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Breaking changes

Starting with Release 2.0 only mruby-3 is supported, if you are on an older version check out a commit from before 2021.


First get a working copy of mruby then add

  conf.gem mgem: 'mruby-simplemsgpack'

to the build_conf.rb of the mruby directory

mruby-simplemsgpack searches for msgpack-c on your system, if it can find it it links against it, otherwise it builds against msgpack-c from source. You need at least msgpack-c 1 and depending on your system also pkg-config.

For building from source you need to have cmake installed on your system, take a look at for more information.


Objects can be packed with Object#to_msgpack or MessagePack.pack:

packed_hash = { a: 'hash', with: [1, 'embedded', 'array'] }.to_msgpack
packed_string = MessagePack.pack('bye')

packed_hash   # => "\x82\xA1a\xA4hash\xA4with\x93\x01\xA8embedded\xA5array"
packed_string # => "\xA3bye"

They are unpacked with MessagePack.unpack:

MessagePack.unpack(packed_hash)   # => { a: 'hash', with: [1, 'embedded', 'array'] }
MessagePack.unpack(packed_string) # => 'bye'

A string with multiple packed values can be unpacked by handing a block to MessagePack.unpack:

packed = packed_string + packed_hash
unpacked = []
MessagePack.unpack(packed) do |result|
  unpacked << result
unpacked # => ['bye', { a: 'hash', with: [1, 'embedded', 'array'] }]

When using MessagePack.unpack with a block and passing it a incomplete packed Message it returns the position of the first offending byte, if it was able to unpack the whole Message it returns self. This is helpful if the given data contains an incomplete last object and we want to continue unpacking after we have more data.

packed = packed_string + packed_hash.slice(0, packed_hash.length/2)
unpacked = []
offending_byte = MessagePack.unpack(packed) do |result|
  unpacked << result
offending_byte # => 19 (length of packed)
unpacked # => ['bye']

Extension Types

To customize how objects are packed, define an extension type.

By default, MessagePack packs symbols as strings and does not convert them back when unpacking them. Symbols can be preserved by registering an extension type for them:

sym_ext_type = 0
MessagePack.register_pack_type(sym_ext_type, Symbol) { |symbol| symbol.to_s }
MessagePack.register_unpack_type(sym_ext_type) { |data| data.to_sym }

MessagePack.unpack(:symbol.to_msgpack) # => :symbol

Other objects like classes can also be preserved:

cls_ext_type = 1
MessagePack.register_pack_type(cls_ext_type, Class) { |cls| cls.to_s }
MessagePack.register_unpack_type(cls_ext_type) { |data| data.constantize }
MessagePack.unpack(Object.to_msgpack) # => Object

For nil, true, false, Integer, Float, String, Array and Hash a registered ext type is ignored. They are always packed according to the MessagePack specification.

Proc, blocks or lambas

If you want to pack and unpack mruby blocks take a look at the mruby-proc-irep-ext gem, it can be registered like the other extension types

Overriding to_msgpack

It's not supported to override to_msgpack, MessagePack.pack ignores it, same when that object is included in a Hash or Array. This gem treats objects like ruby does, if you want to change the way your custom Class gets handled you can add to_hash, to_ary, to_int or to_str methods so it will be packed like a Hash, Array, Integer or String (in that order) then.


This is using code from

Copyright (C) 2008-2015 FURUHASHI Sadayuki

Distributed under the Boost Software License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at


mruby wrapper for msgpack (>= 1.0) /[mruby]





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