[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>
This commit is contained in:
Robert Love
Linus Torvalds
parent
35e422c967
commit
6f97933d0f
1 changed files with 45 additions and 32 deletions
|
|
@ -5,14 +5,18 @@
|
|||
|
||||
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)
|
||||
|
||||
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
|
||||
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.
|
||||
|
||||
|
|
|
|||
Loading…
Reference in a new issue