f.formatOctal(field[:7], chksum) // Never fails since 128776 < 262143
	field[7] = ' '
}

// computeChecksum computes the checksum for the header block.
// POSIX specifies a sum of the unsigned byte values, but the Sun tar used
// signed byte values.
// We compute and return both.
func (b *block) computeChecksum() (unsigned, signed int64) {
	for i, c := range b {
		if 148 <= i && i < 156 {