code: func unlockpt(f *os.File) error {
  	var u int32
  	return ioctl(f.Fd(), unix.TIOCPTYUNLK, uintptr(unsafe.Pointer(&u)))
  }
docs: // unlockpt unlocks the slave pseudoterminal device corresponding to the master pseudoterminal referred to by f.
  // unlockpt should be called before opening the slave side of a pty.