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| -rw-r--r-- | Documentation/filesystems/inotify.txt | 77 |
1 files changed, 45 insertions, 32 deletions
diff --git a/Documentation/filesystems/inotify.txt b/Documentation/filesystems/inotify.txt index 2c716041f57..6d501903f68 100644 --- a/Documentation/filesystems/inotify.txt +++ b/Documentation/filesystems/inotify.txt @@ -1,18 +1,22 @@ - inotify - a powerful yet simple file change notification system + inotify + a powerful yet simple file change notification system Document started 15 Mar 2005 by Robert Love <rml@novell.com> + (i) User Interface -Inotify is controlled by a set of three sys calls +Inotify is controlled by a set of three system calls and normal file I/O on a +returned file descriptor. -First step in using inotify is to initialise an inotify instance +First step in using inotify is to initialise an inotify instance: int fd = inotify_init (); +Each instance is associated with a unique, ordered queue. + Change events are managed by "watches". A watch is an (object,mask) pair where the object is a file or directory and the mask is a bit mask of one or more inotify events that the application wishes to receive. See <linux/inotify.h> @@ -22,43 +26,52 @@ Watches are added via a path to the file. Watches on a directory will return events on any files inside of the directory. -Adding a watch is simple, +Adding a watch is simple: int wd = inotify_add_watch (fd, path, mask); -You can add a large number of files via something like - - for each file to watch { - int wd = inotify_add_watch (fd, file, mask); - } +Where "fd" is the return value from inotify_init(), path is the path to the +object to watch, and mask is the watch mask (see <linux/inotify.h>). You can update an existing watch in the same manner, by passing in a new mask. -An existing watch is removed via the INOTIFY_IGNORE ioctl, for example +An existing watch is removed via - inotify_rm_watch (fd, wd); + int ret = inotify_rm_watch (fd, wd); Events are provided in the form of an inotify_event structure that is read(2) -from a inotify instance fd. The filename is of dynamic length and follows the -struct. It is of size len. The filename is padded with null bytes to ensure -proper alignment. This padding is reflected in len. +from a given inotify instance. The filename is of dynamic length and follows +the struct. It is of size len. The filename is padded with null bytes to +ensure proper alignment. This padding is reflected in len. You can slurp multiple events by passing a large buffer, for example size_t len = read (fd, buf, BUF_LEN); -Will return as many events as are available and fit in BUF_LEN. +Where "buf" is a pointer to an array of "inotify_event" structures at least +BUF_LEN bytes in size. The above example will return as many events as are +available and fit in BUF_LEN. -each inotify instance fd is also select()- and poll()-able. +Each inotify instance fd is also select()- and poll()-able. -You can find the size of the current event queue via the FIONREAD ioctl. +You can find the size of the current event queue via the standard FIONREAD +ioctl on the fd returned by inotify_init(). All watches are destroyed and cleaned up on close. -(ii) Internal Kernel Implementation +(ii) + +Prototypes: + + int inotify_init (void); + int inotify_add_watch (int fd, const char *path, __u32 mask); + int inotify_rm_watch (int fd, __u32 mask); + -Each open inotify instance is associated with an inotify_device structure. +(iii) Internal Kernel Implementation + +Each inotify instance is associated with an inotify_device structure. Each watch is associated with an inotify_watch structure. Watches are chained off of each associated device and each associated inode. @@ -66,7 +79,7 @@ off of each associated device and each associated inode. See fs/inotify.c for the locking and lifetime rules. -(iii) Rationale +(iv) Rationale Q: What is the design decision behind not tying the watch to the open fd of the watched object? @@ -75,9 +88,9 @@ A: Watches are associated with an open inotify device, not an open file. This solves the primary problem with dnotify: keeping the file open pins the file and thus, worse, pins the mount. Dnotify is therefore infeasible for use on a desktop system with removable media as the media cannot be - unmounted. + unmounted. Watching a file should not require that it be open. -Q: What is the design decision behind using an-fd-per-device as opposed to +Q: What is the design decision behind using an-fd-per-instance as opposed to an fd-per-watch? A: An fd-per-watch quickly consumes more file descriptors than are allowed, @@ -86,8 +99,8 @@ A: An fd-per-watch quickly consumes more file descriptors than are allowed, can use epoll, but requiring both is a silly and extraneous requirement. A watch consumes less memory than an open file, separating the number spaces is thus sensible. The current design is what user-space developers - want: Users initialize inotify, once, and add n watches, requiring but one fd - and no twiddling with fd limits. Initializing an inotify instance two + want: Users initialize inotify, once, and add n watches, requiring but one + fd and no twiddling with fd limits. Initializing an inotify instance two thousand times is silly. If we can implement user-space's preferences cleanly--and we can, the idr layer makes stuff like this trivial--then we should. @@ -111,9 +124,6 @@ A: An fd-per-watch quickly consumes more file descriptors than are allowed, example, love it. Trust me, I asked. It is not a surprise: Who'd want to manage and block on 1000 fd's via select? - - You'd have to manage the fd's, as an example: Call close() when you - received a delete event. - - No way to get out of band data. - 1024 is still too low. ;-) @@ -122,6 +132,11 @@ A: An fd-per-watch quickly consumes more file descriptors than are allowed, scales to 1000s of directories, juggling 1000s of fd's just does not seem the right interface. It is too heavy. + Additionally, it _is_ possible to more than one instance and + juggle more than one queue and thus more than one associated fd. There + need not be a one-fd-per-process mapping; it is one-fd-per-queue and a + process can easily want more than one queue. + Q: Why the system call approach? A: The poor user-space interface is the second biggest problem with dnotify. @@ -131,8 +146,6 @@ A: The poor user-space interface is the second biggest problem with dnotify. Obtaining the fd and managing the watches could have been done either via a device file or a family of new system calls. We decided to implement a family of system calls because that is the preffered approach for new kernel - features and it means our user interface requirements. - - Additionally, it _is_ possible to more than one instance and - juggle more than one queue and thus more than one associated fd. + interfaces. The only real difference was whether we wanted to use open(2) + and ioctl(2) or a couple of new system calls. System calls beat ioctls. |