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Package rlp

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Published: Sep 9, 2020 | License: GPL3 | Module:


Package rlp implements the RLP serialization format.

The purpose of RLP (Recursive Linear Prefix) is to encode arbitrarily nested arrays of binary data, and RLP is the main encoding method used to serialize objects in Ethereum. The only purpose of RLP is to encode structure; encoding specific atomic data types (eg. strings, ints, floats) is left up to higher-order protocols. In Ethereum integers must be represented in big endian binary form with no leading zeroes (thus making the integer value zero equivalent to the empty string).

RLP values are distinguished by a type tag. The type tag precedes the value in the input stream and defines the size and kind of the bytes that follow.

Encoding Rules

Package rlp uses reflection and encodes RLP based on the Go type of the value.

If the type implements the Encoder interface, Encode calls EncodeRLP. It does not call EncodeRLP on nil pointer values.

To encode a pointer, the value being pointed to is encoded. A nil pointer to a struct type, slice or array always encodes as an empty RLP list unless the slice or array has elememt type byte. A nil pointer to any other value encodes as the empty string.

Struct values are encoded as an RLP list of all their encoded public fields. Recursive struct types are supported.

To encode slices and arrays, the elements are encoded as an RLP list of the value's elements. Note that arrays and slices with element type uint8 or byte are always encoded as an RLP string.

A Go string is encoded as an RLP string.

An unsigned integer value is encoded as an RLP string. Zero always encodes as an empty RLP string. big.Int values are treated as integers. Signed integers (int, int8, int16, ...) are not supported and will return an error when encoding.

Boolean values are encoded as the unsigned integers zero (false) and one (true).

An interface value encodes as the value contained in the interface.

Floating point numbers, maps, channels and functions are not supported.

Decoding Rules

Decoding uses the following type-dependent rules:

If the type implements the Decoder interface, DecodeRLP is called.

To decode into a pointer, the value will be decoded as the element type of the pointer. If the pointer is nil, a new value of the pointer's element type is allocated. If the pointer is non-nil, the existing value will be reused. Note that package rlp never leaves a pointer-type struct field as nil unless one of the "nil" struct tags is present.

To decode into a struct, decoding expects the input to be an RLP list. The decoded elements of the list are assigned to each public field in the order given by the struct's definition. The input list must contain an element for each decoded field. Decoding returns an error if there are too few or too many elements for the struct.

To decode into a slice, the input must be a list and the resulting slice will contain the input elements in order. For byte slices, the input must be an RLP string. Array types decode similarly, with the additional restriction that the number of input elements (or bytes) must match the array's defined length.

To decode into a Go string, the input must be an RLP string. The input bytes are taken as-is and will not necessarily be valid UTF-8.

To decode into an unsigned integer type, the input must also be an RLP string. The bytes are interpreted as a big endian representation of the integer. If the RLP string is larger than the bit size of the type, decoding will return an error. Decode also supports *big.Int. There is no size limit for big integers.

To decode into a boolean, the input must contain an unsigned integer of value zero (false) or one (true).

To decode into an interface value, one of these types is stored in the value:

[]interface{}, for RLP lists
[]byte, for RLP strings

Non-empty interface types are not supported when decoding. Signed integers, floating point numbers, maps, channels and functions cannot be decoded into.

Struct Tags

Package rlp honours certain struct tags: "-", "tail", "nil", "nilList" and "nilString".

The "-" tag ignores fields.

The "tail" tag, which may only be used on the last exported struct field, allows slurping up any excess list elements into a slice. See examples for more details.

The "nil" tag applies to pointer-typed fields and changes the decoding rules for the field such that input values of size zero decode as a nil pointer. This tag can be useful when decoding recursive types.

type StructWithOptionalFoo struct {
    Foo *[20]byte `rlp:"nil"`

RLP supports two kinds of empty values: empty lists and empty strings. When using the "nil" tag, the kind of empty value allowed for a type is chosen automatically. A struct field whose Go type is a pointer to an unsigned integer, string, boolean or byte array/slice expects an empty RLP string. Any other pointer field type encodes/decodes as an empty RLP list.

The choice of null value can be made explicit with the "nilList" and "nilString" struct tags. Using these tags encodes/decodes a Go nil pointer value as the kind of empty RLP value defined by the tag.




var (
	ErrExpectedString   = errors.New("rlp: expected String or Byte")
	ErrExpectedList     = errors.New("rlp: expected List")
	ErrCanonInt         = errors.New("rlp: non-canonical integer format")
	ErrCanonSize        = errors.New("rlp: non-canonical size information")
	ErrElemTooLarge     = errors.New("rlp: element is larger than containing list")
	ErrValueTooLarge    = errors.New("rlp: value size exceeds available input length")
	ErrMoreThanOneValue = errors.New("rlp: input contains more than one value")
var (
	// Common encoded values.
	// These are useful when implementing EncodeRLP.
	EmptyString = []byte{0x80}
	EmptyList   = []byte{0xC0}
var EOL = errors.New("rlp: end of list")

EOL is returned when the end of the current list has been reached during streaming.

func CountValues

func CountValues(b []byte) (int, error)

CountValues counts the number of encoded values in b.

func Decode

func Decode(r io.Reader, val interface{}) error

Decode parses RLP-encoded data from r and stores the result in the value pointed to by val. Please see package-level documentation for the decoding rules. Val must be a non-nil pointer.

If r does not implement ByteReader, Decode will do its own buffering.

Note that Decode does not set an input limit for all readers and may be vulnerable to panics cause by huge value sizes. If you need an input limit, use

NewStream(r, limit).Decode(val)


input, _ := hex.DecodeString("C90A1486666F6F626172")

type example struct {
	A, B   uint
	String string

var s example
err := Decode(bytes.NewReader(input), &s)
if err != nil {
	fmt.Printf("Error: %v\n", err)
} else {
	fmt.Printf("Decoded value: %#v\n", s)
Decoded value: rlp.example{A:0xa, B:0x14, String:"foobar"}
Example (StructTagNil)


// In this example, we'll use the "nil" struct tag to change
// how a pointer-typed field is decoded. The input contains an RLP
// list of one element, an empty string.
input := []byte{0xC1, 0x80}

// This type uses the normal rules.
// The empty input string is decoded as a pointer to an empty Go string.
var normalRules struct {
	String *string
Decode(bytes.NewReader(input), &normalRules)
fmt.Printf("normal: String = %q\n", *normalRules.String)

// This type uses the struct tag.
// The empty input string is decoded as a nil pointer.
var withEmptyOK struct {
	String *string `rlp:"nil"`
Decode(bytes.NewReader(input), &withEmptyOK)
fmt.Printf("with nil tag: String = %v\n", withEmptyOK.String)
normal: String = ""
with nil tag: String = <nil>
Example (StructTagTail)


package main

import (

type structWithTail struct {
	A, B uint
	C    []uint `rlp:"tail"`

func main() {
	// In this example, the "tail" struct tag is used to decode lists of
	// differing length into a struct.
	var val structWithTail

	err := Decode(bytes.NewReader([]byte{0xC4, 0x01, 0x02, 0x03, 0x04}), &val)
	fmt.Printf("with 4 elements: err=%v val=%v\n", err, val)

	err = Decode(bytes.NewReader([]byte{0xC6, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06}), &val)
	fmt.Printf("with 6 elements: err=%v val=%v\n", err, val)

	// Note that at least two list elements must be present to
	// fill fields A and B:
	err = Decode(bytes.NewReader([]byte{0xC1, 0x01}), &val)
	fmt.Printf("with 1 element: err=%q\n", err)

with 4 elements: err=<nil> val={1 2 [3 4]}
with 6 elements: err=<nil> val={1 2 [3 4 5 6]}
with 1 element: err="rlp: too few elements for rlp.structWithTail"

func DecodeBytes

func DecodeBytes(b []byte, val interface{}) error

DecodeBytes parses RLP data from b into val. Please see package-level documentation for the decoding rules. The input must contain exactly one value and no trailing data.

func Encode

func Encode(w io.Writer, val interface{}) error

Encode writes the RLP encoding of val to w. Note that Encode may perform many small writes in some cases. Consider making w buffered.

Please see package-level documentation of encoding rules.

func EncodeToBytes

func EncodeToBytes(val interface{}) ([]byte, error)

EncodeToBytes returns the RLP encoding of val. Please see package-level documentation for the encoding rules.

func EncodeToReader

func EncodeToReader(val interface{}) (size int, r io.Reader, err error)

EncodeToReader returns a reader from which the RLP encoding of val can be read. The returned size is the total size of the encoded data.

Please see the documentation of Encode for the encoding rules.

func ListSize

func ListSize(contentSize uint64) uint64

ListSize returns the encoded size of an RLP list with the given content size.

func NewListIterator

func NewListIterator(data RawValue) (*listIterator, error)

NewListIterator creates an iterator for the (list) represented by data

func SplitList

func SplitList(b []byte) (content, rest []byte, err error)

SplitList splits b into the content of a list and any remaining bytes after the list.

func SplitString

func SplitString(b []byte) (content, rest []byte, err error)

SplitString splits b into the content of an RLP string and any remaining bytes after the string.

type ByteReader

type ByteReader interface {

ByteReader must be implemented by any input reader for a Stream. It is implemented by e.g. bufio.Reader and bytes.Reader.

type Decoder

type Decoder interface {
	DecodeRLP(*Stream) error

Decoder is implemented by types that require custom RLP decoding rules or need to decode into private fields.

The DecodeRLP method should read one value from the given Stream. It is not forbidden to read less or more, but it might be confusing.

type Encoder

type Encoder interface {
	// EncodeRLP should write the RLP encoding of its receiver to w.
	// If the implementation is a pointer method, it may also be
	// called for nil pointers.
	// Implementations should generate valid RLP. The data written is
	// not verified at the moment, but a future version might. It is
	// recommended to write only a single value but writing multiple
	// values or no value at all is also permitted.
	EncodeRLP(io.Writer) error

Encoder is implemented by types that require custom encoding rules or want to encode private fields.



package main

import (

type MyCoolType struct {
	Name string
	a, b uint

// EncodeRLP writes x as RLP list [a, b] that omits the Name field.
func (x *MyCoolType) EncodeRLP(w io.Writer) (err error) {
	return Encode(w, []uint{x.a, x.b})

func main() {
	var t *MyCoolType // t is nil pointer to MyCoolType
	bytes, _ := EncodeToBytes(t)
	fmt.Printf("%v → %X\n", t, bytes)

	t = &MyCoolType{Name: "foobar", a: 5, b: 6}
	bytes, _ = EncodeToBytes(t)
	fmt.Printf("%v → %X\n", t, bytes)

<nil> → C0
&{foobar 5 6} → C20506

type Kind

type Kind int

Kind represents the kind of value contained in an RLP stream.

const (
	Byte Kind = iota

func Split

func Split(b []byte) (k Kind, content, rest []byte, err error)

Split returns the content of first RLP value and any bytes after the value as subslices of b.

func (Kind) String

func (k Kind) String() string

type RawValue

type RawValue []byte

RawValue represents an encoded RLP value and can be used to delay RLP decoding or to precompute an encoding. Note that the decoder does not verify whether the content of RawValues is valid RLP.

type Stream

type Stream struct {
	// contains filtered or unexported fields

Stream can be used for piecemeal decoding of an input stream. This is useful if the input is very large or if the decoding rules for a type depend on the input structure. Stream does not keep an internal buffer. After decoding a value, the input reader will be positioned just before the type information for the next value.

When decoding a list and the input position reaches the declared length of the list, all operations will return error EOL. The end of the list must be acknowledged using ListEnd to continue reading the enclosing list.

Stream is not safe for concurrent use.



input, _ := hex.DecodeString("C90A1486666F6F626172")
s := NewStream(bytes.NewReader(input), 0)

// Check what kind of value lies ahead
kind, size, _ := s.Kind()
fmt.Printf("Kind: %v size:%d\n", kind, size)

// Enter the list
if _, err := s.List(); err != nil {
	fmt.Printf("List error: %v\n", err)

// Decode elements

// Acknowledge end of list
if err := s.ListEnd(); err != nil {
	fmt.Printf("ListEnd error: %v\n", err)
Kind: List size:9
10 <nil>
20 <nil>
[102 111 111 98 97 114] <nil>

func NewListStream

func NewListStream(r io.Reader, len uint64) *Stream

NewListStream creates a new stream that pretends to be positioned at an encoded list of the given length.

func NewStream

func NewStream(r io.Reader, inputLimit uint64) *Stream

NewStream creates a new decoding stream reading from r.

If r implements the ByteReader interface, Stream will not introduce any buffering.

For non-toplevel values, Stream returns ErrElemTooLarge for values that do not fit into the enclosing list.

Stream supports an optional input limit. If a limit is set, the size of any toplevel value will be checked against the remaining input length. Stream operations that encounter a value exceeding the remaining input length will return ErrValueTooLarge. The limit can be set by passing a non-zero value for inputLimit.

If r is a bytes.Reader or strings.Reader, the input limit is set to the length of r's underlying data unless an explicit limit is provided.

func (*Stream) Bool

func (s *Stream) Bool() (bool, error)

Bool reads an RLP string of up to 1 byte and returns its contents as a boolean. If the input does not contain an RLP string, the returned error will be ErrExpectedString.

func (*Stream) Bytes

func (s *Stream) Bytes() ([]byte, error)

Bytes reads an RLP string and returns its contents as a byte slice. If the input does not contain an RLP string, the returned error will be ErrExpectedString.

func (*Stream) Decode

func (s *Stream) Decode(val interface{}) error

Decode decodes a value and stores the result in the value pointed to by val. Please see the documentation for the Decode function to learn about the decoding rules.

func (*Stream) Kind

func (s *Stream) Kind() (kind Kind, size uint64, err error)

Kind returns the kind and size of the next value in the input stream.

The returned size is the number of bytes that make up the value. For kind == Byte, the size is zero because the value is contained in the type tag.

The first call to Kind will read size information from the input reader and leave it positioned at the start of the actual bytes of the value. Subsequent calls to Kind (until the value is decoded) will not advance the input reader and return cached information.

func (*Stream) List

func (s *Stream) List() (size uint64, err error)

List starts decoding an RLP list. If the input does not contain a list, the returned error will be ErrExpectedList. When the list's end has been reached, any Stream operation will return EOL.

func (*Stream) ListEnd

func (s *Stream) ListEnd() error

ListEnd returns to the enclosing list. The input reader must be positioned at the end of a list.

func (*Stream) Raw

func (s *Stream) Raw() ([]byte, error)

Raw reads a raw encoded value including RLP type information.

func (*Stream) Reset

func (s *Stream) Reset(r io.Reader, inputLimit uint64)

Reset discards any information about the current decoding context and starts reading from r. This method is meant to facilitate reuse of a preallocated Stream across many decoding operations.

If r does not also implement ByteReader, Stream will do its own buffering.

func (*Stream) Uint

func (s *Stream) Uint() (uint64, error)

Uint reads an RLP string of up to 8 bytes and returns its contents as an unsigned integer. If the input does not contain an RLP string, the returned error will be ErrExpectedString.

Package Files

Documentation was rendered with GOOS=linux and GOARCH=amd64.

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