func (h *headerGNU) v7() *headerV7       { return (*headerV7)(h) }
func (h *headerGNU) magic() []byte       { return h[257:][:6] }
func (h *headerGNU) version() []byte     { return h[263:][:2] }
func (h *headerGNU) userName() []byte    { return h[265:][:32] }
func (h *headerGNU) groupName() []byte   { return h[297:][:32] }
func (h *headerGNU) devMajor() []byte    { return h[329:][:8] }
func (h *headerGNU) devMinor() []byte    { return h[337:][:8] }

	fmtNum(v7.mode(), hdr.Mode)
	fmtNum(v7.uid(), int64(hdr.Uid))
	fmtNum(v7.gid(), int64(hdr.Gid))
	fmtNum(v7.size(), hdr.Size)
	fmtNum(v7.modTime(), modTime.Unix())

	ustar := tw.blk.toUSTAR()
	fmtStr(ustar.userName(), hdr.Uname)
	fmtStr(ustar.groupName(), hdr.Gname)
	fmtNum(ustar.devMajor(), hdr.Devmajor)
	fmtNum(ustar.devMinor(), hdr.Devminor)

	return &tw.blk
}

		// (not implemented on that platform, cgo not enabled, etc).
		if u, ok := userMap.Load(h.Uid); ok {
			h.Uname = u.(string)
		} else if u, err := user.LookupId(strconv.Itoa(h.Uid)); err == nil {
			h.Uname = u.Username
			userMap.Store(h.Uid, h.Uname)
		}
		if g, ok := groupMap.Load(h.Gid); ok {
			h.Gname = g.(string)
		} else if g, err := user.LookupGroupId(strconv.Itoa(h.Gid)); err == nil {
			h.Gname = g.Name

	v7 := blk.toV7()
	ustar := blk.toUSTAR()
	gnu := blk.toGNU()
	verifyString(h.Name, len(v7.name()), "Name", paxPath)
	verifyString(h.Linkname, len(v7.linkName()), "Linkname", paxLinkpath)
	verifyString(h.Uname, len(ustar.userName()), "Uname", paxUname)
	verifyString(h.Gname, len(ustar.groupName()), "Gname", paxGname)
	verifyNumeric(h.Mode, len(v7.mode()), "Mode", paxNone)
	verifyNumeric(int64(h.Uid), len(v7.uid()), "Uid", paxUid)

	hdr.ModTime = time.Unix(p.parseNumeric(v7.modTime()), 0)

	// Unpack format specific fields.
	if format > formatV7 {
		ustar := tr.blk.toUSTAR()
		hdr.Uname = p.parseString(ustar.userName())
		hdr.Gname = p.parseString(ustar.groupName())
		hdr.Devmajor = p.parseNumeric(ustar.devMajor())
		hdr.Devminor = p.parseNumeric(ustar.devMinor())

		var prefix string
		switch {