fanotify.c

#define _GNU_SOURCE
#include <errno.h>
#include <fcntl.h>
#include <limits.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <sys/fanotify.h>
#include <unistd.h>

#define BUF_SIZE 256

int
main(int argc, char **argv)
{
    int fd, ret, event_fd;
    ssize_t len, path_len;
    char path[PATH_MAX];
    char procfd_path[PATH_MAX];
    char events_buf[BUF_SIZE];
    struct file_handle *file_handle;
    struct fanotify_event_metadata *metadata;
    struct fanotify_event_info_fid *fid;

    if (argc != 2) {
	fprintf(stderr, "Invalid number of command line arguments.\n");
	exit(EXIT_FAILURE);
    }

    /* Create an fanotify file descriptor with FAN_REPORT_FID as a flag
       so that program can receive fid events. */

    fd = fanotify_init(FAN_CLASS_NOTIF | FAN_REPORT_FID, 0);
    if (fd == -1) {
	perror("fanotify_init");
	exit(EXIT_FAILURE);
    }

    /* Place a mark on the filesystem object supplied in argv[1]. */

    ret = fanotify_mark(fd, FAN_MARK_ADD | FAN_MARK_ONLYDIR,
			FAN_CREATE | FAN_ONDIR,
			AT_FDCWD, argv[1]);
    if (ret == -1) {
	perror("fanotify_mark");
	exit(EXIT_FAILURE);
    }

    printf("Listening for events.\n");

    /* Read events from the event queue into a buffer */

    len = read(fd, (void *) &events_buf, sizeof(events_buf));
    if (len == -1 && errno != EAGAIN) {
	perror("read");
	exit(EXIT_FAILURE);
    }

    /* Process all events within the buffer */

    for (metadata = (struct fanotify_event_metadata *) events_buf;
	 FAN_EVENT_OK(metadata, len);
	 metadata = FAN_EVENT_NEXT(metadata, len)) {
	fid = (struct fanotify_event_info_fid *) (metadata + 1);
	file_handle = (struct file_handle *) fid->handle;

	/* Ensure that the event info is of the correct type */

	if (fid->hdr.info_type != FAN_EVENT_INFO_TYPE_FID) {
	    fprintf(stderr, "Received unexpected event info type.\n");
	    exit(EXIT_FAILURE);
	}

	if (metadata->mask == FAN_CREATE)
	    printf("FAN_CREATE (file created):");

	if (metadata->mask == FAN_CREATE | FAN_ONDIR)
	    printf("FAN_CREATE | FAN_ONDIR (subdirectory created):");

	/* metadata->fd is set to FAN_NOFD when FAN_REPORT_FID is enabled.
	   To obtain a file descriptor for the file object corresponding to
	   an event you can use the struct file_handle that's provided
	   within the fanotify_event_info_fid in conjunction with the
	   open_by_handle_at(2) system call. A check for ESTALE is done
	   to accommodate for the situation where the file handle for the
	   object was deleted prior to this system call. */

	event_fd = open_by_handle_at(AT_FDCWD, file_handle, O_RDONLY);
	if (ret == -1) {
	    if (errno == ESTALE) {
		printf("File handle is no longer valid. "
		       "File has been deleted\n");
		continue;
	    } else {
		perror("open_by_handle_at");
		exit(EXIT_FAILURE);
	    }
	}

	snprintf(procfd_path, sizeof(procfd_path), "/proc/self/fd/%d",
		 event_fd);

	/* Retrieve and print the path of the modified dentry */

	path_len = readlink(procfd_path, path, sizeof(path) - 1);
	if (path_len == -1) {
	    perror("readlink");
	    exit(EXIT_FAILURE);
	}

	path[path_len] = '\0';
	printf("\tDirectory '%s' has been modified.\n", path);

	/* Close associated file descriptor for this event */

	close(event_fd);
    }

    printf("All events processed successfully. Program exiting.\n");
    exit(EXIT_SUCCESS);
}