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- Linux Directory Notification
- ============================
-
- Stephen Rothwell <sfr@canb.auug.org.au>
-
-The intention of directory notification is to allow user applications
-to be notified when a directory, or any of the files in it, are changed.
-The basic mechanism involves the application registering for notification
-on a directory using a fcntl(2) call and the notifications themselves
-being delivered using signals.
-
-The application decides which "events" it wants to be notified about.
-The currently defined events are:
-
- DN_ACCESS A file in the directory was accessed (read)
- DN_MODIFY A file in the directory was modified (write,truncate)
- DN_CREATE A file was created in the directory
- DN_DELETE A file was unlinked from directory
- DN_RENAME A file in the directory was renamed
- DN_ATTRIB A file in the directory had its attributes
- changed (chmod,chown)
-
-Usually, the application must reregister after each notification, but
-if DN_MULTISHOT is or'ed with the event mask, then the registration will
-remain until explicitly removed (by registering for no events).
-
-By default, SIGIO will be delivered to the process and no other useful
-information. However, if the F_SETSIG fcntl(2) call is used to let the
-kernel know which signal to deliver, a siginfo structure will be passed to
-the signal handler and the si_fd member of that structure will contain the
-file descriptor associated with the directory in which the event occurred.
-
-Preferably the application will choose one of the real time signals
-(SIGRTMIN + <n>) so that the notifications may be queued. This is
-especially important if DN_MULTISHOT is specified. Note that SIGRTMIN
-is often blocked, so it is better to use (at least) SIGRTMIN + 1.
-
-Implementation expectations (features and bugs :-))
----------------------------
-
-The notification should work for any local access to files even if the
-actual file system is on a remote server. This implies that remote
-access to files served by local user mode servers should be notified.
-Also, remote accesses to files served by a local kernel NFS server should
-be notified.
-
-In order to make the impact on the file system code as small as possible,
-the problem of hard links to files has been ignored. So if a file (x)
-exists in two directories (a and b) then a change to the file using the
-name "a/x" should be notified to a program expecting notifications on
-directory "a", but will not be notified to one expecting notifications on
-directory "b".
-
-Also, files that are unlinked, will still cause notifications in the
-last directory that they were linked to.
-
-Configuration
--------------
-
-Dnotify is controlled via the CONFIG_DNOTIFY configuration option. When
-disabled, fcntl(fd, F_NOTIFY, ...) will return -EINVAL.
-
-Example
--------
-See Documentation/filesystems/dnotify_test.c for an example.
-
-NOTE
-----
-Beginning with Linux 2.6.13, dnotify has been replaced by inotify.
-See Documentation/filesystems/inotify.txt for more information on it.