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Documentation / filesystems / dnotify.txt

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Based on kernel version 4.15. Page generated on 2018-01-29 10:00 EST.

1			Linux Directory Notification
2			============================
4		   Stephen Rothwell <sfr@canb.auug.org.au>
6	The intention of directory notification is to allow user applications
7	to be notified when a directory, or any of the files in it, are changed.
8	The basic mechanism involves the application registering for notification
9	on a directory using a fcntl(2) call and the notifications themselves
10	being delivered using signals.
12	The application decides which "events" it wants to be notified about.
13	The currently defined events are:
15		DN_ACCESS	A file in the directory was accessed (read)
16		DN_MODIFY	A file in the directory was modified (write,truncate)
17		DN_CREATE	A file was created in the directory
18		DN_DELETE	A file was unlinked from directory
19		DN_RENAME	A file in the directory was renamed
20		DN_ATTRIB	A file in the directory had its attributes
21				changed (chmod,chown)
23	Usually, the application must reregister after each notification, but
24	if DN_MULTISHOT is or'ed with the event mask, then the registration will
25	remain until explicitly removed (by registering for no events).
27	By default, SIGIO will be delivered to the process and no other useful
28	information.  However, if the F_SETSIG fcntl(2) call is used to let the
29	kernel know which signal to deliver, a siginfo structure will be passed to
30	the signal handler and the si_fd member of that structure will contain the
31	file descriptor associated with the directory in which the event occurred.
33	Preferably the application will choose one of the real time signals
34	(SIGRTMIN + <n>) so that the notifications may be queued.  This is
35	especially important if DN_MULTISHOT is specified.  Note that SIGRTMIN
36	is often blocked, so it is better to use (at least) SIGRTMIN + 1.
38	Implementation expectations (features and bugs :-))
39	---------------------------
41	The notification should work for any local access to files even if the
42	actual file system is on a remote server.  This implies that remote
43	access to files served by local user mode servers should be notified.
44	Also, remote accesses to files served by a local kernel NFS server should
45	be notified.
47	In order to make the impact on the file system code as small as possible,
48	the problem of hard links to files has been ignored.  So if a file (x)
49	exists in two directories (a and b) then a change to the file using the
50	name "a/x" should be notified to a program expecting notifications on
51	directory "a", but will not be notified to one expecting notifications on
52	directory "b".
54	Also, files that are unlinked, will still cause notifications in the
55	last directory that they were linked to.
57	Configuration
58	-------------
60	Dnotify is controlled via the CONFIG_DNOTIFY configuration option.  When
61	disabled, fcntl(fd, F_NOTIFY, ...) will return -EINVAL.
63	Example
64	-------
65	See tools/testing/selftests/filesystems/dnotify_test.c for an example.
68	----
69	Beginning with Linux 2.6.13, dnotify has been replaced by inotify.
70	See Documentation/filesystems/inotify.txt for more information on it.
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