[PATCH] inotify: documentation update

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>

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.

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?

   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,

   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.

     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.  ;-)

   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.

   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.