Robert Love | 6f97933 | 2005-07-15 03:56:33 -0700 | [diff] [blame] | 1 | inotify |
| 2 | a powerful yet simple file change notification system |
Robert Love | 0eeca28 | 2005-07-12 17:06:03 -0400 | [diff] [blame] | 3 | |
| 4 | |
| 5 | |
| 6 | Document started 15 Mar 2005 by Robert Love <rml@novell.com> |
| 7 | |
Robert Love | 6f97933 | 2005-07-15 03:56:33 -0700 | [diff] [blame] | 8 | |
Robert Love | 0eeca28 | 2005-07-12 17:06:03 -0400 | [diff] [blame] | 9 | (i) User Interface |
| 10 | |
Robert Love | 6f97933 | 2005-07-15 03:56:33 -0700 | [diff] [blame] | 11 | Inotify is controlled by a set of three system calls and normal file I/O on a |
| 12 | returned file descriptor. |
Robert Love | 0eeca28 | 2005-07-12 17:06:03 -0400 | [diff] [blame] | 13 | |
Robert Love | 6f97933 | 2005-07-15 03:56:33 -0700 | [diff] [blame] | 14 | First step in using inotify is to initialise an inotify instance: |
Robert Love | 0eeca28 | 2005-07-12 17:06:03 -0400 | [diff] [blame] | 15 | |
| 16 | int fd = inotify_init (); |
| 17 | |
Robert Love | 6f97933 | 2005-07-15 03:56:33 -0700 | [diff] [blame] | 18 | Each instance is associated with a unique, ordered queue. |
| 19 | |
Robert Love | 0eeca28 | 2005-07-12 17:06:03 -0400 | [diff] [blame] | 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. |
| 24 | |
| 25 | Watches are added via a path to the file. |
| 26 | |
| 27 | Watches on a directory will return events on any files inside of the directory. |
| 28 | |
Robert Love | 6f97933 | 2005-07-15 03:56:33 -0700 | [diff] [blame] | 29 | Adding a watch is simple: |
Robert Love | 0eeca28 | 2005-07-12 17:06:03 -0400 | [diff] [blame] | 30 | |
| 31 | int wd = inotify_add_watch (fd, path, mask); |
| 32 | |
Robert Love | 6f97933 | 2005-07-15 03:56:33 -0700 | [diff] [blame] | 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>). |
Robert Love | 0eeca28 | 2005-07-12 17:06:03 -0400 | [diff] [blame] | 35 | |
| 36 | You can update an existing watch in the same manner, by passing in a new mask. |
| 37 | |
Robert Love | 6f97933 | 2005-07-15 03:56:33 -0700 | [diff] [blame] | 38 | An existing watch is removed via |
Robert Love | 0eeca28 | 2005-07-12 17:06:03 -0400 | [diff] [blame] | 39 | |
Robert Love | 6f97933 | 2005-07-15 03:56:33 -0700 | [diff] [blame] | 40 | int ret = inotify_rm_watch (fd, wd); |
Robert Love | 0eeca28 | 2005-07-12 17:06:03 -0400 | [diff] [blame] | 41 | |
| 42 | Events are provided in the form of an inotify_event structure that is read(2) |
Robert Love | 6f97933 | 2005-07-15 03:56:33 -0700 | [diff] [blame] | 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. |
Robert Love | 0eeca28 | 2005-07-12 17:06:03 -0400 | [diff] [blame] | 46 | |
| 47 | You can slurp multiple events by passing a large buffer, for example |
| 48 | |
| 49 | size_t len = read (fd, buf, BUF_LEN); |
| 50 | |
Robert Love | 6f97933 | 2005-07-15 03:56:33 -0700 | [diff] [blame] | 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. |
Robert Love | 0eeca28 | 2005-07-12 17:06:03 -0400 | [diff] [blame] | 54 | |
Robert Love | 6f97933 | 2005-07-15 03:56:33 -0700 | [diff] [blame] | 55 | Each inotify instance fd is also select()- and poll()-able. |
Robert Love | 0eeca28 | 2005-07-12 17:06:03 -0400 | [diff] [blame] | 56 | |
Robert Love | 6f97933 | 2005-07-15 03:56:33 -0700 | [diff] [blame] | 57 | You can find the size of the current event queue via the standard FIONREAD |
| 58 | ioctl on the fd returned by inotify_init(). |
Robert Love | 0eeca28 | 2005-07-12 17:06:03 -0400 | [diff] [blame] | 59 | |
| 60 | All watches are destroyed and cleaned up on close. |
| 61 | |
| 62 | |
Robert Love | 6f97933 | 2005-07-15 03:56:33 -0700 | [diff] [blame] | 63 | (ii) |
Robert Love | 0eeca28 | 2005-07-12 17:06:03 -0400 | [diff] [blame] | 64 | |
Robert Love | 6f97933 | 2005-07-15 03:56:33 -0700 | [diff] [blame] | 65 | Prototypes: |
| 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 Griffis | 0edce19 | 2006-06-01 13:11:07 -0700 | [diff] [blame] | 72 | (iii) Kernel Interface |
Robert Love | 6f97933 | 2005-07-15 03:56:33 -0700 | [diff] [blame] | 73 | |
Amy Griffis | 0edce19 | 2006-06-01 13:11:07 -0700 | [diff] [blame] | 74 | Inotify's kernel API consists a set of functions for managing watches and an |
| 75 | event callback. |
| 76 | |
| 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. |
| 80 | |
| 81 | struct inotify_handle *ih = inotify_init(my_event_handler); |
| 82 | |
| 83 | You must provide a function for processing events and a function for destroying |
| 84 | the 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 | |
| 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. |
| 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 | |
| 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(). |
| 116 | |
| 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. |
| 120 | |
| 121 | wd = inotify_find_watch(ih, inode, &watchp); |
| 122 | |
| 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: |
| 127 | |
| 128 | put_inotify_watch(watchp); |
| 129 | |
| 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. |
| 133 | |
| 134 | wd = inotify_find_update_watch(ih, inode, mask); |
| 135 | |
| 136 | An existing watch may be removed by calling either inotify_rm_watch() or |
| 137 | inotify_rm_wd(). |
| 138 | |
| 139 | int ret = inotify_rm_watch(ih, &my_watch->iwatch); |
| 140 | int ret = inotify_rm_wd(ih, wd); |
| 141 | |
| 142 | A watch may be removed while executing your event handler with the following: |
| 143 | |
| 144 | inotify_remove_watch_locked(ih, iwatch); |
| 145 | |
| 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. |
| 149 | |
| 150 | inotify_destroy(ih); |
| 151 | |
| 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. |
| 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 | |
| 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. |
Robert Love | 0eeca28 | 2005-07-12 17:06:03 -0400 | [diff] [blame] | 193 | |
| 194 | Each watch is associated with an inotify_watch structure. Watches are chained |
Amy Griffis | 0edce19 | 2006-06-01 13:11:07 -0700 | [diff] [blame] | 195 | off of each associated inotify_handle and each associated inode. |
Robert Love | 0eeca28 | 2005-07-12 17:06:03 -0400 | [diff] [blame] | 196 | |
Marcos Paulo de Souza | d83fe6b | 2011-11-04 11:22:14 -0700 | [diff] [blame] | 197 | See fs/notify/inotify/inotify_fsnotify.c and fs/notify/inotify/inotify_user.c |
| 198 | for the locking and lifetime rules. |
Robert Love | 0eeca28 | 2005-07-12 17:06:03 -0400 | [diff] [blame] | 199 | |
| 200 | |
Amy Griffis | 0edce19 | 2006-06-01 13:11:07 -0700 | [diff] [blame] | 201 | (vi) Rationale |
Robert Love | 0eeca28 | 2005-07-12 17:06:03 -0400 | [diff] [blame] | 202 | |
| 203 | Q: What is the design decision behind not tying the watch to the open fd of |
| 204 | the watched object? |
| 205 | |
| 206 | A: Watches are associated with an open inotify device, not an open file. |
| 207 | This solves the primary problem with dnotify: keeping the file open pins |
| 208 | the file and thus, worse, pins the mount. Dnotify is therefore infeasible |
| 209 | for use on a desktop system with removable media as the media cannot be |
Robert Love | 6f97933 | 2005-07-15 03:56:33 -0700 | [diff] [blame] | 210 | unmounted. Watching a file should not require that it be open. |
Robert Love | 0eeca28 | 2005-07-12 17:06:03 -0400 | [diff] [blame] | 211 | |
Robert Love | 6f97933 | 2005-07-15 03:56:33 -0700 | [diff] [blame] | 212 | Q: What is the design decision behind using an-fd-per-instance as opposed to |
Robert Love | 0eeca28 | 2005-07-12 17:06:03 -0400 | [diff] [blame] | 213 | an fd-per-watch? |
| 214 | |
| 215 | A: An fd-per-watch quickly consumes more file descriptors than are allowed, |
| 216 | more fd's than are feasible to manage, and more fd's than are optimally |
| 217 | select()-able. Yes, root can bump the per-process fd limit and yes, users |
| 218 | can use epoll, but requiring both is a silly and extraneous requirement. |
| 219 | A watch consumes less memory than an open file, separating the number |
| 220 | spaces is thus sensible. The current design is what user-space developers |
Robert Love | 6f97933 | 2005-07-15 03:56:33 -0700 | [diff] [blame] | 221 | want: Users initialize inotify, once, and add n watches, requiring but one |
| 222 | fd and no twiddling with fd limits. Initializing an inotify instance two |
Robert Love | 0eeca28 | 2005-07-12 17:06:03 -0400 | [diff] [blame] | 223 | thousand times is silly. If we can implement user-space's preferences |
| 224 | cleanly--and we can, the idr layer makes stuff like this trivial--then we |
| 225 | should. |
| 226 | |
| 227 | There are other good arguments. With a single fd, there is a single |
| 228 | item to block on, which is mapped to a single queue of events. The single |
| 229 | fd returns all watch events and also any potential out-of-band data. If |
| 230 | every fd was a separate watch, |
| 231 | |
| 232 | - There would be no way to get event ordering. Events on file foo and |
| 233 | file bar would pop poll() on both fd's, but there would be no way to tell |
| 234 | which happened first. A single queue trivially gives you ordering. Such |
| 235 | ordering is crucial to existing applications such as Beagle. Imagine |
| 236 | "mv a b ; mv b a" events without ordering. |
| 237 | |
| 238 | - We'd have to maintain n fd's and n internal queues with state, |
| 239 | versus just one. It is a lot messier in the kernel. A single, linear |
| 240 | queue is the data structure that makes sense. |
| 241 | |
| 242 | - User-space developers prefer the current API. The Beagle guys, for |
| 243 | example, love it. Trust me, I asked. It is not a surprise: Who'd want |
| 244 | to manage and block on 1000 fd's via select? |
| 245 | |
Robert Love | 0eeca28 | 2005-07-12 17:06:03 -0400 | [diff] [blame] | 246 | - No way to get out of band data. |
| 247 | |
| 248 | - 1024 is still too low. ;-) |
| 249 | |
| 250 | When you talk about designing a file change notification system that |
| 251 | scales to 1000s of directories, juggling 1000s of fd's just does not seem |
| 252 | the right interface. It is too heavy. |
| 253 | |
Robert Love | 6f97933 | 2005-07-15 03:56:33 -0700 | [diff] [blame] | 254 | Additionally, it _is_ possible to more than one instance and |
| 255 | juggle more than one queue and thus more than one associated fd. There |
| 256 | need not be a one-fd-per-process mapping; it is one-fd-per-queue and a |
| 257 | process can easily want more than one queue. |
| 258 | |
Robert Love | 0eeca28 | 2005-07-12 17:06:03 -0400 | [diff] [blame] | 259 | Q: Why the system call approach? |
| 260 | |
| 261 | A: The poor user-space interface is the second biggest problem with dnotify. |
| 262 | Signals are a terrible, terrible interface for file notification. Or for |
| 263 | anything, for that matter. The ideal solution, from all perspectives, is a |
| 264 | file descriptor-based one that allows basic file I/O and poll/select. |
| 265 | Obtaining the fd and managing the watches could have been done either via a |
| 266 | device file or a family of new system calls. We decided to implement a |
Amy Griffis | 0edce19 | 2006-06-01 13:11:07 -0700 | [diff] [blame] | 267 | family of system calls because that is the preferred approach for new kernel |
Robert Love | 6f97933 | 2005-07-15 03:56:33 -0700 | [diff] [blame] | 268 | interfaces. The only real difference was whether we wanted to use open(2) |
| 269 | and ioctl(2) or a couple of new system calls. System calls beat ioctls. |
Robert Love | 0eeca28 | 2005-07-12 17:06:03 -0400 | [diff] [blame] | 270 | |