/*
 * filewait - wait for an event on a file, then exit.
 * by David Simmons, April 7, 2011
 * http://cafbit.com/entry/file_event_notifications_in_macos
 *
 * This program is in the public domain.
 *
 * Compile this program with the following command:
 *     gcc -o filewait filewait.c
 *
 * This simple utility uses the BSD kqueue kernel facility to wait for
 * an event to occur on the specified file, then exit.  An example use
 * of this utility might be a shell script that automatically compiles
 * a program whenever the user's editor saves the source file.
 *
 * This program has only been tested in MacOS Snow Leopard
 * (Darwin Kernel Version 10.7.0)
 *
 * David Simmons
 * April 7, 2011
 */

#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <sys/types.h>
#include <sys/event.h>
#include <sys/time.h>

int main(int argc, char **argv) {

    /* insist on a single argument -- the file to wait on. */
    if (argc != 2) {
        printf("usage: filewait filename\n");
        exit(1);
    }

    /* allocate a kqueue */
    int kq = kqueue();
    if (kq == -1) {
        perror("kqueue");
        exit(1);
    }

    /* open a read-only file descriptor to the specified file */
    int fd = open(argv[1], O_RDONLY);
    if (fd == -1) {
        perror("open");
        exit(1);
    }

    /* define a kevent structure for waiting on file events */
    struct kevent *ke = calloc(1, sizeof(struct kevent));
    ke->ident = fd;
    ke->filter = EVFILT_VNODE;
    ke->flags = EV_ADD|EV_ENABLE|EV_ONESHOT;
    ke->fflags = NOTE_DELETE|NOTE_WRITE|NOTE_ATTRIB|NOTE_RENAME;
    ke->data = 0;
    ke->udata = 0;

    /* wait indefinitely for an event to occur on the file */
    struct kevent events[1];
    kevent(kq, ke, 1, events, 1, NULL);

    exit(0);
}