func blockPadding(offset int64) (n int64) {
	return -offset & (blockSize - 1)
}

var zeroBlock block

type block [blockSize]byte

// Convert block to any number of formats.
func (b *block) toV7() *headerV7       { return (*headerV7)(b) }
func (b *block) toGNU() *headerGNU     { return (*headerGNU)(b) }
func (b *block) toSTAR() *headerSTAR   { return (*headerSTAR)(b) }

	tw.blk.reset()

	modTime := hdr.ModTime
	if modTime.IsZero() {
		modTime = time.Unix(0, 0)
	}

	v7 := tw.blk.toV7()
	v7.typeFlag()[0] = hdr.Typeflag
	fmtStr(v7.name(), hdr.Name)
	fmtStr(v7.linkName(), hdr.Linkname)
	fmtNum(v7.mode(), hdr.Mode)
	fmtNum(v7.uid(), int64(hdr.Uid))
	fmtNum(v7.gid(), int64(hdr.Gid))

				paxHdrs[paxKey] = formatPAXTime(ts)
			}
		}
		if v, ok := h.PAXRecords[paxKey]; ok && v == formatPAXTime(ts) {
			paxHdrs[paxKey] = v
		}
	}

	// Check basic fields.
	var blk block
	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)

	format := tr.blk.getFormat()
	if format == FormatUnknown {
		return nil, nil, ErrHeader
	}

	var p parser
	hdr := new(Header)

	// Unpack the V7 header.
	v7 := tr.blk.toV7()
	hdr.Typeflag = v7.typeFlag()[0]
	hdr.Name = p.parseString(v7.name())
	hdr.Linkname = p.parseString(v7.linkName())
	hdr.Size = p.parseNumeric(v7.size())
	hdr.Mode = p.parseNumeric(v7.mode())