json/docs/mkdocs/docs/features/binary_formats/messagepack.md

MessagePack

MessagePack is an efficient binary serialization format. It lets you exchange data among multiple languages like JSON. But it's faster and smaller. Small integers are encoded into a single byte, and typical short strings require only one extra byte in addition to the strings themselves.

!!! abstract "References"

- [MessagePack website](https://msgpack.org)
- [MessagePack specification](https://github.com/msgpack/msgpack/blob/master/spec.md)

Byte layout

MessagePack encodes each value as a one-byte format marker, optionally followed by a length or the payload. The marker range is partitioned to keep common values small: small non-negative integers (0 to 127) and small negative integers (-32 to -1) are the marker byte itself; short strings, arrays, and maps (up to 31 bytes for strings and up to 15 elements for arrays and maps) hide their length in the marker's lower bits. Larger values use dedicated markers that announce the width (8, 16, 32, or 64 bits) of the length or number that follows, in big-endian byte order. Like CBOR, containers are length-prefixed, so no terminator is needed.

Consider the following JSON value (shown formatted for readability):

{
  "id": 300,
  "name": "json",
  "pi": 3.14,
  "tags": [
    "a",
    "b"
  ]
}

to_msgpack encodes it to 39 bytes (shown in hexadecimal; the encoding contains no whitespace). The colors mark the role of each byte: type/marker data.

JSON	Bytes	Meaning
{	84	map with 4 key/value pairs
"id": 300,	A2 69 64 CD 01 2C	key: fixstr of length 2, then its UTF-8 bytes; value: uint 16 (big-endian)
"name": "json",	A4 6E 61 6D 65 A4 6A 73 6F 6E	key: fixstr of length 4; value: fixstr of length 4
"pi": 3.14,	A2 70 69 CB 40 09 1E B8 51 EB 85 1F	key: fixstr of length 2; value: float 64 (IEEE 754, big-endian)
"tags": [	A4 74 61 67 73 92	key: fixstr of length 4; value: fixarray with 2 elements
"a",	A1 61	fixstr of length 1
"b"	A1 62	fixstr of length 1
]		no end marker: the element count was given up front
}		no end marker: the member count was given up front

Serialization

The library uses the following mapping from JSON values types to MessagePack types according to the MessagePack specification:

JSON value type	value/range	MessagePack type	first byte
null	null	nil	0xC0
boolean	true	true	0xC3
boolean	false	false	0xC2
number_integer	-9223372036854775808..-2147483649	int64	0xD3
number_integer	-2147483648..-32769	int32	0xD2
number_integer	-32768..-129	int16	0xD1
number_integer	-128..-33	int8	0xD0
number_integer	-32..-1	negative fixint	0xE0..0xFF
number_integer	0..127	positive fixint	0x00..0x7F
number_integer	128..255	uint 8	0xCC
number_integer	256..65535	uint 16	0xCD
number_integer	65536..4294967295	uint 32	0xCE
number_integer	4294967296..18446744073709551615	uint 64	0xCF
number_unsigned	0..127	positive fixint	0x00..0x7F
number_unsigned	128..255	uint 8	0xCC
number_unsigned	256..65535	uint 16	0xCD
number_unsigned	65536..4294967295	uint 32	0xCE
number_unsigned	4294967296..18446744073709551615	uint 64	0xCF
number_float	any value representable by a float	float 32	0xCA
number_float	any value NOT representable by a float	float 64	0xCB
string	length: 0..31	fixstr	0xA0..0xBF
string	length: 32..255	str 8	0xD9
string	length: 256..65535	str 16	0xDA
string	length: 65536..4294967295	str 32	0xDB
array	size: 0..15	fixarray	0x90..0x9F
array	size: 16..65535	array 16	0xDC
array	size: 65536..4294967295	array 32	0xDD
object	size: 0..15	fix map	0x80..0x8F
object	size: 16..65535	map 16	0xDE
object	size: 65536..4294967295	map 32	0xDF
binary	size: 0..255	bin 8	0xC4
binary	size: 256..65535	bin 16	0xC5
binary	size: 65536..4294967295	bin 32	0xC6

!!! success "Complete mapping"

The mapping is **complete** in the sense that any JSON value type can be converted to a MessagePack value.

Any MessagePack output created by `to_msgpack` can be successfully parsed by `from_msgpack`.

!!! warning "Size constraints"

The following values can **not** be converted to a MessagePack value:

  - strings with more than 4294967295 bytes
  - byte strings with more than 4294967295 bytes
  - arrays with more than 4294967295 elements
  - objects with more than 4294967295 elements

Serializing such a value throws [`out_of_range.412`](../../home/exceptions.md#jsonexceptionout_of_range412).

!!! info "NaN/infinity handling"

`NaN`, `Infinity`, and `-Infinity` are serialized as a MessagePack float 32 (type 0xCA, 5 bytes total),
regardless of magnitude, in contrast to the [dump](../../api/basic_json/dump.md) function which serializes NaN
or Infinity to `null`.

!!! note

Prior to version 3.13.0, NaN and Infinity were instead serialized as a MessagePack float 64 (type 0xCB, 9 bytes
total), because the check used to select the smaller float 32 encoding failed for NaN (all comparisons are `false`)
and for Infinity and -Infinity (outside the float range checked against `std::numeric_limits<float>::lowest()` and
`max()`), so the float 32 path was skipped.

??? example "Example: serialize a JSON value to MessagePack"

```cpp
--8<-- "examples/to_msgpack.cpp"
```

Output:

```c
--8<-- "examples/to_msgpack.output"
```

Deserialization

The library maps MessagePack types to JSON value types as follows:

MessagePack type	JSON value type	first byte
positive fixint	number_unsigned	0x00..0x7F
fixmap	object	0x80..0x8F
fixarray	array	0x90..0x9F
fixstr	string	0xA0..0xBF
nil	null	0xC0
(never used)	–	0xC1
false	false	0xC2
true	true	0xC3
float 32	number_float	0xCA
float 64	number_float	0xCB
uint 8	number_unsigned	0xCC
uint 16	number_unsigned	0xCD
uint 32	number_unsigned	0xCE
uint 64	number_unsigned	0xCF
int 8	number_integer	0xD0
int 16	number_integer	0xD1
int 32	number_integer	0xD2
int 64	number_integer	0xD3
str 8	string	0xD9
str 16	string	0xDA
str 32	string	0xDB
array 16	array	0xDC
array 32	array	0xDD
map 16	object	0xDE
map 32	object	0xDF
bin 8	binary	0xC4
bin 16	binary	0xC5
bin 32	binary	0xC6
ext 8	binary	0xC7
ext 16	binary	0xC8
ext 32	binary	0xC9
fixext 1	binary	0xD4
fixext 2	binary	0xD5
fixext 4	binary	0xD6
fixext 8	binary	0xD7
fixext 16	binary	0xD8
negative fixint	number_integer	0xE0-0xFF

The byte 0xC1 is reserved as "never used" by the specification. As a value it is rejected with parse_error.112 (invalid byte: 0xC1); as a map key it is rejected with parse_error.113 and the generic "expected length specification" message, because the key is read as a string.

Duplicate keys in a map are accepted; the last value wins.

Extension types (ext, fixext)

MessagePack ext and fixext values are read as binary values, and the ext type byte is stored as the binary subtype. Although the specification defines the type as a signed 8-bit integer (0 to 127 for applications, -128 to -1 reserved, with -1 being the Timestamp extension), the library reads it as an unsigned byte: a negative ext type n becomes subtype 256+n, so ext type -1 is read as subtype 255 and -128 as subtype 128. The Timestamp extension is not interpreted; it is returned as binary with subtype 255. Use has_subtype() to tell a bin value (no subtype) from an ext value (subtype, even if it is 0).

When serializing, a binary value with a subtype is always written as fixext (1, 2, 4, 8, or 16 bytes) or ext, even for subtype 0 (d6 00 01 02 03 04); a binary value without a subtype is written as bin. Subtypes 128 to 255 are written as the reserved negative ext types (255 as -1), so json::binary({1,2,3,4}, 255) becomes d6 ff 01 02 03 04, which other MessagePack libraries decode as a 32-bit timestamp. A subtype above 255 throws out_of_range.415. The payload length is limited to 4294967295 bytes like all other lengths.

!!! info

Any MessagePack output created by `to_msgpack` can be successfully parsed by `from_msgpack`.

!!! warning "Object keys"

MessagePack allows map keys of any type, whereas JSON only allows strings as keys in object values. Like the
JSON-compatible [profile](https://github.com/msgpack/msgpack/blob/master/spec.md#profile) sketched in the
MessagePack specification, this library restricts map keys to `str` values. Maps with keys of any other type are
rejected with a [`parse_error.113`](../../home/exceptions.md#jsonexceptionparse_error113) exception (or, with
`allow_exceptions` set to `false`, a discarded value). For most key types the message names the type that was
found, for instance:

```
[json.exception.parse_error.113] parse error at byte 2: syntax error while parsing MessagePack object key: only string keys are supported, but found nil; last byte: 0xC0
```

This applies to the [SAX interface](../parsing/sax_interface.md) as well, as the key is read before it is passed
on. Such input needs a general-purpose MessagePack library instead.

!!! warning "Ill-formed UTF-8 in string values"

The MessagePack specification explicitly allows a `str` value (`fixstr`, `str 8`, `str 16`, `str 32`) to contain
a byte sequence that is not valid UTF-8, and recommends that a deserializer provide a way to obtain the original bytes.
This library follows that by default: with its
[`error_handler`](../../api/basic_json/from_msgpack.md) parameter left at `keep` (the default),
`from_msgpack()` reads `str` bytes (object keys included) as-is, without validating them, so such a value
round-trips through `from_msgpack(to_msgpack(j))` byte for byte. Passing `error_handler_t::strict` makes
`from_msgpack()` check anyway and throw
[`parse_error.113`](../../home/exceptions.md#jsonexceptionparse_error113) for ill-formed UTF-8, and
`replace`/`ignore` sanitize the string instead of keeping it. `to_msgpack()` also writes `str` bytes as-is by
default, since the specification permits it; its [`error_handler`](../../api/basic_json/to_msgpack.md) parameter
can be set to `strict` to throw [`type_error.316`](../../home/exceptions.md#jsonexceptiontype_error316) instead, or
to `replace`/`ignore` to sanitize the string, for instance for a decoder that rejects ill-formed UTF-8. However,
[`dump()`](../../api/basic_json/dump.md) still requires valid UTF-8 and throws
[`type_error.316`](../../home/exceptions.md#jsonexceptiontype_error316) for a value read this way with the
default `keep` handler, unless an error handler is passed that replaces or ignores the ill-formed bytes.

??? example "Example: deserialize a JSON value from MessagePack"

```cpp
--8<-- "examples/from_msgpack.cpp"
```

Output:

```json
--8<-- "examples/from_msgpack.output"
```