// Exclude TypeLink and TypeSymlink, since they may reference directories.
		if strings.HasSuffix(h.Name, "/") {
			return FormatUnknown, nil, headerError{"filename may not have trailing slash"}
		}
	case TypeXHeader, TypeGNULongName, TypeGNULongLink:
		return FormatUnknown, nil, headerError{"cannot manually encode TypeXHeader, TypeGNULongName, or TypeGNULongLink headers"}
	case TypeXGlobalHeader:

// Constants to identify various tar formats.
const (
	// Deliberately hide the meaning of constants from public API.
	_ Format = (1 << iota) / 4 // Sequence of 0, 0, 1, 2, 4, 8, etc...

	// FormatUnknown indicates that the format is unknown.
	FormatUnknown

	// The format of the original Unix V7 tar tool prior to standardization.
	formatV7

	// FormatUSTAR represents the USTAR header format defined in POSIX.1-1988.
	//

			if bytes.IndexFunc(tr.blk[:], notASCII) >= 0 {
				hdr.Format = FormatUnknown // Non-ASCII characters in block.
			}
			nul := func(b []byte) bool { return int(b[len(b)-1]) == 0 }
			if !(nul(v7.size()) && nul(v7.mode()) && nul(v7.uid()) && nul(v7.gid()) &&
				nul(v7.modTime()) && nul(ustar.devMajor()) && nul(ustar.devMinor())) {
				hdr.Format = FormatUnknown // Numeric fields must end in NUL
			}
		case format.has(formatSTAR):

pkg archive/tar, const FormatPAX = 4
pkg archive/tar, const FormatPAX Format
pkg archive/tar, const FormatUSTAR = 2
pkg archive/tar, const FormatUSTAR Format
pkg archive/tar, const FormatUnknown = 0
pkg archive/tar, const FormatUnknown Format
pkg archive/tar, method (Format) String() string
pkg archive/tar, type Format int
pkg archive/tar, type Header struct, Format Format
pkg archive/tar, type Header struct, PAXRecords map[string]string

		// Write the initial GNU header.
		var blk block
		gnu := blk.toGNU()
		sparse := gnu.sparse()
		copy(gnu.realSize(), size)
		sps = populateSparseMap(sparse, sps)
		if format != FormatUnknown {
			blk.setFormat(format)
		}
		out = append(out, blk[:]...)

		// Write extended sparse blocks.
		for len(sps) > 0 {
			var blk block
			sps = populateSparseMap(blk.toSparse(), sps)

	// This ensures nominal usage of WriteHeader (without specifying the format)
	// does not always result in the PAX format being chosen, which
	// causes a 1KiB increase to every header.
	if tw.hdr.Format == FormatUnknown {
		tw.hdr.ModTime = tw.hdr.ModTime.Round(time.Second)
		tw.hdr.AccessTime = time.Time{}
		tw.hdr.ChangeTime = time.Time{}
	}

	allowedFormats, paxHdrs, err := tw.hdr.allowedFormats()
	switch {