mruby 3.0.0

We are announcing the first stable release of mruby 3.0 series - mruby 3.0.0.

With mruby 3.0, the memory used of the mruby VM has been significantly reduced, and we have succeeded in significantly saving memory. Previous versions required several hundred KB of RAM, but mruby 3.0 can launch an application with about 100 KB of RAM.

Describes the new features and changes in mruby 3.0.
The main changes in mruby 3.0 are also described in doc/

New Features

Core Language Features

Features for mruby Developer

  • Add new C APIs
    • mrbc_cleanup_local_variables() - Clean up local variables on mrbc_context. #4931
    • Add functions that take symbols as arguments.
      • mrb_define_singleton_method_id()
      • mrb_define_class_method_id()
      • mrb_define_module_function_id()
      • mrb_undef_method_id()
      • mrb_undef_class_method_id()
      • mrb_define_const_id()
      • mrb_funcall_id()
    • Add functions for symbol checking that return mrb_sym value.
      • mrb_intern_check()
      • mrb_intern_check_cstr()
      • mrb_intern_check_str()
    • mrb_fiber_resume() - Allow context switch from C module.

Update Features

Reduce memory usage

  • Preallocate Symbols
    The mruby 3.0 can now significantly reduce RAM usage by allocating symbols to static memory area at compile time. See doc/guides/ for more information.
  • Reduce memory usage of Hash objects. f2d8db39
  • Reduce memory usage when calling methods. #5272

Updated build process

The old build configuration file build_config.rb has been reconfigured to a file in the build_config directory.
For example, the following typical build configuration is available.

  • default: the default configuration
  • host-gprof: compiles with gprof for performance tuning
  • host-m32: compiles in gcc 32bit mode on 64bit platforms
  • boxing: compiles all three boxing options
  • clang-asan: compiles with clang’s Address Sanitizer

You can specify the mruby build configuration by building mruby with the rake build, specifying the build configuration file in the environment variable MRUBY_CONFIG.

Core Libraries

  • Add new libraries.
    • mruby-catch - Support Kernel.#catch and Kernel.#throw. 7eaaee54
    • mruby-os-memsize - Support ObjectSpace.memsize_of and ObjectSpace.memsize_of_all. #5032 #5040 #5041
  • Add[]) initialization. (mruby-array)
  • Add Hash#except (mruby-hash-ext)
  • Update IO#popen to use keyword arguments. (mruby-io)
  • Make Module#include and Module#prepend behave like Ruby 3.0. 3972df57


  • Allow to mixed and specify *.rb and *.mrb in mruby interpreter. a045b6b8
  • Add irep C struct dump from mrbc with -S option.

Features for mruby Developer

  • Add new specifier c to mrb_get_args() for receive Class/Module.

Breaking Changes

mruby VM and bytecode

Due to improvements in the binary format, mruby binaries are no longer backward compatible. To run the mruby binaries on mruby 3.0, recompile with the mruby 3.0 mrbc.

  • Upgrade mruby VM version RITE_VM_VER to 0300 (means mruby 3.0).
  • Upgrade mruby binary version RITE_BINARY_FORMAT_VER to 0200.

Recognize Integer system

Integer has been changed to a specification similar to CRuby.

  • Integrate Fixnum and Integer.
  • Remove Integral.
  • int / int -> int
  • Add APIs for Integer. APIs for Fixnum (mrb_fixnum_xxx()) is also left for compatibility.
    • mrb_fixnum() -> mrb_integer()
    • mrb_fixnum_value() -> mrb_int_value()
    • mrb_fixnum_p() -> mrb_integer_p()

Major bug fixes

  • Fix take over file scope variables with mruby and mirb command. #4933
  • Fix performing block_given? from inside an orphan block always returns false. #5039
  • Fix segmentation fault in Mruby interpreter making it crash #5046
  • Fix splat operator to frozen Array raises FrozenError. #5067
  • Fix wrong behavior of File.extname when the filename starts or ends with a period. (mruby-io) #5077
  • Fix #parameters mistakes keyword “splat” for a block parameter. #5066
  • Fix mruby-io doesn’t work in processes started as Windows service. #5114
  • Fix resuming fiber from C seems to loop infinitely. #5261
  • Fix invalid UTF-8 string length. #5269
  • Fix avoid syntax error when EXPR_MID appears in conditional operator. #5290
  • Fix symbols like numbered parameters in blocks get a syntax error. #5295
  • Avoid infinite loops when converting objects to strings. 01a8d849
  • Fix module order of #include. ruby-bug:7844

We have done 983 commits to 274 files with 16,929 additions and 11,685 deletions since mruby 2.1.2. For more detail of the updates, see Commit Log.

Thanks to all the contributors who have worked on bug fixes and improvements in the release of mruby 3.0.0.