Source File
	type.go

Belonging Package
	runtime

// Copyright 2009 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.

// Runtime type representation.

package runtime

import (
	"internal/abi"
	"unsafe"
)

// tflag is documented in reflect/type.go.
//
// tflag values must be kept in sync with copies in:
//	cmd/compile/internal/reflectdata/reflect.go
//	cmd/link/internal/ld/decodesym.go
//	reflect/type.go
//      internal/reflectlite/type.go
type tflag uint8

const (
	tflagUncommon      tflag = 1 << 0
	tflagExtraStar     tflag = 1 << 1
	tflagNamed         tflag = 1 << 2
	tflagRegularMemory tflag = 1 << 3 // equal and hash can treat values of this type as a single region of t.size bytes
)

// Needs to be in sync with ../cmd/link/internal/ld/decodesym.go:/^func.commonsize,
// ../cmd/compile/internal/reflectdata/reflect.go:/^func.dcommontype and
// ../reflect/type.go:/^type.rtype.
// ../internal/reflectlite/type.go:/^type.rtype.
type _type struct {
	size       uintptr
	ptrdata    uintptr // size of memory prefix holding all pointers
	hash       uint32
	tflag      tflag
	align      uint8
	fieldAlign uint8
	kind       uint8
	// function for comparing objects of this type
	// (ptr to object A, ptr to object B) -> ==?
	equal func(unsafe.Pointer, unsafe.Pointer) bool
	// gcdata stores the GC type data for the garbage collector.
	// If the KindGCProg bit is set in kind, gcdata is a GC program.
	// Otherwise it is a ptrmask bitmap. See mbitmap.go for details.
	gcdata    *byte
	str       nameOff
	ptrToThis typeOff
}

func (t *_type) string() string {
	s := t.nameOff(t.str).name()
	if t.tflag&tflagExtraStar != 0 {
		return s[1:]
	}
	return s
}

func (t *_type) uncommon() *uncommontype {
	if t.tflag&tflagUncommon == 0 {
		return nil
	}
	switch t.kind & kindMask {
	case kindStruct:
		type u struct {
			structtype
			u uncommontype
		}
		return &(*u)(unsafe.Pointer(t)).u
	case kindPtr:
		type u struct {
			ptrtype
			u uncommontype
		}
		return &(*u)(unsafe.Pointer(t)).u
	case kindFunc:
		type u struct {
			functype
			u uncommontype
		}
		return &(*u)(unsafe.Pointer(t)).u
	case kindSlice:
		type u struct {
			slicetype
			u uncommontype
		}
		return &(*u)(unsafe.Pointer(t)).u
	case kindArray:
		type u struct {
			arraytype
			u uncommontype
		}
		return &(*u)(unsafe.Pointer(t)).u
	case kindChan:
		type u struct {
			chantype
			u uncommontype
		}
		return &(*u)(unsafe.Pointer(t)).u
	case kindMap:
		type u struct {
			maptype
			u uncommontype
		}
		return &(*u)(unsafe.Pointer(t)).u
	case kindInterface:
		type u struct {
			interfacetype
			u uncommontype
		}
		return &(*u)(unsafe.Pointer(t)).u
	default:
		type u struct {
			_type
			u uncommontype
		}
		return &(*u)(unsafe.Pointer(t)).u
	}
}

func (t *_type) name() string {
	if t.tflag&tflagNamed == 0 {
		return ""
	}
	s := t.string()
	i := len(s) - 1
	for i >= 0 && s[i] != '.' {
		i--
	}
	return s[i+1:]
}

// pkgpath returns the path of the package where t was defined, if
// available. This is not the same as the reflect package's PkgPath
// method, in that it returns the package path for struct and interface
// types, not just named types.
func (t *_type) pkgpath() string {
	if u := t.uncommon(); u != nil {
		return t.nameOff(u.pkgpath).name()
	}
	switch t.kind & kindMask {
	case kindStruct:
		st := (*structtype)(unsafe.Pointer(t))
		return st.pkgPath.name()
	case kindInterface:
		it := (*interfacetype)(unsafe.Pointer(t))
		return it.pkgpath.name()
	}
	return ""
}

// reflectOffs holds type offsets defined at run time by the reflect package.
//
// When a type is defined at run time, its *rtype data lives on the heap.
// There are a wide range of possible addresses the heap may use, that
// may not be representable as a 32-bit offset. Moreover the GC may
// one day start moving heap memory, in which case there is no stable
// offset that can be defined.
//
// To provide stable offsets, we add pin *rtype objects in a global map
// and treat the offset as an identifier. We use negative offsets that
// do not overlap with any compile-time module offsets.
//
// Entries are created by reflect.addReflectOff.
var reflectOffs struct {
	lock mutex
	next int32
	m    map[int32]unsafe.Pointer
	minv map[unsafe.Pointer]int32
}

func reflectOffsLock() {
	lock(&reflectOffs.lock)
	if raceenabled {
		raceacquire(unsafe.Pointer(&reflectOffs.lock))
	}
}

func reflectOffsUnlock() {
	if raceenabled {
		racerelease(unsafe.Pointer(&reflectOffs.lock))
	}
	unlock(&reflectOffs.lock)
}

func resolveNameOff(ptrInModule unsafe.Pointer, off nameOff) name {
	if off == 0 {
		return name{}
	}
	base := uintptr(ptrInModule)
	for md := &firstmoduledata; md != nil; md = md.next {
		if base >= md.types && base < md.etypes {
			res := md.types + uintptr(off)
			if res > md.etypes {
				println("runtime: nameOff", hex(off), "out of range", hex(md.types), "-", hex(md.etypes))
				throw("runtime: name offset out of range")
			}
			return name{(*byte)(unsafe.Pointer(res))}
		}
	}

	// No module found. see if it is a run time name.
	reflectOffsLock()
	res, found := reflectOffs.m[int32(off)]
	reflectOffsUnlock()
	if !found {
		println("runtime: nameOff", hex(off), "base", hex(base), "not in ranges:")
		for next := &firstmoduledata; next != nil; next = next.next {
			println("\ttypes", hex(next.types), "etypes", hex(next.etypes))
		}
		throw("runtime: name offset base pointer out of range")
	}
	return name{(*byte)(res)}
}

func (t *_type) nameOff(off nameOff) name {
	return resolveNameOff(unsafe.Pointer(t), off)
}

func resolveTypeOff(ptrInModule unsafe.Pointer, off typeOff) *_type {
	if off == 0 || off == -1 {
		// -1 is the sentinel value for unreachable code.
		// See cmd/link/internal/ld/data.go:relocsym.
		return nil
	}
	base := uintptr(ptrInModule)
	var md *moduledata
	for next := &firstmoduledata; next != nil; next = next.next {
		if base >= next.types && base < next.etypes {
			md = next
			break
		}
	}
	if md == nil {
		reflectOffsLock()
		res := reflectOffs.m[int32(off)]
		reflectOffsUnlock()
		if res == nil {
			println("runtime: typeOff", hex(off), "base", hex(base), "not in ranges:")
			for next := &firstmoduledata; next != nil; next = next.next {
				println("\ttypes", hex(next.types), "etypes", hex(next.etypes))
			}
			throw("runtime: type offset base pointer out of range")
		}
		return (*_type)(res)
	}
	if t := md.typemap[off]; t != nil {
		return t
	}
	res := md.types + uintptr(off)
	if res > md.etypes {
		println("runtime: typeOff", hex(off), "out of range", hex(md.types), "-", hex(md.etypes))
		throw("runtime: type offset out of range")
	}
	return (*_type)(unsafe.Pointer(res))
}

func (t *_type) typeOff(off typeOff) *_type {
	return resolveTypeOff(unsafe.Pointer(t), off)
}

func (t *_type) textOff(off textOff) unsafe.Pointer {
	if off == -1 {
		// -1 is the sentinel value for unreachable code.
		// See cmd/link/internal/ld/data.go:relocsym.
		return unsafe.Pointer(abi.FuncPCABIInternal(unreachableMethod))
	}
	base := uintptr(unsafe.Pointer(t))
	var md *moduledata
	for next := &firstmoduledata; next != nil; next = next.next {
		if base >= next.types && base < next.etypes {
			md = next
			break
		}
	}
	if md == nil {
		reflectOffsLock()
		res := reflectOffs.m[int32(off)]
		reflectOffsUnlock()
		if res == nil {
			println("runtime: textOff", hex(off), "base", hex(base), "not in ranges:")
			for next := &firstmoduledata; next != nil; next = next.next {
				println("\ttypes", hex(next.types), "etypes", hex(next.etypes))
			}
			throw("runtime: text offset base pointer out of range")
		}
		return res
	}
	res := md.textAddr(uint32(off))
	return unsafe.Pointer(res)
}

func (t *functype) in() []*_type {
	// See funcType in reflect/type.go for details on data layout.
	uadd := uintptr(unsafe.Sizeof(functype{}))
	if t.typ.tflag&tflagUncommon != 0 {
		uadd += unsafe.Sizeof(uncommontype{})
	}
	return (*[1 << 20]*_type)(add(unsafe.Pointer(t), uadd))[:t.inCount]
}

func (t *functype) out() []*_type {
	// See funcType in reflect/type.go for details on data layout.
	uadd := uintptr(unsafe.Sizeof(functype{}))
	if t.typ.tflag&tflagUncommon != 0 {
		uadd += unsafe.Sizeof(uncommontype{})
	}
	outCount := t.outCount & (1<<15 - 1)
	return (*[1 << 20]*_type)(add(unsafe.Pointer(t), uadd))[t.inCount : t.inCount+outCount]
}

func (t *functype) dotdotdot() bool {
	return t.outCount&(1<<15) != 0
}

type nameOff int32
type typeOff int32
type textOff int32

type method struct {
	name nameOff
	mtyp typeOff
	ifn  textOff
	tfn  textOff
}

type uncommontype struct {
	pkgpath nameOff
	mcount  uint16 // number of methods
	xcount  uint16 // number of exported methods
	moff    uint32 // offset from this uncommontype to [mcount]method
	_       uint32 // unused
}

type imethod struct {
	name nameOff
	ityp typeOff
}

type interfacetype struct {
	typ     _type
	pkgpath name
	mhdr    []imethod
}

type maptype struct {
	typ    _type
	key    *_type
	elem   *_type
	bucket *_type // internal type representing a hash bucket
	// function for hashing keys (ptr to key, seed) -> hash
	hasher     func(unsafe.Pointer, uintptr) uintptr
	keysize    uint8  // size of key slot
	elemsize   uint8  // size of elem slot
	bucketsize uint16 // size of bucket
	flags      uint32
}

// Note: flag values must match those used in the TMAP case
// in ../cmd/compile/internal/reflectdata/reflect.go:writeType.
func (mt *maptype) indirectkey() bool { // store ptr to key instead of key itself
	return mt.flags&1 != 0
}
func (mt *maptype) indirectelem() bool { // store ptr to elem instead of elem itself
	return mt.flags&2 != 0
}
func (mt *maptype) reflexivekey() bool { // true if k==k for all keys
	return mt.flags&4 != 0
}
func (mt *maptype) needkeyupdate() bool { // true if we need to update key on an overwrite
	return mt.flags&8 != 0
}
func (mt *maptype) hashMightPanic() bool { // true if hash function might panic
	return mt.flags&16 != 0
}

type arraytype struct {
	typ   _type
	elem  *_type
	slice *_type
	len   uintptr
}

type chantype struct {
	typ  _type
	elem *_type
	dir  uintptr
}

type slicetype struct {
	typ  _type
	elem *_type
}

type functype struct {
	typ      _type
	inCount  uint16
	outCount uint16
}

type ptrtype struct {
	typ  _type
	elem *_type
}

type structfield struct {
	name       name
	typ        *_type
	offsetAnon uintptr
}

func (f *structfield) offset() uintptr {
	return f.offsetAnon >> 1
}

type structtype struct {
	typ     _type
	pkgPath name
	fields  []structfield
}

// name is an encoded type name with optional extra data.
// See reflect/type.go for details.
type name struct {
	bytes *byte
}

func (n name) data(off int) *byte {
	return (*byte)(add(unsafe.Pointer(n.bytes), uintptr(off)))
}

func (n name) isExported() bool {
	return (*n.bytes)&(1<<0) != 0
}

func (n name) readvarint(off int) (int, int) {
	v := 0
	for i := 0; ; i++ {
		x := *n.data(off + i)
		v += int(x&0x7f) << (7 * i)
		if x&0x80 == 0 {
			return i + 1, v
		}
	}
}

func (n name) name() (s string) {
	if n.bytes == nil {
		return ""
	}
	i, l := n.readvarint(1)
	if l == 0 {
		return ""
	}
	hdr := (*stringStruct)(unsafe.Pointer(&s))
	hdr.str = unsafe.Pointer(n.data(1 + i))
	hdr.len = l
	return
}

func (n name) tag() (s string) {
	if *n.data(0)&(1<<1) == 0 {
		return ""
	}
	i, l := n.readvarint(1)
	i2, l2 := n.readvarint(1 + i + l)
	hdr := (*stringStruct)(unsafe.Pointer(&s))
	hdr.str = unsafe.Pointer(n.data(1 + i + l + i2))
	hdr.len = l2
	return
}

func (n name) pkgPath() string {
	if n.bytes == nil || *n.data(0)&(1<<2) == 0 {
		return ""
	}
	i, l := n.readvarint(1)
	off := 1 + i + l
	if *n.data(0)&(1<<1) != 0 {
		i2, l2 := n.readvarint(off)
		off += i2 + l2
	}
	var nameOff nameOff
	copy((*[4]byte)(unsafe.Pointer(&nameOff))[:], (*[4]byte)(unsafe.Pointer(n.data(off)))[:])
	pkgPathName := resolveNameOff(unsafe.Pointer(n.bytes), nameOff)
	return pkgPathName.name()
}

func (n name) isBlank() bool {
	if n.bytes == nil {
		return false
	}
	_, l := n.readvarint(1)
	return l == 1 && *n.data(2) == '_'
}

// typelinksinit scans the types from extra modules and builds the
// moduledata typemap used to de-duplicate type pointers.
func typelinksinit() {
	if firstmoduledata.next == nil {
		return
	}
	typehash := make(map[uint32][]*_type, len(firstmoduledata.typelinks))

	modules := activeModules()
	prev := modules[0]
	for _, md := range modules[1:] {
		// Collect types from the previous module into typehash.
	collect:
		for _, tl := range prev.typelinks {
			var t *_type
			if prev.typemap == nil {
				t = (*_type)(unsafe.Pointer(prev.types + uintptr(tl)))
			} else {
				t = prev.typemap[typeOff(tl)]
			}
			// Add to typehash if not seen before.
			tlist := typehash[t.hash]
			for _, tcur := range tlist {
				if tcur == t {
					continue collect
				}
			}
			typehash[t.hash] = append(tlist, t)
		}

		if md.typemap == nil {
			// If any of this module's typelinks match a type from a
			// prior module, prefer that prior type by adding the offset
			// to this module's typemap.
			tm := make(map[typeOff]*_type, len(md.typelinks))
			pinnedTypemaps = append(pinnedTypemaps, tm)
			md.typemap = tm
			for _, tl := range md.typelinks {
				t := (*_type)(unsafe.Pointer(md.types + uintptr(tl)))
				for _, candidate := range typehash[t.hash] {
					seen := map[_typePair]struct{}{}
					if typesEqual(t, candidate, seen) {
						t = candidate
						break
					}
				}
				md.typemap[typeOff(tl)] = t
			}
		}

		prev = md
	}
}

type _typePair struct {
	t1 *_type
	t2 *_type
}

// typesEqual reports whether two types are equal.
//
// Everywhere in the runtime and reflect packages, it is assumed that
// there is exactly one *_type per Go type, so that pointer equality
// can be used to test if types are equal. There is one place that
// breaks this assumption: buildmode=shared. In this case a type can
// appear as two different pieces of memory. This is hidden from the
// runtime and reflect package by the per-module typemap built in
// typelinksinit. It uses typesEqual to map types from later modules
// back into earlier ones.
//
// Only typelinksinit needs this function.
func typesEqual(t, v *_type, seen map[_typePair]struct{}) bool {
	tp := _typePair{t, v}
	if _, ok := seen[tp]; ok {
		return true
	}

	// mark these types as seen, and thus equivalent which prevents an infinite loop if
	// the two types are identical, but recursively defined and loaded from
	// different modules
	seen[tp] = struct{}{}

	if t == v {
		return true
	}
	kind := t.kind & kindMask
	if kind != v.kind&kindMask {
		return false
	}
	if t.string() != v.string() {
		return false
	}
	ut := t.uncommon()
	uv := v.uncommon()
	if ut != nil || uv != nil {
		if ut == nil || uv == nil {
			return false
		}
		pkgpatht := t.nameOff(ut.pkgpath).name()
		pkgpathv := v.nameOff(uv.pkgpath).name()
		if pkgpatht != pkgpathv {
			return false
		}
	}
	if kindBool <= kind && kind <= kindComplex128 {
		return true
	}
	switch kind {
	case kindString, kindUnsafePointer:
		return true
	case kindArray:
		at := (*arraytype)(unsafe.Pointer(t))
		av := (*arraytype)(unsafe.Pointer(v))
		return typesEqual(at.elem, av.elem, seen) && at.len == av.len
	case kindChan:
		ct := (*chantype)(unsafe.Pointer(t))
		cv := (*chantype)(unsafe.Pointer(v))
		return ct.dir == cv.dir && typesEqual(ct.elem, cv.elem, seen)
	case kindFunc:
		ft := (*functype)(unsafe.Pointer(t))
		fv := (*functype)(unsafe.Pointer(v))
		if ft.outCount != fv.outCount || ft.inCount != fv.inCount {
			return false
		}
		tin, vin := ft.in(), fv.in()
		for i := 0; i < len(tin); i++ {
			if !typesEqual(tin[i], vin[i], seen) {
				return false
			}
		}
		tout, vout := ft.out(), fv.out()
		for i := 0; i < len(tout); i++ {
			if !typesEqual(tout[i], vout[i], seen) {
				return false
			}
		}
		return true
	case kindInterface:
		it := (*interfacetype)(unsafe.Pointer(t))
		iv := (*interfacetype)(unsafe.Pointer(v))
		if it.pkgpath.name() != iv.pkgpath.name() {
			return false
		}
		if len(it.mhdr) != len(iv.mhdr) {
			return false
		}
		for i := range it.mhdr {
			tm := &it.mhdr[i]
			vm := &iv.mhdr[i]
			// Note the mhdr array can be relocated from
			// another module. See #17724.
			tname := resolveNameOff(unsafe.Pointer(tm), tm.name)
			vname := resolveNameOff(unsafe.Pointer(vm), vm.name)
			if tname.name() != vname.name() {
				return false
			}
			if tname.pkgPath() != vname.pkgPath() {
				return false
			}
			tityp := resolveTypeOff(unsafe.Pointer(tm), tm.ityp)
			vityp := resolveTypeOff(unsafe.Pointer(vm), vm.ityp)
			if !typesEqual(tityp, vityp, seen) {
				return false
			}
		}
		return true
	case kindMap:
		mt := (*maptype)(unsafe.Pointer(t))
		mv := (*maptype)(unsafe.Pointer(v))
		return typesEqual(mt.key, mv.key, seen) && typesEqual(mt.elem, mv.elem, seen)
	case kindPtr:
		pt := (*ptrtype)(unsafe.Pointer(t))
		pv := (*ptrtype)(unsafe.Pointer(v))
		return typesEqual(pt.elem, pv.elem, seen)
	case kindSlice:
		st := (*slicetype)(unsafe.Pointer(t))
		sv := (*slicetype)(unsafe.Pointer(v))
		return typesEqual(st.elem, sv.elem, seen)
	case kindStruct:
		st := (*structtype)(unsafe.Pointer(t))
		sv := (*structtype)(unsafe.Pointer(v))
		if len(st.fields) != len(sv.fields) {
			return false
		}
		if st.pkgPath.name() != sv.pkgPath.name() {
			return false
		}
		for i := range st.fields {
			tf := &st.fields[i]
			vf := &sv.fields[i]
			if tf.name.name() != vf.name.name() {
				return false
			}
			if !typesEqual(tf.typ, vf.typ, seen) {
				return false
			}
			if tf.name.tag() != vf.name.tag() {
				return false
			}
			if tf.offsetAnon != vf.offsetAnon {
				return false
			}
		}
		return true
	default:
		println("runtime: impossible type kind", kind)
		throw("runtime: impossible type kind")
		return false
	}
}

The pages are generated with Golds v0.4.9. (GOOS=linux GOARCH=amd64)