view Documentation/filesystems/inotify.txt @ 524:7f8b544237bf

netfront: Allow netfront in domain 0.

This is useful if your physical network device is in a utility domain.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
author Keir Fraser <keir.fraser@citrix.com>
date Tue Apr 15 15:18:58 2008 +0100 (2008-04-15)
parents 831230e53067
line source
1 inotify
2 a powerful yet simple file change notification system
6 Document started 15 Mar 2005 by Robert Love <rml@novell.com>
9 (i) User Interface
11 Inotify is controlled by a set of three system calls and normal file I/O on a
12 returned file descriptor.
14 First step in using inotify is to initialise an inotify instance:
16 int fd = inotify_init ();
18 Each instance is associated with a unique, ordered queue.
20 Change events are managed by "watches". A watch is an (object,mask) pair where
21 the object is a file or directory and the mask is a bit mask of one or more
22 inotify events that the application wishes to receive. See <linux/inotify.h>
23 for valid events. A watch is referenced by a watch descriptor, or wd.
25 Watches are added via a path to the file.
27 Watches on a directory will return events on any files inside of the directory.
29 Adding a watch is simple:
31 int wd = inotify_add_watch (fd, path, mask);
33 Where "fd" is the return value from inotify_init(), path is the path to the
34 object to watch, and mask is the watch mask (see <linux/inotify.h>).
36 You can update an existing watch in the same manner, by passing in a new mask.
38 An existing watch is removed via
40 int ret = inotify_rm_watch (fd, wd);
42 Events are provided in the form of an inotify_event structure that is read(2)
43 from a given inotify instance. The filename is of dynamic length and follows
44 the struct. It is of size len. The filename is padded with null bytes to
45 ensure proper alignment. This padding is reflected in len.
47 You can slurp multiple events by passing a large buffer, for example
49 size_t len = read (fd, buf, BUF_LEN);
51 Where "buf" is a pointer to an array of "inotify_event" structures at least
52 BUF_LEN bytes in size. The above example will return as many events as are
53 available and fit in BUF_LEN.
55 Each inotify instance fd is also select()- and poll()-able.
57 You can find the size of the current event queue via the standard FIONREAD
58 ioctl on the fd returned by inotify_init().
60 All watches are destroyed and cleaned up on close.
63 (ii)
65 Prototypes:
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);
72 (iii) Kernel Interface
74 Inotify's kernel API consists a set of functions for managing watches and an
75 event callback.
77 To use the kernel API, you must first initialize an inotify instance with a set
78 of inotify_operations. You are given an opaque inotify_handle, which you use
79 for any further calls to inotify.
81 struct inotify_handle *ih = inotify_init(my_event_handler);
83 You must provide a function for processing events and a function for destroying
84 the inotify watch.
86 void handle_event(struct inotify_watch *watch, u32 wd, u32 mask,
87 u32 cookie, const char *name, struct inode *inode)
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
96 void destroy_watch(struct inotify_watch *watch)
98 You may add watches by providing a pre-allocated and initialized inotify_watch
99 structure and specifying the inode to watch along with an inotify event mask.
100 You must pin the inode during the call. You will likely wish to embed the
101 inotify_watch structure in a structure of your own which contains other
102 information about the watch. Once you add an inotify watch, it is immediately
103 subject to removal depending on filesystem events. You must grab a reference if
104 you depend on the watch hanging around after the call.
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
111 You may use the watch descriptor (wd) or the address of the inotify_watch for
112 other inotify operations. You must not directly read or manipulate data in the
113 inotify_watch. Additionally, you must not call inotify_add_watch() more than
114 once for a given inotify_watch structure, unless you have first called either
115 inotify_rm_watch() or inotify_rm_wd().
117 To determine if you have already registered a watch for a given inode, you may
118 call inotify_find_watch(), which gives you both the wd and the watch pointer for
119 the inotify_watch, or an error if the watch does not exist.
121 wd = inotify_find_watch(ih, inode, &watchp);
123 You may use container_of() on the watch pointer to access your own data
124 associated with a given watch. When an existing watch is found,
125 inotify_find_watch() bumps the refcount before releasing its locks. You must
126 put that reference with:
128 put_inotify_watch(watchp);
130 Call inotify_find_update_watch() to update the event mask for an existing watch.
131 inotify_find_update_watch() returns the wd of the updated watch, or an error if
132 the watch does not exist.
134 wd = inotify_find_update_watch(ih, inode, mask);
136 An existing watch may be removed by calling either inotify_rm_watch() or
137 inotify_rm_wd().
139 int ret = inotify_rm_watch(ih, &my_watch->iwatch);
140 int ret = inotify_rm_wd(ih, wd);
142 A watch may be removed while executing your event handler with the following:
144 inotify_remove_watch_locked(ih, iwatch);
146 Call inotify_destroy() to remove all watches from your inotify instance and
147 release it. If there are no outstanding references, inotify_destroy() will call
148 your destroy_watch op for each watch.
150 inotify_destroy(ih);
152 When inotify removes a watch, it sends an IN_IGNORED event to your callback.
153 You may use this event as an indication to free the watch memory. Note that
154 inotify may remove a watch due to filesystem events, as well as by your request.
155 If you use IN_ONESHOT, inotify will remove the watch after the first event, at
156 which point you may call the final inotify_put_watch.
158 (iv) Kernel Interface Prototypes
160 struct inotify_handle *inotify_init(struct inotify_operations *ops);
162 inotify_init_watch(struct inotify_watch *watch);
164 s32 inotify_add_watch(struct inotify_handle *ih,
165 struct inotify_watch *watch,
166 struct inode *inode, u32 mask);
168 s32 inotify_find_watch(struct inotify_handle *ih, struct inode *inode,
169 struct inotify_watch **watchp);
171 s32 inotify_find_update_watch(struct inotify_handle *ih,
172 struct inode *inode, u32 mask);
174 int inotify_rm_wd(struct inotify_handle *ih, u32 wd);
176 int inotify_rm_watch(struct inotify_handle *ih,
177 struct inotify_watch *watch);
179 void inotify_remove_watch_locked(struct inotify_handle *ih,
180 struct inotify_watch *watch);
182 void inotify_destroy(struct inotify_handle *ih);
184 void get_inotify_watch(struct inotify_watch *watch);
185 void put_inotify_watch(struct inotify_watch *watch);
188 (v) Internal Kernel Implementation
190 Each inotify instance is represented by an inotify_handle structure.
191 Inotify's userspace consumers also have an inotify_device which is
192 associated with the inotify_handle, and on which events are queued.
194 Each watch is associated with an inotify_watch structure. Watches are chained
195 off of each associated inotify_handle and each associated inode.
197 See fs/inotify.c and fs/inotify_user.c for the locking and lifetime rules.
200 (vi) Rationale
202 Q: What is the design decision behind not tying the watch to the open fd of
203 the watched object?
205 A: 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
209 unmounted. Watching a file should not require that it be open.
211 Q: What is the design decision behind using an-fd-per-instance as opposed to
212 an fd-per-watch?
214 A: 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
220 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
222 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.
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,
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.
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.
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?
245 - No way to get out of band data.
247 - 1024 is still too low. ;-)
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.
253 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.
258 Q: Why the system call approach?
260 A: 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
266 family of system calls because that is the preferred approach for new kernel
267 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.