Source file src/pkg/gob/encode.go
// 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. /* The gob package manages streams of gobs - binary values exchanged between an Encoder (transmitter) and a Decoder (receiver). A typical use is transporting arguments and results of remote procedure calls (RPCs) such as those provided by package "rpc". A stream of gobs is self-describing. Each data item in the stream is preceded by a specification of its type, expressed in terms of a small set of predefined types. Pointers are not transmitted, but the things they point to are transmitted; that is, the values are flattened. Recursive types work fine, but recursive values (data with cycles) are problematic. This may change. To use gobs, create an Encoder and present it with a series of data items as values or addresses that can be dereferenced to values. The Encoder makes sure all type information is sent before it is needed. At the receive side, a Decoder retrieves values from the encoded stream and unpacks them into local variables. The source and destination values/types need not correspond exactly. For structs, fields (identified by name) that are in the source but absent from the receiving variable will be ignored. Fields that are in the receiving variable but missing from the transmitted type or value will be ignored in the destination. If a field with the same name is present in both, their types must be compatible. Both the receiver and transmitter will do all necessary indirection and dereferencing to convert between gobs and actual Go values. For instance, a gob type that is schematically, struct { a, b int } can be sent from or received into any of these Go types: struct { a, b int } // the same *struct { a, b int } // extra indirection of the struct struct { *a, **b int } // extra indirection of the fields struct { a, b int64 } // different concrete value type; see below It may also be received into any of these: struct { a, b int } // the same struct { b, a int } // ordering doesn't matter; matching is by name struct { a, b, c int } // extra field (c) ignored struct { b int } // missing field (a) ignored; data will be dropped struct { b, c int } // missing field (a) ignored; extra field (c) ignored. Attempting to receive into these types will draw a decode error: struct { a int; b uint } // change of signedness for b struct { a int; b float } // change of type for b struct { } // no field names in common struct { c, d int } // no field names in common Integers are transmitted two ways: arbitrary precision signed integers or arbitrary precision unsigned integers. There is no int8, int16 etc. discrimination in the gob format; there are only signed and unsigned integers. As described below, the transmitter sends the value in a variable-length encoding; the receiver accepts the value and stores it in the destination variable. Floating-point numbers are always sent using IEEE-754 64-bit precision (see below). Signed integers may be received into any signed integer variable: int, int16, etc.; unsigned integers may be received into any unsigned integer variable; and floating point values may be received into any floating point variable. However, the destination variable must be able to represent the value or the decode operation will fail. Structs, arrays and slices are also supported. Strings and arrays of bytes are supported with a special, efficient representation (see below). Interfaces, functions, and channels cannot be sent in a gob. Attempting to encode a value that contains one will fail. The rest of this comment documents the encoding, details that are not important for most users. Details are presented bottom-up. An unsigned integer is sent one of two ways. If it is less than 128, it is sent as a byte with that value. Otherwise it is sent as a minimal-length big-endian (high byte first) byte stream holding the value, preceded by one byte holding the byte count, negated. Thus 0 is transmitted as (00), 7 is transmitted as (07) and 256 is transmitted as (FE 01 00). A boolean is encoded within an unsigned integer: 0 for false, 1 for true. A signed integer, i, is encoded within an unsigned integer, u. Within u, bits 1 upward contain the value; bit 0 says whether they should be complemented upon receipt. The encode algorithm looks like this: uint u; if i < 0 { u = (^i << 1) | 1 // complement i, bit 0 is 1 } else { u = (i << 1) // do not complement i, bit 0 is 0 } encodeUnsigned(u) The low bit is therefore analogous to a sign bit, but making it the complement bit instead guarantees that the largest negative integer is not a special case. For example, -129=^128=(^256>>1) encodes as (FE 01 01). Floating-point numbers are always sent as a representation of a float64 value. That value is converted to a uint64 using math.Float64bits. The uint64 is then byte-reversed and sent as a regular unsigned integer. The byte-reversal means the exponent and high-precision part of the mantissa go first. Since the low bits are often zero, this can save encoding bytes. For instance, 17.0 is encoded in only three bytes (FE 31 40). Strings and slices of bytes are sent as an unsigned count followed by that many uninterpreted bytes of the value. All other slices and arrays are sent as an unsigned count followed by that many elements using the standard gob encoding for their type, recursively. Structs are sent as a sequence of (field number, field value) pairs. The field value is sent using the standard gob encoding for its type, recursively. If a field has the zero value for its type, it is omitted from the transmission. The field number is defined by the type of the encoded struct: the first field of the encoded type is field 0, the second is field 1, etc. When encoding a value, the field numbers are delta encoded for efficiency and the fields are always sent in order of increasing field number; the deltas are therefore unsigned. The initialization for the delta encoding sets the field number to -1, so an unsigned integer field 0 with value 7 is transmitted as unsigned delta = 1, unsigned value = 7 or (01 0E). Finally, after all the fields have been sent a terminating mark denotes the end of the struct. That mark is a delta=0 value, which has representation (00). The representation of types is described below. When a type is defined on a given connection between an Encoder and Decoder, it is assigned a signed integer type id. When Encoder.Encode(v) is called, it makes sure there is an id assigned for the type of v and all its elements and then it sends the pair (typeid, encoded-v) where typeid is the type id of the encoded type of v and encoded-v is the gob encoding of the value v. To define a type, the encoder chooses an unused, positive type id and sends the pair (-type id, encoded-type) where encoded-type is the gob encoding of a wireType description, constructed from these types: type wireType struct { s structType; } type fieldType struct { name string; // the name of the field. id int; // the type id of the field, which must be already defined } type commonType { name string; // the name of the struct type id int; // the id of the type, repeated for so it's inside the type } type structType struct { commonType; field []fieldType; // the fields of the struct. } If there are nested type ids, the types for all inner type ids must be defined before the top-level type id is used to describe an encoded-v. For simplicity in setup, the connection is defined to understand these types a priori, as well as the basic gob types int, uint, etc. Their ids are: bool 1 int 2 uint 3 float 4 []byte 5 string 6 wireType 7 structType 8 commonType 9 fieldType 10 In summary, a gob stream looks like ((-type id, encoding of a wireType)* (type id, encoding of a value))* where * signifies zero or more repetitions and the type id of a value must be predefined or be defined before the value in the stream. */ package gob /* For implementers and the curious, here is an encoded example. Given type Point {x, y int} and the value p := Point{22, 33} the bytes transmitted that encode p will be: 1f ff 81 03 01 01 05 50 6f 69 6e 74 01 ff 82 00 01 02 01 01 78 01 04 00 01 01 79 01 04 00 00 00 07 ff 82 01 2c 01 42 00 07 ff 82 01 2c 01 42 00 They are determined as follows. Since this is the first transmission of type Point, the type descriptor for Point itself must be sent before the value. This is the first type we've sent on this Encoder, so it has type id 65 (0 through 64 are reserved). 1f // This item (a type descriptor) is 31 bytes long. ff 81 // The negative of the id for the type we're defining, -65. // This is one byte (indicated by FF = -1) followed by // ^-65<<1 | 1. The low 1 bit signals to complement the // rest upon receipt. // Now we send a type descriptor, which is itself a struct (wireType). // The type of wireType itself is known (it's built in, as is the type of // all its components), so we just need to send a *value* of type wireType // that represents type "Point". // Here starts the encoding of that value. // Set the field number implicitly to zero; this is done at the beginning // of every struct, including nested structs. 03 // Add 3 to field number; now 3 (wireType.structType; this is a struct). // structType starts with an embedded commonType, which appears // as a regular structure here too. 01 // add 1 to field number (now 1); start of embedded commonType. 01 // add one to field number (now 1, the name of the type) 05 // string is (unsigned) 5 bytes long 50 6f 69 6e 74 // wireType.structType.commonType.name = "Point" 01 // add one to field number (now 2, the id of the type) ff 82 // wireType.structType.commonType._id = 65 00 // end of embedded wiretype.structType.commonType struct 01 // add one to field number (now 2, the Field array in wireType.structType) 02 // There are two fields in the type (len(structType.field)) 01 // Start of first field structure; add 1 to get field number 1: field[0].name 01 // 1 byte 78 // structType.field[0].name = "x" 01 // Add 1 to get field number 2: field[0].id 04 // structType.field[0].typeId is 2 (signed int). 00 // End of structType.field[0]; start structType.field[1]; set field number to 0. 01 // Add 1 to get field number 1: field[1].name 01 // 1 byte 79 // structType.field[1].name = "y" 01 // Add 1 to get field number 2: field[0].id 04 // struct.Type.field[1].typeId is 2 (signed int). 00 // End of structType.field[1]; end of structType.field. 00 // end of wireType.structType structure 00 // end of wireType structure Now we can send the Point value. Again the field number resets to zero: 07 // this value is 7 bytes long ff 82 // the type number, 65 (1 byte (-FF) followed by 65<<1) 01 // add one to field number, yielding field 1 2c // encoding of signed "22" (0x22 = 44 = 22<<1); Point.x = 22 01 // add one to field number, yielding field 2 42 // encoding of signed "33" (0x42 = 66 = 33<<1); Point.y = 33 00 // end of structure The type encoding is long and fairly intricate but we send it only once. If p is transmitted a second time, the type is already known so the output will be just: 07 ff 82 01 2c 01 42 00 A single non-struct value at top level is transmitted like a field with delta tag 0. For instance, a signed integer with value 3 presented as the argument to Encode will emit: 03 04 00 06 Which represents: 03 // this value is 3 bytes long 04 // the type number, 2, represents an integer 00 // tag delta 0 06 // value 3 */ import ( "bytes" "io" "math" "os" "reflect" "unsafe" ) const uint64Size = unsafe.Sizeof(uint64(0)) // The global execution state of an instance of the encoder. // Field numbers are delta encoded and always increase. The field // number is initialized to -1 so 0 comes out as delta(1). A delta of // 0 terminates the structure. type encoderState struct { b *bytes.Buffer err os.Error // error encountered during encoding. sendZero bool // encoding an array element or map key/value pair; send zero values fieldnum int // the last field number written. buf [1 + uint64Size]byte // buffer used by the encoder; here to avoid allocation. } // Unsigned integers have a two-state encoding. If the number is less // than 128 (0 through 0x7F), its value is written directly. // Otherwise the value is written in big-endian byte order preceded // by the byte length, negated. // encodeUint writes an encoded unsigned integer to state.b. Sets state.err. // If state.err is already non-nil, it does nothing. func encodeUint(state *encoderState, x uint64) { if state.err != nil { return } if x <= 0x7F { state.err = state.b.WriteByte(uint8(x)) return } var n, m int m = uint64Size for n = 1; x > 0; n++ { state.buf[m] = uint8(x & 0xFF) x >>= 8 m-- } state.buf[m] = uint8(-(n - 1)) n, state.err = state.b.Write(state.buf[m : uint64Size+1]) } // encodeInt writes an encoded signed integer to state.w. // The low bit of the encoding says whether to bit complement the (other bits of the) uint to recover the int. // Sets state.err. If state.err is already non-nil, it does nothing. func encodeInt(state *encoderState, i int64) { var x uint64 if i < 0 { x = uint64(^i<<1) | 1 } else { x = uint64(i << 1) } encodeUint(state, uint64(x)) } type encOp func(i *encInstr, state *encoderState, p unsafe.Pointer) // The 'instructions' of the encoding machine type encInstr struct { op encOp field int // field number indir int // how many pointer indirections to reach the value in the struct offset uintptr // offset in the structure of the field to encode } // Emit a field number and update the state to record its value for delta encoding. // If the instruction pointer is nil, do nothing func (state *encoderState) update(instr *encInstr) { if instr != nil { encodeUint(state, uint64(instr.field-state.fieldnum)) state.fieldnum = instr.field } } // Each encoder is responsible for handling any indirections associated // with the data structure. If any pointer so reached is nil, no bytes are written. // If the data item is zero, no bytes are written. // Otherwise, the output (for a scalar) is the field number, as an encoded integer, // followed by the field data in its appropriate format. func encIndirect(p unsafe.Pointer, indir int) unsafe.Pointer { for ; indir > 0; indir-- { p = *(*unsafe.Pointer)(p) if p == nil { return unsafe.Pointer(nil) } } return p } func encBool(i *encInstr, state *encoderState, p unsafe.Pointer) { b := *(*bool)(p) if b || state.sendZero { state.update(i) if b { encodeUint(state, 1) } else { encodeUint(state, 0) } } } func encInt(i *encInstr, state *encoderState, p unsafe.Pointer) { v := int64(*(*int)(p)) if v != 0 || state.sendZero { state.update(i) encodeInt(state, v) } } func encUint(i *encInstr, state *encoderState, p unsafe.Pointer) { v := uint64(*(*uint)(p)) if v != 0 || state.sendZero { state.update(i) encodeUint(state, v) } } func encInt8(i *encInstr, state *encoderState, p unsafe.Pointer) { v := int64(*(*int8)(p)) if v != 0 || state.sendZero { state.update(i) encodeInt(state, v) } } func encUint8(i *encInstr, state *encoderState, p unsafe.Pointer) { v := uint64(*(*uint8)(p)) if v != 0 || state.sendZero { state.update(i) encodeUint(state, v) } } func encInt16(i *encInstr, state *encoderState, p unsafe.Pointer) { v := int64(*(*int16)(p)) if v != 0 || state.sendZero { state.update(i) encodeInt(state, v) } } func encUint16(i *encInstr, state *encoderState, p unsafe.Pointer) { v := uint64(*(*uint16)(p)) if v != 0 || state.sendZero { state.update(i) encodeUint(state, v) } } func encInt32(i *encInstr, state *encoderState, p unsafe.Pointer) { v := int64(*(*int32)(p)) if v != 0 || state.sendZero { state.update(i) encodeInt(state, v) } } func encUint32(i *encInstr, state *encoderState, p unsafe.Pointer) { v := uint64(*(*uint32)(p)) if v != 0 || state.sendZero { state.update(i) encodeUint(state, v) } } func encInt64(i *encInstr, state *encoderState, p unsafe.Pointer) { v := *(*int64)(p) if v != 0 || state.sendZero { state.update(i) encodeInt(state, v) } } func encUint64(i *encInstr, state *encoderState, p unsafe.Pointer) { v := *(*uint64)(p) if v != 0 || state.sendZero { state.update(i) encodeUint(state, v) } } func encUintptr(i *encInstr, state *encoderState, p unsafe.Pointer) { v := uint64(*(*uintptr)(p)) if v != 0 || state.sendZero { state.update(i) encodeUint(state, v) } } // Floating-point numbers are transmitted as uint64s holding the bits // of the underlying representation. They are sent byte-reversed, with // the exponent end coming out first, so integer floating point numbers // (for example) transmit more compactly. This routine does the // swizzling. func floatBits(f float64) uint64 { u := math.Float64bits(f) var v uint64 for i := 0; i < 8; i++ { v <<= 8 v |= u & 0xFF u >>= 8 } return v } func encFloat(i *encInstr, state *encoderState, p unsafe.Pointer) { f := *(*float)(p) if f != 0 || state.sendZero { v := floatBits(float64(f)) state.update(i) encodeUint(state, v) } } func encFloat32(i *encInstr, state *encoderState, p unsafe.Pointer) { f := *(*float32)(p) if f != 0 || state.sendZero { v := floatBits(float64(f)) state.update(i) encodeUint(state, v) } } func encFloat64(i *encInstr, state *encoderState, p unsafe.Pointer) { f := *(*float64)(p) if f != 0 || state.sendZero { state.update(i) v := floatBits(f) encodeUint(state, v) } } // Complex numbers are just a pair of floating-point numbers, real part first. func encComplex(i *encInstr, state *encoderState, p unsafe.Pointer) { c := *(*complex)(p) if c != 0+0i || state.sendZero { rpart := floatBits(float64(real(c))) ipart := floatBits(float64(imag(c))) state.update(i) encodeUint(state, rpart) encodeUint(state, ipart) } } func encComplex64(i *encInstr, state *encoderState, p unsafe.Pointer) { c := *(*complex64)(p) if c != 0+0i || state.sendZero { rpart := floatBits(float64(real(c))) ipart := floatBits(float64(imag(c))) state.update(i) encodeUint(state, rpart) encodeUint(state, ipart) } } func encComplex128(i *encInstr, state *encoderState, p unsafe.Pointer) { c := *(*complex128)(p) if c != 0+0i || state.sendZero { rpart := floatBits(real(c)) ipart := floatBits(imag(c)) state.update(i) encodeUint(state, rpart) encodeUint(state, ipart) } } // Byte arrays are encoded as an unsigned count followed by the raw bytes. func encUint8Array(i *encInstr, state *encoderState, p unsafe.Pointer) { b := *(*[]byte)(p) if len(b) > 0 || state.sendZero { state.update(i) encodeUint(state, uint64(len(b))) state.b.Write(b) } } // Strings are encoded as an unsigned count followed by the raw bytes. func encString(i *encInstr, state *encoderState, p unsafe.Pointer) { s := *(*string)(p) if len(s) > 0 || state.sendZero { state.update(i) encodeUint(state, uint64(len(s))) io.WriteString(state.b, s) } } // The end of a struct is marked by a delta field number of 0. func encStructTerminator(i *encInstr, state *encoderState, p unsafe.Pointer) { encodeUint(state, 0) } // Execution engine // The encoder engine is an array of instructions indexed by field number of the encoding // data, typically a struct. It is executed top to bottom, walking the struct. type encEngine struct { instr []encInstr } const singletonField = 0 func encodeSingle(engine *encEngine, b *bytes.Buffer, basep uintptr) os.Error { state := new(encoderState) state.b = b state.fieldnum = singletonField // There is no surrounding struct to frame the transmission, so we must // generate data even if the item is zero. To do this, set sendZero. state.sendZero = true instr := &engine.instr[singletonField] p := unsafe.Pointer(basep) // offset will be zero if instr.indir > 0 { if p = encIndirect(p, instr.indir); p == nil { return nil } } instr.op(instr, state, p) return state.err } func encodeStruct(engine *encEngine, b *bytes.Buffer, basep uintptr) os.Error { state := new(encoderState) state.b = b state.fieldnum = -1 for i := 0; i < len(engine.instr); i++ { instr := &engine.instr[i] p := unsafe.Pointer(basep + instr.offset) if instr.indir > 0 { if p = encIndirect(p, instr.indir); p == nil { continue } } instr.op(instr, state, p) if state.err != nil { break } } return state.err } func encodeArray(b *bytes.Buffer, p uintptr, op encOp, elemWid uintptr, elemIndir int, length int) os.Error { state := new(encoderState) state.b = b state.fieldnum = -1 state.sendZero = true encodeUint(state, uint64(length)) for i := 0; i < length && state.err == nil; i++ { elemp := p up := unsafe.Pointer(elemp) if elemIndir > 0 { if up = encIndirect(up, elemIndir); up == nil { state.err = os.ErrorString("gob: encodeArray: nil element") break } elemp = uintptr(up) } op(nil, state, unsafe.Pointer(elemp)) p += uintptr(elemWid) } return state.err } func encodeReflectValue(state *encoderState, v reflect.Value, op encOp, indir int) { for i := 0; i < indir && v != nil; i++ { v = reflect.Indirect(v) } if v == nil { state.err = os.ErrorString("gob: encodeReflectValue: nil element") return } op(nil, state, unsafe.Pointer(v.Addr())) } func encodeMap(b *bytes.Buffer, mv *reflect.MapValue, keyOp, elemOp encOp, keyIndir, elemIndir int) os.Error { state := new(encoderState) state.b = b state.fieldnum = -1 state.sendZero = true keys := mv.Keys() encodeUint(state, uint64(len(keys))) for _, key := range keys { if state.err != nil { break } encodeReflectValue(state, key, keyOp, keyIndir) encodeReflectValue(state, mv.Elem(key), elemOp, elemIndir) } return state.err } var encOpMap = []encOp{ reflect.Bool: encBool, reflect.Int: encInt, reflect.Int8: encInt8, reflect.Int16: encInt16, reflect.Int32: encInt32, reflect.Int64: encInt64, reflect.Uint: encUint, reflect.Uint8: encUint8, reflect.Uint16: encUint16, reflect.Uint32: encUint32, reflect.Uint64: encUint64, reflect.Uintptr: encUintptr, reflect.Float: encFloat, reflect.Float32: encFloat32, reflect.Float64: encFloat64, reflect.Complex: encComplex, reflect.Complex64: encComplex64, reflect.Complex128: encComplex128, reflect.String: encString, } // Return the encoding op for the base type under rt and // the indirection count to reach it. func encOpFor(rt reflect.Type) (encOp, int, os.Error) { typ, indir := indirect(rt) var op encOp k := typ.Kind() if int(k) < len(encOpMap) { op = encOpMap[k] } if op == nil { // Special cases switch t := typ.(type) { case *reflect.SliceType: if t.Elem().Kind() == reflect.Uint8 { op = encUint8Array break } // Slices have a header; we decode it to find the underlying array. elemOp, indir, err := encOpFor(t.Elem()) if err != nil { return nil, 0, err } op = func(i *encInstr, state *encoderState, p unsafe.Pointer) { slice := (*reflect.SliceHeader)(p) if slice.Len == 0 { return } state.update(i) state.err = encodeArray(state.b, slice.Data, elemOp, t.Elem().Size(), indir, int(slice.Len)) } case *reflect.ArrayType: // True arrays have size in the type. elemOp, indir, err := encOpFor(t.Elem()) if err != nil { return nil, 0, err } op = func(i *encInstr, state *encoderState, p unsafe.Pointer) { slice := (*reflect.SliceHeader)(p) if slice.Len == 0 { return } state.update(i) state.err = encodeArray(state.b, uintptr(p), elemOp, t.Elem().Size(), indir, t.Len()) } case *reflect.MapType: keyOp, keyIndir, err := encOpFor(t.Key()) if err != nil { return nil, 0, err } elemOp, elemIndir, err := encOpFor(t.Elem()) if err != nil { return nil, 0, err } op = func(i *encInstr, state *encoderState, p unsafe.Pointer) { // Maps cannot be accessed by moving addresses around the way // that slices etc. can. We must recover a full reflection value for // the iteration. v := reflect.NewValue(unsafe.Unreflect(t, unsafe.Pointer((p)))) mv := reflect.Indirect(v).(*reflect.MapValue) if mv.Len() == 0 { return } state.update(i) state.err = encodeMap(state.b, mv, keyOp, elemOp, keyIndir, elemIndir) } case *reflect.StructType: // Generate a closure that calls out to the engine for the nested type. _, err := getEncEngine(typ) if err != nil { return nil, 0, err } info := mustGetTypeInfo(typ) op = func(i *encInstr, state *encoderState, p unsafe.Pointer) { state.update(i) // indirect through info to delay evaluation for recursive structs state.err = encodeStruct(info.encoder, state.b, uintptr(p)) } } } if op == nil { return op, indir, os.ErrorString("gob enc: can't happen: encode type " + rt.String()) } return op, indir, nil } // The local Type was compiled from the actual value, so we know it's compatible. func compileEnc(rt reflect.Type) (*encEngine, os.Error) { srt, isStruct := rt.(*reflect.StructType) engine := new(encEngine) if isStruct { engine.instr = make([]encInstr, srt.NumField()+1) // +1 for terminator for fieldnum := 0; fieldnum < srt.NumField(); fieldnum++ { f := srt.Field(fieldnum) op, indir, err := encOpFor(f.Type) if err != nil { return nil, err } engine.instr[fieldnum] = encInstr{op, fieldnum, indir, uintptr(f.Offset)} } engine.instr[srt.NumField()] = encInstr{encStructTerminator, 0, 0, 0} } else { engine.instr = make([]encInstr, 1) op, indir, err := encOpFor(rt) if err != nil { return nil, err } engine.instr[0] = encInstr{op, singletonField, indir, 0} // offset is zero } return engine, nil } // typeLock must be held (or we're in initialization and guaranteed single-threaded). // The reflection type must have all its indirections processed out. func getEncEngine(rt reflect.Type) (*encEngine, os.Error) { info, err := getTypeInfo(rt) if err != nil { return nil, err } if info.encoder == nil { // mark this engine as underway before compiling to handle recursive types. info.encoder = new(encEngine) info.encoder, err = compileEnc(rt) } return info.encoder, err } func encode(b *bytes.Buffer, value reflect.Value) os.Error { // Dereference down to the underlying object. rt, indir := indirect(value.Type()) for i := 0; i < indir; i++ { value = reflect.Indirect(value) } typeLock.Lock() engine, err := getEncEngine(rt) typeLock.Unlock() if err != nil { return err } if value.Type().Kind() == reflect.Struct { return encodeStruct(engine, b, value.Addr()) } return encodeSingle(engine, b, value.Addr()) }