dump addresses from btmp via stdin

#define _GNU_SOURCE

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <stddef.h>
#include <utmp.h>
#include <arpa/inet.h>

const int utmp_len = sizeof(struct utmp);

int main(int argc, const char * argv[]) {
  struct utmp z;
  const int32_t * x = z.ut_addr_v6;
  char buf[INET6_ADDRSTRLEN];
  const char *text;

  while(read(STDIN_FILENO, &z, utmp_len) == utmp_len) {
    text = inet_ntop(
      (x[1] || x[2] || x[3]) ? (AF_INET6) : (AF_INET),
      x,
      buf, sizeof(buf));
    printf("%s\n", text);
  }
  return 0;
}