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26 .TH INOTIFY 7 2011-12-07 "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 only used 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 only present 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 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(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,
163 .BR inotify_add_watch (2)
169 structure returned when
171 an inotify file descriptor are both bit masks identifying
173 The following bits can be specified in
176 .BR inotify_add_watch (2)
177 and may be returned in the
186 File was accessed (read) (*).
189 Metadata changed, e.g., permissions, timestamps, extended attributes,
190 link count (since Linux 2.6.25), UID, GID, etc. (*).
193 File opened for writing was closed (*).
196 File not opened for writing was closed (*).
199 File/directory created in watched directory (*).
202 File/directory deleted from watched directory (*).
205 Watched file/directory was itself deleted.
208 File was modified (*).
211 Watched file/directory was itself moved.
214 File moved out of watched directory (*).
217 File moved into watched directory (*).
224 When monitoring a directory,
225 the events marked with an asterisk (*) above can occur for
226 files in the directory, in which case the
228 field in the returned
230 structure identifies the name of the file within the directory.
234 macro is defined as a bit mask of all of the above events.
235 This macro can be used as the
237 argument when calling
238 .BR inotify_add_watch (2).
240 Two additional convenience macros are
243 IN_MOVED_FROM|IN_MOVED_TO,
247 IN_CLOSE_WRITE|IN_CLOSE_NOWRITE.
249 The following further bits can be specified in
252 .BR inotify_add_watch (2):
257 .BR IN_DONT_FOLLOW " (since Linux 2.6.15)"
258 Don't dereference \fIpathname\fP if it is a symbolic link.
260 .BR IN_EXCL_UNLINK " (since Linux 2.6.36)"
261 .\" commit 8c1934c8d70b22ca8333b216aec6c7d09fdbd6a6
262 By default, when watching events on the children of a directory,
263 events are generated for children even after they have been unlinked
265 This can result in large numbers of uninteresting events for
266 some applications (e.g., if watching
268 in which many applications create temporary files whose
269 names are immediately unlinked).
272 changes the default behavior,
273 so that events are not generated for children after
274 they have been unlinked from the watched directory.
277 Add (OR) events to watch mask for this pathname if
278 it already exists (instead of replacing mask).
281 Monitor \fIpathname\fP for one event, then remove from
284 .BR IN_ONLYDIR " (since Linux 2.6.15)"
285 Only watch \fIpathname\fP if it is a directory.
289 The following bits may be set in the
298 Watch was removed explicitly (\fBinotify_rm_watch\fP(2))
299 or automatically (file was deleted, or file system was unmounted).
302 Subject of this event is a directory.
305 Event queue overflowed (\fIwd\fP is \-1 for this event).
308 File system containing watched object was unmounted.
312 The following interfaces can be used to limit the amount of
313 kernel memory consumed by inotify:
315 .I /proc/sys/fs/inotify/max_queued_events
316 The value in this file is used when an application calls
318 to set an upper limit on the number of events that can be
319 queued to the corresponding inotify instance.
320 Events in excess of this limit are dropped, but an
322 event is always generated.
324 .I /proc/sys/fs/inotify/max_user_instances
325 This specifies an upper limit on the number of inotify instances
326 that can be created per real user ID.
328 .I /proc/sys/fs/inotify/max_user_watches
329 This specifies an upper limit on the number of watches
330 that can be created per real user ID.
332 Inotify was merged into the 2.6.13 Linux kernel.
333 The required library interfaces were added to glibc in version 2.4.
334 .RB ( IN_DONT_FOLLOW ,
338 were only added in version 2.5.)
340 The inotify API is Linux-specific.
342 Inotify file descriptors can be monitored using
347 When an event is available, the file descriptor indicates as readable.
350 signal-driven I/O notification is available for inotify file descriptors;
351 see the discussion of
363 structure (described in
365 that is passed to the signal handler has the following fields set:
367 is set to the inotify file descriptor number;
369 is set to the signal number;
378 If successive output inotify events produced on the
379 inotify file descriptor are identical (same
385 then they are coalesced into a single event if the
386 older event has not yet been read (but see BUGS).
388 The events returned by reading from an inotify file descriptor
389 form an ordered queue.
390 Thus, for example, it is guaranteed that when renaming from
391 one directory to another, events will be produced in the
392 correct order on the inotify file descriptor.
397 returns the number of bytes available to read from an
398 inotify file descriptor.
399 .SS Limitations and caveats
400 Inotify monitoring of directories is not recursive:
401 to monitor subdirectories under a directory,
402 additional watches must be created.
403 This can take a significant amount time for large directory trees.
405 The inotify API provides no information about the user or process that
406 triggered the inotify event.
408 Note that the event queue can overflow.
409 In this case, events are lost.
410 Robust applications should handle the possibility of
411 lost events gracefully.
413 The inotify API identifies affected files by filename.
414 However, by the time an application processes an inotify event,
415 the filename may already have been deleted or renamed.
417 If monitoring an entire directory subtree,
418 and a new subdirectory is created in that tree,
419 be aware that by the time you create a watch for the new subdirectory,
420 new files may already have been created in the subdirectory.
421 Therefore, you may want to scan the contents of the subdirectory
422 immediately after adding the watch.
424 In kernels before 2.6.16, the
429 Before kernel 2.6.25,
430 the kernel code that was intended to coalesce successive identical events
431 (i.e., the two most recent events could potentially be coalesced
432 if the older had not yet been read)
433 instead checked if the most recent event could be coalesced with the
437 .BR inotify_add_watch (2),
438 .BR inotify_init (2),
439 .BR inotify_init1 (2),
440 .BR inotify_rm_watch (2),
443 .IR Documentation/filesystems/inotify.txt .