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[PATCH] inotify: documentation update

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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>
This commit is contained in:
Robert Love 2005-07-15 03:56:33 -07:00 committed by Linus Torvalds
parent 35e422c967
commit 6f97933d0f
1 changed files with 45 additions and 32 deletions
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77
Documentation/filesystems/inotify.txt
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@ -1,18 +1,22 @@
inotify inotify
a powerful yet simple file change notification system a powerful yet simple file change notification system
Document started 15 Mar 2005 by Robert Love <rml@novell.com> Document started 15 Mar 2005 by Robert Love <rml@novell.com>
(i) User Interface (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 (); 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 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 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> 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. 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); int wd = inotify_add_watch (fd, path, mask);
You can add a large number of files via something like 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>).
for each file to watch {
int wd = inotify_add_watch (fd, file, mask);
}
You can update an existing watch in the same manner, by passing in a new mask. 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) 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 from a given inotify instance. The filename is of dynamic length and follows
struct. It is of size len. The filename is padded with null bytes to ensure the struct. It is of size len. The filename is padded with null bytes to
proper alignment. This padding is reflected in len. ensure proper alignment. This padding is reflected in len.
You can slurp multiple events by passing a large buffer, for example You can slurp multiple events by passing a large buffer, for example
size_t len = read (fd, buf, BUF_LEN); 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. All watches are destroyed and cleaned up on close.
(ii) Internal Kernel Implementation (ii)
Each open inotify instance is associated with an inotify_device structure. Prototypes:
int inotify_init (void);
int inotify_add_watch (int fd, const char *path, __u32 mask);
int inotify_rm_watch (int fd, __u32 mask);
(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 Each watch is associated with an inotify_watch structure. Watches are chained
off of each associated device and each associated inode. 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. 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 Q: What is the design decision behind not tying the watch to the open fd of
the watched object? 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 This solves the primary problem with dnotify: keeping the file open pins
the file and thus, worse, pins the mount. Dnotify is therefore infeasible 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 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? an fd-per-watch?
A: An fd-per-watch quickly consumes more file descriptors than are allowed, 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. can use epoll, but requiring both is a silly and extraneous requirement.
A watch consumes less memory than an open file, separating the number A watch consumes less memory than an open file, separating the number
spaces is thus sensible. The current design is what user-space developers 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 want: Users initialize inotify, once, and add n watches, requiring but one
and no twiddling with fd limits. Initializing an inotify instance two fd and no twiddling with fd limits. Initializing an inotify instance two
thousand times is silly. If we can implement user-space's preferences 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 cleanly--and we can, the idr layer makes stuff like this trivial--then we
should. 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 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? 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. - No way to get out of band data.
- 1024 is still too low. ;-) - 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 scales to 1000s of directories, juggling 1000s of fd's just does not seem
the right interface. It is too heavy. 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? Q: Why the system call approach?
A: The poor user-space interface is the second biggest problem with dnotify. 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 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 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 family of system calls because that is the preffered approach for new kernel
features and it means our user interface requirements. 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.
Additionally, it _is_ possible to more than one instance and
juggle more than one queue and thus more than one associated fd.