x                  A pointer to an object of type DIR.
    NULL               Indicates an error; errno is set to one of the
                       following values:

                       o  EACCES - Search permission is denied
                          for any component of dir_name or read
                          permission is denied for dir_name.

                       o  ENAMETOOLONG - The length of the dir_name
                          string exceeds PATH_MAX, or a pathname
                          component is longer than NAME_MAX.

                       o  ENOENT - The dir_name argument points to
                          the name of a file that does not exist, or
                          is an empty string.