[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>

diff --git a/Documentation/filesystems/inotify.txt b/Documentation/filesystems/inotify.txt
index 2c716041f57872bca1cd69f1e72f6b847de29fc3..6d501903f68ed823c12fc788c5f2be6e3d556338 100644 (file)
--- 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>
 
 
 
 
 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
 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>
 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.
 
 
 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
-
-       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.
 
 
 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 
-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);
 
 
 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.
 
 
 
 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.
 
 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.
 
 
 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?
 
 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
    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,
    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
    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.
    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?
 
      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.  ;-)
    - 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.
 
    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.
 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
    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.