encoding/gob/doc.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.

/*
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 "net/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 the top 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:

	var u uint
	if i < 0 {
		u = (^uint(i) << 1) | 1 // complement i, bit 0 is 1
	} else {
		u = (uint(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 receiver has not allocated
one already, one will always be allocated on receipt unless the transmitted map
is nil and not at the top level.

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:
https://blog.golang.org/gobs-of-data
*/
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

*/