for _, v := range vectors {
		got, err := formatPAXRecord(v.inKey, v.inVal)
		ok := (err == nil)
		if ok != v.ok {
			if v.ok {
				t.Errorf("formatPAXRecord(%q, %q): got format failure, want success", v.inKey, v.inVal)
			} else {
				t.Errorf("formatPAXRecord(%q, %q): got format success, want failure", v.inKey, v.inVal)
			}
		}
		if got != v.want {
			t.Errorf("formatPAXRecord(%q, %q): got %q, want %q",

	if !ok {
		return "", "", s, ErrHeader
	}

	if !validPAXRecord(k, v) {
		return "", "", s, ErrHeader
	}
	return k, v, rem, nil
}

// formatPAXRecord formats a single PAX record, prefixing it with the
// appropriate length.
func formatPAXRecord(k, v string) (string, error) {
	if !validPAXRecord(k, v) {
		return "", ErrHeader
	}

	const padding = 3 // Extra padding for ' ', '=', and '\n'

	if len(paxHdrs) > 0 || isGlobal {
		// Write each record to a buffer.
		var buf strings.Builder
		// Sort keys for deterministic ordering.
		for _, k := range slices.Sorted(maps.Keys(paxHdrs)) {
			rec, err := formatPAXRecord(k, paxHdrs[k])
			if err != nil {
				return err
			}
			buf.WriteString(rec)
		}

		// Write the extended header file.
		var name string
		var flag byte
		if isGlobal {