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// 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 gob
import (
// A Decoder manages the receipt of type and data information read from the
// remote side of a connection.
type Decoder struct {
mutex sync.Mutex // each item must be received atomically
r io.Reader // source of the data
wireType map[typeId]*wireType // map from remote ID to local description
decoderCache map[reflect.Type]map[typeId]**decEngine // cache of compiled engines
ignorerCache map[typeId]**decEngine // ditto for ignored objects
state *decodeState // reads data from in-memory buffer
countState *decodeState // reads counts from wire
buf []byte
countBuf [9]byte // counts may be uint64s (unlikely!), require 9 bytes
byteBuffer *bytes.Buffer
err os.Error
// NewDecoder returns a new decoder that reads from the io.Reader.
func NewDecoder(r io.Reader) *Decoder {
dec := new(Decoder)
dec.r = r
dec.wireType = make(map[typeId]*wireType)
dec.state = newDecodeState(dec, &dec.byteBuffer) // buffer set in Decode()
dec.decoderCache = make(map[reflect.Type]map[typeId]**decEngine)
dec.ignorerCache = make(map[typeId]**decEngine)
return dec
// recvType loads the definition of a type and reloads the Decoder's buffer.
func (dec *Decoder) recvType(id typeId) {
// Have we already seen this type? That's an error
if dec.wireType[id] != nil {
dec.err = os.ErrorString("gob: duplicate type received")
// Type:
wire := new(wireType)
dec.err = dec.decode(tWireType, reflect.NewValue(wire))
if dec.err != nil {
// Remember we've seen this type.
dec.wireType[id] = wire
// Load the next parcel.
// Decode reads the next value from the connection and stores
// it in the data represented by the empty interface value.
// The value underlying e must be the correct type for the next
// data item received, and must be a pointer.
func (dec *Decoder) Decode(e interface{}) os.Error {
value := reflect.NewValue(e)
// If e represents a value as opposed to a pointer, the answer won't
// get back to the caller. Make sure it's a pointer.
if value.Type().Kind() != reflect.Ptr {
dec.err = os.ErrorString("gob: attempt to decode into a non-pointer")
return dec.err
return dec.DecodeValue(value)
// recv reads the next count-delimited item from the input. It is the converse
// of Encoder.send.
func (dec *Decoder) recv() {
// Read a count.
var nbytes uint64
nbytes, dec.err = decodeUintReader(dec.r, dec.countBuf[0:])
if dec.err != nil {
// Allocate the buffer.
if nbytes > uint64(len(dec.buf)) {
dec.buf = make([]byte, nbytes+1000)
dec.byteBuffer = bytes.NewBuffer(dec.buf[0:nbytes])
// Read the data
_, dec.err = io.ReadFull(dec.r, dec.buf[0:nbytes])
if dec.err != nil {
if dec.err == os.EOF {
dec.err = io.ErrUnexpectedEOF
// decodeValueFromBuffer grabs the next value from the input. The Decoder's
// buffer already contains data. If the next item in the buffer is a type
// descriptor, it may be necessary to reload the buffer, but recvType does that.
func (dec *Decoder) decodeValueFromBuffer(value reflect.Value, ignoreInterfaceValue, countPresent bool) {
for dec.state.b.Len() > 0 {
// Receive a type id.
id := typeId(decodeInt(dec.state))
// Is it a new type?
if id < 0 { // 0 is the error state, handled above
// If the id is negative, we have a type.
if dec.err != nil {
// Make sure the type has been defined already or is a builtin type (for
// top-level singleton values).
if dec.wireType[id] == nil && builtinIdToType[id] == nil {
dec.err = errBadType
// An interface value is preceded by a byte count.
if countPresent {
count := int(decodeUint(dec.state))
if ignoreInterfaceValue {
// An interface value is preceded by a byte count. Just skip that many bytes.
// Otherwise fall through and decode it.
dec.err = dec.decode(id, value)
// DecodeValue reads the next value from the connection and stores
// it in the data represented by the reflection value.
// The value must be the correct type for the next
// data item received.
func (dec *Decoder) DecodeValue(value reflect.Value) os.Error {
// Make sure we're single-threaded through here.
defer dec.mutex.Unlock()
dec.err = nil
if dec.err != nil {
return dec.err
dec.decodeValueFromBuffer(value, false, false)
return dec.err
// If debug.go is compiled into the program , debugFunc prints a human-readable
// representation of the gob data read from r by calling that file's Debug function.
// Otherwise it is nil.
var debugFunc func(io.Reader)