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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
-------

        #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);
                fcntl(fd, F_NOTIFY, DN_MODIFY|DN_CREATE|DN_MULTISHOT);
                /* 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);
                }
        }