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. Nil pointers are not permitted,
as they have no value. 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.
In general, if allocation is required, the decoder will allocate memory. If not,
it will update the destination variables with values read from the stream. It does
not initialize them first, so if the destination is a compound value such as a
map, struct, or slice, the decoded values will be merged elementwise into the
existing variables.
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.
In slices and arrays, as well as maps, all elements, even zero-valued elements,
are transmitted, even if all the elements are zero.
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 (except for arrays; see above), 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.
If a value is passed to Encode and the type is not a struct (or pointer to struct,
etc.), for simplicity of processing it is represented as a struct of one field.
The only visible effect of this is to encode a zero byte after the value, just as
after the last field of an encoded struct, so that the decode algorithm knows when
the top-level value is complete.
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
GobEncoderT *gobEncoderType
BinaryMarshalerT *gobEncoderType
TextMarshalerT *gobEncoderType
}
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
}
type gobEncoderType struct {
CommonType
}
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.
Compatibility: Any future changes to the package will endeavor to maintain
compatibility with streams encoded using previous versions. That is, any released
version of this package should be able to decode data written with any previously
released version, subject to issues such as security fixes. See the Go compatibility
document for background: https://golang.org/doc/go1compat
See "Gobs of data" for a design discussion of the gob wire format:
https://blog.golang.org/gobs-of-data
Example (Basic)
This example shows the basic usage of the package: Create an encoder,
transmit some values, receive them with a decoder.
Code:
package gob_test
import (
"bytes"
"encoding/gob"
"fmt"
"log"
)
type P struct {
X, Y, Z int
Name string
}
type Q struct {
X, Y *int32
Name string
}
func Example_basic() {
var network bytes.Buffer
enc := gob.NewEncoder(&network)
dec := gob.NewDecoder(&network)
err := enc.Encode(P{3, 4, 5, "Pythagoras"})
if err != nil {
log.Fatal("encode error:", err)
}
err = enc.Encode(P{1782, 1841, 1922, "Treehouse"})
if err != nil {
log.Fatal("encode error:", err)
}
var q Q
err = dec.Decode(&q)
if err != nil {
log.Fatal("decode error 1:", err)
}
fmt.Printf("%q: {%d, %d}\n", q.Name, *q.X, *q.Y)
err = dec.Decode(&q)
if err != nil {
log.Fatal("decode error 2:", err)
}
fmt.Printf("%q: {%d, %d}\n", q.Name, *q.X, *q.Y)
}
Example (EncodeDecode)
This example transmits a value that implements the custom encoding and decoding methods.
Code:
package gob_test
import (
"bytes"
"encoding/gob"
"fmt"
"log"
)
type Vector struct {
x, y, z int
}
func (v Vector) MarshalBinary() ([]byte, error) {
var b bytes.Buffer
fmt.Fprintln(&b, v.x, v.y, v.z)
return b.Bytes(), nil
}
func (v *Vector) UnmarshalBinary(data []byte) error {
b := bytes.NewBuffer(data)
_, err := fmt.Fscanln(b, &v.x, &v.y, &v.z)
return err
}
func Example_encodeDecode() {
var network bytes.Buffer
enc := gob.NewEncoder(&network)
err := enc.Encode(Vector{3, 4, 5})
if err != nil {
log.Fatal("encode:", err)
}
dec := gob.NewDecoder(&network)
var v Vector
err = dec.Decode(&v)
if err != nil {
log.Fatal("decode:", err)
}
fmt.Println(v)
}
Example (Interface)
This example shows how to encode an interface value. The key
distinction from regular types is to register the concrete type that
implements the interface.
Code:
package gob_test
import (
"bytes"
"encoding/gob"
"fmt"
"log"
"math"
)
type Point struct {
X, Y int
}
func (p Point) Hypotenuse() float64 {
return math.Hypot(float64(p.X), float64(p.Y))
}
type Pythagoras interface {
Hypotenuse() float64
}
func Example_interface() {
var network bytes.Buffer
gob.Register(Point{})
enc := gob.NewEncoder(&network)
for i := 1; i <= 3; i++ {
interfaceEncode(enc, Point{3 * i, 4 * i})
}
dec := gob.NewDecoder(&network)
for i := 1; i <= 3; i++ {
result := interfaceDecode(dec)
fmt.Println(result.Hypotenuse())
}
}
func interfaceEncode(enc *gob.Encoder, p Pythagoras) {
err := enc.Encode(&p)
if err != nil {
log.Fatal("encode:", err)
}
}
func interfaceDecode(dec *gob.Decoder) Pythagoras {
var p Pythagoras
err := dec.Decode(&p)
if err != nil {
log.Fatal("decode:", err)
}
return p
}