const char buf[] = "hello world";
  int fd;
  int fd2;
  FILE *f;

  /* Prepare a seekable file. */
  fd = open (NAME, O_RDWR | O_CREAT | O_TRUNC, 0600);
  assert (0 <= fd);
  assert (write (fd, buf, sizeof buf) == sizeof buf);
  assert (lseek (fd, 1, SEEK_SET) == 1);

  /* Create a second handle visiting the file */
  fd2 = dup (fd);
  assert (0 <= fd2);
  f = fdopen (fd2, "w"); // stream offset of f must be at 1
  assert (f);
  assert(lseek(fd, 4, SEEK_SET) == 4); // fd is now the active handle
  assert (fclose (f) == 0); // The f/fd2 handle was never repositioned or made active
  assert (lseek (fd, 0, SEEK_CUR) == 4); // so fd should still be at 4

the file offset of the underlying open file description shall be set to
the file position of the stream if the stream is the active handle to the
underlying file description.