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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.
+Dnotify is controlled via the CONFIG_DNOTIFY configuration option. When
+disabled, fcntl(fd, F_NOTIFY, ...) will return -EINVAL.
+ #define _GNU_SOURCE /* needed to get the defines */
+ #include <fcntl.h> /* in glibc 2.2 this has the needed
+ values defined */
+ #include <signal.h>
+ #include <stdio.h>
+ #include <unistd.h>
+ static volatile int event_fd;
+ static void handler(int sig, siginfo_t *si, void *data)
+ {
+ event_fd = si->si_fd;
+ }
+ int main(void)
+ {
+ struct sigaction act;
+ int fd;
+ act.sa_sigaction = handler;
+ sigemptyset(&act.sa_mask);
+ act.sa_flags = SA_SIGINFO;
+ sigaction(SIGRTMIN + 1, &act, NULL);
+ fd = open(".", O_RDONLY);
+ fcntl(fd, F_SETSIG, SIGRTMIN + 1);
+ /* we will now be notified if any of the files
+ in "." is modified or new files are created */
+ while (1) {
+ pause();
+ printf("Got event on fd=%d\n", event_fd);
+ }
+ }