if (!_M_file)
      return 0.0;

    const int fd = fileno(static_cast<FILE*>(_M_file));
    if (fd < 0)
      return 0.0;

    int ent;
    if (ioctl(fd, RNDGETENTCNT, &ent) < 0)
      return 0.0;

    if (ent < 0)
      return 0.0;

    const int max = sizeof(result_type) * __CHAR_BIT__;
    if (ent > max)
      ent = max;

    return static_cast<double>(ent);