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