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diff --git a/src/encoding/gob/doc.go b/src/encoding/gob/doc.go new file mode 100644 index 000000000..d0acaba1a --- /dev/null +++ b/src/encoding/gob/doc.go @@ -0,0 +1,386 @@ +// 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 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". + +The implementation compiles a custom codec for each data type in the stream and +is most efficient when a single Encoder is used to transmit a stream of values, +amortizing the cost of compilation. + +Basics + +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. + +Types and Values + +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. Structs encode and decode only +exported fields. Strings and arrays of bytes are supported with a special, +efficient representation (see below). When a slice is decoded, if the existing +slice has capacity the slice will be extended in place; if not, a new array is +allocated. Regardless, the length of the resulting slice reports the number of +elements decoded. + +Functions and channels will not be sent in a gob. Attempting to encode such a value +at top the level will fail. A struct field of chan or func type is treated exactly +like an unexported field and is ignored. + +Gob can encode a value of any type implementing the GobEncoder or +encoding.BinaryMarshaler interfaces by calling the corresponding method, +in that order of preference. + +Gob can decode a value of any type implementing the GobDecoder or +encoding.BinaryUnmarshaler interfaces by calling the corresponding method, +again in that order of preference. + +Encoding Details + +This section 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. + +Maps are sent as an unsigned count followed by that many key, element +pairs. Empty but non-nil maps are sent, so if the sender has allocated +a map, the receiver will allocate a map even if no elements are +transmitted. + +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 07). 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). + +Interface types are not checked for compatibility; all interface types are +treated, for transmission, as members of a single "interface" type, analogous to +int or []byte - in effect they're all treated as interface{}. Interface values +are transmitted as a string identifying the concrete type being sent (a name +that must be pre-defined by calling Register), followed by a byte count of the +length of the following data (so the value can be skipped if it cannot be +stored), followed by the usual encoding of concrete (dynamic) value stored in +the interface value. (A nil interface value is identified by the empty string +and transmits no value.) Upon receipt, the decoder verifies that the unpacked +concrete item satisfies the interface of the receiving variable. + +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 { + ArrayT *ArrayType + SliceT *SliceType + StructT *StructType + MapT *MapType + } + type arrayType struct { + CommonType + Elem typeId + Len int + } + type CommonType struct { + Name string // the name of the struct type + Id int // the id of the type, repeated so it's inside the type + } + type sliceType struct { + CommonType + Elem typeId + } + type structType struct { + CommonType + Field []*fieldType // the fields of the struct. + } + type fieldType struct { + Name string // the name of the field. + Id int // the type id of the field, which must be already defined + } + type mapType struct { + CommonType + Key typeId + Elem typeId + } + +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 + complex 7 + interface 8 + // gap for reserved ids. + WireType 16 + ArrayType 17 + CommonType 18 + SliceType 19 + StructType 20 + FieldType 21 + // 22 is slice of fieldType. + MapType 23 + +Finally, each message created by a call to Encode is preceded by an encoded +unsigned integer count of the number of bytes remaining in the message. After +the initial type name, interface values are wrapped the same way; in effect, the +interface value acts like a recursive invocation of Encode. + +In summary, a gob stream looks like + + (byteCount (-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. + +See "Gobs of data" for a design discussion of the gob wire format: +http://golang.org/doc/articles/gobs_of_data.html +*/ +package gob + +/* +Grammar: + +Tokens starting with a lower case letter are terminals; int(n) +and uint(n) represent the signed/unsigned encodings of the value n. + +GobStream: + DelimitedMessage* +DelimitedMessage: + uint(lengthOfMessage) Message +Message: + TypeSequence TypedValue +TypeSequence + (TypeDefinition DelimitedTypeDefinition*)? +DelimitedTypeDefinition: + uint(lengthOfTypeDefinition) TypeDefinition +TypedValue: + int(typeId) Value +TypeDefinition: + int(-typeId) encodingOfWireType +Value: + SingletonValue | StructValue +SingletonValue: + uint(0) FieldValue +FieldValue: + builtinValue | ArrayValue | MapValue | SliceValue | StructValue | InterfaceValue +InterfaceValue: + NilInterfaceValue | NonNilInterfaceValue +NilInterfaceValue: + uint(0) +NonNilInterfaceValue: + ConcreteTypeName TypeSequence InterfaceContents +ConcreteTypeName: + uint(lengthOfName) [already read=n] name +InterfaceContents: + int(concreteTypeId) DelimitedValue +DelimitedValue: + uint(length) Value +ArrayValue: + uint(n) FieldValue*n [n elements] +MapValue: + uint(n) (FieldValue FieldValue)*n [n (key, value) pairs] +SliceValue: + uint(n) FieldValue*n [n elements] +StructValue: + (uint(fieldDelta) FieldValue)* +*/ + +/* +For implementers and the curious, here is an encoded example. Given + type Point struct {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 58 01 04 00 01 01 59 01 04 00 00 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 -1; this is done at the beginning + // of every struct, including nested structs. + 03 // Add 3 to field number; now 2 (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 0); start of embedded CommonType. + 01 // add 1 to field number (now 0, the name of the type) + 05 // string is (unsigned) 5 bytes long + 50 6f 69 6e 74 // wireType.structType.CommonType.name = "Point" + 01 // add 1 to field number (now 1, the id of the type) + ff 82 // wireType.structType.CommonType._id = 65 + 00 // end of embedded wiretype.structType.CommonType struct + 01 // add 1 to field number (now 1, 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 0: field[0].name + 01 // 1 byte + 58 // structType.field[0].name = "X" + 01 // Add 1 to get field number 1: 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 -1. + 01 // Add 1 to get field number 0: field[1].name + 01 // 1 byte + 59 // structType.field[1].name = "Y" + 01 // Add 1 to get field number 1: field[1].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 -1: + + 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 0 + 2c // encoding of signed "22" (0x22 = 44 = 22<<1); Point.x = 22 + 01 // add one to field number, yielding field 1 + 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 + +*/ |