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26 .TH INOTIFY 7 2013-06-21 "Linux" "Linux Programmer's Manual"
28 inotify \- monitoring file system events
32 API provides a mechanism for monitoring file system events.
33 Inotify can be used to monitor individual files,
34 or to monitor directories.
35 When a directory is monitored, inotify will return events
36 for the directory itself, and for files inside the directory.
38 The following system calls are used with this API:
41 .BR inotify_init1 (2)),
42 .BR inotify_add_watch (2),
43 .BR inotify_rm_watch (2),
49 creates an inotify instance and returns a file descriptor
50 referring to the inotify instance.
55 but provides some extra functionality.
57 .BR inotify_add_watch (2)
58 manipulates the "watch list" associated with an inotify instance.
59 Each item ("watch") in the watch list specifies the pathname of
61 along with some set of events that the kernel should monitor for the
62 file referred to by that pathname.
63 .BR inotify_add_watch (2)
64 either creates a new watch item, or modifies an existing watch.
65 Each watch has a unique "watch descriptor", an integer
67 .BR inotify_add_watch (2)
68 when the watch is created.
70 .BR inotify_rm_watch (2)
71 removes an item from an inotify watch list.
73 When all file descriptors referring to an inotify
74 instance have been closed,
75 the underlying object and its resources are
76 freed for reuse by the kernel;
77 all associated watches are automatically freed.
79 To determine what events have occurred, an application
81 from the inotify file descriptor.
82 If no events have so far occurred, then,
83 assuming a blocking file descriptor,
85 will block until at least one event occurs
86 (unless interrupted by a signal,
87 in which case the call fails with the error
94 returns a buffer containing one or more of the following structures:
98 struct inotify_event {
99 int wd; /* Watch descriptor */
100 .\" FIXME . The type of the 'wd' field should probably be "int32_t".
101 .\" I submitted a patch to fix this. See the LKML thread
102 .\" "[patch] Fix type errors in inotify interfaces", 18 Nov 2008
103 .\" Glibc bug filed: http://sources.redhat.com/bugzilla/show_bug.cgi?id=7040
104 uint32_t mask; /* Mask of events */
105 uint32_t cookie; /* Unique cookie associating related
106 events (for rename(2)) */
107 uint32_t len; /* Size of \fIname\fP field */
108 char name[]; /* Optional null-terminated name */
114 identifies the watch for which this event occurs.
115 It is one of the watch descriptors returned by a previous call to
116 .BR inotify_add_watch (2).
119 contains bits that describe the event that occurred (see below).
122 is a unique integer that connects related events.
123 Currently this is used only for rename events, and
124 allows the resulting pair of
128 events to be connected by the application.
129 For all other event types,
135 field is present only when an event is returned
136 for a file inside a watched directory;
137 it identifies the file pathname relative to the watched directory.
138 This pathname is null-terminated,
139 and may include further null bytes (\(aq\\0\(aq) to align subsequent reads to a
140 suitable address boundary.
144 field counts all of the bytes in
146 including the null bytes;
150 .IR "sizeof(struct inotify_event)+len" .
152 The behavior when the buffer given to
154 is too small to return information about the next event depends
155 on the kernel version: in kernels before 2.6.21,
157 returns 0; since kernel 2.6.21,
161 Specifying a buffer of size
163 sizeof(struct inotify_event) + NAME_MAX + 1
165 will be sufficient to read at least one event.
168 .BR inotify_add_watch (2)
174 structure returned when
176 an inotify file descriptor are both bit masks identifying
178 The following bits can be specified in
181 .BR inotify_add_watch (2)
182 and may be returned in the
191 File was accessed (read) (*).
194 Metadata changed, e.g., permissions, timestamps, extended attributes,
195 link count (since Linux 2.6.25), UID, GID, etc. (*).
198 File opened for writing was closed (*).
201 File not opened for writing was closed (*).
204 File/directory created in watched directory (*).
207 File/directory deleted from watched directory (*).
210 Watched file/directory was itself deleted.
213 File was modified (*).
216 Watched file/directory was itself moved.
219 File moved out of watched directory (*).
222 File moved into watched directory (*).
229 When monitoring a directory,
230 the events marked with an asterisk (*) above can occur for
231 files in the directory, in which case the
233 field in the returned
235 structure identifies the name of the file within the directory.
239 macro is defined as a bit mask of all of the above events.
240 This macro can be used as the
242 argument when calling
243 .BR inotify_add_watch (2).
245 Two additional convenience macros are
248 IN_MOVED_FROM|IN_MOVED_TO,
252 IN_CLOSE_WRITE|IN_CLOSE_NOWRITE.
254 The following further bits can be specified in
257 .BR inotify_add_watch (2):
262 .BR IN_DONT_FOLLOW " (since Linux 2.6.15)"
265 if it is a symbolic link.
267 .BR IN_EXCL_UNLINK " (since Linux 2.6.36)"
268 .\" commit 8c1934c8d70b22ca8333b216aec6c7d09fdbd6a6
269 By default, when watching events on the children of a directory,
270 events are generated for children even after they have been unlinked
272 This can result in large numbers of uninteresting events for
273 some applications (e.g., if watching
275 in which many applications create temporary files whose
276 names are immediately unlinked).
279 changes the default behavior,
280 so that events are not generated for children after
281 they have been unlinked from the watched directory.
284 Add (OR) events to watch mask for this pathname if
285 it already exists (instead of replacing mask).
290 for one event, then remove from
293 .BR IN_ONLYDIR " (since Linux 2.6.15)"
296 if it is a directory.
300 The following bits may be set in the
309 Watch was removed explicitly
310 .RB ( inotify_rm_watch (2))
311 or automatically (file was deleted, or file system was unmounted).
314 Subject of this event is a directory.
317 Event queue overflowed
319 is \-1 for this event).
322 File system containing watched object was unmounted.
326 The following interfaces can be used to limit the amount of
327 kernel memory consumed by inotify:
329 .I /proc/sys/fs/inotify/max_queued_events
330 The value in this file is used when an application calls
332 to set an upper limit on the number of events that can be
333 queued to the corresponding inotify instance.
334 Events in excess of this limit are dropped, but an
336 event is always generated.
338 .I /proc/sys/fs/inotify/max_user_instances
339 This specifies an upper limit on the number of inotify instances
340 that can be created per real user ID.
342 .I /proc/sys/fs/inotify/max_user_watches
343 This specifies an upper limit on the number of watches
344 that can be created per real user ID.
346 Inotify was merged into the 2.6.13 Linux kernel.
347 The required library interfaces were added to glibc in version 2.4.
348 .RB ( IN_DONT_FOLLOW ,
352 were added in version 2.5.)
354 The inotify API is Linux-specific.
356 Inotify file descriptors can be monitored using
361 When an event is available, the file descriptor indicates as readable.
364 signal-driven I/O notification is available for inotify file descriptors;
365 see the discussion of
377 structure (described in
379 that is passed to the signal handler has the following fields set:
381 is set to the inotify file descriptor number;
383 is set to the signal number;
392 If successive output inotify events produced on the
393 inotify file descriptor are identical (same
399 then they are coalesced into a single event if the
400 older event has not yet been read (but see BUGS).
402 The events returned by reading from an inotify file descriptor
403 form an ordered queue.
404 Thus, for example, it is guaranteed that when renaming from
405 one directory to another, events will be produced in the
406 correct order on the inotify file descriptor.
411 returns the number of bytes available to read from an
412 inotify file descriptor.
413 .SS Limitations and caveats
414 Inotify monitoring of directories is not recursive:
415 to monitor subdirectories under a directory,
416 additional watches must be created.
417 This can take a significant amount time for large directory trees.
419 The inotify API provides no information about the user or process that
420 triggered the inotify event.
421 In particular, there is no easy
422 way for a process that is monitoring events via inotify
423 to distinguish events that it triggers
424 itself from those that are triggered by other processes.
426 Note that the event queue can overflow.
427 In this case, events are lost.
428 Robust applications should handle the possibility of
429 lost events gracefully.
431 The inotify API identifies affected files by filename.
432 However, by the time an application processes an inotify event,
433 the filename may already have been deleted or renamed.
435 If monitoring an entire directory subtree,
436 and a new subdirectory is created in that tree,
437 be aware that by the time you create a watch for the new subdirectory,
438 new files may already have been created in the subdirectory.
439 Therefore, you may want to scan the contents of the subdirectory
440 immediately after adding the watch.
442 In kernels before 2.6.16, the
447 Before kernel 2.6.25,
448 the kernel code that was intended to coalesce successive identical events
449 (i.e., the two most recent events could potentially be coalesced
450 if the older had not yet been read)
451 instead checked if the most recent event could be coalesced with the
455 .BR inotify_add_watch (2),
456 .BR inotify_init (2),
457 .BR inotify_init1 (2),
458 .BR inotify_rm_watch (2),
462 .IR Documentation/filesystems/inotify.txt
463 in the Linux kernel source tree
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