/* cc -o t-journal-tail t-journal-tail.c -lsystemd-journal
 */

#include <stdio.h>
#include <stdlib.h>
#include <systemd/sd-journal.h>

void
check (int r)
{
  if (r < 0)
    {
      fprintf (stderr, "%s\n", strerror(-r));
      exit (1);
    }
}

void
checkv (const char *label, int r)
{
  if (r < 0)
    {
      fprintf (stderr, "%s: %s\n", label, strerror(-r));
      exit (1);
    }
  fprintf (stderr, "%s: %d\n", label, r);
}

char *
dump (sd_journal *j, int count)
{
  uint64_t last_usec = 0;
  char *cursor;
  int i = 0;

  while (i < count) {
    const void *data;
    size_t size;
    uint64_t usec;
    int r;

    check (sd_journal_get_realtime_usec (j, &usec));
    check (sd_journal_get_data (j, "MESSAGE", &data, &size));
    // printf ("%lld %.*s\n", usec, size, data);

    if (usec < last_usec)
      printf ("back\n");
    last_usec = usec;

    i++;
    if (i >= count)
      break;

    check (r = sd_journal_next (j));
    if (r == 0) {
        printf("%d entries, EOF\n", i);
        return NULL;
    }
  }

  check (sd_journal_get_cursor (j, &cursor));

  printf("%d entries, %s\n", i, cursor);
  return cursor;
}

void
match (sd_journal *j, const char *m)
{
  check (sd_journal_add_match (j, m, strlen (m)));
}

void
main ()
{
  sd_journal *j;

  check (sd_journal_open_directory (&j, "/home/mvo/work/XXX/var/log/journal", 0));
  match (j, "_SYSTEMD_UNIT=NetworkManager.service");

  check (sd_journal_seek_tail (j));
  check (sd_journal_previous_skip (j, 10));

  while (1) {
    char *c = dump (j, 2000);
    if (c == NULL)
      break;
    check (sd_journal_seek_cursor (j, c));
    check (sd_journal_next (j));
    check (sd_journal_test_cursor (j, c));
    free (c);
  }

}