blob: 59a919f16144df84dc6e3934dcdc22c26611a6ea [file] [log] [blame]
Robert Love6f979332005-07-15 03:56:33 -07001 inotify
2 a powerful yet simple file change notification system
Robert Love0eeca282005-07-12 17:06:03 -04003
4
5
6Document started 15 Mar 2005 by Robert Love <rml@novell.com>
7
Robert Love6f979332005-07-15 03:56:33 -07008
Robert Love0eeca282005-07-12 17:06:03 -04009(i) User Interface
10
Robert Love6f979332005-07-15 03:56:33 -070011Inotify is controlled by a set of three system calls and normal file I/O on a
12returned file descriptor.
Robert Love0eeca282005-07-12 17:06:03 -040013
Robert Love6f979332005-07-15 03:56:33 -070014First step in using inotify is to initialise an inotify instance:
Robert Love0eeca282005-07-12 17:06:03 -040015
16 int fd = inotify_init ();
17
Robert Love6f979332005-07-15 03:56:33 -070018Each instance is associated with a unique, ordered queue.
19
Robert Love0eeca282005-07-12 17:06:03 -040020Change events are managed by "watches". A watch is an (object,mask) pair where
21the object is a file or directory and the mask is a bit mask of one or more
22inotify events that the application wishes to receive. See <linux/inotify.h>
23for valid events. A watch is referenced by a watch descriptor, or wd.
24
25Watches are added via a path to the file.
26
27Watches on a directory will return events on any files inside of the directory.
28
Robert Love6f979332005-07-15 03:56:33 -070029Adding a watch is simple:
Robert Love0eeca282005-07-12 17:06:03 -040030
31 int wd = inotify_add_watch (fd, path, mask);
32
Robert Love6f979332005-07-15 03:56:33 -070033Where "fd" is the return value from inotify_init(), path is the path to the
34object to watch, and mask is the watch mask (see <linux/inotify.h>).
Robert Love0eeca282005-07-12 17:06:03 -040035
36You can update an existing watch in the same manner, by passing in a new mask.
37
Robert Love6f979332005-07-15 03:56:33 -070038An existing watch is removed via
Robert Love0eeca282005-07-12 17:06:03 -040039
Robert Love6f979332005-07-15 03:56:33 -070040 int ret = inotify_rm_watch (fd, wd);
Robert Love0eeca282005-07-12 17:06:03 -040041
42Events are provided in the form of an inotify_event structure that is read(2)
Robert Love6f979332005-07-15 03:56:33 -070043from a given inotify instance. The filename is of dynamic length and follows
44the struct. It is of size len. The filename is padded with null bytes to
45ensure proper alignment. This padding is reflected in len.
Robert Love0eeca282005-07-12 17:06:03 -040046
47You can slurp multiple events by passing a large buffer, for example
48
49 size_t len = read (fd, buf, BUF_LEN);
50
Robert Love6f979332005-07-15 03:56:33 -070051Where "buf" is a pointer to an array of "inotify_event" structures at least
52BUF_LEN bytes in size. The above example will return as many events as are
53available and fit in BUF_LEN.
Robert Love0eeca282005-07-12 17:06:03 -040054
Robert Love6f979332005-07-15 03:56:33 -070055Each inotify instance fd is also select()- and poll()-able.
Robert Love0eeca282005-07-12 17:06:03 -040056
Robert Love6f979332005-07-15 03:56:33 -070057You can find the size of the current event queue via the standard FIONREAD
58ioctl on the fd returned by inotify_init().
Robert Love0eeca282005-07-12 17:06:03 -040059
60All watches are destroyed and cleaned up on close.
61
62
Robert Love6f979332005-07-15 03:56:33 -070063(ii)
Robert Love0eeca282005-07-12 17:06:03 -040064
Robert Love6f979332005-07-15 03:56:33 -070065Prototypes:
66
67 int inotify_init (void);
68 int inotify_add_watch (int fd, const char *path, __u32 mask);
69 int inotify_rm_watch (int fd, __u32 mask);
70
71
Amy Griffis0edce1972006-06-01 13:11:07 -070072(iii) Kernel Interface
Robert Love6f979332005-07-15 03:56:33 -070073
Amy Griffis0edce1972006-06-01 13:11:07 -070074Inotify's kernel API consists a set of functions for managing watches and an
75event callback.
76
77To use the kernel API, you must first initialize an inotify instance with a set
78of inotify_operations. You are given an opaque inotify_handle, which you use
79for any further calls to inotify.
80
81 struct inotify_handle *ih = inotify_init(my_event_handler);
82
83You must provide a function for processing events and a function for destroying
84the inotify watch.
85
86 void handle_event(struct inotify_watch *watch, u32 wd, u32 mask,
87 u32 cookie, const char *name, struct inode *inode)
88
89 watch - the pointer to the inotify_watch that triggered this call
90 wd - the watch descriptor
91 mask - describes the event that occurred
92 cookie - an identifier for synchronizing events
93 name - the dentry name for affected files in a directory-based event
94 inode - the affected inode in a directory-based event
95
96 void destroy_watch(struct inotify_watch *watch)
97
98You may add watches by providing a pre-allocated and initialized inotify_watch
99structure and specifying the inode to watch along with an inotify event mask.
100You must pin the inode during the call. You will likely wish to embed the
101inotify_watch structure in a structure of your own which contains other
102information about the watch. Once you add an inotify watch, it is immediately
103subject to removal depending on filesystem events. You must grab a reference if
104you depend on the watch hanging around after the call.
105
106 inotify_init_watch(&my_watch->iwatch);
107 inotify_get_watch(&my_watch->iwatch); // optional
108 s32 wd = inotify_add_watch(ih, &my_watch->iwatch, inode, mask);
109 inotify_put_watch(&my_watch->iwatch); // optional
110
111You may use the watch descriptor (wd) or the address of the inotify_watch for
112other inotify operations. You must not directly read or manipulate data in the
113inotify_watch. Additionally, you must not call inotify_add_watch() more than
114once for a given inotify_watch structure, unless you have first called either
115inotify_rm_watch() or inotify_rm_wd().
116
117To determine if you have already registered a watch for a given inode, you may
118call inotify_find_watch(), which gives you both the wd and the watch pointer for
119the inotify_watch, or an error if the watch does not exist.
120
121 wd = inotify_find_watch(ih, inode, &watchp);
122
123You may use container_of() on the watch pointer to access your own data
124associated with a given watch. When an existing watch is found,
125inotify_find_watch() bumps the refcount before releasing its locks. You must
126put that reference with:
127
128 put_inotify_watch(watchp);
129
130Call inotify_find_update_watch() to update the event mask for an existing watch.
131inotify_find_update_watch() returns the wd of the updated watch, or an error if
132the watch does not exist.
133
134 wd = inotify_find_update_watch(ih, inode, mask);
135
136An existing watch may be removed by calling either inotify_rm_watch() or
137inotify_rm_wd().
138
139 int ret = inotify_rm_watch(ih, &my_watch->iwatch);
140 int ret = inotify_rm_wd(ih, wd);
141
142A watch may be removed while executing your event handler with the following:
143
144 inotify_remove_watch_locked(ih, iwatch);
145
146Call inotify_destroy() to remove all watches from your inotify instance and
147release it. If there are no outstanding references, inotify_destroy() will call
148your destroy_watch op for each watch.
149
150 inotify_destroy(ih);
151
152When inotify removes a watch, it sends an IN_IGNORED event to your callback.
153You may use this event as an indication to free the watch memory. Note that
154inotify may remove a watch due to filesystem events, as well as by your request.
155If you use IN_ONESHOT, inotify will remove the watch after the first event, at
156which point you may call the final inotify_put_watch.
157
158(iv) Kernel Interface Prototypes
159
160 struct inotify_handle *inotify_init(struct inotify_operations *ops);
161
162 inotify_init_watch(struct inotify_watch *watch);
163
164 s32 inotify_add_watch(struct inotify_handle *ih,
165 struct inotify_watch *watch,
166 struct inode *inode, u32 mask);
167
168 s32 inotify_find_watch(struct inotify_handle *ih, struct inode *inode,
169 struct inotify_watch **watchp);
170
171 s32 inotify_find_update_watch(struct inotify_handle *ih,
172 struct inode *inode, u32 mask);
173
174 int inotify_rm_wd(struct inotify_handle *ih, u32 wd);
175
176 int inotify_rm_watch(struct inotify_handle *ih,
177 struct inotify_watch *watch);
178
179 void inotify_remove_watch_locked(struct inotify_handle *ih,
180 struct inotify_watch *watch);
181
182 void inotify_destroy(struct inotify_handle *ih);
183
184 void get_inotify_watch(struct inotify_watch *watch);
185 void put_inotify_watch(struct inotify_watch *watch);
186
187
188(v) Internal Kernel Implementation
189
190Each inotify instance is represented by an inotify_handle structure.
191Inotify's userspace consumers also have an inotify_device which is
192associated with the inotify_handle, and on which events are queued.
Robert Love0eeca282005-07-12 17:06:03 -0400193
194Each watch is associated with an inotify_watch structure. Watches are chained
Amy Griffis0edce1972006-06-01 13:11:07 -0700195off of each associated inotify_handle and each associated inode.
Robert Love0eeca282005-07-12 17:06:03 -0400196
Amy Griffis0edce1972006-06-01 13:11:07 -0700197See fs/inotify.c and fs/inotify_user.c for the locking and lifetime rules.
Robert Love0eeca282005-07-12 17:06:03 -0400198
199
Amy Griffis0edce1972006-06-01 13:11:07 -0700200(vi) Rationale
Robert Love0eeca282005-07-12 17:06:03 -0400201
202Q: What is the design decision behind not tying the watch to the open fd of
203 the watched object?
204
205A: Watches are associated with an open inotify device, not an open file.
206 This solves the primary problem with dnotify: keeping the file open pins
207 the file and thus, worse, pins the mount. Dnotify is therefore infeasible
208 for use on a desktop system with removable media as the media cannot be
Robert Love6f979332005-07-15 03:56:33 -0700209 unmounted. Watching a file should not require that it be open.
Robert Love0eeca282005-07-12 17:06:03 -0400210
Robert Love6f979332005-07-15 03:56:33 -0700211Q: What is the design decision behind using an-fd-per-instance as opposed to
Robert Love0eeca282005-07-12 17:06:03 -0400212 an fd-per-watch?
213
214A: An fd-per-watch quickly consumes more file descriptors than are allowed,
215 more fd's than are feasible to manage, and more fd's than are optimally
216 select()-able. Yes, root can bump the per-process fd limit and yes, users
217 can use epoll, but requiring both is a silly and extraneous requirement.
218 A watch consumes less memory than an open file, separating the number
219 spaces is thus sensible. The current design is what user-space developers
Robert Love6f979332005-07-15 03:56:33 -0700220 want: Users initialize inotify, once, and add n watches, requiring but one
221 fd and no twiddling with fd limits. Initializing an inotify instance two
Robert Love0eeca282005-07-12 17:06:03 -0400222 thousand times is silly. If we can implement user-space's preferences
223 cleanly--and we can, the idr layer makes stuff like this trivial--then we
224 should.
225
226 There are other good arguments. With a single fd, there is a single
227 item to block on, which is mapped to a single queue of events. The single
228 fd returns all watch events and also any potential out-of-band data. If
229 every fd was a separate watch,
230
231 - There would be no way to get event ordering. Events on file foo and
232 file bar would pop poll() on both fd's, but there would be no way to tell
233 which happened first. A single queue trivially gives you ordering. Such
234 ordering is crucial to existing applications such as Beagle. Imagine
235 "mv a b ; mv b a" events without ordering.
236
237 - We'd have to maintain n fd's and n internal queues with state,
238 versus just one. It is a lot messier in the kernel. A single, linear
239 queue is the data structure that makes sense.
240
241 - User-space developers prefer the current API. The Beagle guys, for
242 example, love it. Trust me, I asked. It is not a surprise: Who'd want
243 to manage and block on 1000 fd's via select?
244
Robert Love0eeca282005-07-12 17:06:03 -0400245 - No way to get out of band data.
246
247 - 1024 is still too low. ;-)
248
249 When you talk about designing a file change notification system that
250 scales to 1000s of directories, juggling 1000s of fd's just does not seem
251 the right interface. It is too heavy.
252
Robert Love6f979332005-07-15 03:56:33 -0700253 Additionally, it _is_ possible to more than one instance and
254 juggle more than one queue and thus more than one associated fd. There
255 need not be a one-fd-per-process mapping; it is one-fd-per-queue and a
256 process can easily want more than one queue.
257
Robert Love0eeca282005-07-12 17:06:03 -0400258Q: Why the system call approach?
259
260A: The poor user-space interface is the second biggest problem with dnotify.
261 Signals are a terrible, terrible interface for file notification. Or for
262 anything, for that matter. The ideal solution, from all perspectives, is a
263 file descriptor-based one that allows basic file I/O and poll/select.
264 Obtaining the fd and managing the watches could have been done either via a
265 device file or a family of new system calls. We decided to implement a
Amy Griffis0edce1972006-06-01 13:11:07 -0700266 family of system calls because that is the preferred approach for new kernel
Robert Love6f979332005-07-15 03:56:33 -0700267 interfaces. The only real difference was whether we wanted to use open(2)
268 and ioctl(2) or a couple of new system calls. System calls beat ioctls.
Robert Love0eeca282005-07-12 17:06:03 -0400269