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4d2b74dd | 1 | '\" t |
415f63be | 2 | .\" Copyright (C) 2006, 2014 Michael Kerrisk <mtk.manpages@gmail.com> |
ecd96f7c | 3 | .\" Copyright (C) 2014 Heinrich Schuchardt <xypron.glpk@gmx.de> |
4d2b74dd | 4 | .\" |
93015253 | 5 | .\" %%%LICENSE_START(VERBATIM) |
4d2b74dd MK |
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c13182ef | 14 | .\" |
4d2b74dd MK |
15 | .\" Since the Linux kernel and libraries are constantly changing, this |
16 | .\" manual page may be incorrect or out-of-date. The author(s) assume no | |
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10d76543 MK |
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c13182ef | 22 | .\" |
4d2b74dd MK |
23 | .\" Formatted or processed versions of this manual, if unaccompanied by |
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4b72fb64 | 25 | .\" %%%LICENSE_END |
4d2b74dd | 26 | .\" |
0649afd4 | 27 | .TH INOTIFY 7 2014-12-31 "Linux" "Linux Programmer's Manual" |
4d2b74dd | 28 | .SH NAME |
9ee4a2b6 | 29 | inotify \- monitoring filesystem events |
4d2b74dd MK |
30 | .SH DESCRIPTION |
31 | The | |
c13182ef | 32 | .I inotify |
9ee4a2b6 | 33 | API provides a mechanism for monitoring filesystem events. |
4d2b74dd MK |
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 | ||
c13182ef | 39 | The following system calls are used with this API: |
356911f6 | 40 | .IP * 3 |
4d2b74dd | 41 | .BR inotify_init (2) |
c13182ef | 42 | creates an inotify instance and returns a file descriptor |
a2cc46ca | 43 | referring to the inotify instance. |
43bb5faf MK |
44 | The more recent |
45 | .BR inotify_init1 (2) | |
46 | is like | |
47 | .BR inotify_init (2), | |
356911f6 MK |
48 | but has a |
49 | .IR flags | |
50 | argument that provides access to some extra functionality. | |
51 | .IP * | |
4d2b74dd | 52 | .BR inotify_add_watch (2) |
a2cc46ca | 53 | manipulates the "watch list" associated with an inotify instance. |
3a065ac0 | 54 | Each item ("watch") in the watch list specifies the pathname of |
c13182ef | 55 | a file or directory, |
4d2b74dd MK |
56 | along with some set of events that the kernel should monitor for the |
57 | file referred to by that pathname. | |
63f6a20a | 58 | .BR inotify_add_watch (2) |
4d2b74dd MK |
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 | |
63f6a20a | 62 | .BR inotify_add_watch (2) |
4d2b74dd | 63 | when the watch is created. |
356911f6 MK |
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 * | |
4d2b74dd MK |
71 | .BR inotify_rm_watch (2) |
72 | removes an item from an inotify watch list. | |
356911f6 | 73 | .IP * |
c13182ef | 74 | When all file descriptors referring to an inotify |
356911f6 MK |
75 | instance have been closed (using |
76 | .BR close (2)), | |
c13182ef | 77 | the underlying object and its resources are |
3b777aff | 78 | freed for reuse by the kernel; |
4d2b74dd | 79 | all associated watches are automatically freed. |
f070e954 | 80 | .PP |
6b1cc2c9 MK |
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. | |
e3a0ac0e | 87 | It is probably wise to do some consistency checking, |
6b1cc2c9 | 88 | and rebuild the cache when inconsistencies are detected. |
ff6e2397 | 89 | .SS Reading events from an inotify file descriptor |
4d2b74dd MK |
90 | To determine what events have occurred, an application |
91 | .BR read (2)s | |
92 | from the inotify file descriptor. | |
c13182ef | 93 | If no events have so far occurred, then, |
11da88fb | 94 | assuming a blocking file descriptor, |
63f6a20a | 95 | .BR read (2) |
01538d0d MK |
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)). | |
4d2b74dd MK |
102 | |
103 | Each successful | |
63f6a20a | 104 | .BR read (2) |
4d2b74dd | 105 | returns a buffer containing one or more of the following structures: |
a08ea57c | 106 | .in +4n |
4d2b74dd MK |
107 | .nf |
108 | ||
109 | struct inotify_event { | |
110 | int wd; /* Watch descriptor */ | |
24bbe02c MK |
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 | |
6e6231c1 | 114 | .\" Glibc bug filed: http://sources.redhat.com/bugzilla/show_bug.cgi?id=7040 |
425ad621 | 115 | uint32_t mask; /* Mask describing event */ |
c13182ef | 116 | uint32_t cookie; /* Unique cookie associating related |
4d2b74dd | 117 | events (for rename(2)) */ |
84c517a4 | 118 | uint32_t len; /* Size of \fIname\fP field */ |
4d2b74dd MK |
119 | char name[]; /* Optional null-terminated name */ |
120 | }; | |
121 | .fi | |
a08ea57c | 122 | .in |
4d2b74dd MK |
123 | |
124 | .I wd | |
125 | identifies the watch for which this event occurs. | |
c13182ef | 126 | It is one of the watch descriptors returned by a previous call to |
63f6a20a | 127 | .BR inotify_add_watch (2). |
4d2b74dd MK |
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. | |
33a0ccb2 | 134 | Currently this is used only for rename events, and |
4d2b74dd | 135 | allows the resulting pair of |
bc636d8a | 136 | .B IN_MOVED_FROM |
c13182ef | 137 | and |
bc636d8a | 138 | .B IN_MOVED_TO |
4d2b74dd | 139 | events to be connected by the application. |
591b7a5f MK |
140 | For all other event types, |
141 | .I cookie | |
142 | is set to 0. | |
4d2b74dd | 143 | |
c13182ef | 144 | The |
4d2b74dd | 145 | .I name |
33a0ccb2 | 146 | field is present only when an event is returned |
c13182ef | 147 | for a file inside a watched directory; |
4d2b74dd | 148 | it identifies the file pathname relative to the watched directory. |
c13182ef | 149 | This pathname is null-terminated, |
1aff5804 | 150 | and may include further null bytes (\(aq\\0\(aq) to align subsequent reads to a |
4d2b74dd MK |
151 | suitable address boundary. |
152 | ||
153 | The | |
154 | .I len | |
c13182ef | 155 | field counts all of the bytes in |
4d2b74dd | 156 | .IR name , |
c13182ef | 157 | including the null bytes; |
4d2b74dd MK |
158 | the length of each |
159 | .I inotify_event | |
160 | structure is thus | |
655684a9 | 161 | .IR "sizeof(struct inotify_event)+len" . |
c7e3ee6f | 162 | |
988db661 | 163 | The behavior when the buffer given to |
c7e3ee6f | 164 | .BR read (2) |
988db661 | 165 | is too small to return information about the next event depends |
c7e3ee6f MK |
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 . | |
4ba272b3 MK |
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. | |
4d2b74dd | 177 | .SS inotify events |
c13182ef | 178 | The |
4d2b74dd MK |
179 | .BR inotify_add_watch (2) |
180 | .I mask | |
c13182ef | 181 | argument and the |
4d2b74dd MK |
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 | |
63f6a20a | 192 | .BR inotify_add_watch (2) |
c13182ef | 193 | and may be returned in the |
4d2b74dd MK |
194 | .I mask |
195 | field returned by | |
63f6a20a | 196 | .BR read (2): |
64aa9bcb | 197 | .RS 4 |
c577b95c | 198 | .TP |
f279ad49 | 199 | .BR IN_ACCESS " (+)" |
70f70c9d MK |
200 | File was accessed (e.g., |
201 | .BR read (2), | |
f23fc716 | 202 | .BR execve (2)). |
3f174f7d | 203 | .TP |
f23fc716 | 204 | .BR IN_ATTRIB " (*)" |
b0e1ac43 MK |
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)). | |
3f174f7d | 218 | .TP |
858bbaaa | 219 | .BR IN_CLOSE_WRITE " (+)" |
f23fc716 | 220 | File opened for writing was closed. |
3f174f7d | 221 | .TP |
f23fc716 | 222 | .BR IN_CLOSE_NOWRITE " (*)" |
7465ffa6 | 223 | File or directory not opened for writing was closed. |
3f174f7d | 224 | .TP |
e95b1911 | 225 | .BR IN_CREATE " (+)" |
7a64793b MK |
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), | |
1a737afd | 231 | .BR symlink (2), |
7a64793b MK |
232 | .BR bind (2) |
233 | on a UNIX domain socket). | |
3f174f7d | 234 | .TP |
e95b1911 | 235 | .BR IN_DELETE " (+)" |
f23fc716 | 236 | File/directory deleted from watched directory. |
3f174f7d MK |
237 | .TP |
238 | .B IN_DELETE_SELF | |
239 | Watched file/directory was itself deleted. | |
4a1e4cca MK |
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 | |
49b07b8f | 244 | then deletes it from the original filesystem.) |
39f43968 MK |
245 | In addition, an |
246 | .B IN_IGNORED | |
247 | event will subsequently be generated for the watch descriptor. | |
3f174f7d | 248 | .TP |
e06fc20c | 249 | .BR IN_MODIFY " (+)" |
f54a1255 MK |
250 | File was modified (e.g., |
251 | .BR write (2), | |
252 | .BR truncate (2)). | |
3f174f7d MK |
253 | .TP |
254 | .B IN_MOVE_SELF | |
255 | Watched file/directory was itself moved. | |
256 | .TP | |
e95b1911 | 257 | .BR IN_MOVED_FROM " (+)" |
107375cf | 258 | Generated for the directory containing the old filename |
f23fc716 | 259 | when a file is renamed. |
3f174f7d | 260 | .TP |
e95b1911 | 261 | .BR IN_MOVED_TO " (+)" |
107375cf | 262 | Generated for the directory containing the new filename |
f23fc716 | 263 | when a file is renamed. |
3f174f7d | 264 | .TP |
f23fc716 | 265 | .BR IN_OPEN " (*)" |
7465ffa6 | 266 | File or directory was opened. |
64aa9bcb | 267 | .RE |
4d2b74dd | 268 | .PP |
e95b1911 MK |
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 | |
4d2b74dd MK |
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 | |
63f6a20a | 289 | .BR inotify_add_watch (2). |
4d2b74dd | 290 | |
dede00fe | 291 | Two additional convenience macros are defined: |
64aa9bcb | 292 | .RS 4 |
dede00fe MK |
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" . | |
64aa9bcb | 301 | .RE |
4d2b74dd | 302 | .PP |
c13182ef | 303 | The following further bits can be specified in |
4d2b74dd MK |
304 | .I mask |
305 | when calling | |
63f6a20a | 306 | .BR inotify_add_watch (2): |
64aa9bcb | 307 | .RS 4 |
c577b95c | 308 | .TP |
31daf529 | 309 | .BR IN_DONT_FOLLOW " (since Linux 2.6.15)" |
aeb9b6a6 MK |
310 | Don't dereference |
311 | .I pathname | |
312 | if it is a symbolic link. | |
dda869a4 | 313 | .TP |
0ff2cc88 | 314 | .BR IN_EXCL_UNLINK " (since Linux 2.6.36)" |
b3ad7609 MK |
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 | |
dda869a4 | 330 | .B IN_MASK_ADD |
7747b84b MK |
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). | |
dda869a4 MK |
336 | .TP |
337 | .B IN_ONESHOT | |
dcaff23c | 338 | Monitor the filesystem object corresponding to |
aeb9b6a6 MK |
339 | .I pathname |
340 | for one event, then remove from | |
dda869a4 MK |
341 | watch list. |
342 | .TP | |
31daf529 | 343 | .BR IN_ONLYDIR " (since Linux 2.6.15)" |
aeb9b6a6 MK |
344 | Only watch |
345 | .I pathname | |
346 | if it is a directory. | |
18d92a14 | 347 | Using this flag provides an application with a race-free way of |
9dcdfde4 | 348 | ensuring that the monitored object is a directory. |
64aa9bcb | 349 | .RE |
4d2b74dd MK |
350 | .PP |
351 | The following bits may be set in the | |
352 | .I mask | |
353 | field returned by | |
63f6a20a | 354 | .BR read (2): |
64aa9bcb | 355 | .RS 4 |
c577b95c | 356 | .TP |
dda869a4 | 357 | .B IN_IGNORED |
aeb9b6a6 MK |
358 | Watch was removed explicitly |
359 | .RB ( inotify_rm_watch (2)) | |
9ee4a2b6 | 360 | or automatically (file was deleted, or filesystem was unmounted). |
7b5151b7 | 361 | See also BUGS. |
dda869a4 MK |
362 | .TP |
363 | .B IN_ISDIR | |
364 | Subject of this event is a directory. | |
365 | .TP | |
366 | .B IN_Q_OVERFLOW | |
aeb9b6a6 MK |
367 | Event queue overflowed |
368 | .RI ( wd | |
369 | is \-1 for this event). | |
dda869a4 MK |
370 | .TP |
371 | .B IN_UNMOUNT | |
9ee4a2b6 | 372 | Filesystem containing watched object was unmounted. |
b01c936e MK |
373 | In addition, an |
374 | .B IN_IGNORED | |
375 | event will subsequently be generated for the watch descriptor. | |
64aa9bcb | 376 | .RE |
415f63be MK |
377 | .SS Examples |
378 | Suppose an application is watching the directory | |
379 | .I dir | |
380 | and the file | |
381 | .IR dir/myfile | |
382 | for all events. | |
383 | The examples below show some events that will be generated | |
384 | for these two objects. | |
385 | .RS 4 | |
386 | .TP | |
387 | fd = open("dir/myfile", O_RDWR); | |
388 | Generates | |
389 | .B IN_OPEN | |
390 | events for both | |
391 | .I dir | |
392 | and | |
393 | .IR dir/myfile . | |
394 | .TP | |
395 | read(fd, buf, count); | |
396 | Generates | |
397 | .B IN_ACCESS | |
398 | events for both | |
399 | .I dir | |
400 | and | |
401 | .IR dir/myfile . | |
402 | .TP | |
403 | write(fd, buf, count); | |
404 | Generates | |
405 | .B IN_MODIFY | |
406 | events for both | |
407 | .I dir | |
408 | and | |
409 | .IR dir/myfile . | |
410 | .TP | |
411 | fchmod(fd, mode); | |
412 | Generates | |
413 | .B IN_ATTRIB | |
414 | events for both | |
415 | .I dir | |
416 | and | |
417 | .IR dir/myfile . | |
418 | .TP | |
419 | close(fd); | |
420 | Generates | |
421 | .B IN_CLOSE_WRITE | |
422 | events for both | |
423 | .I dir | |
424 | and | |
425 | .IR dir/myfile . | |
426 | .RE | |
427 | .PP | |
428 | Suppose an application is watching the directories | |
429 | .I dir1 | |
430 | and | |
d2c3d8a8 | 431 | .IR dir2 , |
415f63be MK |
432 | and the file |
433 | .IR dir1/myfile . | |
434 | The following examples show some events that may be generated. | |
435 | .RS 4 | |
436 | .TP | |
d2c3d8a8 | 437 | link("dir1/myfile", "dir2/new"); |
415f63be MK |
438 | Generates an |
439 | .B IN_ATTRIB | |
440 | event for | |
441 | .IR myfile | |
442 | and an | |
443 | .B IN_CREATE | |
444 | event for | |
445 | .IR dir2 . | |
446 | .TP | |
447 | rename("dir1/myfile", "dir2/myfile"); | |
448 | Generates an | |
449 | .B IN_MOVED_FROM | |
450 | event for | |
451 | .IR dir1 , | |
452 | an | |
453 | .B IN_MOVED_TO | |
454 | event for | |
455 | .IR dir2 , | |
456 | and an | |
457 | .B IN_MOVE_SELF | |
458 | event for | |
459 | .IR myfile . | |
460 | The | |
461 | .B IN_MOVED_FROM | |
462 | and | |
463 | .B IN_MOVED_TO | |
464 | events will have the same | |
465 | .I cookie | |
466 | value. | |
467 | .RE | |
468 | .PP | |
469 | Suppose that | |
470 | .IR dir1/xx | |
471 | and | |
472 | .IR dir2/yy | |
473 | are (the only) links to the same file, and an application is watching | |
474 | .IR dir1 , | |
475 | .IR dir2 , | |
476 | .IR dir1/xx , | |
477 | and | |
478 | .IR dir2/yy . | |
479 | Executing the following calls in the order given below will generate | |
480 | the following events: | |
481 | .RS 4 | |
482 | .TP | |
483 | unlink("dir2/yy"); | |
d2c3d8a8 | 484 | Generates an |
415f63be MK |
485 | .BR IN_ATTRIB |
486 | event for | |
487 | .IR xx | |
488 | (because its link count changes) | |
489 | and an | |
490 | .B IN_DELETE | |
491 | event for | |
492 | .IR dir2 . | |
493 | .TP | |
494 | unlink("dir1/xx"); | |
495 | Generates | |
496 | .BR IN_ATTRIB , | |
497 | .BR IN_DELETE_SELF , | |
498 | and | |
499 | .BR IN_IGNORED | |
500 | events for | |
501 | .IR xx , | |
502 | and an | |
503 | .BR IN_DELETE | |
d2c3d8a8 | 504 | event for |
415f63be MK |
505 | .IR dir1 . |
506 | .RE | |
507 | .PP | |
508 | Suppose an application is watching the directory | |
509 | .IR dir | |
510 | and (the empty) directory | |
511 | .IR dir/subdir . | |
512 | The following examples show some events that may be generated. | |
513 | .RS 4 | |
514 | .TP | |
515 | mkdir("dir/new", mode); | |
516 | Generates an | |
517 | .B "IN_CREATE | IN_ISDIR" | |
518 | event for | |
519 | .IR dir . | |
520 | .TP | |
d2c3d8a8 | 521 | rmdir("dir/subdir"); |
415f63be MK |
522 | Generates |
523 | .B IN_DELETE_SELF | |
524 | and | |
525 | .B IN_IGNORED | |
526 | events for | |
527 | .IR subdir , | |
528 | and an | |
529 | .B "IN_DELETE | IN_ISDIR" | |
530 | event for | |
531 | .IR dir . | |
532 | .RE | |
4d2b74dd | 533 | .SS /proc interfaces |
c13182ef | 534 | The following interfaces can be used to limit the amount of |
4d2b74dd MK |
535 | kernel memory consumed by inotify: |
536 | .TP | |
0daa9e92 | 537 | .I /proc/sys/fs/inotify/max_queued_events |
4d2b74dd MK |
538 | The value in this file is used when an application calls |
539 | .BR inotify_init (2) | |
c13182ef | 540 | to set an upper limit on the number of events that can be |
4d2b74dd MK |
541 | queued to the corresponding inotify instance. |
542 | Events in excess of this limit are dropped, but an | |
543 | .B IN_Q_OVERFLOW | |
544 | event is always generated. | |
545 | .TP | |
0daa9e92 | 546 | .I /proc/sys/fs/inotify/max_user_instances |
c13182ef | 547 | This specifies an upper limit on the number of inotify instances |
4d2b74dd MK |
548 | that can be created per real user ID. |
549 | .TP | |
0daa9e92 | 550 | .I /proc/sys/fs/inotify/max_user_watches |
31546b46 VN |
551 | This specifies an upper limit on the number of watches |
552 | that can be created per real user ID. | |
47297adb | 553 | .SH VERSIONS |
2b2581ee MK |
554 | Inotify was merged into the 2.6.13 Linux kernel. |
555 | The required library interfaces were added to glibc in version 2.4. | |
556 | .RB ( IN_DONT_FOLLOW , | |
557 | .BR IN_MASK_ADD , | |
558 | and | |
559 | .B IN_ONLYDIR | |
64aa9bcb | 560 | were added in glibc version 2.5.) |
47297adb | 561 | .SH CONFORMING TO |
8382f16d | 562 | The inotify API is Linux-specific. |
47297adb | 563 | .SH NOTES |
4d2b74dd MK |
564 | Inotify file descriptors can be monitored using |
565 | .BR select (2), | |
566 | .BR poll (2), | |
c13182ef | 567 | and |
2315114c | 568 | .BR epoll (7). |
0000daa5 MK |
569 | When an event is available, the file descriptor indicates as readable. |
570 | ||
571 | Since Linux 2.6.25, | |
572 | signal-driven I/O notification is available for inotify file descriptors; | |
573 | see the discussion of | |
574 | .B F_SETFL | |
575 | (for setting the | |
576 | .B O_ASYNC | |
577 | flag), | |
578 | .BR F_SETOWN , | |
579 | and | |
580 | .B F_SETSIG | |
581 | in | |
582 | .BR fcntl (2). | |
583 | The | |
584 | .I siginfo_t | |
585 | structure (described in | |
586 | .BR sigaction (2)) | |
587 | that is passed to the signal handler has the following fields set: | |
588 | .IR si_fd | |
589 | is set to the inotify file descriptor number; | |
590 | .IR si_signo | |
591 | is set to the signal number; | |
592 | .IR si_code | |
593 | is set to | |
594 | .BR POLL_IN ; | |
595 | and | |
596 | .B POLLIN | |
597 | is set in | |
598 | .IR si_band . | |
4d2b74dd | 599 | |
c13182ef MK |
600 | If successive output inotify events produced on the |
601 | inotify file descriptor are identical (same | |
602 | .IR wd , | |
603 | .IR mask , | |
4d2b74dd MK |
604 | .IR cookie , |
605 | and | |
3f3698d8 | 606 | .IR name ), |
6f0ab035 MK |
607 | then they are coalesced into a single event if the |
608 | older event has not yet been read (but see BUGS). | |
8856aab8 MK |
609 | This reduces the amount of kernel memory required for the event queue, |
610 | but also means that an application can't use inotify to reliably count | |
611 | file events. | |
4d2b74dd | 612 | |
c13182ef MK |
613 | The events returned by reading from an inotify file descriptor |
614 | form an ordered queue. | |
615 | Thus, for example, it is guaranteed that when renaming from | |
616 | one directory to another, events will be produced in the | |
4d2b74dd MK |
617 | correct order on the inotify file descriptor. |
618 | ||
619 | The | |
620 | .B FIONREAD | |
63f6a20a | 621 | .BR ioctl (2) |
c13182ef | 622 | returns the number of bytes available to read from an |
4d2b74dd | 623 | inotify file descriptor. |
613836aa | 624 | .SS Limitations and caveats |
67898fdf MK |
625 | The inotify API provides no information about the user or process that |
626 | triggered the inotify event. | |
627 | In particular, there is no easy | |
628 | way for a process that is monitoring events via inotify | |
629 | to distinguish events that it triggers | |
630 | itself from those that are triggered by other processes. | |
631 | ||
e226bed7 MK |
632 | Inotify reports only events that a user-space program triggers through |
633 | the filesystem API. | |
634 | As a result, it does not catch remote events that occur | |
635 | on network filesystems. | |
636 | (Applications must fall back to polling the filesystem | |
637 | to catch such events.) | |
fa1d49a6 | 638 | Furthermore, various pseudo-filesystems such as |
e226bed7 MK |
639 | .IR /proc , |
640 | .IR /sys , | |
641 | and | |
642 | .IR /dev/pts | |
643 | are not monitorable with inotify. | |
644 | ||
e449e5f1 MK |
645 | The inotify API does not report file accesses and modifications that |
646 | may occur because of | |
7a8110f6 HS |
647 | .BR mmap (2), |
648 | .BR msync (2), | |
e449e5f1 | 649 | and |
7a8110f6 | 650 | .BR munmap (2). |
e449e5f1 | 651 | |
67898fdf MK |
652 | The inotify API identifies affected files by filename. |
653 | However, by the time an application processes an inotify event, | |
654 | the filename may already have been deleted or renamed. | |
655 | ||
da977073 | 656 | The inotify API identifies events via watch descriptors. |
264276c6 MK |
657 | It is the application's responsibility to cache a mapping |
658 | (if one is needed) between watch descriptors and pathnames. | |
659 | Be aware that directory renamings may affect multiple cached pathnames. | |
660 | ||
4d2b74dd MK |
661 | Inotify monitoring of directories is not recursive: |
662 | to monitor subdirectories under a directory, | |
663 | additional watches must be created. | |
613836aa MK |
664 | This can take a significant amount time for large directory trees. |
665 | ||
a79d28b2 MK |
666 | If monitoring an entire directory subtree, |
667 | and a new subdirectory is created in that tree or an existing directory | |
668 | is renamed into that tree, | |
669 | be aware that by the time you create a watch for the new subdirectory, | |
670 | new files (and subdirectories) may already exist inside the subdirectory. | |
671 | Therefore, you may want to scan the contents of the subdirectory | |
672 | immediately after adding the watch (and, if desired, | |
673 | recursively add watches for any subdirectories that it contains). | |
674 | ||
613836aa MK |
675 | Note that the event queue can overflow. |
676 | In this case, events are lost. | |
09fa72fa | 677 | Robust applications should handle the possibility of |
613836aa | 678 | lost events gracefully. |
94d52c15 MK |
679 | For example, it may be necessary to rebuild part or all of |
680 | the application cache. | |
681 | (One simple, but possibly expensive, | |
682 | approach is to close the inotify file descriptor, empty the cache, | |
85e179c5 | 683 | create a new inotify file descriptor, |
94d52c15 MK |
684 | and then re-create watches and cache entries |
685 | for the objects to be monitored.) | |
85e179c5 | 686 | .SS Dealing with rename() events |
fa51f4d9 | 687 | As noted above, the |
85e179c5 MK |
688 | .B IN_MOVED_FROM |
689 | and | |
690 | .B IN_MOVED_TO | |
fa51f4d9 | 691 | event pair that is generated by |
85e179c5 | 692 | .BR rename (2) |
fa51f4d9 MK |
693 | can be matched up via their shared cookie value. |
694 | However, the task of matching has some challenges. | |
695 | ||
696 | These two events are usually consecutive in the event stream available | |
697 | when reading from the inotify file descriptor. | |
85e179c5 MK |
698 | However, this is not guaranteed. |
699 | If multiple processes are triggering events for monitored objects, | |
700 | then (on rare occasions) an arbitrary number of | |
701 | other events may appear between the | |
702 | .B IN_MOVED_FROM | |
703 | and | |
704 | .B IN_MOVED_TO | |
705 | events. | |
003798c7 MK |
706 | Furthermore, it is not guaranteed that the event pair is atomically |
707 | inserted into the queue: there may be a brief interval where the | |
708 | .B IN_MOVED_FROM | |
709 | has appeared, but the | |
710 | .B IN_MOVED_TO | |
711 | has not. | |
85e179c5 MK |
712 | |
713 | Matching up the | |
714 | .B IN_MOVED_FROM | |
715 | and | |
716 | .B IN_MOVED_TO | |
717 | event pair generated by | |
718 | .BR rename (2) | |
719 | is thus inherently racy. | |
720 | (Don't forget that if an object is renamed outside of a monitored directory, | |
721 | there may not even be an | |
722 | .BR IN_MOVED_TO | |
723 | event.) | |
724 | Heuristic approaches (e.g., assume the events are always consecutive) | |
725 | can be used to ensure a match in most cases, | |
726 | but will inevitably miss some cases, | |
727 | causing the application to perceive the | |
728 | .B IN_MOVED_FROM | |
729 | and | |
730 | .B IN_MOVED_TO | |
731 | events as being unrelated. | |
732 | If watch descriptors are destroyed and re-created as a result, | |
733 | then those watch descriptors will be inconsistent with | |
734 | the watch descriptors in any pending events. | |
6f1a4954 | 735 | (Re-creating the inotify file descriptor and rebuilding the cache may |
85e179c5 MK |
736 | be useful to deal with this scenario.) |
737 | ||
738 | Applications should also allow for the possibility that the | |
739 | .B IN_MOVED_FROM | |
740 | event was the last event that could fit in the buffer | |
741 | returned by the current call to | |
742 | .BR read (2), | |
743 | and the accompanying | |
744 | .B IN_MOVED_TO | |
745 | event might be fetched only on the next | |
3da91159 MK |
746 | .BR read (2), |
747 | which should be done with a (small) timeout to allow for the fact that | |
748 | insertion of the | |
749 | .BR IN_MOVED_FROM - IN_MOVED_TO | |
750 | event pair is not atomic, | |
751 | and also the possibility that there may not be any | |
752 | .B IN_MOVED_TO | |
753 | event. | |
47297adb | 754 | .SH BUGS |
e75c6d07 | 755 | As of kernel 3.17, calling |
360d3116 | 756 | .BR fallocate (2) |
e75c6d07 HS |
757 | does not create any inotify events. |
758 | ||
bea08fec | 759 | .\" FIXME . kernel commit 611da04f7a31b2208e838be55a42c7a1310ae321 |
a15ead5e MK |
760 | .\" implies that unmount events were buggy 2.6.11 to 2.6.36 |
761 | .\" | |
ed7b0235 MK |
762 | In kernels before 2.6.16, the |
763 | .B IN_ONESHOT | |
c13182ef | 764 | .I mask |
ed7b0235 | 765 | flag does not work. |
6f0ab035 | 766 | |
7b5151b7 MK |
767 | As originally designed and implemented, the |
768 | .B IN_ONESHOT | |
769 | flag did not cause an | |
770 | .B IN_IGNORED | |
771 | event to be generated when the watch was dropped after one event. | |
772 | However, as an unintended effect of other changes, | |
773 | since Linux 2.6.36, an | |
774 | .B IN_IGNORED | |
775 | event is generated in this case. | |
776 | ||
6f0ab035 | 777 | Before kernel 2.6.25, |
22129aa9 | 778 | .\" commit 1c17d18e3775485bf1e0ce79575eb637a94494a2 |
9ed6b517 | 779 | the kernel code that was intended to coalesce successive identical events |
6f0ab035 MK |
780 | (i.e., the two most recent events could potentially be coalesced |
781 | if the older had not yet been read) | |
782 | instead checked if the most recent event could be coalesced with the | |
783 | .I oldest | |
784 | unread event. | |
ec976a53 | 785 | |
ec976a53 | 786 | When a watch descriptor is removed by calling |
e530e367 MK |
787 | .BR inotify_rm_watch (2) |
788 | (or because a watch file is deleted or the filesystem | |
789 | that contains it is unmounted), | |
ec976a53 HS |
790 | any pending unread events for that watch descriptor remain available to read. |
791 | As watch descriptors are subsequently allocated with | |
792 | .BR inotify_add_watch (2), | |
793 | the kernel cycles through the range of possible watch descriptors (0 to | |
794 | .BR INT_MAX ) | |
795 | incrementally. | |
796 | When allocating a free watch descriptor, no check is made to see whether that | |
797 | watch descriptor number has any pending unread events in the inotify queue. | |
798 | Thus, it can happen that a watch descriptor is reallocated even | |
799 | when pending unread events exist for a previous incarnation of | |
800 | that watch descriptor number, with the result that the application | |
cbe0e644 | 801 | might then read those events and interpret them as belonging to |
ec976a53 | 802 | the file associated with the newly recycled watch descriptor. |
2bbf2299 MK |
803 | In practice, the likelihood of hitting this bug may be extremely low, |
804 | since it requires that an application cycle through | |
805 | .B INT_MAX | |
806 | watch descriptors, | |
807 | release a watch descriptor while leaving unread events for that | |
08063d8f | 808 | watch descriptor in the queue, |
2bbf2299 MK |
809 | and then recycle that watch descriptor. |
810 | For this reason, and because there have been no reports | |
811 | of the bug occurring in real-world applications, | |
812 | as of Linux 3.15, | |
bea08fec | 813 | .\" FIXME . https://bugzilla.kernel.org/show_bug.cgi?id=77111 |
2bbf2299 | 814 | no kernel changes have yet been made to eliminate this possible bug. |
ecd96f7c HS |
815 | .SH EXAMPLE |
816 | The following program demonstrates the usage of the inotify API. | |
817 | It marks the directories passed as a command-line arguments | |
818 | and waits for events of type | |
819 | .BR IN_OPEN , | |
2ef6778b | 820 | .BR IN_CLOSE_NOWRITE |
ecd96f7c HS |
821 | and |
822 | .BR IN_CLOSE_WRITE . | |
823 | .PP | |
824 | The following output was recorded while editing the file | |
825 | .I /home/user/temp/foo | |
826 | and listing directory | |
827 | .IR /tmp . | |
828 | Before the file and the directory were opened, | |
829 | .B IN_OPEN | |
830 | events occurred. | |
831 | After the file was closed, an | |
832 | .B IN_CLOSE_WRITE | |
833 | event occurred. | |
834 | After the directory was closed, an | |
835 | .B IN_CLOSE_NOWRITE | |
836 | event occurred. | |
837 | Execution of the program ended when the user pressed the ENTER key. | |
838 | .SS Example output | |
839 | .in +4n | |
840 | .nf | |
2483209a | 841 | $ \fB./a.out /tmp /home/user/temp\fP |
ecd96f7c HS |
842 | Press enter key to terminate. |
843 | Listening for events. | |
844 | IN_OPEN: /home/user/temp/foo [file] | |
845 | IN_CLOSE_WRITE: /home/user/temp/foo [file] | |
846 | IN_OPEN: /tmp/ [directory] | |
847 | IN_CLOSE_NOWRITE: /tmp/ [directory] | |
848 | ||
849 | Listening for events stopped. | |
850 | .fi | |
851 | .in | |
852 | .SS Program source | |
853 | .nf | |
854 | #include <errno.h> | |
855 | #include <poll.h> | |
856 | #include <stdio.h> | |
857 | #include <stdlib.h> | |
858 | #include <sys/inotify.h> | |
859 | #include <unistd.h> | |
860 | ||
861 | /* Read all available inotify events from the file descriptor 'fd'. | |
862 | wd is the table of watch descriptors for the directories in argv. | |
863 | argc is the length of wd and argv. | |
864 | argv is the list of watched directories. | |
865 | Entry 0 of wd and argv is unused. */ | |
866 | ||
867 | static void | |
868 | handle_events(int fd, int *wd, int argc, char* argv[]) | |
869 | { | |
2483209a MK |
870 | /* Some systems cannot read integer variables if they are not |
871 | properly aligned. On other systems, incorrect alignment may | |
872 | decrease performance. Hence, the buffer used for reading from | |
873 | the inotify file descriptor should have the same alignment as | |
ecd96f7c | 874 | struct inotify_event. */ |
2483209a | 875 | |
ecd96f7c | 876 | char buf[4096] |
2483209a | 877 | __attribute__ ((aligned(__alignof__(struct inotify_event)))); |
ecd96f7c HS |
878 | const struct inotify_event *event; |
879 | int i; | |
880 | ssize_t len; | |
881 | char *ptr; | |
882 | ||
883 | /* Loop while events can be read from inotify file descriptor. */ | |
884 | ||
885 | for (;;) { | |
886 | ||
887 | /* Read some events. */ | |
888 | ||
889 | len = read(fd, buf, sizeof buf); | |
890 | if (len == \-1 && errno != EAGAIN) { | |
891 | perror("read"); | |
892 | exit(EXIT_FAILURE); | |
893 | } | |
894 | ||
895 | /* If the nonblocking read() found no events to read, then | |
896 | it returns \-1 with errno set to EAGAIN. In that case, | |
897 | we exit the loop. */ | |
898 | ||
899 | if (len <= 0) | |
900 | break; | |
901 | ||
902 | /* Loop over all events in the buffer */ | |
903 | ||
904 | for (ptr = buf; ptr < buf + len; | |
2483209a | 905 | ptr += sizeof(struct inotify_event) + event\->len) { |
ecd96f7c HS |
906 | |
907 | event = (const struct inotify_event *) ptr; | |
908 | ||
909 | /* Print event type */ | |
910 | ||
911 | if (event\->mask & IN_OPEN) | |
912 | printf("IN_OPEN: "); | |
913 | if (event\->mask & IN_CLOSE_NOWRITE) | |
914 | printf("IN_CLOSE_NOWRITE: "); | |
915 | if (event\->mask & IN_CLOSE_WRITE) | |
916 | printf("IN_CLOSE_WRITE: "); | |
917 | ||
918 | /* Print the name of the watched directory */ | |
919 | ||
920 | for (i = 1; i < argc; ++i) { | |
921 | if (wd[i] == event\->wd) { | |
922 | printf("%s/", argv[i]); | |
923 | break; | |
924 | } | |
925 | } | |
926 | ||
927 | /* Print the name of the file */ | |
928 | ||
929 | if (event\->len) | |
930 | printf("%s", event\->name); | |
931 | ||
932 | /* Print type of filesystem object */ | |
933 | ||
934 | if (event\->mask & IN_ISDIR) | |
935 | printf(" [directory]\\n"); | |
936 | else | |
937 | printf(" [file]\\n"); | |
ecd96f7c HS |
938 | } |
939 | } | |
940 | } | |
941 | ||
942 | int | |
943 | main(int argc, char* argv[]) | |
944 | { | |
945 | char buf; | |
946 | int fd, i, poll_num; | |
947 | int *wd; | |
948 | nfds_t nfds; | |
949 | struct pollfd fds[2]; | |
950 | ||
951 | if (argc < 2) { | |
952 | printf("Usage: %s PATH [PATH ...]\\n", argv[0]); | |
953 | exit(EXIT_FAILURE); | |
954 | } | |
955 | ||
956 | printf("Press ENTER key to terminate.\\n"); | |
957 | ||
958 | /* Create the file descriptor for accessing the inotify API */ | |
959 | ||
960 | fd = inotify_init1(IN_NONBLOCK); | |
961 | if (fd == \-1) { | |
962 | perror("inotify_init1"); | |
963 | exit(EXIT_FAILURE); | |
964 | } | |
965 | ||
966 | /* Allocate memory for watch descriptors */ | |
967 | ||
968 | wd = calloc(argc, sizeof(int)); | |
969 | if (wd == NULL) { | |
970 | perror("calloc"); | |
971 | exit(EXIT_FAILURE); | |
972 | } | |
973 | ||
974 | /* Mark directories for events | |
975 | \- file was opened | |
976 | \- file was closed */ | |
977 | ||
978 | for (i = 1; i < argc; i++) { | |
979 | wd[i] = inotify_add_watch(fd, argv[i], | |
980 | IN_OPEN | IN_CLOSE); | |
981 | if (wd[i] == \-1) { | |
982 | fprintf(stderr, "Cannot watch '%s'\\n", argv[i]); | |
983 | perror("inotify_add_watch"); | |
984 | exit(EXIT_FAILURE); | |
985 | } | |
986 | } | |
987 | ||
988 | /* Prepare for polling */ | |
989 | ||
990 | nfds = 2; | |
991 | ||
992 | /* Console input */ | |
993 | ||
994 | fds[0].fd = STDIN_FILENO; | |
995 | fds[0].events = POLLIN; | |
996 | ||
997 | /* Inotify input */ | |
998 | ||
999 | fds[1].fd = fd; | |
1000 | fds[1].events = POLLIN; | |
1001 | ||
1002 | /* Wait for events and/or terminal input */ | |
1003 | ||
1004 | printf("Listening for events.\\n"); | |
1005 | while (1) { | |
1006 | poll_num = poll(fds, nfds, \-1); | |
1007 | if (poll_num == \-1) { | |
1008 | if (errno == EINTR) | |
1009 | continue; | |
1010 | perror("poll"); | |
1011 | exit(EXIT_FAILURE); | |
1012 | } | |
2483209a | 1013 | |
ecd96f7c HS |
1014 | if (poll_num > 0) { |
1015 | ||
1016 | if (fds[0].revents & POLLIN) { | |
1017 | ||
1018 | /* Console input is available. Empty stdin and quit */ | |
1019 | ||
1020 | while (read(STDIN_FILENO, &buf, 1) > 0 && buf != '\\n') | |
1021 | continue; | |
1022 | break; | |
1023 | } | |
2483209a | 1024 | |
ecd96f7c HS |
1025 | if (fds[1].revents & POLLIN) { |
1026 | ||
1027 | /* Inotify events are available */ | |
2483209a | 1028 | |
ecd96f7c HS |
1029 | handle_events(fd, wd, argc, argv); |
1030 | } | |
1031 | } | |
1032 | } | |
1033 | ||
2483209a MK |
1034 | printf("Listening for events stopped.\\n"); |
1035 | ||
ecd96f7c HS |
1036 | /* Close inotify file descriptor */ |
1037 | ||
1038 | close(fd); | |
2483209a | 1039 | |
ecd96f7c | 1040 | free(wd); |
ecd96f7c HS |
1041 | exit(EXIT_SUCCESS); |
1042 | } | |
1043 | .fi | |
47297adb | 1044 | .SH SEE ALSO |
f0afb16a MK |
1045 | .BR inotifywait (1), |
1046 | .BR inotifywatch (1), | |
4d2b74dd MK |
1047 | .BR inotify_add_watch (2), |
1048 | .BR inotify_init (2), | |
43bb5faf | 1049 | .BR inotify_init1 (2), |
4d2b74dd MK |
1050 | .BR inotify_rm_watch (2), |
1051 | .BR read (2), | |
f75d27e6 HS |
1052 | .BR stat (2), |
1053 | .BR fanotify (7) | |
173fe7e7 DP |
1054 | |
1055 | .IR Documentation/filesystems/inotify.txt | |
1056 | in the Linux kernel source tree |