src/pkg/debug/gosym/symtab.go - go - Git at Google

 // 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.

 // Package gosym implements access to the Go symbol
 // and line number tables embedded in Go binaries generated
 // by the gc compilers.
 package gosym

 // The table format is a variant of the format used in Plan 9's a.out
 // format, documented at http://plan9.bell-labs.com/magic/man2html/6/a.out.
 // The best reference for the differences between the Plan 9 format
 // and the Go format is the runtime source, specifically ../../runtime/symtab.c.

 import (
 	"encoding/binary"
 	"fmt"
 	"os"
 	"strconv"
 	"strings"
 )

 /*
  * Symbols
  */

 // A Sym represents a single symbol table entry.
 type Sym struct {
 	Value  uint64
 	Type   byte
 	Name   string
 	GoType uint64
 	// If this symbol if a function symbol, the corresponding Func
 	Func *Func
 }

 // Static returns whether this symbol is static (not visible outside its file).
 func (s *Sym) Static() bool { return s.Type >= 'a' }

 // PackageName returns the package part of the symbol name,
 // or the empty string if there is none.
 func (s *Sym) PackageName() string {
 	if i := strings.Index(s.Name, "."); i != -1 {
 		return s.Name[0:i]
 	}
 	return ""
 }

 // ReceiverName returns the receiver type name of this symbol,
 // or the empty string if there is none.
 func (s *Sym) ReceiverName() string {
 	l := strings.Index(s.Name, ".")
 	r := strings.LastIndex(s.Name, ".")
 	if l == -1 || r == -1 || l == r {
 		return ""
 	}
 	return s.Name[l+1 : r]
 }

 // BaseName returns the symbol name without the package or receiver name.
 func (s *Sym) BaseName() string {
 	if i := strings.LastIndex(s.Name, "."); i != -1 {
 		return s.Name[i+1:]
 	}
 	return s.Name
 }

 // A Func collects information about a single function.
 type Func struct {
 	Entry uint64
 	*Sym
 	End       uint64
 	Params    []*Sym
 	Locals    []*Sym
 	FrameSize int
 	LineTable *LineTable
 	Obj       *Obj
 }

 // An Obj represents a single object file.
 type Obj struct {
 	Funcs []Func
 	Paths []Sym
 }

 /*
  * Symbol tables
  */

 // Table represents a Go symbol table.  It stores all of the
 // symbols decoded from the program and provides methods to translate
 // between symbols, names, and addresses.
 type Table struct {
 	Syms  []Sym
 	Funcs []Func
 	Files map[string]*Obj
 	Objs  []Obj
 	//	textEnd uint64;
 }

 type sym struct {
 	value  uint32
 	gotype uint32
 	typ    byte
 	name   []byte
 }

 func walksymtab(data []byte, fn func(sym) os.Error) os.Error {
 	var s sym
 	p := data
 	for len(p) >= 6 {
 		s.value = binary.BigEndian.Uint32(p[0:4])
 		typ := p[4]
 		if typ&0x80 == 0 {
 			return &DecodingError{len(data) - len(p) + 4, "bad symbol type", typ}
 		}
 		typ &^= 0x80
 		s.typ = typ
 		p = p[5:]
 		var i int
 		var nnul int
 		for i = 0; i < len(p); i++ {
 			if p[i] == 0 {
 				nnul = 1
 				break
 			}
 		}
 		switch typ {
 		case 'z', 'Z':
 			p = p[i+nnul:]
 			for i = 0; i+2 <= len(p); i += 2 {
 				if p[i] == 0 && p[i+1] == 0 {
 					nnul = 2
 					break
 				}
 			}
 		}
 		if i+nnul+4 > len(p) {
 			return &DecodingError{len(data), "unexpected EOF", nil}
 		}
 		s.name = p[0:i]
 		i += nnul
 		s.gotype = binary.BigEndian.Uint32(p[i : i+4])
 		p = p[i+4:]
 		fn(s)
 	}
 	return nil
 }

 // NewTable decodes the Go symbol table in data,
 // returning an in-memory representation.
 func NewTable(symtab []byte, pcln *LineTable) (*Table, os.Error) {
 	var n int
 	err := walksymtab(symtab, func(s sym) os.Error {
 		n++
 		return nil
 	})
 	if err != nil {
 		return nil, err
 	}

 	var t Table
 	fname := make(map[uint16]string)
 	t.Syms = make([]Sym, 0, n)
 	nf := 0
 	nz := 0
 	lasttyp := uint8(0)
 	err = walksymtab(symtab, func(s sym) os.Error {
 		n := len(t.Syms)
 		t.Syms = t.Syms[0 : n+1]
 		ts := &t.Syms[n]
 		ts.Type = s.typ
 		ts.Value = uint64(s.value)
 		ts.GoType = uint64(s.gotype)
 		switch s.typ {
 		default:
 			// rewrite name to use . instead of · (c2 b7)
 			w := 0
 			b := s.name
 			for i := 0; i < len(b); i++ {
 				if b[i] == 0xc2 && i+1 < len(b) && b[i+1] == 0xb7 {
 					i++
 					b[i] = '.'
 				}
 				b[w] = b[i]
 				w++
 			}
 			ts.Name = string(s.name[0:w])
 		case 'z', 'Z':
 			if lasttyp != 'z' && lasttyp != 'Z' {
 				nz++
 			}
 			for i := 0; i < len(s.name); i += 2 {
 				eltIdx := binary.BigEndian.Uint16(s.name[i : i+2])
 				elt, ok := fname[eltIdx]
 				if !ok {
 					return &DecodingError{-1, "bad filename code", eltIdx}
 				}
 				if n := len(ts.Name); n > 0 && ts.Name[n-1] != '/' {
 					ts.Name += "/"
 				}
 				ts.Name += elt
 			}
 		}
 		switch s.typ {
 		case 'T', 't', 'L', 'l':
 			nf++
 		case 'f':
 			fname[uint16(s.value)] = ts.Name
 		}
 		lasttyp = s.typ
 		return nil
 	})
 	if err != nil {
 		return nil, err
 	}

 	t.Funcs = make([]Func, 0, nf)
 	t.Objs = make([]Obj, 0, nz)
 	t.Files = make(map[string]*Obj)

 	// Count text symbols and attach frame sizes, parameters, and
 	// locals to them.  Also, find object file boundaries.
 	var obj *Obj
 	lastf := 0
 	for i := 0; i < len(t.Syms); i++ {
 		sym := &t.Syms[i]
 		switch sym.Type {
 		case 'Z', 'z': // path symbol
 			// Finish the current object
 			if obj != nil {
 				obj.Funcs = t.Funcs[lastf:]
 			}
 			lastf = len(t.Funcs)

 			// Start new object
 			n := len(t.Objs)
 			t.Objs = t.Objs[0 : n+1]
 			obj = &t.Objs[n]

 			// Count & copy path symbols
 			var end int
 			for end = i + 1; end < len(t.Syms); end++ {
 				if c := t.Syms[end].Type; c != 'Z' && c != 'z' {
 					break
 				}
 			}
 			obj.Paths = t.Syms[i:end]
 			i = end - 1 // loop will i++

 			// Record file names
 			depth := 0
 			for j := range obj.Paths {
 				s := &obj.Paths[j]
 				if s.Name == "" {
 					depth--
 				} else {
 					if depth == 0 {
 						t.Files[s.Name] = obj
 					}
 					depth++
 				}
 			}

 		case 'T', 't', 'L', 'l': // text symbol
 			if n := len(t.Funcs); n > 0 {
 				t.Funcs[n-1].End = sym.Value
 			}
 			if sym.Name == "etext" {
 				continue
 			}

 			// Count parameter and local (auto) syms
 			var np, na int
 			var end int
 		countloop:
 			for end = i + 1; end < len(t.Syms); end++ {
 				switch t.Syms[end].Type {
 				case 'T', 't', 'L', 'l', 'Z', 'z':
 					break countloop
 				case 'p':
 					np++
 				case 'a':
 					na++
 				}
 			}

 			// Fill in the function symbol
 			n := len(t.Funcs)
 			t.Funcs = t.Funcs[0 : n+1]
 			fn := &t.Funcs[n]
 			sym.Func = fn
 			fn.Params = make([]*Sym, 0, np)
 			fn.Locals = make([]*Sym, 0, na)
 			fn.Sym = sym
 			fn.Entry = sym.Value
 			fn.Obj = obj
 			if pcln != nil {
 				fn.LineTable = pcln.slice(fn.Entry)
 				pcln = fn.LineTable
 			}
 			for j := i; j < end; j++ {
 				s := &t.Syms[j]
 				switch s.Type {
 				case 'm':
 					fn.FrameSize = int(s.Value)
 				case 'p':
 					n := len(fn.Params)
 					fn.Params = fn.Params[0 : n+1]
 					fn.Params[n] = s
 				case 'a':
 					n := len(fn.Locals)
 					fn.Locals = fn.Locals[0 : n+1]
 					fn.Locals[n] = s
 				}
 			}
 			i = end - 1 // loop will i++
 		}
 	}
 	if obj != nil {
 		obj.Funcs = t.Funcs[lastf:]
 	}
 	return &t, nil
 }

 // PCToFunc returns the function containing the program counter pc,
 // or nil if there is no such function.
 func (t *Table) PCToFunc(pc uint64) *Func {
 	funcs := t.Funcs
 	for len(funcs) > 0 {
 		m := len(funcs) / 2
 		fn := &funcs[m]
 		switch {
 		case pc < fn.Entry:
 			funcs = funcs[0:m]
 		case fn.Entry <= pc && pc < fn.End:
 			return fn
 		default:
 			funcs = funcs[m+1:]
 		}
 	}
 	return nil
 }

 // PCToLine looks up line number information for a program counter.
 // If there is no information, it returns fn == nil.
 func (t *Table) PCToLine(pc uint64) (file string, line int, fn *Func) {
 	if fn = t.PCToFunc(pc); fn == nil {
 		return
 	}
 	file, line = fn.Obj.lineFromAline(fn.LineTable.PCToLine(pc))
 	return
 }

 // LineToPC looks up the first program counter on the given line in
 // the named file.  Returns UnknownPathError or UnknownLineError if
 // there is an error looking up this line.
 func (t *Table) LineToPC(file string, line int) (pc uint64, fn *Func, err os.Error) {
 	obj, ok := t.Files[file]
 	if !ok {
 		return 0, nil, UnknownFileError(file)
 	}
 	abs, err := obj.alineFromLine(file, line)
 	if err != nil {
 		return
 	}
 	for i := range obj.Funcs {
 		f := &obj.Funcs[i]
 		pc := f.LineTable.LineToPC(abs, f.End)
 		if pc != 0 {
 			return pc, f, nil
 		}
 	}
 	return 0, nil, &UnknownLineError{file, line}
 }

 // LookupSym returns the text, data, or bss symbol with the given name,
 // or nil if no such symbol is found.
 func (t *Table) LookupSym(name string) *Sym {
 	// TODO(austin) Maybe make a map
 	for i := range t.Syms {
 		s := &t.Syms[i]
 		switch s.Type {
 		case 'T', 't', 'L', 'l', 'D', 'd', 'B', 'b':
 			if s.Name == name {
 				return s
 			}
 		}
 	}
 	return nil
 }

 // LookupFunc returns the text, data, or bss symbol with the given name,
 // or nil if no such symbol is found.
 func (t *Table) LookupFunc(name string) *Func {
 	for i := range t.Funcs {
 		f := &t.Funcs[i]
 		if f.Sym.Name == name {
 			return f
 		}
 	}
 	return nil
 }

 // SymByAddr returns the text, data, or bss symbol starting at the given address.
 // TODO(rsc): Allow lookup by any address within the symbol.
 func (t *Table) SymByAddr(addr uint64) *Sym {
 	// TODO(austin) Maybe make a map
 	for i := range t.Syms {
 		s := &t.Syms[i]
 		switch s.Type {
 		case 'T', 't', 'L', 'l', 'D', 'd', 'B', 'b':
 			if s.Value == addr {
 				return s
 			}
 		}
 	}
 	return nil
 }

 /*
  * Object files
  */

 func (o *Obj) lineFromAline(aline int) (string, int) {
 	type stackEnt struct {
 		path   string
 		start  int
 		offset int
 		prev   *stackEnt
 	}

 	noPath := &stackEnt{"", 0, 0, nil}
 	tos := noPath

 	// TODO(austin) I have no idea how 'Z' symbols work, except
 	// that they pop the stack.
 pathloop:
 	for _, s := range o.Paths {
 		val := int(s.Value)
 		switch {
 		case val > aline:
 			break pathloop

 		case val == 1:
 			// Start a new stack
 			tos = &stackEnt{s.Name, val, 0, noPath}

 		case s.Name == "":
 			// Pop
 			if tos == noPath {
 				return "<malformed symbol table>", 0
 			}
 			tos.prev.offset += val - tos.start
 			tos = tos.prev

 		default:
 			// Push
 			tos = &stackEnt{s.Name, val, 0, tos}
 		}
 	}

 	if tos == noPath {
 		return "", 0
 	}
 	return tos.path, aline - tos.start - tos.offset + 1
 }

 func (o *Obj) alineFromLine(path string, line int) (int, os.Error) {
 	if line < 1 {
 		return 0, &UnknownLineError{path, line}
 	}

 	for i, s := range o.Paths {
 		// Find this path
 		if s.Name != path {
 			continue
 		}

 		// Find this line at this stack level
 		depth := 0
 		var incstart int
 		line += int(s.Value)
 	pathloop:
 		for _, s := range o.Paths[i:] {
 			val := int(s.Value)
 			switch {
 			case depth == 1 && val >= line:
 				return line - 1, nil

 			case s.Name == "":
 				depth--
 				if depth == 0 {
 					break pathloop
 				} else if depth == 1 {
 					line += val - incstart
 				}

 			default:
 				if depth == 1 {
 					incstart = val
 				}
 				depth++
 			}
 		}
 		return 0, &UnknownLineError{path, line}
 	}
 	return 0, UnknownFileError(path)
 }

 /*
  * Errors
  */

 // UnknownFileError represents a failure to find the specific file in
 // the symbol table.
 type UnknownFileError string

 func (e UnknownFileError) String() string { return "unknown file: " + string(e) }

 // UnknownLineError represents a failure to map a line to a program
 // counter, either because the line is beyond the bounds of the file
 // or because there is no code on the given line.
 type UnknownLineError struct {
 	File string
 	Line int
 }

 func (e *UnknownLineError) String() string {
 	return "no code at " + e.File + ":" + strconv.Itoa(e.Line)
 }

 // DecodingError represents an error during the decoding of
 // the symbol table.
 type DecodingError struct {
 	off int
 	msg string
 	val interface{}
 }

 func (e *DecodingError) String() string {
 	msg := e.msg
 	if e.val != nil {
 		msg += fmt.Sprintf(" '%v'", e.val)
 	}
 	msg += fmt.Sprintf(" at byte %#x", e.off)
 	return msg
 }
	// 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.

	// Package gosym implements access to the Go symbol
	// and line number tables embedded in Go binaries generated
	// by the gc compilers.
	package gosym

	// The table format is a variant of the format used in Plan 9's a.out
	// format, documented at http://plan9.bell-labs.com/magic/man2html/6/a.out.
	// The best reference for the differences between the Plan 9 format
	// and the Go format is the runtime source, specifically ../../runtime/symtab.c.

	import (
	"encoding/binary"
	"fmt"
	"os"
	"strconv"
	"strings"
	)

	/*
	* Symbols
	*/

	// A Sym represents a single symbol table entry.
	type Sym struct {
	Value uint64
	Type byte
	Name string
	GoType uint64
	// If this symbol if a function symbol, the corresponding Func
	Func *Func
	}

	// Static returns whether this symbol is static (not visible outside its file).
	func (s *Sym) Static() bool { return s.Type >= 'a' }

	// PackageName returns the package part of the symbol name,
	// or the empty string if there is none.
	func (s *Sym) PackageName() string {
	if i := strings.Index(s.Name, "."); i != -1 {
	return s.Name[0:i]
	}
	return ""
	}

	// ReceiverName returns the receiver type name of this symbol,
	// or the empty string if there is none.
	func (s *Sym) ReceiverName() string {
	l := strings.Index(s.Name, ".")
	r := strings.LastIndex(s.Name, ".")
	if l == -1 \|\| r == -1 \|\| l == r {
	return ""
	}
	return s.Name[l+1 : r]
	}

	// BaseName returns the symbol name without the package or receiver name.
	func (s *Sym) BaseName() string {
	if i := strings.LastIndex(s.Name, "."); i != -1 {
	return s.Name[i+1:]
	}
	return s.Name
	}

	// A Func collects information about a single function.
	type Func struct {
	Entry uint64
	*Sym
	End uint64
	Params []*Sym
	Locals []*Sym
	FrameSize int
	LineTable *LineTable
	Obj *Obj
	}

	// An Obj represents a single object file.
	type Obj struct {
	Funcs []Func
	Paths []Sym
	}

	/*
	* Symbol tables
	*/

	// Table represents a Go symbol table. It stores all of the
	// symbols decoded from the program and provides methods to translate
	// between symbols, names, and addresses.
	type Table struct {
	Syms []Sym
	Funcs []Func
	Files map[string]*Obj
	Objs []Obj
	// textEnd uint64;
	}

	type sym struct {
	value uint32
	gotype uint32
	typ byte
	name []byte
	}

	func walksymtab(data []byte, fn func(sym) os.Error) os.Error {
	var s sym
	p := data
	for len(p) >= 6 {
	s.value = binary.BigEndian.Uint32(p[0:4])
	typ := p[4]
	if typ&0x80 == 0 {
	return &DecodingError{len(data) - len(p) + 4, "bad symbol type", typ}
	}
	typ &^= 0x80
	s.typ = typ
	p = p[5:]
	var i int
	var nnul int
	for i = 0; i < len(p); i++ {
	if p[i] == 0 {
	nnul = 1
	break
	}
	}
	switch typ {
	case 'z', 'Z':
	p = p[i+nnul:]
	for i = 0; i+2 <= len(p); i += 2 {
	if p[i] == 0 && p[i+1] == 0 {
	nnul = 2
	break
	}
	}
	}
	if i+nnul+4 > len(p) {
	return &DecodingError{len(data), "unexpected EOF", nil}
	}
	s.name = p[0:i]
	i += nnul
	s.gotype = binary.BigEndian.Uint32(p[i : i+4])
	p = p[i+4:]
	fn(s)
	}
	return nil
	}

	// NewTable decodes the Go symbol table in data,
	// returning an in-memory representation.
	func NewTable(symtab []byte, pcln LineTable) (Table, os.Error) {
	var n int
	err := walksymtab(symtab, func(s sym) os.Error {
	n++
	return nil
	})
	if err != nil {
	return nil, err
	}

	var t Table
	fname := make(map[uint16]string)
	t.Syms = make([]Sym, 0, n)
	nf := 0
	nz := 0
	lasttyp := uint8(0)
	err = walksymtab(symtab, func(s sym) os.Error {
	n := len(t.Syms)
	t.Syms = t.Syms[0 : n+1]
	ts := &t.Syms[n]
	ts.Type = s.typ
	ts.Value = uint64(s.value)
	ts.GoType = uint64(s.gotype)
	switch s.typ {
	default:
	// rewrite name to use . instead of · (c2 b7)
	w := 0
	b := s.name
	for i := 0; i < len(b); i++ {
	if b[i] == 0xc2 && i+1 < len(b) && b[i+1] == 0xb7 {
	i++
	b[i] = '.'
	}
	b[w] = b[i]
	w++
	}
	ts.Name = string(s.name[0:w])
	case 'z', 'Z':
	if lasttyp != 'z' && lasttyp != 'Z' {
	nz++
	}
	for i := 0; i < len(s.name); i += 2 {
	eltIdx := binary.BigEndian.Uint16(s.name[i : i+2])
	elt, ok := fname[eltIdx]
	if !ok {
	return &DecodingError{-1, "bad filename code", eltIdx}
	}
	if n := len(ts.Name); n > 0 && ts.Name[n-1] != '/' {
	ts.Name += "/"
	}
	ts.Name += elt
	}
	}
	switch s.typ {
	case 'T', 't', 'L', 'l':
	nf++
	case 'f':
	fname[uint16(s.value)] = ts.Name
	}
	lasttyp = s.typ
	return nil
	})
	if err != nil {
	return nil, err
	}

	t.Funcs = make([]Func, 0, nf)
	t.Objs = make([]Obj, 0, nz)
	t.Files = make(map[string]*Obj)

	// Count text symbols and attach frame sizes, parameters, and
	// locals to them. Also, find object file boundaries.
	var obj *Obj
	lastf := 0
	for i := 0; i < len(t.Syms); i++ {
	sym := &t.Syms[i]
	switch sym.Type {
	case 'Z', 'z': // path symbol
	// Finish the current object
	if obj != nil {
	obj.Funcs = t.Funcs[lastf:]
	}
	lastf = len(t.Funcs)

	// Start new object
	n := len(t.Objs)
	t.Objs = t.Objs[0 : n+1]
	obj = &t.Objs[n]

	// Count & copy path symbols
	var end int
	for end = i + 1; end < len(t.Syms); end++ {
	if c := t.Syms[end].Type; c != 'Z' && c != 'z' {
	break
	}
	}
	obj.Paths = t.Syms[i:end]
	i = end - 1 // loop will i++

	// Record file names
	depth := 0
	for j := range obj.Paths {
	s := &obj.Paths[j]
	if s.Name == "" {
	depth--
	} else {
	if depth == 0 {
	t.Files[s.Name] = obj
	}
	depth++
	}
	}

	case 'T', 't', 'L', 'l': // text symbol
	if n := len(t.Funcs); n > 0 {
	t.Funcs[n-1].End = sym.Value
	}
	if sym.Name == "etext" {
	continue
	}

	// Count parameter and local (auto) syms
	var np, na int
	var end int
	countloop:
	for end = i + 1; end < len(t.Syms); end++ {
	switch t.Syms[end].Type {
	case 'T', 't', 'L', 'l', 'Z', 'z':
	break countloop
	case 'p':
	np++
	case 'a':
	na++
	}
	}

	// Fill in the function symbol
	n := len(t.Funcs)
	t.Funcs = t.Funcs[0 : n+1]
	fn := &t.Funcs[n]
	sym.Func = fn
	fn.Params = make([]*Sym, 0, np)
	fn.Locals = make([]*Sym, 0, na)
	fn.Sym = sym
	fn.Entry = sym.Value
	fn.Obj = obj
	if pcln != nil {
	fn.LineTable = pcln.slice(fn.Entry)
	pcln = fn.LineTable
	}
	for j := i; j < end; j++ {
	s := &t.Syms[j]
	switch s.Type {
	case 'm':
	fn.FrameSize = int(s.Value)
	case 'p':
	n := len(fn.Params)
	fn.Params = fn.Params[0 : n+1]
	fn.Params[n] = s
	case 'a':
	n := len(fn.Locals)
	fn.Locals = fn.Locals[0 : n+1]
	fn.Locals[n] = s
	}
	}
	i = end - 1 // loop will i++
	}
	}
	if obj != nil {
	obj.Funcs = t.Funcs[lastf:]
	}
	return &t, nil
	}

	// PCToFunc returns the function containing the program counter pc,
	// or nil if there is no such function.
	func (t Table) PCToFunc(pc uint64) Func {
	funcs := t.Funcs
	for len(funcs) > 0 {
	m := len(funcs) / 2
	fn := &funcs[m]
	switch {
	case pc < fn.Entry:
	funcs = funcs[0:m]
	case fn.Entry <= pc && pc < fn.End:
	return fn
	default:
	funcs = funcs[m+1:]
	}
	}
	return nil
	}

	// PCToLine looks up line number information for a program counter.
	// If there is no information, it returns fn == nil.
	func (t Table) PCToLine(pc uint64) (file string, line int, fn Func) {
	if fn = t.PCToFunc(pc); fn == nil {
	return
	}
	file, line = fn.Obj.lineFromAline(fn.LineTable.PCToLine(pc))
	return
	}

	// LineToPC looks up the first program counter on the given line in
	// the named file. Returns UnknownPathError or UnknownLineError if
	// there is an error looking up this line.
	func (t Table) LineToPC(file string, line int) (pc uint64, fn Func, err os.Error) {
	obj, ok := t.Files[file]
	if !ok {
	return 0, nil, UnknownFileError(file)
	}
	abs, err := obj.alineFromLine(file, line)
	if err != nil {
	return
	}
	for i := range obj.Funcs {
	f := &obj.Funcs[i]
	pc := f.LineTable.LineToPC(abs, f.End)
	if pc != 0 {
	return pc, f, nil
	}
	}
	return 0, nil, &UnknownLineError{file, line}
	}

	// LookupSym returns the text, data, or bss symbol with the given name,
	// or nil if no such symbol is found.
	func (t Table) LookupSym(name string) Sym {
	// TODO(austin) Maybe make a map
	for i := range t.Syms {
	s := &t.Syms[i]
	switch s.Type {
	case 'T', 't', 'L', 'l', 'D', 'd', 'B', 'b':
	if s.Name == name {
	return s
	}
	}
	}
	return nil
	}

	// LookupFunc returns the text, data, or bss symbol with the given name,
	// or nil if no such symbol is found.
	func (t Table) LookupFunc(name string) Func {
	for i := range t.Funcs {
	f := &t.Funcs[i]
	if f.Sym.Name == name {
	return f
	}
	}
	return nil
	}

	// SymByAddr returns the text, data, or bss symbol starting at the given address.
	// TODO(rsc): Allow lookup by any address within the symbol.
	func (t Table) SymByAddr(addr uint64) Sym {
	// TODO(austin) Maybe make a map
	for i := range t.Syms {
	s := &t.Syms[i]
	switch s.Type {
	case 'T', 't', 'L', 'l', 'D', 'd', 'B', 'b':
	if s.Value == addr {
	return s
	}
	}
	}
	return nil
	}

	/*
	* Object files
	*/

	func (o *Obj) lineFromAline(aline int) (string, int) {
	type stackEnt struct {
	path string
	start int
	offset int
	prev *stackEnt
	}

	noPath := &stackEnt{"", 0, 0, nil}
	tos := noPath

	// TODO(austin) I have no idea how 'Z' symbols work, except
	// that they pop the stack.
	pathloop:
	for _, s := range o.Paths {
	val := int(s.Value)
	switch {
	case val > aline:
	break pathloop

	case val == 1:
	// Start a new stack
	tos = &stackEnt{s.Name, val, 0, noPath}

	case s.Name == "":
	// Pop
	if tos == noPath {
	return "<malformed symbol table>", 0
	}
	tos.prev.offset += val - tos.start
	tos = tos.prev

	default:
	// Push
	tos = &stackEnt{s.Name, val, 0, tos}
	}
	}

	if tos == noPath {
	return "", 0
	}
	return tos.path, aline - tos.start - tos.offset + 1
	}

	func (o *Obj) alineFromLine(path string, line int) (int, os.Error) {
	if line < 1 {
	return 0, &UnknownLineError{path, line}
	}

	for i, s := range o.Paths {
	// Find this path
	if s.Name != path {
	continue
	}

	// Find this line at this stack level
	depth := 0
	var incstart int
	line += int(s.Value)
	pathloop:
	for _, s := range o.Paths[i:] {
	val := int(s.Value)
	switch {
	case depth == 1 && val >= line:
	return line - 1, nil

	case s.Name == "":
	depth--
	if depth == 0 {
	break pathloop
	} else if depth == 1 {
	line += val - incstart
	}

	default:
	if depth == 1 {
	incstart = val
	}
	depth++
	}
	}
	return 0, &UnknownLineError{path, line}
	}
	return 0, UnknownFileError(path)
	}

	/*
	* Errors
	*/

	// UnknownFileError represents a failure to find the specific file in
	// the symbol table.
	type UnknownFileError string

	func (e UnknownFileError) String() string { return "unknown file: " + string(e) }

	// UnknownLineError represents a failure to map a line to a program
	// counter, either because the line is beyond the bounds of the file
	// or because there is no code on the given line.
	type UnknownLineError struct {
	File string
	Line int
	}

	func (e *UnknownLineError) String() string {
	return "no code at " + e.File + ":" + strconv.Itoa(e.Line)
	}

	// DecodingError represents an error during the decoding of
	// the symbol table.
	type DecodingError struct {
	off int
	msg string
	val interface{}
	}

	func (e *DecodingError) String() string {
	msg := e.msg
	if e.val != nil {
	msg += fmt.Sprintf(" '%v'", e.val)
	}
	msg += fmt.Sprintf(" at byte %#x", e.off)
	return msg
	}