Source: string.go in package runtime


package runtime

import (
	"internal/bytealg"
	"runtime/internal/sys"
	"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)
	for _, x := range a {
		copy(b, x)
		b = b[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[:])
}

slicebytetostring converts a byte slice to a string. It is inserted by the compiler into generated code. ptr is a pointer to the first element of the slice; n is the length of the slice. Buf is a fixed-size buffer for the result, it is not nil if the result does not escape.

func slicebytetostring(buf *tmpBuf, ptr *byte, n int) (str string) {

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 {
		racereadrangepc(unsafe.Pointer(ptr),
			uintptr(n),
			getcallerpc(),
			funcPC(slicebytetostring))
	}
	if msanenabled {
		msanread(unsafe.Pointer(ptr), uintptr(n))
	}
	if n == 1 {
		p := unsafe.Pointer(&staticuint64s[*ptr])
		if sys.BigEndian {
			p = add(p, 7)
		}
		stringStructOf(&str).str = p
		stringStructOf(&str).len = 1
		return
	}

	var p unsafe.Pointer
	if buf != nil && n <= len(buf) {
		p = unsafe.Pointer(buf)
	} else {
		p = mallocgc(uintptr(n), nil, false)
	}
	stringStructOf(&str).str = p
	stringStructOf(&str).len = n
	memmove(p, unsafe.Pointer(ptr), uintptr(n))
	return
}

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[0], len(b))
	} else {
		s, b = rawstring(l)
	}
	return
}

slicebytetostringtmp returns a "string" referring to the actual []byte bytes. Callers need to ensure that the returned string will not be used after the calling goroutine modifies the original slice or synchronizes with another goroutine. The function is only called when instrumenting and otherwise intrinsified by the compiler. Some internal compiler optimizations use this function. - Used for m[T1{... Tn{..., string(k), ...} ...}] and m[string(k)] where k is []byte, T1 to Tn is a nesting of struct and array literals. - Used for "<"+string(b)+">" concatenation where b is []byte. - Used for string(b)=="foo" comparison where b is []byte.

func slicebytetostringtmp(ptr *byte, n int) (str string) {
	if raceenabled && n > 0 {
		racereadrangepc(unsafe.Pointer(ptr),
			uintptr(n),
			getcallerpc(),
			funcPC(slicebytetostringtmp))
	}
	if msanenabled && n > 0 {
		msanread(unsafe.Pointer(ptr), uintptr(n))
	}
	stringStructOf(&str).str = unsafe.Pointer(ptr)
	stringStructOf(&str).len = n
	return
}

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
}

two passes. unlike slicerunetostring, no race because strings are immutable.

	n := 0
	for range s {
		n++
	}

	var a []rune
	if buf != nil && n <= len(buf) {
		*buf = [tmpStringBufSize]rune{}
		a = buf[:n]
	} else {
		a = rawruneslice(n)
	}

	n = 0
	for _, r := range s {
		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(),
			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 += encoderune(dum[:], r)
	}
	s, b := rawstringtmp(buf, size1+3)
	size2 := 0

check for race

		if size2 >= size1 {
			break
		}
		size2 += encoderune(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) (s string) {
	var b []byte
	if buf != nil {
		b = buf[:]
		s = slicebytetostringtmp(&b[0], len(b))
	} else {
		s, b = rawstring(4)
	}
	if int64(rune(v)) != v {
		v = runeError
	}
	n := encoderune(b, rune(v))
	return s[:n]
}

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}

	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) {
		memclrNoHeapPointers(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) > maxAlloc/4 {
		throw("out of memory")
	}
	mem := roundupsize(uintptr(size) * 4)
	p := mallocgc(mem, nil, false)
	if mem != uintptr(size)*4 {
		memclrNoHeapPointers(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) (b []byte) {
	if n == 0 {
		return make([]byte, 0)
	}

	if n < 0 || uintptr(n) > maxAlloc {
		panic(errorString("gobytes: length out of range"))
	}

	bp := mallocgc(uintptr(n), nil, false)
	memmove(bp, unsafe.Pointer(p), uintptr(n))

	*(*slice)(unsafe.Pointer(&b)) = slice{bp, n, n}
	return
}

This is exported via linkname to assembly in syscall (for Plan9).go:linkname gostring

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 hasPrefix(s, prefix string) bool {
	return len(s) >= len(prefix) && s[:len(prefix)] == prefix
}

const (
	maxUint = ^uint(0)
	maxInt  = int(maxUint >> 1)
)

atoi parses an int from a string s. The bool result reports whether s is a number representable by a value of type int.

func atoi(s string) (int, bool) {
	if s == "" {
		return 0, false
	}

	neg := false
	if s[0] == '-' {
		neg = true
		s = s[1:]
	}

	un := uint(0)
	for i := 0; i < len(s); i++ {
		c := s[i]
		if c < '0' || c > '9' {
			return 0, false
		}

overflow

			return 0, false
		}
		un *= 10
		un1 := un + uint(c) - '0'

overflow

			return 0, false
		}
		un = un1
	}

	if !neg && un > uint(maxInt) {
		return 0, false
	}
	if neg && un > uint(maxInt)+1 {
		return 0, false
	}

	n := int(un)
	if neg {
		n = -n
	}

	return n, true
}

atoi32 is like atoi but for integers that fit into an int32.

func atoi32(s string) (int32, bool) {
	if n, ok := atoi(s); n == int(int32(n)) {
		return int32(n), ok
	}
	return 0, false
}

go:nosplit

func findnull(s *byte) int {
	if s == nil {
		return 0
	}

Avoid IndexByteString on Plan 9 because it uses SSE instructions on x86 machines, and those are classified as floating point instructions, which are illegal in a note handler.

	if GOOS == "plan9" {
		p := (*[maxAlloc/2 - 1]byte)(unsafe.Pointer(s))
		l := 0
		for p[l] != 0 {
			l++
		}
		return l
	}

pageSize is the unit we scan at a time looking for NULL. It must be the minimum page size for any architecture Go runs on. It's okay (just a minor performance loss) if the actual system page size is larger than this value.

	const pageSize = 4096

	offset := 0

IndexByteString uses wide reads, so we need to be careful with page boundaries. Call IndexByteString on [ptr, endOfPage) interval.

	safeLen := int(pageSize - uintptr(ptr)%pageSize)

	for {

Check one page at a time.

		if i := bytealg.IndexByteString(t, 0); i != -1 {
			return offset + i

Move to next page

		ptr = unsafe.Pointer(uintptr(ptr) + uintptr(safeLen))
		offset += safeLen
		safeLen = pageSize
	}
}

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

go:nosplit

func gostringnocopy(str *byte) string {
	ss := stringStruct{str: unsafe.Pointer(str), len: findnull(str)}
	s := *(*string)(unsafe.Pointer(&ss))
	return s
}

func gostringw(strw *uint16) string {
	var buf [8]byte
	str := (*[maxAlloc/2/2 - 1]uint16)(unsafe.Pointer(strw))
	n1 := 0
	for i := 0; str[i] != 0; i++ {
		n1 += encoderune(buf[:], rune(str[i]))
	}
	s, b := rawstring(n1 + 4)
	n2 := 0

check for race

		if n2 >= n1 {
			break
		}
		n2 += encoderune(b[n2:], rune(str[i]))
	}
	b[n2] = 0 // for luck
	return s[:n2]