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1 | '\" t | |
2 | .\" Copyright (C) 2006, 2014 Michael Kerrisk <mtk.manpages@gmail.com> | |
3 | .\" Copyright (C) 2014 Heinrich Schuchardt <xypron.glpk@gmx.de> | |
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26 | .\" | |
27 | .TH INOTIFY 7 2016-12-12 "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 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 filename within to the watched directory. | |
149 | This filename 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 | .\" FIXME . | |
213 | .\" Events do not occur for link count changes on a file inside a monitored | |
214 | .\" directory. This differs from other metadata changes for files inside | |
215 | .\" a monitored directory. | |
216 | for the target of | |
217 | .BR link (2) | |
218 | and for | |
219 | .BR unlink (2)), | |
220 | and user/group ID (e.g., | |
221 | .BR chown (2)). | |
222 | .TP | |
223 | .BR IN_CLOSE_WRITE " (+)" | |
224 | File opened for writing was closed. | |
225 | .TP | |
226 | .BR IN_CLOSE_NOWRITE " (*)" | |
227 | File or directory not opened for writing was closed. | |
228 | .TP | |
229 | .BR IN_CREATE " (+)" | |
230 | File/directory created in watched directory (e.g., | |
231 | .BR open (2) | |
232 | .BR O_CREAT , | |
233 | .BR mkdir (2), | |
234 | .BR link (2), | |
235 | .BR symlink (2), | |
236 | .BR bind (2) | |
237 | on a UNIX domain socket). | |
238 | .TP | |
239 | .BR IN_DELETE " (+)" | |
240 | File/directory deleted from watched directory. | |
241 | .TP | |
242 | .B IN_DELETE_SELF | |
243 | Watched file/directory was itself deleted. | |
244 | (This event also occurs if an object is moved to another filesystem, | |
245 | since | |
246 | .BR mv (1) | |
247 | in effect copies the file to the other filesystem and | |
248 | then deletes it from the original filesystem.) | |
249 | In addition, an | |
250 | .B IN_IGNORED | |
251 | event will subsequently be generated for the watch descriptor. | |
252 | .TP | |
253 | .BR IN_MODIFY " (+)" | |
254 | File was modified (e.g., | |
255 | .BR write (2), | |
256 | .BR truncate (2)). | |
257 | .TP | |
258 | .B IN_MOVE_SELF | |
259 | Watched file/directory was itself moved. | |
260 | .TP | |
261 | .BR IN_MOVED_FROM " (+)" | |
262 | Generated for the directory containing the old filename | |
263 | when a file is renamed. | |
264 | .TP | |
265 | .BR IN_MOVED_TO " (+)" | |
266 | Generated for the directory containing the new filename | |
267 | when a file is renamed. | |
268 | .TP | |
269 | .BR IN_OPEN " (*)" | |
270 | File or directory was opened. | |
271 | .RE | |
272 | .PP | |
273 | When monitoring a directory: | |
274 | .IP * 3 | |
275 | the events marked above with an asterisk (*) can occur both | |
276 | for the directory itself and for objects inside the directory; and | |
277 | .IP * | |
278 | the events marked with a plus sign (+) occur only for objects | |
279 | inside the directory (not for the directory itself). | |
280 | .PP | |
281 | .IR Note : | |
282 | when monitoring a directory, | |
283 | events are not generated for the files inside the directory | |
284 | when the events are performed via a pathname (i.e., a link) | |
285 | that lies outside the monitored directory. | |
286 | ||
287 | When events are generated for objects inside a watched directory, the | |
288 | .I name | |
289 | field in the returned | |
290 | .I inotify_event | |
291 | structure identifies the name of the file within the directory. | |
292 | .PP | |
293 | The | |
294 | .B IN_ALL_EVENTS | |
295 | macro is defined as a bit mask of all of the above events. | |
296 | This macro can be used as the | |
297 | .I mask | |
298 | argument when calling | |
299 | .BR inotify_add_watch (2). | |
300 | ||
301 | Two additional convenience macros are defined: | |
302 | .RS 4 | |
303 | .TP | |
304 | .BR IN_MOVE | |
305 | Equates to | |
306 | .BR "IN_MOVED_FROM | IN_MOVED_TO" . | |
307 | .TP | |
308 | .BR IN_CLOSE | |
309 | Equates to | |
310 | .BR "IN_CLOSE_WRITE | IN_CLOSE_NOWRITE" . | |
311 | .RE | |
312 | .PP | |
313 | The following further bits can be specified in | |
314 | .I mask | |
315 | when calling | |
316 | .BR inotify_add_watch (2): | |
317 | .RS 4 | |
318 | .TP | |
319 | .BR IN_DONT_FOLLOW " (since Linux 2.6.15)" | |
320 | Don't dereference | |
321 | .I pathname | |
322 | if it is a symbolic link. | |
323 | .TP | |
324 | .BR IN_EXCL_UNLINK " (since Linux 2.6.36)" | |
325 | .\" commit 8c1934c8d70b22ca8333b216aec6c7d09fdbd6a6 | |
326 | By default, when watching events on the children of a directory, | |
327 | events are generated for children even after they have been unlinked | |
328 | from the directory. | |
329 | This can result in large numbers of uninteresting events for | |
330 | some applications (e.g., if watching | |
331 | .IR /tmp , | |
332 | in which many applications create temporary files whose | |
333 | names are immediately unlinked). | |
334 | Specifying | |
335 | .B IN_EXCL_UNLINK | |
336 | changes the default behavior, | |
337 | so that events are not generated for children after | |
338 | they have been unlinked from the watched directory. | |
339 | .TP | |
340 | .B IN_MASK_ADD | |
341 | If a watch instance already exists for the filesystem object corresponding to | |
342 | .IR pathname , | |
343 | add (OR) the events in | |
344 | .I mask | |
345 | to the watch mask (instead of replacing the mask). | |
346 | .TP | |
347 | .B IN_ONESHOT | |
348 | Monitor the filesystem object corresponding to | |
349 | .I pathname | |
350 | for one event, then remove from | |
351 | watch list. | |
352 | .TP | |
353 | .BR IN_ONLYDIR " (since Linux 2.6.15)" | |
354 | Watch | |
355 | .I pathname | |
356 | only if it is a directory. | |
357 | Using this flag provides an application with a race-free way of | |
358 | ensuring that the monitored object is a directory. | |
359 | .RE | |
360 | .PP | |
361 | The following bits may be set in the | |
362 | .I mask | |
363 | field returned by | |
364 | .BR read (2): | |
365 | .RS 4 | |
366 | .TP | |
367 | .B IN_IGNORED | |
368 | Watch was removed explicitly | |
369 | .RB ( inotify_rm_watch (2)) | |
370 | or automatically (file was deleted, or filesystem was unmounted). | |
371 | See also BUGS. | |
372 | .TP | |
373 | .B IN_ISDIR | |
374 | Subject of this event is a directory. | |
375 | .TP | |
376 | .B IN_Q_OVERFLOW | |
377 | Event queue overflowed | |
378 | .RI ( wd | |
379 | is \-1 for this event). | |
380 | .TP | |
381 | .B IN_UNMOUNT | |
382 | Filesystem containing watched object was unmounted. | |
383 | In addition, an | |
384 | .B IN_IGNORED | |
385 | event will subsequently be generated for the watch descriptor. | |
386 | .RE | |
387 | .SS Examples | |
388 | Suppose an application is watching the directory | |
389 | .I dir | |
390 | and the file | |
391 | .IR dir/myfile | |
392 | for all events. | |
393 | The examples below show some events that will be generated | |
394 | for these two objects. | |
395 | .RS 4 | |
396 | .TP | |
397 | fd = open("dir/myfile", O_RDWR); | |
398 | Generates | |
399 | .B IN_OPEN | |
400 | events for both | |
401 | .I dir | |
402 | and | |
403 | .IR dir/myfile . | |
404 | .TP | |
405 | read(fd, buf, count); | |
406 | Generates | |
407 | .B IN_ACCESS | |
408 | events for both | |
409 | .I dir | |
410 | and | |
411 | .IR dir/myfile . | |
412 | .TP | |
413 | write(fd, buf, count); | |
414 | Generates | |
415 | .B IN_MODIFY | |
416 | events for both | |
417 | .I dir | |
418 | and | |
419 | .IR dir/myfile . | |
420 | .TP | |
421 | fchmod(fd, mode); | |
422 | Generates | |
423 | .B IN_ATTRIB | |
424 | events for both | |
425 | .I dir | |
426 | and | |
427 | .IR dir/myfile . | |
428 | .TP | |
429 | close(fd); | |
430 | Generates | |
431 | .B IN_CLOSE_WRITE | |
432 | events for both | |
433 | .I dir | |
434 | and | |
435 | .IR dir/myfile . | |
436 | .RE | |
437 | .PP | |
438 | Suppose an application is watching the directories | |
439 | .I dir1 | |
440 | and | |
441 | .IR dir2 , | |
442 | and the file | |
443 | .IR dir1/myfile . | |
444 | The following examples show some events that may be generated. | |
445 | .RS 4 | |
446 | .TP | |
447 | link("dir1/myfile", "dir2/new"); | |
448 | Generates an | |
449 | .B IN_ATTRIB | |
450 | event for | |
451 | .IR myfile | |
452 | and an | |
453 | .B IN_CREATE | |
454 | event for | |
455 | .IR dir2 . | |
456 | .TP | |
457 | rename("dir1/myfile", "dir2/myfile"); | |
458 | Generates an | |
459 | .B IN_MOVED_FROM | |
460 | event for | |
461 | .IR dir1 , | |
462 | an | |
463 | .B IN_MOVED_TO | |
464 | event for | |
465 | .IR dir2 , | |
466 | and an | |
467 | .B IN_MOVE_SELF | |
468 | event for | |
469 | .IR myfile . | |
470 | The | |
471 | .B IN_MOVED_FROM | |
472 | and | |
473 | .B IN_MOVED_TO | |
474 | events will have the same | |
475 | .I cookie | |
476 | value. | |
477 | .RE | |
478 | .PP | |
479 | Suppose that | |
480 | .IR dir1/xx | |
481 | and | |
482 | .IR dir2/yy | |
483 | are (the only) links to the same file, and an application is watching | |
484 | .IR dir1 , | |
485 | .IR dir2 , | |
486 | .IR dir1/xx , | |
487 | and | |
488 | .IR dir2/yy . | |
489 | Executing the following calls in the order given below will generate | |
490 | the following events: | |
491 | .RS 4 | |
492 | .TP | |
493 | unlink("dir2/yy"); | |
494 | Generates an | |
495 | .BR IN_ATTRIB | |
496 | event for | |
497 | .IR xx | |
498 | (because its link count changes) | |
499 | and an | |
500 | .B IN_DELETE | |
501 | event for | |
502 | .IR dir2 . | |
503 | .TP | |
504 | unlink("dir1/xx"); | |
505 | Generates | |
506 | .BR IN_ATTRIB , | |
507 | .BR IN_DELETE_SELF , | |
508 | and | |
509 | .BR IN_IGNORED | |
510 | events for | |
511 | .IR xx , | |
512 | and an | |
513 | .BR IN_DELETE | |
514 | event for | |
515 | .IR dir1 . | |
516 | .RE | |
517 | .PP | |
518 | Suppose an application is watching the directory | |
519 | .IR dir | |
520 | and (the empty) directory | |
521 | .IR dir/subdir . | |
522 | The following examples show some events that may be generated. | |
523 | .RS 4 | |
524 | .TP | |
525 | mkdir("dir/new", mode); | |
526 | Generates an | |
527 | .B "IN_CREATE | IN_ISDIR" | |
528 | event for | |
529 | .IR dir . | |
530 | .TP | |
531 | rmdir("dir/subdir"); | |
532 | Generates | |
533 | .B IN_DELETE_SELF | |
534 | and | |
535 | .B IN_IGNORED | |
536 | events for | |
537 | .IR subdir , | |
538 | and an | |
539 | .B "IN_DELETE | IN_ISDIR" | |
540 | event for | |
541 | .IR dir . | |
542 | .RE | |
543 | .SS /proc interfaces | |
544 | The following interfaces can be used to limit the amount of | |
545 | kernel memory consumed by inotify: | |
546 | .TP | |
547 | .I /proc/sys/fs/inotify/max_queued_events | |
548 | The value in this file is used when an application calls | |
549 | .BR inotify_init (2) | |
550 | to set an upper limit on the number of events that can be | |
551 | queued to the corresponding inotify instance. | |
552 | Events in excess of this limit are dropped, but an | |
553 | .B IN_Q_OVERFLOW | |
554 | event is always generated. | |
555 | .TP | |
556 | .I /proc/sys/fs/inotify/max_user_instances | |
557 | This specifies an upper limit on the number of inotify instances | |
558 | that can be created per real user ID. | |
559 | .TP | |
560 | .I /proc/sys/fs/inotify/max_user_watches | |
561 | This specifies an upper limit on the number of watches | |
562 | that can be created per real user ID. | |
563 | .SH VERSIONS | |
564 | Inotify was merged into the 2.6.13 Linux kernel. | |
565 | The required library interfaces were added to glibc in version 2.4. | |
566 | .RB ( IN_DONT_FOLLOW , | |
567 | .BR IN_MASK_ADD , | |
568 | and | |
569 | .B IN_ONLYDIR | |
570 | were added in glibc version 2.5.) | |
571 | .SH CONFORMING TO | |
572 | The inotify API is Linux-specific. | |
573 | .SH NOTES | |
574 | Inotify file descriptors can be monitored using | |
575 | .BR select (2), | |
576 | .BR poll (2), | |
577 | and | |
578 | .BR epoll (7). | |
579 | When an event is available, the file descriptor indicates as readable. | |
580 | ||
581 | Since Linux 2.6.25, | |
582 | signal-driven I/O notification is available for inotify file descriptors; | |
583 | see the discussion of | |
584 | .B F_SETFL | |
585 | (for setting the | |
586 | .B O_ASYNC | |
587 | flag), | |
588 | .BR F_SETOWN , | |
589 | and | |
590 | .B F_SETSIG | |
591 | in | |
592 | .BR fcntl (2). | |
593 | The | |
594 | .I siginfo_t | |
595 | structure (described in | |
596 | .BR sigaction (2)) | |
597 | that is passed to the signal handler has the following fields set: | |
598 | .IR si_fd | |
599 | is set to the inotify file descriptor number; | |
600 | .IR si_signo | |
601 | is set to the signal number; | |
602 | .IR si_code | |
603 | is set to | |
604 | .BR POLL_IN ; | |
605 | and | |
606 | .B POLLIN | |
607 | is set in | |
608 | .IR si_band . | |
609 | ||
610 | If successive output inotify events produced on the | |
611 | inotify file descriptor are identical (same | |
612 | .IR wd , | |
613 | .IR mask , | |
614 | .IR cookie , | |
615 | and | |
616 | .IR name ), | |
617 | then they are coalesced into a single event if the | |
618 | older event has not yet been read (but see BUGS). | |
619 | This reduces the amount of kernel memory required for the event queue, | |
620 | but also means that an application can't use inotify to reliably count | |
621 | file events. | |
622 | ||
623 | The events returned by reading from an inotify file descriptor | |
624 | form an ordered queue. | |
625 | Thus, for example, it is guaranteed that when renaming from | |
626 | one directory to another, events will be produced in the | |
627 | correct order on the inotify file descriptor. | |
628 | ||
629 | The set of watch descriptors that is being monitored via | |
630 | an inotify file descriptor can be viewed via the entry for | |
631 | the inotify file descriptor in the process's | |
632 | .IR /proc/[pid]/fdinfo | |
633 | directory. | |
634 | See | |
635 | .BR proc (5) | |
636 | for further details. | |
637 | The | |
638 | .B FIONREAD | |
639 | .BR ioctl (2) | |
640 | returns the number of bytes available to read from an | |
641 | inotify file descriptor. | |
642 | .SS Limitations and caveats | |
643 | The inotify API provides no information about the user or process that | |
644 | triggered the inotify event. | |
645 | In particular, there is no easy | |
646 | way for a process that is monitoring events via inotify | |
647 | to distinguish events that it triggers | |
648 | itself from those that are triggered by other processes. | |
649 | ||
650 | Inotify reports only events that a user-space program triggers through | |
651 | the filesystem API. | |
652 | As a result, it does not catch remote events that occur | |
653 | on network filesystems. | |
654 | (Applications must fall back to polling the filesystem | |
655 | to catch such events.) | |
656 | Furthermore, various pseudo-filesystems such as | |
657 | .IR /proc , | |
658 | .IR /sys , | |
659 | and | |
660 | .IR /dev/pts | |
661 | are not monitorable with inotify. | |
662 | ||
663 | The inotify API does not report file accesses and modifications that | |
664 | may occur because of | |
665 | .BR mmap (2), | |
666 | .BR msync (2), | |
667 | and | |
668 | .BR munmap (2). | |
669 | ||
670 | The inotify API identifies affected files by filename. | |
671 | However, by the time an application processes an inotify event, | |
672 | the filename may already have been deleted or renamed. | |
673 | ||
674 | The inotify API identifies events via watch descriptors. | |
675 | It is the application's responsibility to cache a mapping | |
676 | (if one is needed) between watch descriptors and pathnames. | |
677 | Be aware that directory renamings may affect multiple cached pathnames. | |
678 | ||
679 | Inotify monitoring of directories is not recursive: | |
680 | to monitor subdirectories under a directory, | |
681 | additional watches must be created. | |
682 | This can take a significant amount time for large directory trees. | |
683 | ||
684 | If monitoring an entire directory subtree, | |
685 | and a new subdirectory is created in that tree or an existing directory | |
686 | is renamed into that tree, | |
687 | be aware that by the time you create a watch for the new subdirectory, | |
688 | new files (and subdirectories) may already exist inside the subdirectory. | |
689 | Therefore, you may want to scan the contents of the subdirectory | |
690 | immediately after adding the watch (and, if desired, | |
691 | recursively add watches for any subdirectories that it contains). | |
692 | ||
693 | Note that the event queue can overflow. | |
694 | In this case, events are lost. | |
695 | Robust applications should handle the possibility of | |
696 | lost events gracefully. | |
697 | For example, it may be necessary to rebuild part or all of | |
698 | the application cache. | |
699 | (One simple, but possibly expensive, | |
700 | approach is to close the inotify file descriptor, empty the cache, | |
701 | create a new inotify file descriptor, | |
702 | and then re-create watches and cache entries | |
703 | for the objects to be monitored.) | |
704 | .SS Dealing with rename() events | |
705 | As noted above, the | |
706 | .B IN_MOVED_FROM | |
707 | and | |
708 | .B IN_MOVED_TO | |
709 | event pair that is generated by | |
710 | .BR rename (2) | |
711 | can be matched up via their shared cookie value. | |
712 | However, the task of matching has some challenges. | |
713 | ||
714 | These two events are usually consecutive in the event stream available | |
715 | when reading from the inotify file descriptor. | |
716 | However, this is not guaranteed. | |
717 | If multiple processes are triggering events for monitored objects, | |
718 | then (on rare occasions) an arbitrary number of | |
719 | other events may appear between the | |
720 | .B IN_MOVED_FROM | |
721 | and | |
722 | .B IN_MOVED_TO | |
723 | events. | |
724 | Furthermore, it is not guaranteed that the event pair is atomically | |
725 | inserted into the queue: there may be a brief interval where the | |
726 | .B IN_MOVED_FROM | |
727 | has appeared, but the | |
728 | .B IN_MOVED_TO | |
729 | has not. | |
730 | ||
731 | Matching up the | |
732 | .B IN_MOVED_FROM | |
733 | and | |
734 | .B IN_MOVED_TO | |
735 | event pair generated by | |
736 | .BR rename (2) | |
737 | is thus inherently racy. | |
738 | (Don't forget that if an object is renamed outside of a monitored directory, | |
739 | there may not even be an | |
740 | .BR IN_MOVED_TO | |
741 | event.) | |
742 | Heuristic approaches (e.g., assume the events are always consecutive) | |
743 | can be used to ensure a match in most cases, | |
744 | but will inevitably miss some cases, | |
745 | causing the application to perceive the | |
746 | .B IN_MOVED_FROM | |
747 | and | |
748 | .B IN_MOVED_TO | |
749 | events as being unrelated. | |
750 | If watch descriptors are destroyed and re-created as a result, | |
751 | then those watch descriptors will be inconsistent with | |
752 | the watch descriptors in any pending events. | |
753 | (Re-creating the inotify file descriptor and rebuilding the cache may | |
754 | be useful to deal with this scenario.) | |
755 | ||
756 | Applications should also allow for the possibility that the | |
757 | .B IN_MOVED_FROM | |
758 | event was the last event that could fit in the buffer | |
759 | returned by the current call to | |
760 | .BR read (2), | |
761 | and the accompanying | |
762 | .B IN_MOVED_TO | |
763 | event might be fetched only on the next | |
764 | .BR read (2), | |
765 | which should be done with a (small) timeout to allow for the fact that | |
766 | insertion of the | |
767 | .BR IN_MOVED_FROM - IN_MOVED_TO | |
768 | event pair is not atomic, | |
769 | and also the possibility that there may not be any | |
770 | .B IN_MOVED_TO | |
771 | event. | |
772 | .SH BUGS | |
773 | Before Linux 3.19, | |
774 | .BR fallocate (2) | |
775 | did not create any inotify events. | |
776 | Since Linux 3.19, | |
777 | .\" commit 820c12d5d6c0890bc93dd63893924a13041fdc35 | |
778 | calls to | |
779 | .BR fallocate (2) | |
780 | generate | |
781 | .B IN_MODIFY | |
782 | events. | |
783 | ||
784 | .\" FIXME . kernel commit 611da04f7a31b2208e838be55a42c7a1310ae321 | |
785 | .\" implies that unmount events were buggy 2.6.11 to 2.6.36 | |
786 | .\" | |
787 | In kernels before 2.6.16, the | |
788 | .B IN_ONESHOT | |
789 | .I mask | |
790 | flag does not work. | |
791 | ||
792 | As originally designed and implemented, the | |
793 | .B IN_ONESHOT | |
794 | flag did not cause an | |
795 | .B IN_IGNORED | |
796 | event to be generated when the watch was dropped after one event. | |
797 | However, as an unintended effect of other changes, | |
798 | since Linux 2.6.36, an | |
799 | .B IN_IGNORED | |
800 | event is generated in this case. | |
801 | ||
802 | Before kernel 2.6.25, | |
803 | .\" commit 1c17d18e3775485bf1e0ce79575eb637a94494a2 | |
804 | the kernel code that was intended to coalesce successive identical events | |
805 | (i.e., the two most recent events could potentially be coalesced | |
806 | if the older had not yet been read) | |
807 | instead checked if the most recent event could be coalesced with the | |
808 | .I oldest | |
809 | unread event. | |
810 | ||
811 | When a watch descriptor is removed by calling | |
812 | .BR inotify_rm_watch (2) | |
813 | (or because a watch file is deleted or the filesystem | |
814 | that contains it is unmounted), | |
815 | any pending unread events for that watch descriptor remain available to read. | |
816 | As watch descriptors are subsequently allocated with | |
817 | .BR inotify_add_watch (2), | |
818 | the kernel cycles through the range of possible watch descriptors (0 to | |
819 | .BR INT_MAX ) | |
820 | incrementally. | |
821 | When allocating a free watch descriptor, no check is made to see whether that | |
822 | watch descriptor number has any pending unread events in the inotify queue. | |
823 | Thus, it can happen that a watch descriptor is reallocated even | |
824 | when pending unread events exist for a previous incarnation of | |
825 | that watch descriptor number, with the result that the application | |
826 | might then read those events and interpret them as belonging to | |
827 | the file associated with the newly recycled watch descriptor. | |
828 | In practice, the likelihood of hitting this bug may be extremely low, | |
829 | since it requires that an application cycle through | |
830 | .B INT_MAX | |
831 | watch descriptors, | |
832 | release a watch descriptor while leaving unread events for that | |
833 | watch descriptor in the queue, | |
834 | and then recycle that watch descriptor. | |
835 | For this reason, and because there have been no reports | |
836 | of the bug occurring in real-world applications, | |
837 | as of Linux 3.15, | |
838 | .\" FIXME . https://bugzilla.kernel.org/show_bug.cgi?id=77111 | |
839 | no kernel changes have yet been made to eliminate this possible bug. | |
840 | .SH EXAMPLE | |
841 | The following program demonstrates the usage of the inotify API. | |
842 | It marks the directories passed as a command-line arguments | |
843 | and waits for events of type | |
844 | .BR IN_OPEN , | |
845 | .BR IN_CLOSE_NOWRITE | |
846 | and | |
847 | .BR IN_CLOSE_WRITE . | |
848 | .PP | |
849 | The following output was recorded while editing the file | |
850 | .I /home/user/temp/foo | |
851 | and listing directory | |
852 | .IR /tmp . | |
853 | Before the file and the directory were opened, | |
854 | .B IN_OPEN | |
855 | events occurred. | |
856 | After the file was closed, an | |
857 | .B IN_CLOSE_WRITE | |
858 | event occurred. | |
859 | After the directory was closed, an | |
860 | .B IN_CLOSE_NOWRITE | |
861 | event occurred. | |
862 | Execution of the program ended when the user pressed the ENTER key. | |
863 | .SS Example output | |
864 | .in +4n | |
865 | .nf | |
866 | $ \fB./a.out /tmp /home/user/temp\fP | |
867 | Press enter key to terminate. | |
868 | Listening for events. | |
869 | IN_OPEN: /home/user/temp/foo [file] | |
870 | IN_CLOSE_WRITE: /home/user/temp/foo [file] | |
871 | IN_OPEN: /tmp/ [directory] | |
872 | IN_CLOSE_NOWRITE: /tmp/ [directory] | |
873 | ||
874 | Listening for events stopped. | |
875 | .fi | |
876 | .in | |
877 | .SS Program source | |
878 | .nf | |
879 | #include <errno.h> | |
880 | #include <poll.h> | |
881 | #include <stdio.h> | |
882 | #include <stdlib.h> | |
883 | #include <sys/inotify.h> | |
884 | #include <unistd.h> | |
885 | ||
886 | /* Read all available inotify events from the file descriptor 'fd'. | |
887 | wd is the table of watch descriptors for the directories in argv. | |
888 | argc is the length of wd and argv. | |
889 | argv is the list of watched directories. | |
890 | Entry 0 of wd and argv is unused. */ | |
891 | ||
892 | static void | |
893 | handle_events(int fd, int *wd, int argc, char* argv[]) | |
894 | { | |
895 | /* Some systems cannot read integer variables if they are not | |
896 | properly aligned. On other systems, incorrect alignment may | |
897 | decrease performance. Hence, the buffer used for reading from | |
898 | the inotify file descriptor should have the same alignment as | |
899 | struct inotify_event. */ | |
900 | ||
901 | char buf[4096] | |
902 | __attribute__ ((aligned(__alignof__(struct inotify_event)))); | |
903 | const struct inotify_event *event; | |
904 | int i; | |
905 | ssize_t len; | |
906 | char *ptr; | |
907 | ||
908 | /* Loop while events can be read from inotify file descriptor. */ | |
909 | ||
910 | for (;;) { | |
911 | ||
912 | /* Read some events. */ | |
913 | ||
914 | len = read(fd, buf, sizeof buf); | |
915 | if (len == \-1 && errno != EAGAIN) { | |
916 | perror("read"); | |
917 | exit(EXIT_FAILURE); | |
918 | } | |
919 | ||
920 | /* If the nonblocking read() found no events to read, then | |
921 | it returns \-1 with errno set to EAGAIN. In that case, | |
922 | we exit the loop. */ | |
923 | ||
924 | if (len <= 0) | |
925 | break; | |
926 | ||
927 | /* Loop over all events in the buffer */ | |
928 | ||
929 | for (ptr = buf; ptr < buf + len; | |
930 | ptr += sizeof(struct inotify_event) + event\->len) { | |
931 | ||
932 | event = (const struct inotify_event *) ptr; | |
933 | ||
934 | /* Print event type */ | |
935 | ||
936 | if (event\->mask & IN_OPEN) | |
937 | printf("IN_OPEN: "); | |
938 | if (event\->mask & IN_CLOSE_NOWRITE) | |
939 | printf("IN_CLOSE_NOWRITE: "); | |
940 | if (event\->mask & IN_CLOSE_WRITE) | |
941 | printf("IN_CLOSE_WRITE: "); | |
942 | ||
943 | /* Print the name of the watched directory */ | |
944 | ||
945 | for (i = 1; i < argc; ++i) { | |
946 | if (wd[i] == event\->wd) { | |
947 | printf("%s/", argv[i]); | |
948 | break; | |
949 | } | |
950 | } | |
951 | ||
952 | /* Print the name of the file */ | |
953 | ||
954 | if (event\->len) | |
955 | printf("%s", event\->name); | |
956 | ||
957 | /* Print type of filesystem object */ | |
958 | ||
959 | if (event\->mask & IN_ISDIR) | |
960 | printf(" [directory]\\n"); | |
961 | else | |
962 | printf(" [file]\\n"); | |
963 | } | |
964 | } | |
965 | } | |
966 | ||
967 | int | |
968 | main(int argc, char* argv[]) | |
969 | { | |
970 | char buf; | |
971 | int fd, i, poll_num; | |
972 | int *wd; | |
973 | nfds_t nfds; | |
974 | struct pollfd fds[2]; | |
975 | ||
976 | if (argc < 2) { | |
977 | printf("Usage: %s PATH [PATH ...]\\n", argv[0]); | |
978 | exit(EXIT_FAILURE); | |
979 | } | |
980 | ||
981 | printf("Press ENTER key to terminate.\\n"); | |
982 | ||
983 | /* Create the file descriptor for accessing the inotify API */ | |
984 | ||
985 | fd = inotify_init1(IN_NONBLOCK); | |
986 | if (fd == \-1) { | |
987 | perror("inotify_init1"); | |
988 | exit(EXIT_FAILURE); | |
989 | } | |
990 | ||
991 | /* Allocate memory for watch descriptors */ | |
992 | ||
993 | wd = calloc(argc, sizeof(int)); | |
994 | if (wd == NULL) { | |
995 | perror("calloc"); | |
996 | exit(EXIT_FAILURE); | |
997 | } | |
998 | ||
999 | /* Mark directories for events | |
1000 | \- file was opened | |
1001 | \- file was closed */ | |
1002 | ||
1003 | for (i = 1; i < argc; i++) { | |
1004 | wd[i] = inotify_add_watch(fd, argv[i], | |
1005 | IN_OPEN | IN_CLOSE); | |
1006 | if (wd[i] == \-1) { | |
1007 | fprintf(stderr, "Cannot watch '%s'\\n", argv[i]); | |
1008 | perror("inotify_add_watch"); | |
1009 | exit(EXIT_FAILURE); | |
1010 | } | |
1011 | } | |
1012 | ||
1013 | /* Prepare for polling */ | |
1014 | ||
1015 | nfds = 2; | |
1016 | ||
1017 | /* Console input */ | |
1018 | ||
1019 | fds[0].fd = STDIN_FILENO; | |
1020 | fds[0].events = POLLIN; | |
1021 | ||
1022 | /* Inotify input */ | |
1023 | ||
1024 | fds[1].fd = fd; | |
1025 | fds[1].events = POLLIN; | |
1026 | ||
1027 | /* Wait for events and/or terminal input */ | |
1028 | ||
1029 | printf("Listening for events.\\n"); | |
1030 | while (1) { | |
1031 | poll_num = poll(fds, nfds, \-1); | |
1032 | if (poll_num == \-1) { | |
1033 | if (errno == EINTR) | |
1034 | continue; | |
1035 | perror("poll"); | |
1036 | exit(EXIT_FAILURE); | |
1037 | } | |
1038 | ||
1039 | if (poll_num > 0) { | |
1040 | ||
1041 | if (fds[0].revents & POLLIN) { | |
1042 | ||
1043 | /* Console input is available. Empty stdin and quit */ | |
1044 | ||
1045 | while (read(STDIN_FILENO, &buf, 1) > 0 && buf != '\\n') | |
1046 | continue; | |
1047 | break; | |
1048 | } | |
1049 | ||
1050 | if (fds[1].revents & POLLIN) { | |
1051 | ||
1052 | /* Inotify events are available */ | |
1053 | ||
1054 | handle_events(fd, wd, argc, argv); | |
1055 | } | |
1056 | } | |
1057 | } | |
1058 | ||
1059 | printf("Listening for events stopped.\\n"); | |
1060 | ||
1061 | /* Close inotify file descriptor */ | |
1062 | ||
1063 | close(fd); | |
1064 | ||
1065 | free(wd); | |
1066 | exit(EXIT_SUCCESS); | |
1067 | } | |
1068 | .fi | |
1069 | .SH SEE ALSO | |
1070 | .BR inotifywait (1), | |
1071 | .BR inotifywatch (1), | |
1072 | .BR inotify_add_watch (2), | |
1073 | .BR inotify_init (2), | |
1074 | .BR inotify_init1 (2), | |
1075 | .BR inotify_rm_watch (2), | |
1076 | .BR read (2), | |
1077 | .BR stat (2), | |
1078 | .BR fanotify (7) | |
1079 | ||
1080 | .IR Documentation/filesystems/inotify.txt | |
1081 | in the Linux kernel source tree |