Source File
	type.go

Belonging Package
	debug/dwarf

// 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.

// DWARF type information structures.
// The format is heavily biased toward C, but for simplicity
// the String methods use a pseudo-Go syntax.

package dwarf

import "strconv"

// A Type conventionally represents a pointer to any of the
// specific Type structures ([CharType], [StructType], etc.).
type Type interface {
	Common() *CommonType
	String() string
	Size() int64
}

// A CommonType holds fields common to multiple types.
// If a field is not known or not applicable for a given type,
// the zero value is used.
type CommonType struct {
	ByteSize int64  // size of value of this type, in bytes
	Name     string // name that can be used to refer to type
}

func (c *CommonType) Common() *CommonType { return c }

func (c *CommonType) Size() int64 { return c.ByteSize }

// Basic types

// A BasicType holds fields common to all basic types.
//
// See the documentation for [StructField] for more info on the interpretation of
// the BitSize/BitOffset/DataBitOffset fields.
type BasicType struct {
	CommonType
	BitSize       int64
	BitOffset     int64
	DataBitOffset int64
}

func (b *BasicType) Basic() *BasicType { return b }

func (t *BasicType) String() string {
	if t.Name != "" {
		return t.Name
	}
	return "?"
}

// A CharType represents a signed character type.
type CharType struct {
	BasicType
}

// A UcharType represents an unsigned character type.
type UcharType struct {
	BasicType
}

// An IntType represents a signed integer type.
type IntType struct {
	BasicType
}

// A UintType represents an unsigned integer type.
type UintType struct {
	BasicType
}

// A FloatType represents a floating point type.
type FloatType struct {
	BasicType
}

// A ComplexType represents a complex floating point type.
type ComplexType struct {
	BasicType
}

// A BoolType represents a boolean type.
type BoolType struct {
	BasicType
}

// An AddrType represents a machine address type.
type AddrType struct {
	BasicType
}

// An UnspecifiedType represents an implicit, unknown, ambiguous or nonexistent type.
type UnspecifiedType struct {
	BasicType
}

// qualifiers

// A QualType represents a type that has the C/C++ "const", "restrict", or "volatile" qualifier.
type QualType struct {
	CommonType
	Qual string
	Type Type
}

func (t *QualType) String() string { return t.Qual + " " + t.Type.String() }

func (t *QualType) Size() int64 { return t.Type.Size() }

// An ArrayType represents a fixed size array type.
type ArrayType struct {
	CommonType
	Type          Type
	StrideBitSize int64 // if > 0, number of bits to hold each element
	Count         int64 // if == -1, an incomplete array, like char x[].
}

func (t *ArrayType) String() string {
	return "[" + strconv.FormatInt(t.Count, 10) + "]" + t.Type.String()
}

func (t *ArrayType) Size() int64 {
	if t.Count == -1 {
		return 0
	}
	return t.Count * t.Type.Size()
}

// A VoidType represents the C void type.
type VoidType struct {
	CommonType
}

func (t *VoidType) String() string { return "void" }

// A PtrType represents a pointer type.
type PtrType struct {
	CommonType
	Type Type
}

func (t *PtrType) String() string { return "*" + t.Type.String() }

// A StructType represents a struct, union, or C++ class type.
type StructType struct {
	CommonType
	StructName string
	Kind       string // "struct", "union", or "class".
	Field      []*StructField
	Incomplete bool // if true, struct, union, class is declared but not defined
}

// A StructField represents a field in a struct, union, or C++ class type.
//
// # Bit Fields
//
// The BitSize, BitOffset, and DataBitOffset fields describe the bit
// size and offset of data members declared as bit fields in C/C++
// struct/union/class types.
//
// BitSize is the number of bits in the bit field.
//
// DataBitOffset, if non-zero, is the number of bits from the start of
// the enclosing entity (e.g. containing struct/class/union) to the
// start of the bit field. This corresponds to the DW_AT_data_bit_offset
// DWARF attribute that was introduced in DWARF 4.
//
// BitOffset, if non-zero, is the number of bits between the most
// significant bit of the storage unit holding the bit field to the
// most significant bit of the bit field. Here "storage unit" is the
// type name before the bit field (for a field "unsigned x:17", the
// storage unit is "unsigned"). BitOffset values can vary depending on
// the endianness of the system. BitOffset corresponds to the
// DW_AT_bit_offset DWARF attribute that was deprecated in DWARF 4 and
// removed in DWARF 5.
//
// At most one of DataBitOffset and BitOffset will be non-zero;
// DataBitOffset/BitOffset will only be non-zero if BitSize is
// non-zero. Whether a C compiler uses one or the other
// will depend on compiler vintage and command line options.
//
// Here is an example of C/C++ bit field use, along with what to
// expect in terms of DWARF bit offset info. Consider this code:
//
//	struct S {
//		int q;
//		int j:5;
//		int k:6;
//		int m:5;
//		int n:8;
//	} s;
//
// For the code above, one would expect to see the following for
// DW_AT_bit_offset values (using GCC 8):
//
//	       Little   |     Big
//	       Endian   |    Endian
//	                |
//	"j":     27     |     0
//	"k":     21     |     5
//	"m":     16     |     11
//	"n":     8      |     16
//
// Note that in the above the offsets are purely with respect to the
// containing storage unit for j/k/m/n -- these values won't vary based
// on the size of prior data members in the containing struct.
//
// If the compiler emits DW_AT_data_bit_offset, the expected values
// would be:
//
//	"j":     32
//	"k":     37
//	"m":     43
//	"n":     48
//
// Here the value 32 for "j" reflects the fact that the bit field is
// preceded by other data members (recall that DW_AT_data_bit_offset
// values are relative to the start of the containing struct). Hence
// DW_AT_data_bit_offset values can be quite large for structs with
// many fields.
//
// DWARF also allow for the possibility of base types that have
// non-zero bit size and bit offset, so this information is also
// captured for base types, but it is worth noting that it is not
// possible to trigger this behavior using mainstream languages.
type StructField struct {
	Name          string
	Type          Type
	ByteOffset    int64
	ByteSize      int64 // usually zero; use Type.Size() for normal fields
	BitOffset     int64
	DataBitOffset int64
	BitSize       int64 // zero if not a bit field
}

func (t *StructType) String() string {
	if t.StructName != "" {
		return t.Kind + " " + t.StructName
	}
	return t.Defn()
}

func (f *StructField) bitOffset() int64 {
	if f.BitOffset != 0 {
		return f.BitOffset
	}
	return f.DataBitOffset
}

func (t *StructType) Defn() string {
	s := t.Kind
	if t.StructName != "" {
		s += " " + t.StructName
	}
	if t.Incomplete {
		s += " /*incomplete*/"
		return s
	}
	s += " {"
	for i, f := range t.Field {
		if i > 0 {
			s += "; "
		}
		s += f.Name + " " + f.Type.String()
		s += "@" + strconv.FormatInt(f.ByteOffset, 10)
		if f.BitSize > 0 {
			s += " : " + strconv.FormatInt(f.BitSize, 10)
			s += "@" + strconv.FormatInt(f.bitOffset(), 10)
		}
	}
	s += "}"
	return s
}

// An EnumType represents an enumerated type.
// The only indication of its native integer type is its ByteSize
// (inside [CommonType]).
type EnumType struct {
	CommonType
	EnumName string
	Val      []*EnumValue
}

// An EnumValue represents a single enumeration value.
type EnumValue struct {
	Name string
	Val  int64
}

func (t *EnumType) String() string {
	s := "enum"
	if t.EnumName != "" {
		s += " " + t.EnumName
	}
	s += " {"
	for i, v := range t.Val {
		if i > 0 {
			s += "; "
		}
		s += v.Name + "=" + strconv.FormatInt(v.Val, 10)
	}
	s += "}"
	return s
}

// A FuncType represents a function type.
type FuncType struct {
	CommonType
	ReturnType Type
	ParamType  []Type
}

func (t *FuncType) String() string {
	s := "func("
	for i, t := range t.ParamType {
		if i > 0 {
			s += ", "
		}
		s += t.String()
	}
	s += ")"
	if t.ReturnType != nil {
		s += " " + t.ReturnType.String()
	}
	return s
}

// A DotDotDotType represents the variadic ... function parameter.
type DotDotDotType struct {
	CommonType
}

func (t *DotDotDotType) String() string { return "..." }

// A TypedefType represents a named type.
type TypedefType struct {
	CommonType
	Type Type
}

func (t *TypedefType) String() string { return t.Name }

func (t *TypedefType) Size() int64 { return t.Type.Size() }

// An UnsupportedType is a placeholder returned in situations where we
// encounter a type that isn't supported.
type UnsupportedType struct {
	CommonType
	Tag Tag
}

func (t *UnsupportedType) String() string {
	if t.Name != "" {
		return t.Name
	}
	return t.Name + "(unsupported type " + t.Tag.String() + ")"
}

// typeReader is used to read from either the info section or the
// types section.
type typeReader interface {
	Seek(Offset)
	Next() (*Entry, error)
	clone() typeReader
	offset() Offset
	// AddressSize returns the size in bytes of addresses in the current
	// compilation unit.
	AddressSize() int
}

// Type reads the type at off in the DWARF “info” section.
func (d *Data) Type(off Offset) (Type, error) {
	return d.readType("info", d.Reader(), off, d.typeCache, nil)
}

type typeFixer struct {
	typedefs   []*TypedefType
	arraytypes []*Type
}

func (tf *typeFixer) recordArrayType(t *Type) {
	if t == nil {
		return
	}
	_, ok := (*t).(*ArrayType)
	if ok {
		tf.arraytypes = append(tf.arraytypes, t)
	}
}

func (tf *typeFixer) apply() {
	for _, t := range tf.typedefs {
		t.Common().ByteSize = t.Type.Size()
	}
	for _, t := range tf.arraytypes {
		zeroArray(t)
	}
}

// readType reads a type from r at off of name. It adds types to the
// type cache, appends new typedef types to typedefs, and computes the
// sizes of types. Callers should pass nil for typedefs; this is used
// for internal recursion.
func (d *Data) readType(name string, r typeReader, off Offset, typeCache map[Offset]Type, fixups *typeFixer) (Type, error) {
	if t, ok := typeCache[off]; ok {
		return t, nil
	}
	r.Seek(off)
	e, err := r.Next()
	if err != nil {
		return nil, err
	}
	addressSize := r.AddressSize()
	if e == nil || e.Offset != off {
		return nil, DecodeError{name, off, "no type at offset"}
	}

	// If this is the root of the recursion, prepare to resolve
	// typedef sizes and perform other fixups once the recursion is
	// done. This must be done after the type graph is constructed
	// because it may need to resolve cycles in a different order than
	// readType encounters them.
	if fixups == nil {
		var fixer typeFixer
		defer func() {
			fixer.apply()
		}()
		fixups = &fixer
	}

	// Parse type from Entry.
	// Must always set typeCache[off] before calling
	// d.readType recursively, to handle circular types correctly.
	var typ Type

	nextDepth := 0

	// Get next child; set err if error happens.
	next := func() *Entry {
		if !e.Children {
			return nil
		}
		// Only return direct children.
		// Skip over composite entries that happen to be nested
		// inside this one. Most DWARF generators wouldn't generate
		// such a thing, but clang does.
		// See golang.org/issue/6472.
		for {
			kid, err1 := r.Next()
			if err1 != nil {
				err = err1
				return nil
			}
			if kid == nil {
				err = DecodeError{name, r.offset(), "unexpected end of DWARF entries"}
				return nil
			}
			if kid.Tag == 0 {
				if nextDepth > 0 {
					nextDepth--
					continue
				}
				return nil
			}
			if kid.Children {
				nextDepth++
			}
			if nextDepth > 0 {
				continue
			}
			return kid
		}
	}

	// Get Type referred to by Entry's AttrType field.
	// Set err if error happens. Not having a type is an error.
	typeOf := func(e *Entry) Type {
		tval := e.Val(AttrType)
		var t Type
		switch toff := tval.(type) {
		case Offset:
			if t, err = d.readType(name, r.clone(), toff, typeCache, fixups); err != nil {
				return nil
			}
		case uint64:
			if t, err = d.sigToType(toff); err != nil {
				return nil
			}
		default:
			// It appears that no Type means "void".
			return new(VoidType)
		}
		return t
	}

	switch e.Tag {
	case TagArrayType:
		// Multi-dimensional array.  (DWARF v2 §5.4)
		// Attributes:
		//	AttrType:subtype [required]
		//	AttrStrideSize: size in bits of each element of the array
		//	AttrByteSize: size of entire array
		// Children:
		//	TagSubrangeType or TagEnumerationType giving one dimension.
		//	dimensions are in left to right order.
		t := new(ArrayType)
		typ = t
		typeCache[off] = t
		if t.Type = typeOf(e); err != nil {
			goto Error
		}
		t.StrideBitSize, _ = e.Val(AttrStrideSize).(int64)

		// Accumulate dimensions,
		var dims []int64
		for kid := next(); kid != nil; kid = next() {
			// TODO(rsc): Can also be TagEnumerationType
			// but haven't seen that in the wild yet.
			switch kid.Tag {
			case TagSubrangeType:
				count, ok := kid.Val(AttrCount).(int64)
				if !ok {
					// Old binaries may have an upper bound instead.
					count, ok = kid.Val(AttrUpperBound).(int64)
					if ok {
						count++ // Length is one more than upper bound.
					} else if len(dims) == 0 {
						count = -1 // As in x[].
					}
				}
				dims = append(dims, count)
			case TagEnumerationType:
				err = DecodeError{name, kid.Offset, "cannot handle enumeration type as array bound"}
				goto Error
			}
		}
		if len(dims) == 0 {
			// LLVM generates this for x[].
			dims = []int64{-1}
		}

		t.Count = dims[0]
		for i := len(dims) - 1; i >= 1; i-- {
			t.Type = &ArrayType{Type: t.Type, Count: dims[i]}
		}

	case TagBaseType:
		// Basic type.  (DWARF v2 §5.1)
		// Attributes:
		//	AttrName: name of base type in programming language of the compilation unit [required]
		//	AttrEncoding: encoding value for type (encFloat etc) [required]
		//	AttrByteSize: size of type in bytes [required]
		//	AttrBitOffset: bit offset of value within containing storage unit
		//	AttrDataBitOffset: bit offset of value within containing storage unit
		//	AttrBitSize: size in bits
		//
		// For most languages BitOffset/DataBitOffset/BitSize will not be present
		// for base types.
		name, _ := e.Val(AttrName).(string)
		enc, ok := e.Val(AttrEncoding).(int64)
		if !ok {
			err = DecodeError{name, e.Offset, "missing encoding attribute for " + name}
			goto Error
		}
		switch enc {
		default:
			err = DecodeError{name, e.Offset, "unrecognized encoding attribute value"}
			goto Error

		case encAddress:
			typ = new(AddrType)
		case encBoolean:
			typ = new(BoolType)
		case encComplexFloat:
			typ = new(ComplexType)
			if name == "complex" {
				// clang writes out 'complex' instead of 'complex float' or 'complex double'.
				// clang also writes out a byte size that we can use to distinguish.
				// See issue 8694.
				switch byteSize, _ := e.Val(AttrByteSize).(int64); byteSize {
				case 8:
					name = "complex float"
				case 16:
					name = "complex double"
				}
			}
		case encFloat:
			typ = new(FloatType)
		case encSigned:
			typ = new(IntType)
		case encUnsigned:
			typ = new(UintType)
		case encSignedChar:
			typ = new(CharType)
		case encUnsignedChar:
			typ = new(UcharType)
		}
		typeCache[off] = typ
		t := typ.(interface {
			Basic() *BasicType
		}).Basic()
		t.Name = name
		t.BitSize, _ = e.Val(AttrBitSize).(int64)
		haveBitOffset := false
		haveDataBitOffset := false
		t.BitOffset, haveBitOffset = e.Val(AttrBitOffset).(int64)
		t.DataBitOffset, haveDataBitOffset = e.Val(AttrDataBitOffset).(int64)
		if haveBitOffset && haveDataBitOffset {
			err = DecodeError{name, e.Offset, "duplicate bit offset attributes"}
			goto Error
		}

	case TagClassType, TagStructType, TagUnionType:
		// Structure, union, or class type.  (DWARF v2 §5.5)
		// Attributes:
		//	AttrName: name of struct, union, or class
		//	AttrByteSize: byte size [required]
		//	AttrDeclaration: if true, struct/union/class is incomplete
		// Children:
		//	TagMember to describe one member.
		//		AttrName: name of member [required]
		//		AttrType: type of member [required]
		//		AttrByteSize: size in bytes
		//		AttrBitOffset: bit offset within bytes for bit fields
		//		AttrDataBitOffset: field bit offset relative to struct start
		//		AttrBitSize: bit size for bit fields
		//		AttrDataMemberLoc: location within struct [required for struct, class]
		// There is much more to handle C++, all ignored for now.
		t := new(StructType)
		typ = t
		typeCache[off] = t
		switch e.Tag {
		case TagClassType:
			t.Kind = "class"
		case TagStructType:
			t.Kind = "struct"
		case TagUnionType:
			t.Kind = "union"
		}
		t.StructName, _ = e.Val(AttrName).(string)
		t.Incomplete = e.Val(AttrDeclaration) != nil
		t.Field = make([]*StructField, 0, 8)
		var lastFieldType *Type
		var lastFieldBitSize int64
		var lastFieldByteOffset int64
		for kid := next(); kid != nil; kid = next() {
			if kid.Tag != TagMember {
				continue
			}
			f := new(StructField)
			if f.Type = typeOf(kid); err != nil {
				goto Error
			}
			switch loc := kid.Val(AttrDataMemberLoc).(type) {
			case []byte:
				// TODO: Should have original compilation
				// unit here, not unknownFormat.
				b := makeBuf(d, unknownFormat{}, "location", 0, loc)
				if b.uint8() != opPlusUconst {
					err = DecodeError{name, kid.Offset, "unexpected opcode"}
					goto Error
				}
				f.ByteOffset = int64(b.uint())
				if b.err != nil {
					err = b.err
					goto Error
				}
			case int64:
				f.ByteOffset = loc
			}

			f.Name, _ = kid.Val(AttrName).(string)
			f.ByteSize, _ = kid.Val(AttrByteSize).(int64)
			haveBitOffset := false
			haveDataBitOffset := false
			f.BitOffset, haveBitOffset = kid.Val(AttrBitOffset).(int64)
			f.DataBitOffset, haveDataBitOffset = kid.Val(AttrDataBitOffset).(int64)
			if haveBitOffset && haveDataBitOffset {
				err = DecodeError{name, e.Offset, "duplicate bit offset attributes"}
				goto Error
			}
			f.BitSize, _ = kid.Val(AttrBitSize).(int64)
			t.Field = append(t.Field, f)

			if lastFieldBitSize == 0 && lastFieldByteOffset == f.ByteOffset && t.Kind != "union" {
				// Last field was zero width. Fix array length.
				// (DWARF writes out 0-length arrays as if they were 1-length arrays.)
				fixups.recordArrayType(lastFieldType)
			}
			lastFieldType = &f.Type
			lastFieldByteOffset = f.ByteOffset
			lastFieldBitSize = f.BitSize
		}
		if t.Kind != "union" {
			b, ok := e.Val(AttrByteSize).(int64)
			if ok && b == lastFieldByteOffset {
				// Final field must be zero width. Fix array length.
				fixups.recordArrayType(lastFieldType)
			}
		}

	case TagConstType, TagVolatileType, TagRestrictType:
		// Type modifier (DWARF v2 §5.2)
		// Attributes:
		//	AttrType: subtype
		t := new(QualType)
		typ = t
		typeCache[off] = t
		if t.Type = typeOf(e); err != nil {
			goto Error
		}
		switch e.Tag {
		case TagConstType:
			t.Qual = "const"
		case TagRestrictType:
			t.Qual = "restrict"
		case TagVolatileType:
			t.Qual = "volatile"
		}

	case TagEnumerationType:
		// Enumeration type (DWARF v2 §5.6)
		// Attributes:
		//	AttrName: enum name if any
		//	AttrByteSize: bytes required to represent largest value
		// Children:
		//	TagEnumerator:
		//		AttrName: name of constant
		//		AttrConstValue: value of constant
		t := new(EnumType)
		typ = t
		typeCache[off] = t
		t.EnumName, _ = e.Val(AttrName).(string)
		t.Val = make([]*EnumValue, 0, 8)
		for kid := next(); kid != nil; kid = next() {
			if kid.Tag == TagEnumerator {
				f := new(EnumValue)
				f.Name, _ = kid.Val(AttrName).(string)
				f.Val, _ = kid.Val(AttrConstValue).(int64)
				n := len(t.Val)
				if n >= cap(t.Val) {
					val := make([]*EnumValue, n, n*2)
					copy(val, t.Val)
					t.Val = val
				}
				t.Val = t.Val[0 : n+1]
				t.Val[n] = f
			}
		}

	case TagPointerType:
		// Type modifier (DWARF v2 §5.2)
		// Attributes:
		//	AttrType: subtype [not required!  void* has no AttrType]
		//	AttrAddrClass: address class [ignored]
		t := new(PtrType)
		typ = t
		typeCache[off] = t
		if e.Val(AttrType) == nil {
			t.Type = &VoidType{}
			break
		}
		t.Type = typeOf(e)

	case TagSubroutineType:
		// Subroutine type.  (DWARF v2 §5.7)
		// Attributes:
		//	AttrType: type of return value if any
		//	AttrName: possible name of type [ignored]
		//	AttrPrototyped: whether used ANSI C prototype [ignored]
		// Children:
		//	TagFormalParameter: typed parameter
		//		AttrType: type of parameter
		//	TagUnspecifiedParameter: final ...
		t := new(FuncType)
		typ = t
		typeCache[off] = t
		if t.ReturnType = typeOf(e); err != nil {
			goto Error
		}
		t.ParamType = make([]Type, 0, 8)
		for kid := next(); kid != nil; kid = next() {
			var tkid Type
			switch kid.Tag {
			default:
				continue
			case TagFormalParameter:
				if tkid = typeOf(kid); err != nil {
					goto Error
				}
			case TagUnspecifiedParameters:
				tkid = &DotDotDotType{}
			}
			t.ParamType = append(t.ParamType, tkid)
		}

	case TagTypedef:
		// Typedef (DWARF v2 §5.3)
		// Attributes:
		//	AttrName: name [required]
		//	AttrType: type definition [required]
		t := new(TypedefType)
		typ = t
		typeCache[off] = t
		t.Name, _ = e.Val(AttrName).(string)
		t.Type = typeOf(e)

	case TagUnspecifiedType:
		// Unspecified type (DWARF v3 §5.2)
		// Attributes:
		//	AttrName: name
		t := new(UnspecifiedType)
		typ = t
		typeCache[off] = t
		t.Name, _ = e.Val(AttrName).(string)

	default:
		// This is some other type DIE that we're currently not
		// equipped to handle. Return an abstract "unsupported type"
		// object in such cases.
		t := new(UnsupportedType)
		typ = t
		typeCache[off] = t
		t.Tag = e.Tag
		t.Name, _ = e.Val(AttrName).(string)
	}

	if err != nil {
		goto Error
	}

	{
		b, ok := e.Val(AttrByteSize).(int64)
		if !ok {
			b = -1
			switch t := typ.(type) {
			case *TypedefType:
				// Record that we need to resolve this
				// type's size once the type graph is
				// constructed.
				fixups.typedefs = append(fixups.typedefs, t)
			case *PtrType:
				b = int64(addressSize)
			}
		}
		typ.Common().ByteSize = b
	}
	return typ, nil

Error:
	// If the parse fails, take the type out of the cache
	// so that the next call with this offset doesn't hit
	// the cache and return success.
	delete(typeCache, off)
	return nil, err
}

func zeroArray(t *Type) {
	at := (*t).(*ArrayType)
	if at.Type.Size() == 0 {
		return
	}
	// Make a copy to avoid invalidating typeCache.
	tt := *at
	tt.Count = 0
	*t = &tt
}

The pages are generated with Golds v0.7.0-preview. (GOOS=linux GOARCH=amd64)
Golds is a Go 101 project developed by Tapir Liu.
PR and bug reports are welcome and can be submitted to the issue list.
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