t
 577 structure (described in
 578 .BR sigaction (2))
 579 that is passed to the signal handler has the following fields set:
 580 .IR si_fd
 581 is set to the inotify file descriptor number;
 582 .IR si_signo
 583 is set to the signal number;
 584 .IR si_code
 585 is set to
 586 .BR POLL_IN ;
 587 and
 588 .B POLLIN
 589 is set in
 590 .IR si_band .
 591
 592 If successive output inotify events produced on the
 593 inotify file descriptor are identical (same
 594 .IR wd ,
 595 .IR mask ,
 596 .IR cookie ,
 597 and
 598 .IR name ),
 599 then they are coalesced into a single event if the
 600 older event has not yet been read (but see BUGS).
 601 This reduces the amount of kernel memory required for the event queue,
 602 but also means that an application can't use inotify to reliably count
 603 file events.
 604
 605 The events returned by reading from an inotify file descriptor
 606 form an ordered queue.
 607 Thus, for example, it is guaranteed that when renaming from
 608 one directory to another, events will be produced in the
 609 correct order on the inotify file descriptor.
 610
 611 The
 612 .B FIONREAD
 613 .BR ioctl (2)
 614 returns the number of bytes available to read from an
 615 inotify file descriptor.
 616 .SS Limitations and caveats
 617 The inotify API provides no information about the user or process that
 618 triggered the inotify event.
 619 In particular, there is no easy
 620 way for a process that is monitoring events via inotify
 621 to distinguish events that it triggers
 622 itself from those that are triggered by other processes.
 623
 624 Inotify reports only events that a user-space program triggers through
 625 the filesystem API.
 626 As a result, it does not catch remote events that occur
 627 on network filesystems.
 628 (Applications must fall back to polling the filesystem
 629 to catch such events.)
 630 Furthermore, various pseudo-filesystems such as
 631 .IR /proc ,
 632 .IR /sys ,
 633 and
 634 .IR /dev/pts
 635 are not monitorable with inotify.
 636
 637 The inotify API does not report file accesses and modifications that
 638 may occur because of
 639 .BR mmap (2),
 640 .BR msync (2),
 641 and
 642 .BR munmap (2).
 643
 644 The inotify API identifies affected files by filename.
 645 However, by the time an application processes an inotify event,
 646 the filename may already have been deleted or renamed.
 647
 648 The inotify API identifies events via watch descriptors.
 649 It is the application's responsibility to cache a mapping
 650 (if one is needed) between watch descriptors and pathnames.
 651 Be aware that directory renamings may affect multiple cached pathnames.
 652
 653 Inotify monitoring of directories is not recursive:
 654 to monitor subdirectories under a directory,
 655 additional watches must be created.
 656 This can take a significant amount time for large directory trees.
 657
 658 If monitoring an entire directory subtree,
 659 and a new subdirectory is created in that tree or an existing directory
 660 is renamed into that tree,
 661 be aware that by the time you create a watch for the new subdirectory,
 662 new files (and subdirectories) may already exist inside the subdirectory.
 663 Therefore, you may want to scan the contents of the subdirectory
 664 immediately after adding the watch (and, if desired,
 665 recursively add watches for any subdirectories that it contains).
 666
 667 Note that the event queue can overflow.
 668 In this case, events are lost.
 669 Robust applications should handle the possibility of
 670 lost events gracefully.
 671 For example, it may be necessary to rebuild part or all of
 672 the application cache.
 673 (One simple, but possibly expensive,
 674 approach is to close the inotify file descriptor, empty the cache,
 675 create a new inotify file descriptor,
 676 and then re-create watches and cache entries
 677 for the objects to be monitored.)
 678 .SS Dealing with rename() events
 679 As noted above, the
 680 .B IN_MOVED_FROM
 681 and
 682 .B IN_MOVED_TO
 683 event pair that is generated by
 684 .BR rename (2)
 685 can be matched up via their shared cookie value.
 686 However, the task of matching has some challenges.
 687
 688 These two events are usually consecutive in the event stream available
 689 when reading from the inotify file descriptor.
 690 However, this is not guaranteed.
 691 If multiple processes are triggering events for monitored objects,
 692 then (on rare occasions) an arbitrary number of
 693 other events may appear between the
 694 .B IN_MOVED_FROM
 695 and
 696 .B IN_MOVED_TO
 697 events.
 698
 699 Matching up the
 700 .B IN_MOVED_FROM
 701 and
 702 .B IN_MOVED_TO
 703 event pair generated by
 704 .BR rename (2)
 705 is thus inherently racy.
 706 (Don't forget that if an object is renamed outside of a monitored directory,
 707 there may not even be an
 708 .BR IN_MOVED_TO
 709 event.)
 710 Heuristic approaches (e.g., assume the events are always consecutive)
 711 can be used to ensure a match in most cases,
 712 but will inevitably miss some cases,
 713 causing the application to perceive the
 714 .B IN_MOVED_FROM
 715 and
 716 .B IN_MOVED_TO
 717 events as being unrelated.
 718 If watch descriptors are destroyed and re-created as a result,
 719 then those watch descriptors will be inconsistent with
 720 the watch descriptors in any pending events.
 721 (Re-creating the inotify file descriptor and rebuilding the cache may
 722 be useful to deal with this scenario.)
 723
 724 Applications should also allow for the possibility that the
 725 .B IN_MOVED_FROM
 726 event was the last event that could fit in the buffer
 727 returned by the current call to
 728 .BR read (2),
 729 and the accompanying
 730 .B IN_MOVED_TO
 731 event might be fetched only on the next
 732 .BR read (2).
 733 .SH BUGS
 734 .\" FIXME kernel commit 611da04f7a31b2208e838be55a42c7a1310ae321
 735 .\" implies that unmount events were buggy 2.6.11 to 2.6.36
 736 .\"
 737 In kernels before 2.6.16, the
 738 .B IN_ONESHOT
 739 .I mask
 740 flag does not work.
 741
 742 As originally designed and implemented, the
 743 .B IN_ONESHOT
 744 flag did not cause an
 745 .B IN_IGNORED
 746 event to be generated when the watch was dropped after one event.
 747 However, as an unintended effect of other changes,
 748 since Linux 2.6.36, an
 749 .B IN_IGNORED
 750 event is generated in this case.
 751
 752 Before kernel 2.6.25,
 753 .\" commit 1c17d18e3775485bf1e0ce79575eb637a94494a2
 754 the kernel code that was intended to coalesce successive identical events
 755 (i.e., the two most recent events could potentially be coalesced
 756 if the older had not yet been read)
 757 instead checked if the most recent event could be coalesced with the
 758 .I oldest
 759 unread event.
 760
 761 When a watch descriptor is removed by calling
 762 .BR inotify_rm_watch (2)
 763 (or because a watch file is deleted or the filesystem
 764 that contains it is unmounted),
 765 any pending unread events for that watch descriptor remain available to read.
 766 As watch descriptors are subsequently allocated with
 767 .BR inotify_add_watch (2),
 768 the kernel cycles through the range of possible watch descriptors (0 to
 769 .BR INT_MAX )
 770 incrementally.
 771 When allocating a free watch descriptor, no check is made to see whether that
 772 watch descriptor number has any pending unread events in the inotify queue.
 773 Thus, it can happen that a watch descriptor is reallocated even
 774 when pending unread events exist for a previous incarnation of
 775 that watch descriptor number, with the result that the application
 776 might then read those events and interpret them as belonging to
 777 the file associated with the newly recycled watch descriptor.
 778 In practice, the likelihood of hitting this bug may be extremely low,
 779 since it requires that an application cycle through
 780 .B INT_MAX
 781 watch descriptors,
 782 release a watch descriptor while leaving unread events for that
 783 watch descriptor in the queue in the queue,
 784 and then recycle that watch descriptor.
 785 For this reason, and because there have been no reports
 786 of the bug occurring in real-world applications,
 787 as of Linux 3.15,
 788 .\" FIXME https://bugzilla.kernel.org/show_bug.cgi?id=77111
 789 no kernel changes have yet been made to eliminate this possible bug.
 790 .SH EXAMPLE
 791 The following program demonstrates the usage of the inotify API.
 792 It marks the directories passed as a command-line arguments
 793 and waits for events of type
 794 .BR IN_OPEN ,
 795 .BR IN_CLOSE_NOWRITE
 796 and
 797 .BR IN_CLOSE_WRITE .
 798 .PP
 799 The following output was recorded while editing the file
 800 .I /home/user/temp/foo
 801 and listing directory
 802 .IR /tmp .
 803 Before the file and the directory were opened,
 804 .B IN_OPEN
 805 events occurred.
 806 After the file was closed, an
 807 .B IN_CLOSE_WRITE
 808 event occurred.
 809 After the directory was closed, an
 810 .B IN_CLOSE_NOWRITE
 811 event occurred.
 812 Execution of the program ended when the user pressed the ENTER key.
 813 .SS Example output
 814 .in +4n
 815 .nf
 816 $ \fB./a.out /tmp /home/user/temp\fP
 817 Press enter key to terminate.
 818 Listening for events.
 819 IN_OPEN: /home/user/temp/foo [file]
 820 IN_CLOSE_WRITE: /home/user/temp/foo [file]
 821 IN_OPEN: /tmp/ [directory]
 822 IN_CLOSE_NOWRITE: /tmp/ [directory]
 823
 824 Listening for events stopped.
 825 .fi
 826 .in
 827 .SS Program source
 828 .nf
 829 #include <errno.h>
 830 #include <poll.h>
 831 #include <stdio.h>
 832 #include <stdlib.h>
 833 #include <sys/inotify.h>
 834 #include <unistd.h>
 835
 836 /* Read all available inotify events from the file descriptor 'fd'.
 837    wd is the table of watch descriptors for the directories in argv.
 838    argc is the length of wd and argv.
 839    argv is the list of watched directories.
 840    Entry 0 of wd and argv is unused. */
 841
 842 static void
 843 handle_events(int fd, int *wd, int argc, char* argv[])
 844 {
 845     /* Some systems cannot read integer variables if they are not
 846        properly aligned. On other systems, incorrect alignment may
 847        decrease performance. Hence, the buffer used for reading from
 848        the inotify file descriptor should have the same alignment as
 849        struct inotify_event. */
 850
 851     char buf[4096]
 852         __attribute__ ((aligned(__alignof__(struct inotify_event))));
 853     const struct inotify_event *event;
 854     int i;
 855     ssize_t len;
 856     char *ptr;
 857
 858     /* Loop while events can be read from inotify file descriptor. */
 859
 860     for (;;) {
 861
 862         /* Read some events. */
 863
 864         len = read(fd, buf, sizeof buf);
 865         if (len == \-1 && errno != EAGAIN) {
 866             perror("read");
 867             exit(EXIT_FAILURE);
 868         }
 869
 870         /* If the nonblocking read() found no events to read, then
 871            it returns \-1 with errno set to EAGAIN. In that case,
 872            we exit the loop. */
 873
 874         if (len <= 0)
 875             break;
 876
 877         /* Loop over all events in the buffer */
 878
 879         for (ptr = buf; ptr < buf + len;
 880                 ptr += sizeof(struct inotify_event) + event\->len) {
 881
 882             event = (const struct inotify_event *) ptr;
 883
 884             /* Print event type */
 885
 886             if (event\->mask & IN_OPEN)
 887                 printf("IN_OPEN: ");
 888             if (event\->mask & IN_CLOSE_NOWRITE)
 889                 printf("IN_CLOSE_NOWRITE: ");
 890             if (event\->mask & IN_CLOSE_WRITE)
 891                 printf("IN_CLOSE_WRITE: ");
 892
 893             /* Print the name of the watched directory */
 894
 895             for (i = 1; i < argc; ++i) {
 896                 if (wd[i] == event\->wd) {
 897                     printf("%s/", argv[i]);
 898                     break;
 899                 }
 900             }
 901
 902             /* Print the name of the file */
 903
 904             if (event\->len)
 905                 printf("%s", event\->name);
 906
 907             /* Print type of filesystem object */
 908
 909             if (event\->mask & IN_ISDIR)
 910                 printf(" [directory]\\n");
 911             else
 912                 printf(" [file]\\n");
 913         }
 914     }
 915 }
 916
 917 int
 918 main(int argc, char* argv[])
 919 {
 920     char buf;
 921     int fd, i, poll_num;
 922     int *wd;
 923     nfds_t nfds;
 924     struct pollfd fds[2];
 925
 926     if (argc < 2) {
 927         printf("Usage: %s PATH [PATH ...]\\n", argv[0]);
 928         exit(EXIT_FAILURE);
 929     }
 930
 931     printf("Press ENTER key to terminate.\\n");
 932
 933     /* Create the file descriptor for accessing the inotify API */
 934
 935     fd = inotify_init1(IN_NONBLOCK);
 936     if (fd == \-1) {
 937         perror("inotify_init1");
 938         exit(EXIT_FAILURE);
 939     }
 940
 941     /* Allocate memory for watch descriptors */
 942
 943     wd = calloc(argc, sizeof(int));
 944     if (wd == NULL) {
 945         perror("calloc");
 946         exit(EXIT_FAILURE);
 947     }
 948
 949     /* Mark directories for events
 950        \- file was opened
 951        \- file was closed */
 952
 953     for (i = 1; i < argc; i++) {
 954         wd[i] = inotify_add_watch(fd, argv[i],
 955                                   IN_OPEN | IN_CLOSE);
 956         if (wd[i] == \-1) {
 957             fprintf(stderr, "Cannot watch '%s'\\n", argv[i]);
 958             perror("inotify_add_watch");
 959             exit(EXIT_FAILURE);
 960         }
 961     }
 962
 963     /* Prepare for polling */
 964
 965     nfds = 2;
 966
 967     /* Console input */
 968
 969     fds[0].fd = STDIN_FILENO;
 970     fds[0].events = POLLIN;
 971
 972     /* Inotify input */
 973
 974     fds[1].fd = fd;
 975     fds[1].events = POLLIN;
 976
 977     /* Wait for events and/or terminal input */
 978
 979     printf("Listening for events.\\n");
 980     while (1) {
 981         poll_num = poll(fds, nfds, \-1);
 982         if (poll_num == \-1) {
 983             if (errno == EINTR)
 984                 continue;
 985             perror("poll");
 986             exit(EXIT_FAILURE);
 987         }
 988
 989         if (poll_num > 0) {
 990
 991             if (fds[0].revents & POLLIN) {
 992
 993                 /* Console input is available. Empty stdin and quit */
 994
 995                 while (read(STDIN_FILENO, &buf, 1) > 0 && buf != '\\n')
 996                     continue;
 997                 break;
 998             }
 999
1000             if (fds[1].revents & POLLIN) {
1001
1002                 /* Inotify events are available */
1003
1004                 handle_events(fd, wd, argc, argv);
1005             }
1006         }
1007     }
1008
1009     printf("Listening for events stopped.\\n");
1010
1011     /* Close inotify file descriptor */
1012
1013     close(fd);
1014
1015     free(wd);
1016     exit(EXIT_SUCCESS);
1017 }
1018 .fi
1019 .SH SEE ALSO
1020 .BR inotifywait (1),
1021 .BR inotifywatch (1),
1022 .BR inotify_add_watch (2),
1023 .BR inotify_init (2),
1024 .BR inotify_init1 (2),
1025 .BR inotify_rm_watch (2),
1026 .BR read (2),
1027 .BR stat (2),
1028 .BR fanotify (7)
1029
1030 .IR Documentation/filesystems/inotify.txt
1031 in the Linux kernel source tree