src/runtime/string.go - go - Git at Google

 // Copyright 2014 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.

 package runtime

 import (
 	"runtime/internal/atomic"
 	"unsafe"
 )

 // The constant is known to the compiler.
 // There is no fundamental theory behind this number.
 const tmpStringBufSize = 32

 type tmpBuf [tmpStringBufSize]byte

 // concatstrings implements a Go string concatenation x+y+z+...
 // The operands are passed in the slice a.
 // If buf != nil, the compiler has determined that the result does not
 // escape the calling function, so the string data can be stored in buf
 // if small enough.
 func concatstrings(buf *tmpBuf, a []string) string {
 	idx := 0
 	l := 0
 	count := 0
 	for i, x := range a {
 		n := len(x)
 		if n == 0 {
 			continue
 		}
 		if l+n < l {
 			throw("string concatenation too long")
 		}
 		l += n
 		count++
 		idx = i
 	}
 	if count == 0 {
 		return ""
 	}

 	// If there is just one string and either it is not on the stack
 	// or our result does not escape the calling frame (buf != nil),
 	// then we can return that string directly.
 	if count == 1 && (buf != nil || !stringDataOnStack(a[idx])) {
 		return a[idx]
 	}
 	s, b := rawstringtmp(buf, l)
 	l = 0
 	for _, x := range a {
 		copy(b[l:], x)
 		l += len(x)
 	}
 	return s
 }

 func concatstring2(buf *tmpBuf, a [2]string) string {
 	return concatstrings(buf, a[:])
 }

 func concatstring3(buf *tmpBuf, a [3]string) string {
 	return concatstrings(buf, a[:])
 }

 func concatstring4(buf *tmpBuf, a [4]string) string {
 	return concatstrings(buf, a[:])
 }

 func concatstring5(buf *tmpBuf, a [5]string) string {
 	return concatstrings(buf, a[:])
 }

 // Buf is a fixed-size buffer for the result,
 // it is not nil if the result does not escape.
 func slicebytetostring(buf *tmpBuf, b []byte) string {
 	l := len(b)
 	if l == 0 {
 		// Turns out to be a relatively common case.
 		// Consider that you want to parse out data between parens in "foo()bar",
 		// you find the indices and convert the subslice to string.
 		return ""
 	}
 	if raceenabled && l > 0 {
 		racereadrangepc(unsafe.Pointer(&b[0]),
 			uintptr(l),
 			getcallerpc(unsafe.Pointer(&buf)),
 			funcPC(slicebytetostring))
 	}
 	if msanenabled && l > 0 {
 		msanread(unsafe.Pointer(&b[0]), uintptr(l))
 	}
 	s, c := rawstringtmp(buf, l)
 	copy(c, b)
 	return s
 }

 // stringDataOnStack reports whether the string's data is
 // stored on the current goroutine's stack.
 func stringDataOnStack(s string) bool {
 	ptr := uintptr(stringStructOf(&s).str)
 	stk := getg().stack
 	return stk.lo <= ptr && ptr < stk.hi
 }

 func rawstringtmp(buf *tmpBuf, l int) (s string, b []byte) {
 	if buf != nil && l <= len(buf) {
 		b = buf[:l]
 		s = slicebytetostringtmp(b)
 	} else {
 		s, b = rawstring(l)
 	}
 	return
 }

 func slicebytetostringtmp(b []byte) string {
 	// Return a "string" referring to the actual []byte bytes.
 	// This is only for use by internal compiler optimizations
 	// that know that the string form will be discarded before
 	// the calling goroutine could possibly modify the original
 	// slice or synchronize with another goroutine.
 	// First such case is a m[string(k)] lookup where
 	// m is a string-keyed map and k is a []byte.
 	// Second such case is "<"+string(b)+">" concatenation where b is []byte.
 	// Third such case is string(b)=="foo" comparison where b is []byte.

 	if raceenabled && len(b) > 0 {
 		racereadrangepc(unsafe.Pointer(&b[0]),
 			uintptr(len(b)),
 			getcallerpc(unsafe.Pointer(&b)),
 			funcPC(slicebytetostringtmp))
 	}
 	if msanenabled && len(b) > 0 {
 		msanread(unsafe.Pointer(&b[0]), uintptr(len(b)))
 	}
 	return *(*string)(unsafe.Pointer(&b))
 }

 func stringtoslicebyte(buf *tmpBuf, s string) []byte {
 	var b []byte
 	if buf != nil && len(s) <= len(buf) {
 		*buf = tmpBuf{}
 		b = buf[:len(s)]
 	} else {
 		b = rawbyteslice(len(s))
 	}
 	copy(b, s)
 	return b
 }

 func stringtoslicebytetmp(s string) []byte {
 	// Return a slice referring to the actual string bytes.
 	// This is only for use by internal compiler optimizations
 	// that know that the slice won't be mutated.
 	// The only such case today is:
 	// for i, c := range []byte(str)

 	str := stringStructOf(&s)
 	ret := slice{array: str.str, len: str.len, cap: str.len}
 	return *(*[]byte)(unsafe.Pointer(&ret))
 }

 func stringtoslicerune(buf *[tmpStringBufSize]rune, s string) []rune {
 	// two passes.
 	// unlike slicerunetostring, no race because strings are immutable.
 	n := 0
 	t := s
 	for len(s) > 0 {
 		_, k := charntorune(s)
 		s = s[k:]
 		n++
 	}
 	var a []rune
 	if buf != nil && n <= len(buf) {
 		*buf = [tmpStringBufSize]rune{}
 		a = buf[:n]
 	} else {
 		a = rawruneslice(n)
 	}
 	n = 0
 	for len(t) > 0 {
 		r, k := charntorune(t)
 		t = t[k:]
 		a[n] = r
 		n++
 	}
 	return a
 }

 func slicerunetostring(buf *tmpBuf, a []rune) string {
 	if raceenabled && len(a) > 0 {
 		racereadrangepc(unsafe.Pointer(&a[0]),
 			uintptr(len(a))*unsafe.Sizeof(a[0]),
 			getcallerpc(unsafe.Pointer(&buf)),
 			funcPC(slicerunetostring))
 	}
 	if msanenabled && len(a) > 0 {
 		msanread(unsafe.Pointer(&a[0]), uintptr(len(a))*unsafe.Sizeof(a[0]))
 	}
 	var dum [4]byte
 	size1 := 0
 	for _, r := range a {
 		size1 += runetochar(dum[:], r)
 	}
 	s, b := rawstringtmp(buf, size1+3)
 	size2 := 0
 	for _, r := range a {
 		// check for race
 		if size2 >= size1 {
 			break
 		}
 		size2 += runetochar(b[size2:], r)
 	}
 	return s[:size2]
 }

 type stringStruct struct {
 	str unsafe.Pointer
 	len int
 }

 // Variant with *byte pointer type for DWARF debugging.
 type stringStructDWARF struct {
 	str *byte
 	len int
 }

 func stringStructOf(sp *string) *stringStruct {
 	return (*stringStruct)(unsafe.Pointer(sp))
 }

 func intstring(buf *[4]byte, v int64) string {
 	var s string
 	var b []byte
 	if buf != nil {
 		b = buf[:]
 		s = slicebytetostringtmp(b)
 	} else {
 		s, b = rawstring(4)
 	}
 	if int64(rune(v)) != v {
 		v = runeerror
 	}
 	n := runetochar(b, rune(v))
 	return s[:n]
 }

 // stringiter returns the index of the next
 // rune after the rune that starts at s[k].
 func stringiter(s string, k int) int {
 	if k >= len(s) {
 		// 0 is end of iteration
 		return 0
 	}

 	c := s[k]
 	if c < runeself {
 		return k + 1
 	}

 	// multi-char rune
 	_, n := charntorune(s[k:])
 	return k + n
 }

 // stringiter2 returns the rune that starts at s[k]
 // and the index where the next rune starts.
 func stringiter2(s string, k int) (int, rune) {
 	if k >= len(s) {
 		// 0 is end of iteration
 		return 0, 0
 	}

 	c := s[k]
 	if c < runeself {
 		return k + 1, rune(c)
 	}

 	// multi-char rune
 	r, n := charntorune(s[k:])
 	return k + n, r
 }

 // rawstring allocates storage for a new string. The returned
 // string and byte slice both refer to the same storage.
 // The storage is not zeroed. Callers should use
 // b to set the string contents and then drop b.
 func rawstring(size int) (s string, b []byte) {
 	p := mallocgc(uintptr(size), nil, false)

 	stringStructOf(&s).str = p
 	stringStructOf(&s).len = size

 	*(*slice)(unsafe.Pointer(&b)) = slice{p, size, size}

 	for {
 		ms := maxstring
 		if uintptr(size) <= ms || atomic.Casuintptr((*uintptr)(unsafe.Pointer(&maxstring)), ms, uintptr(size)) {
 			return
 		}
 	}
 }

 // rawbyteslice allocates a new byte slice. The byte slice is not zeroed.
 func rawbyteslice(size int) (b []byte) {
 	cap := roundupsize(uintptr(size))
 	p := mallocgc(cap, nil, false)
 	if cap != uintptr(size) {
 		memclr(add(p, uintptr(size)), cap-uintptr(size))
 	}

 	*(*slice)(unsafe.Pointer(&b)) = slice{p, size, int(cap)}
 	return
 }

 // rawruneslice allocates a new rune slice. The rune slice is not zeroed.
 func rawruneslice(size int) (b []rune) {
 	if uintptr(size) > _MaxMem/4 {
 		throw("out of memory")
 	}
 	mem := roundupsize(uintptr(size) * 4)
 	p := mallocgc(mem, nil, false)
 	if mem != uintptr(size)*4 {
 		memclr(add(p, uintptr(size)*4), mem-uintptr(size)*4)
 	}

 	*(*slice)(unsafe.Pointer(&b)) = slice{p, size, int(mem / 4)}
 	return
 }

 // used by cmd/cgo
 func gobytes(p *byte, n int) []byte {
 	if n == 0 {
 		return make([]byte, 0)
 	}
 	x := make([]byte, n)
 	memmove(unsafe.Pointer(&x[0]), unsafe.Pointer(p), uintptr(n))
 	return x
 }

 func gostring(p *byte) string {
 	l := findnull(p)
 	if l == 0 {
 		return ""
 	}
 	s, b := rawstring(l)
 	memmove(unsafe.Pointer(&b[0]), unsafe.Pointer(p), uintptr(l))
 	return s
 }

 func gostringn(p *byte, l int) string {
 	if l == 0 {
 		return ""
 	}
 	s, b := rawstring(l)
 	memmove(unsafe.Pointer(&b[0]), unsafe.Pointer(p), uintptr(l))
 	return s
 }

 func index(s, t string) int {
 	if len(t) == 0 {
 		return 0
 	}
 	for i := 0; i < len(s); i++ {
 		if s[i] == t[0] && hasprefix(s[i:], t) {
 			return i
 		}
 	}
 	return -1
 }

 func contains(s, t string) bool {
 	return index(s, t) >= 0
 }

 func hasprefix(s, t string) bool {
 	return len(s) >= len(t) && s[:len(t)] == t
 }

 func atoi(s string) int {
 	n := 0
 	for len(s) > 0 && '0' <= s[0] && s[0] <= '9' {
 		n = n*10 + int(s[0]) - '0'
 		s = s[1:]
 	}
 	return n
 }

 //go:nosplit
 func findnull(s *byte) int {
 	if s == nil {
 		return 0
 	}
 	p := (*[_MaxMem/2 - 1]byte)(unsafe.Pointer(s))
 	l := 0
 	for p[l] != 0 {
 		l++
 	}
 	return l
 }

 func findnullw(s *uint16) int {
 	if s == nil {
 		return 0
 	}
 	p := (*[_MaxMem/2/2 - 1]uint16)(unsafe.Pointer(s))
 	l := 0
 	for p[l] != 0 {
 		l++
 	}
 	return l
 }

 var maxstring uintptr = 256 // a hint for print

 //go:nosplit
 func gostringnocopy(str *byte) string {
 	ss := stringStruct{str: unsafe.Pointer(str), len: findnull(str)}
 	s := *(*string)(unsafe.Pointer(&ss))
 	for {
 		ms := maxstring
 		if uintptr(len(s)) <= ms || atomic.Casuintptr(&maxstring, ms, uintptr(len(s))) {
 			break
 		}
 	}
 	return s
 }

 func gostringw(strw *uint16) string {
 	var buf [8]byte
 	str := (*[_MaxMem/2/2 - 1]uint16)(unsafe.Pointer(strw))
 	n1 := 0
 	for i := 0; str[i] != 0; i++ {
 		n1 += runetochar(buf[:], rune(str[i]))
 	}
 	s, b := rawstring(n1 + 4)
 	n2 := 0
 	for i := 0; str[i] != 0; i++ {
 		// check for race
 		if n2 >= n1 {
 			break
 		}
 		n2 += runetochar(b[n2:], rune(str[i]))
 	}
 	b[n2] = 0 // for luck
 	return s[:n2]
 }
	// Copyright 2014 The Go Authors. All rights reserved.
	// Use of this source code is governed by a BSD-style
	// license that can be found in the LICENSE file.

	package runtime

	import (
	"runtime/internal/atomic"
	"unsafe"
	)

	// The constant is known to the compiler.
	// There is no fundamental theory behind this number.
	const tmpStringBufSize = 32

	type tmpBuf [tmpStringBufSize]byte

	// concatstrings implements a Go string concatenation x+y+z+...
	// The operands are passed in the slice a.
	// If buf != nil, the compiler has determined that the result does not
	// escape the calling function, so the string data can be stored in buf
	// if small enough.
	func concatstrings(buf *tmpBuf, a []string) string {
	idx := 0
	l := 0
	count := 0
	for i, x := range a {
	n := len(x)
	if n == 0 {
	continue
	}
	if l+n < l {
	throw("string concatenation too long")
	}
	l += n
	count++
	idx = i
	}
	if count == 0 {
	return ""
	}

	// If there is just one string and either it is not on the stack
	// or our result does not escape the calling frame (buf != nil),
	// then we can return that string directly.
	if count == 1 && (buf != nil \|\| !stringDataOnStack(a[idx])) {
	return a[idx]
	}
	s, b := rawstringtmp(buf, l)
	l = 0
	for _, x := range a {
	copy(b[l:], x)
	l += len(x)
	}
	return s
	}

	func concatstring2(buf *tmpBuf, a [2]string) string {
	return concatstrings(buf, a[:])
	}

	func concatstring3(buf *tmpBuf, a [3]string) string {
	return concatstrings(buf, a[:])
	}

	func concatstring4(buf *tmpBuf, a [4]string) string {
	return concatstrings(buf, a[:])
	}

	func concatstring5(buf *tmpBuf, a [5]string) string {
	return concatstrings(buf, a[:])
	}

	// Buf is a fixed-size buffer for the result,
	// it is not nil if the result does not escape.
	func slicebytetostring(buf *tmpBuf, b []byte) string {
	l := len(b)
	if l == 0 {
	// Turns out to be a relatively common case.
	// Consider that you want to parse out data between parens in "foo()bar",
	// you find the indices and convert the subslice to string.
	return ""
	}
	if raceenabled && l > 0 {
	racereadrangepc(unsafe.Pointer(&b[0]),
	uintptr(l),
	getcallerpc(unsafe.Pointer(&buf)),
	funcPC(slicebytetostring))
	}
	if msanenabled && l > 0 {
	msanread(unsafe.Pointer(&b[0]), uintptr(l))
	}
	s, c := rawstringtmp(buf, l)
	copy(c, b)
	return s
	}

	// stringDataOnStack reports whether the string's data is
	// stored on the current goroutine's stack.
	func stringDataOnStack(s string) bool {
	ptr := uintptr(stringStructOf(&s).str)
	stk := getg().stack
	return stk.lo <= ptr && ptr < stk.hi
	}

	func rawstringtmp(buf *tmpBuf, l int) (s string, b []byte) {
	if buf != nil && l <= len(buf) {
	b = buf[:l]
	s = slicebytetostringtmp(b)
	} else {
	s, b = rawstring(l)
	}
	return
	}

	func slicebytetostringtmp(b []byte) string {
	// Return a "string" referring to the actual []byte bytes.
	// This is only for use by internal compiler optimizations
	// that know that the string form will be discarded before
	// the calling goroutine could possibly modify the original
	// slice or synchronize with another goroutine.
	// First such case is a m[string(k)] lookup where
	// m is a string-keyed map and k is a []byte.
	// Second such case is "<"+string(b)+">" concatenation where b is []byte.
	// Third such case is string(b)=="foo" comparison where b is []byte.

	if raceenabled && len(b) > 0 {
	racereadrangepc(unsafe.Pointer(&b[0]),
	uintptr(len(b)),
	getcallerpc(unsafe.Pointer(&b)),
	funcPC(slicebytetostringtmp))
	}
	if msanenabled && len(b) > 0 {
	msanread(unsafe.Pointer(&b[0]), uintptr(len(b)))
	}
	return (string)(unsafe.Pointer(&b))
	}

	func stringtoslicebyte(buf *tmpBuf, s string) []byte {
	var b []byte
	if buf != nil && len(s) <= len(buf) {
	*buf = tmpBuf{}
	b = buf[:len(s)]
	} else {
	b = rawbyteslice(len(s))
	}
	copy(b, s)
	return b
	}

	func stringtoslicebytetmp(s string) []byte {
	// Return a slice referring to the actual string bytes.
	// This is only for use by internal compiler optimizations
	// that know that the slice won't be mutated.
	// The only such case today is:
	// for i, c := range []byte(str)

	str := stringStructOf(&s)
	ret := slice{array: str.str, len: str.len, cap: str.len}
	return ([]byte)(unsafe.Pointer(&ret))
	}

	func stringtoslicerune(buf *[tmpStringBufSize]rune, s string) []rune {
	// two passes.
	// unlike slicerunetostring, no race because strings are immutable.
	n := 0
	t := s
	for len(s) > 0 {
	_, k := charntorune(s)
	s = s[k:]
	n++
	}
	var a []rune
	if buf != nil && n <= len(buf) {
	*buf = [tmpStringBufSize]rune{}
	a = buf[:n]
	} else {
	a = rawruneslice(n)
	}
	n = 0
	for len(t) > 0 {
	r, k := charntorune(t)
	t = t[k:]
	a[n] = r
	n++
	}
	return a
	}

	func slicerunetostring(buf *tmpBuf, a []rune) string {
	if raceenabled && len(a) > 0 {
	racereadrangepc(unsafe.Pointer(&a[0]),
	uintptr(len(a))*unsafe.Sizeof(a[0]),
	getcallerpc(unsafe.Pointer(&buf)),
	funcPC(slicerunetostring))
	}
	if msanenabled && len(a) > 0 {
	msanread(unsafe.Pointer(&a[0]), uintptr(len(a))*unsafe.Sizeof(a[0]))
	}
	var dum [4]byte
	size1 := 0
	for _, r := range a {
	size1 += runetochar(dum[:], r)
	}
	s, b := rawstringtmp(buf, size1+3)
	size2 := 0
	for _, r := range a {
	// check for race
	if size2 >= size1 {
	break
	}
	size2 += runetochar(b[size2:], r)
	}
	return s[:size2]
	}

	type stringStruct struct {
	str unsafe.Pointer
	len int
	}

	// Variant with *byte pointer type for DWARF debugging.
	type stringStructDWARF struct {
	str *byte
	len int
	}

	func stringStructOf(sp string) stringStruct {
	return (*stringStruct)(unsafe.Pointer(sp))
	}

	func intstring(buf *[4]byte, v int64) string {
	var s string
	var b []byte
	if buf != nil {
	b = buf[:]
	s = slicebytetostringtmp(b)
	} else {
	s, b = rawstring(4)
	}
	if int64(rune(v)) != v {
	v = runeerror
	}
	n := runetochar(b, rune(v))
	return s[:n]
	}

	// stringiter returns the index of the next
	// rune after the rune that starts at s[k].
	func stringiter(s string, k int) int {
	if k >= len(s) {
	// 0 is end of iteration
	return 0
	}

	c := s[k]
	if c < runeself {
	return k + 1
	}

	// multi-char rune
	_, n := charntorune(s[k:])
	return k + n
	}

	// stringiter2 returns the rune that starts at s[k]
	// and the index where the next rune starts.
	func stringiter2(s string, k int) (int, rune) {
	if k >= len(s) {
	// 0 is end of iteration
	return 0, 0
	}

	c := s[k]
	if c < runeself {
	return k + 1, rune(c)
	}

	// multi-char rune
	r, n := charntorune(s[k:])
	return k + n, r
	}

	// rawstring allocates storage for a new string. The returned
	// string and byte slice both refer to the same storage.
	// The storage is not zeroed. Callers should use
	// b to set the string contents and then drop b.
	func rawstring(size int) (s string, b []byte) {
	p := mallocgc(uintptr(size), nil, false)

	stringStructOf(&s).str = p
	stringStructOf(&s).len = size

	(slice)(unsafe.Pointer(&b)) = slice{p, size, size}

	for {
	ms := maxstring
	if uintptr(size) <= ms \|\| atomic.Casuintptr((*uintptr)(unsafe.Pointer(&maxstring)), ms, uintptr(size)) {
	return
	}
	}
	}

	// rawbyteslice allocates a new byte slice. The byte slice is not zeroed.
	func rawbyteslice(size int) (b []byte) {
	cap := roundupsize(uintptr(size))
	p := mallocgc(cap, nil, false)
	if cap != uintptr(size) {
	memclr(add(p, uintptr(size)), cap-uintptr(size))
	}

	(slice)(unsafe.Pointer(&b)) = slice{p, size, int(cap)}
	return
	}

	// rawruneslice allocates a new rune slice. The rune slice is not zeroed.
	func rawruneslice(size int) (b []rune) {
	if uintptr(size) > _MaxMem/4 {
	throw("out of memory")
	}
	mem := roundupsize(uintptr(size) * 4)
	p := mallocgc(mem, nil, false)
	if mem != uintptr(size)*4 {
	memclr(add(p, uintptr(size)4), mem-uintptr(size)4)
	}

	(slice)(unsafe.Pointer(&b)) = slice{p, size, int(mem / 4)}
	return
	}

	// used by cmd/cgo
	func gobytes(p *byte, n int) []byte {
	if n == 0 {
	return make([]byte, 0)
	}
	x := make([]byte, n)
	memmove(unsafe.Pointer(&x[0]), unsafe.Pointer(p), uintptr(n))
	return x
	}

	func gostring(p *byte) string {
	l := findnull(p)
	if l == 0 {
	return ""
	}
	s, b := rawstring(l)
	memmove(unsafe.Pointer(&b[0]), unsafe.Pointer(p), uintptr(l))
	return s
	}

	func gostringn(p *byte, l int) string {
	if l == 0 {
	return ""
	}
	s, b := rawstring(l)
	memmove(unsafe.Pointer(&b[0]), unsafe.Pointer(p), uintptr(l))
	return s
	}

	func index(s, t string) int {
	if len(t) == 0 {
	return 0
	}
	for i := 0; i < len(s); i++ {
	if s[i] == t[0] && hasprefix(s[i:], t) {
	return i
	}
	}
	return -1
	}

	func contains(s, t string) bool {
	return index(s, t) >= 0
	}

	func hasprefix(s, t string) bool {
	return len(s) >= len(t) && s[:len(t)] == t
	}

	func atoi(s string) int {
	n := 0
	for len(s) > 0 && '0' <= s[0] && s[0] <= '9' {
	n = n*10 + int(s[0]) - '0'
	s = s[1:]
	}
	return n
	}

	//go:nosplit
	func findnull(s *byte) int {
	if s == nil {
	return 0
	}
	p := (*[_MaxMem/2 - 1]byte)(unsafe.Pointer(s))
	l := 0
	for p[l] != 0 {
	l++
	}
	return l
	}

	func findnullw(s *uint16) int {
	if s == nil {
	return 0
	}
	p := (*[_MaxMem/2/2 - 1]uint16)(unsafe.Pointer(s))
	l := 0
	for p[l] != 0 {
	l++
	}
	return l
	}

	var maxstring uintptr = 256 // a hint for print

	//go:nosplit
	func gostringnocopy(str *byte) string {
	ss := stringStruct{str: unsafe.Pointer(str), len: findnull(str)}
	s := (string)(unsafe.Pointer(&ss))
	for {
	ms := maxstring
	if uintptr(len(s)) <= ms \|\| atomic.Casuintptr(&maxstring, ms, uintptr(len(s))) {
	break
	}
	}
	return s
	}

	func gostringw(strw *uint16) string {
	var buf [8]byte
	str := (*[_MaxMem/2/2 - 1]uint16)(unsafe.Pointer(strw))
	n1 := 0
	for i := 0; str[i] != 0; i++ {
	n1 += runetochar(buf[:], rune(str[i]))
	}
	s, b := rawstring(n1 + 4)
	n2 := 0
	for i := 0; str[i] != 0; i++ {
	// check for race
	if n2 >= n1 {
	break
	}
	n2 += runetochar(b[n2:], rune(str[i]))
	}
	b[n2] = 0 // for luck
	return s[:n2]
	}