func (w *window) appendTo(buf []byte, from, to uint32) []byte {
	dataLen := uint32(len(w.data))
	from += uint32(w.off)
	to += uint32(w.off)

	wrap := false
	if from > dataLen {
		from -= dataLen
		wrap = !wrap
	}
	if to > dataLen {
		to -= dataLen
		wrap = !wrap
	}

	if wrap {
		buf = append(buf, w.data[from:]...)
		return append(buf, w.data[:to]...)

// structure, taking into account any necessary alignment.
func CmsgLen(datalen int) int {
	return cmsgAlignOf(SizeofCmsghdr) + datalen
}

// CmsgSpace returns the number of bytes an ancillary element with
// payload of the passed data length occupies.
func CmsgSpace(datalen int) int {
	return cmsgAlignOf(SizeofCmsghdr) + cmsgAlignOf(datalen)
}

func (h *Cmsghdr) data(offset uintptr) unsafe.Pointer {