1 Linux Directory Notification
2 ============================
3
4 Stephen Rothwell <sfr@linuxcare.com.au>
5
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.
11
12 The application decides which "events" it wants to be notified about.
13 The 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
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).
26
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 direcory in which the event occured.
32
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.
36
37 Implementation expectations (features and bugs :-))
38 ---------------------------
39
40 The notification should work for any local access to files even if the
41 actual file system is on a remote server. This implies that remote
42 access to files served by local user mode servers should be notified.
43 Also, remote accesses to files served by a local kernel NFS server should
44 be notified.
45
46 In order to make the impact on the file system code as small as possible,
47 the problem of hard links to files has been ignored. So if a file (x)
48 exists in two directories (a and b) then a change to the file using the
49 name "a/x" should be notified to a program expecting notifications on
50 directory "a", but will not be notified to one expecting notifications on
51 directory "b".
52
53 Also, files that are unlinked, will still cause notifications in the
54 last directory that they were linked to.
55
56 Example
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 }
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