func (b *block) getFormat() Format {
	// Verify checksum.
	var p parser
	value := p.parseOctal(b.toV7().chksum())
	chksum1, chksum2 := b.computeChecksum()
	if p.err != nil || (value != chksum1 && value != chksum2) {
		return FormatUnknown
	}

	// Guess the magic values.
	magic := string(b.toUSTAR().magic())
	version := string(b.toUSTAR().version())

		if s.Flags&SHF_COMPRESSED == 0 {
			s.ReaderAt = s.sr
			s.Size = s.FileSize
		} else {
			// Read the compression header.
			switch f.Class {
			case ELFCLASS32:
				var ch Chdr32
				chdata := make([]byte, unsafe.Sizeof(ch))
				if _, err := s.sr.ReadAt(chdata, 0); err != nil {
					return nil, err
				}
				s.compressionType = CompressionType(bo.Uint32(chdata[unsafe.Offsetof(ch.Type):]))