1INOTIFY(7) Linux Programmer's Manual INOTIFY(7)
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6 inotify - monitoring file system events
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9 The inotify API provides a mechanism for monitoring file system events.
10 Inotify can be used to monitor individual files, or to monitor directo‐
11 ries. When a directory is monitored, inotify will return events for
12 the directory itself, and for files inside the directory.
13 The following system calls are used with this API: inotify_init(2) (or
15 inotify_init1(2)), inotify_add_watch(2), inotify_rm_watch(2), read(2),
16 and close(2).
17 inotify_init(2) creates an inotify instance and returns a file descrip‐
19 tor referring to the inotify instance. The more recent ino‐
20 tify_init1(2) is like inotify_init(2), but provides some extra func‐
21 tionality.
22 inotify_add_watch(2) manipulates the "watch list" associated with an
24 inotify instance. Each item ("watch") in the watch list specifies the
25 pathname of a file or directory, along with some set of events that the
26 kernel should monitor for the file referred to by that pathname. ino‐
27 tify_add_watch(2) either creates a new watch item, or modifies an
28 existing watch. Each watch has a unique "watch descriptor", an integer
29 returned by inotify_add_watch(2) when the watch is created.
30 inotify_rm_watch(2) removes an item from an inotify watch list.
32 When all file descriptors referring to an inotify instance have been
34 closed, the underlying object and its resources are freed for re-use by
35 the kernel; all associated watches are automatically freed.
36 To determine what events have occurred, an application read(2)s from
38 the inotify file descriptor. If no events have so far occurred, then,
39 assuming a blocking file descriptor, read(2) will block until at least
40 one event occurs (unless interrupted by a signal, in which case the
41 call fails with the error EINTR; see signal(7)).
42 Each successful read(2) returns a buffer containing one or more of the
44 following structures:
45 struct inotify_event {
47 int wd; /* Watch descriptor */
48 uint32_t mask; /* Mask of events */
49 uint32_t cookie; /* Unique cookie associating related
50 events (for rename(2)) */
51 uint32_t len; /* Size of name field */
52 char name[]; /* Optional null-terminated name */
53 };
54 wd identifies the watch for which this event occurs. It is one of the
56 watch descriptors returned by a previous call to inotify_add_watch(2).
57 mask contains bits that describe the event that occurred (see below).
59 cookie is a unique integer that connects related events. Currently
61 this is only used for rename events, and allows the resulting pair of
62 IN_MOVE_FROM and IN_MOVE_TO events to be connected by the application.
63 The name field is only present when an event is returned for a file
65 inside a watched directory; it identifies the file pathname relative to
66 the watched directory. This pathname is null-terminated, and may
67 include further null bytes to align subsequent reads to a suitable
68 address boundary.
69 The len field counts all of the bytes in name, including the null
71 bytes; the length of each inotify_event structure is thus sizeof(ino‐
72 tify_event)+len.
73 The behavior when the buffer given to read(2) is too small to return
75 information about the next event depends on the kernel version: in ker‐
76 nels before 2.6.21, read(2) returns 0; since kernel 2.6.21, read(2)
77 fails with the error EINVAL.
78 inotify events
80 The inotify_add_watch(2) mask argument and the mask field of the ino‐
81 tify_event structure returned when read(2)ing an inotify file descrip‐
82 tor are both bit masks identifying inotify events. The following bits
83 can be specified in mask when calling inotify_add_watch(2) and may be
84 returned in the mask field returned by read(2):
85 IN_ACCESS File was accessed (read) (*).
87 IN_ATTRIB Metadata changed, e.g., permissions, timestamps,
88 extended attributes, link count (since Linux
89 2.6.25), UID, GID, etc. (*).
90 IN_CLOSE_WRITE File opened for writing was closed (*).
91 IN_CLOSE_NOWRITE File not opened for writing was closed (*).
92 IN_CREATE File/directory created in watched directory (*).
93 IN_DELETE File/directory deleted from watched directory
94 (*).
95 IN_DELETE_SELF Watched file/directory was itself deleted.
96 IN_MODIFY File was modified (*).
97 IN_MOVE_SELF Watched file/directory was itself moved.
98 IN_MOVED_FROM File moved out of watched directory (*).
99 IN_MOVED_TO File moved into watched directory (*).
100 IN_OPEN File was opened (*).
101 When monitoring a directory, the events marked with an asterisk (*)
103 above can occur for files in the directory, in which case the name
104 field in the returned inotify_event structure identifies the name of
105 the file within the directory.
106 The IN_ALL_EVENTS macro is defined as a bit mask of all of the above
108 events. This macro can be used as the mask argument when calling ino‐
109 tify_add_watch(2).
110 Two additional convenience macros are IN_MOVE, which equates to
112 IN_MOVED_FROM|IN_MOVED_TO, and IN_CLOSE which equates to
113 IN_CLOSE_WRITE|IN_CLOSE_NOWRITE.
114 The following further bits can be specified in mask when calling ino‐
116 tify_add_watch(2):
117 IN_DONT_FOLLOW (since Linux 2.6.15)
119 Don't dereference pathname if it is a symbolic
120 link.
121 IN_MASK_ADD Add (OR) events to watch mask for this pathname
122 if it already exists (instead of replacing mask).
123 IN_ONESHOT Monitor pathname for one event, then remove from
124 watch list.
125 IN_ONLYDIR (since Linux 2.6.15)
126 Only watch pathname if it is a directory.
127 The following bits may be set in the mask field returned by read(2):
129 IN_IGNORED Watch was removed explicitly (ino‐
131 tify_rm_watch(2)) or automatically (file was
132 deleted, or file system was unmounted).
133 IN_ISDIR Subject of this event is a directory.
134 IN_Q_OVERFLOW Event queue overflowed (wd is -1 for this event).
135 IN_UNMOUNT File system containing watched object was
136 unmounted.
137 /proc interfaces
139 The following interfaces can be used to limit the amount of kernel mem‐
140 ory consumed by inotify:
141 /proc/sys/fs/inotify/max_queued_events
143 The value in this file is used when an application calls ino‐
144 tify_init(2) to set an upper limit on the number of events that
145 can be queued to the corresponding inotify instance. Events in
146 excess of this limit are dropped, but an IN_Q_OVERFLOW event is
147 always generated.
148 /proc/sys/fs/inotify/max_user_instances
150 This specifies an upper limit on the number of inotify instances
151 that can be created per real user ID.
152 /proc/sys/fs/inotify/max_user_watches
154 This specifies an upper limit on the number of watches that can
155 be created per real user ID.
156
158 Inotify was merged into the 2.6.13 Linux kernel. The required library
159 interfaces were added to glibc in version 2.4. (IN_DONT_FOLLOW,
160 IN_MASK_ADD, and IN_ONLYDIR were only added in version 2.5.)
161
163 The inotify API is Linux-specific.
164
166 Inotify file descriptors can be monitored using select(2), poll(2), and
167 epoll(7). When an event is available, the file descriptor indicates as
168 readable.
169 Since Linux 2.6.25, signal-driven I/O notification is available for
171 inotify file descriptors; see the discussion of F_SETFL (for setting
172 the O_ASYNC flag), F_SETOWN, and F_SETSIG in fcntl(2). The siginfo_t
173 structure (described in sigaction(2)) that is passed to the signal han‐
174 dler has the following fields set: si_fd is set to the inotify file
175 descriptor number; si_signo is set to the signal number; si_code is set
176 to POLL_IN; and POLLIN is set in si_band.
177 If successive output inotify events produced on the inotify file
179 descriptor are identical (same wd, mask, cookie, and name) then they
180 are coalesced into a single event if the older event has not yet been
181 read (but see BUGS).
182 The events returned by reading from an inotify file descriptor form an
184 ordered queue. Thus, for example, it is guaranteed that when renaming
185 from one directory to another, events will be produced in the correct
186 order on the inotify file descriptor.
187 The FIONREAD ioctl(2) returns the number of bytes available to read
189 from an inotify file descriptor.
190 Inotify monitoring of directories is not recursive: to monitor subdi‐
192 rectories under a directory, additional watches must be created.
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195 In kernels before 2.6.16, the IN_ONESHOT mask flag does not work.
196 Before kernel 2.6.25, the kernel code that was intended to coalesce
198 successive identical events (i.e., the two most recent events could
199 potentially be coalesced if the older had not yet been read) instead
200 checked if the most recent event could be coalesced with the oldest
201 unread event.
202
204 inotify_add_watch(2), inotify_init(2), inotify_init1(2), ino‐
205 tify_rm_watch(2), read(2), stat(2), Documentation/filesystems/ino‐
206 tify.txt.
207
209 This page is part of release 3.22 of the Linux man-pages project. A
210 description of the project, and information about reporting bugs, can
211 be found at http://www.kernel.org/doc/man-pages/.
212Linux 2008-11-18 INOTIFY(7)