1		Linux Directory Notification
2		============================
3
4	   Stephen Rothwell <sfr@canb.auug.org.au>
5
6The intention of directory notification is to allow user applications
7to be notified when a directory, or any of the files in it, are changed.
8The basic mechanism involves the application registering for notification
9on a directory using a fcntl(2) call and the notifications themselves
10being delivered using signals.
11
12The application decides which "events" it wants to be notified about.
13The currently defined events are:
14
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)
22
23Usually, the application must reregister after each notification, but
24if DN_MULTISHOT is or'ed with the event mask, then the registration will
25remain until explicitly removed (by registering for no events).
26
27By default, SIGIO will be delivered to the process and no other useful
28information.  However, if the F_SETSIG fcntl(2) call is used to let the
29kernel know which signal to deliver, a siginfo structure will be passed to
30the signal handler and the si_fd member of that structure will contain the
31file descriptor associated with the directory in which the event occurred.
32
33Preferably the application will choose one of the real time signals
34(SIGRTMIN + <n>) so that the notifications may be queued.  This is
35especially important if DN_MULTISHOT is specified.
36
37Implementation expectations (features and bugs :-))
38---------------------------
39
40The notification should work for any local access to files even if the
41actual file system is on a remote server.  This implies that remote
42access to files served by local user mode servers should be notified.
43Also, remote accesses to files served by a local kernel NFS server should
44be notified.
45
46In order to make the impact on the file system code as small as possible,
47the problem of hard links to files has been ignored.  So if a file (x)
48exists in two directories (a and b) then a change to the file using the
49name "a/x" should be notified to a program expecting notifications on
50directory "a", but will not be notified to one expecting notifications on
51directory "b".
52
53Also, files that are unlinked, will still cause notifications in the
54last directory that they were linked to.
55
56Example
57-------
58
59	#define _GNU_SOURCE	/* needed to get the defines */
60	#include <fcntl.h>	/* in glibc 2.2 this has the needed
61					   values defined */
62	#include <signal.h>
63	#include <stdio.h>
64	#include <unistd.h>
65	
66	static volatile int event_fd;
67	
68	static void handler(int sig, siginfo_t *si, void *data)
69	{
70		event_fd = si->si_fd;
71	}
72	
73	int main(void)
74	{
75		struct sigaction act;
76		int fd;
77		
78		act.sa_sigaction = handler;
79		sigemptyset(&act.sa_mask);
80		act.sa_flags = SA_SIGINFO;
81		sigaction(SIGRTMIN, &act, NULL);
82		
83		fd = open(".", O_RDONLY);
84		fcntl(fd, F_SETSIG, SIGRTMIN);
85		fcntl(fd, F_NOTIFY, DN_MODIFY|DN_CREATE|DN_MULTISHOT);
86		/* we will now be notified if any of the files
87		   in "." is modified or new files are created */
88		while (1) {
89			pause();
90			printf("Got event on fd=%d\n", event_fd);
91		}
92	}
93