Purpose

Detailed specification of the MessagePack binary format.

References

Format overview

MessagePack uses a compact binary format where the first byte contains both format information and often the value itself.

Integer types

Positive fixint (0x00 - 0x7F)

Stores integers 0-127 in a single byte.

+--------+
|0XXXXXXX|
+--------+

stores 0-127 in one byte

Negative fixint (0xE0 - 0xFF)

Stores integers -1 to -32 in a single byte.

+--------+
|111XXXXX|
+--------+

stores -32 to -1 in one byte

uint8 (0xCC)

Stores unsigned 8-bit integer.

+--------+--------+
|  0xCC  |XXXXXXX|
+--------+--------+

stores 0-255 in two bytes

uint16 (0xCD)

Stores unsigned 16-bit integer (big-endian).

+--------+--------+--------+
|  0xCD  |ZZZZZZZZ|ZZZZZZZZ|
+--------+--------+--------+

stores 0-65535 in three bytes

uint32 (0xCE)

Stores unsigned 32-bit integer (big-endian).

+--------+--------+--------+--------+--------+
|  0xCE  |AAAAAAAA|AAAAAAAA|AAAAAAAA|AAAAAAAA|
+--------+--------+--------+--------+--------+

stores 0-4294967295 in five bytes

uint64 (0xCF)

Stores unsigned 64-bit integer (big-endian).

+--------+--------+--------+--------+--------+--------+--------+--------+--------+
|  0xCF  |AAAAAAAA|AAAAAAAA|AAAAAAAA|AAAAAAAA|AAAAAAAA|AAAAAAAA|AAAAAAAA|AAAAAAAA|
+--------+--------+--------+--------+--------+--------+--------+--------+--------+

stores 0-18446744073709551615 in nine bytes

int8 (0xD0)

Stores signed 8-bit integer.

+--------+--------+
|  0xD0  |ZZZZZZZZ|
+--------+--------+

stores -128 to 127 in two bytes

int16 (0xD1)

Stores signed 16-bit integer (big-endian).

int32 (0xD2)

Stores signed 32-bit integer (big-endian).

int64 (0xD3)

Stores signed 64-bit integer (big-endian).

Floating point types

float32 (0xCA)

Stores IEEE 754 single precision floating point (big-endian).

+--------+--------+--------+--------+--------+
|  0xCA  |FFFFFFFF|FFFFFFFF|FFFFFFFF|FFFFFFFF|
+--------+--------+--------+--------+--------+

stores float32 in five bytes

float64 (0xCB)

Stores IEEE 754 double precision floating point (big-endian).

+--------+--------+--------+--------+--------+--------+--------+--------+--------+
|  0xCB  |FFFFFFFF|FFFFFFFF|FFFFFFFF|FFFFFFFF|FFFFFFFF|FFFFFFFF|FFFFFFFF|FFFFFFFF|
+--------+--------+--------+--------+--------+--------+--------+--------+--------+

stores float64 in nine bytes

String types

fixstr (0xA0 - 0xBF)

Stores strings with length 0-31.

+--------+========+
|101XXXXX|  data |
+--------+========+

stores 0-31 byte strings in 2-32 bytes

str8 (0xD9)

Stores strings with length 0-255.

+--------+--------+========+
|  0xD9  |YYYYYYYY|  data |
+--------+--------+========+

stores 0-255 byte strings in 2-257 bytes

str16 (0xDA)

Stores strings with length 0-65535.

+--------+--------+--------+========+
|  0xDA  |ZZZZZZZZ|ZZZZZZZZ|  data |
+--------+--------+--------+========+

stores 0-65535 byte strings in 4-65539 bytes

str32 (0xDB)

Stores strings with length 0-4294967295.

Binary types

bin8 (0xC4)

Stores binary data with length 0-255.

bin16 (0xC5)

Stores binary data with length 0-65535.

bin32 (0xC6)

Stores binary data with length 0-4294967295.

Array types

fixarray (0x90 - 0x9F)

Stores arrays with 0-15 elements.

+--------+~~~~~~~~~~~~~~~~~+
|1001XXXX|    N objects    |
+--------+~~~~~~~~~~~~~~~~~+

stores 0-15 element arrays

array16 (0xDC)

Stores arrays with 0-65535 elements.

array32 (0xDD)

Stores arrays with 0-4294967295 elements.

Map types

fixmap (0x80 - 0x8F)

Stores maps with 0-15 key-value pairs.

map16 (0xDE)

Stores maps with 0-65535 key-value pairs.

map32 (0xDF)

Stores maps with 0-4294967295 key-value pairs.

Extension types

fixext1 (0xD4)

Stores extension with 1 byte data.

fixext2 (0xD5)

Stores extension with 2 bytes data.

fixext4 (0xD6)

Stores extension with 4 bytes data.

fixext8 (0xD7)

Stores extension with 8 bytes data.

fixext16 (0xD8)

Stores extension with 16 bytes data.

ext8 (0xC7)

Stores extension with length 0-255.

ext16 (0xC8)

Stores extension with length 0-65535.

ext32 (0xC9)

Stores extension with length 0-4294967295.

Reserved types

nil (0xC0)

Represents nil/null.

+--------+
|  0xC0  |
+--------+

false (0xC2)

Represents boolean false.

true (0xC3)

Represents boolean true.