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How to contribute

mruby is an open-source project which is looking forward to each contribution. Contributors agree to license their contribution(s) under MIT license.

Your Pull Request

To make it easy to review and understand your change please keep the following things in mind before submitting your pull request:

  • Work on the latest possible state of mruby/master
  • Create a branch which is dedicated to your change
  • Test your changes before creating a pull request (rake test)
  • If possible write a test case which confirms your change
  • Don't mix several features or bug-fixes in one pull request
  • Create a meaningful commit message
  • Explain your change (i.e. with a link to the issue you are fixing)
  • Use mrbgem to provide non ISO features (classes, modules and methods) unless you have a special reason to implement them in the core


A framework for managing and maintaining multi-language pre-commit hooks. pre-commit can be installed with pip, curl, brew or conda.

You need to first install pre-commit and then install the pre-commit hooks with pre-commit install. Now pre-commit will run automatically on git commit!

It's usually a good idea to run the hooks against all the files when adding new hooks (usually pre-commit will only run on the changed files during git hooks). Use pre-commit run --all-files to check all files.

To run a single hook use pre-commit run --all-files <hook_id>

To update use pre-commit autoupdate

Spell Checking

We are running misspell which is mainly written in Golang to check spelling with GitHub Actions. Correct commonly misspelled English words quickly with misspell. You can run misspell locally against all files with:

find . -type f | xargs ./misspell -error

Notable misspell help options or flags are:

  • -i string: ignore the following corrections, comma separated
  • -w: Overwrite file with corrections (default is just to display)

Coding conventions

How to style your C and Ruby code which you want to submit.

C code

The core part (parser, bytecode-interpreter, core-lib, etc.) of mruby is written in the C programming language. Please note the following hints for your C code:

Comply with C99 (ISO/IEC 9899:1999)

mruby should be highly portable to other systems and compilers. For this it is recommended to keep your code as close as possible to the C99 standard (

Visual C++ is also an important target for mruby (supported version is 2013 or later). For this reason features that are not supported by Visual C++ may not be used (e.g. %z of strftime()).

NOTE: Old GCC requires -std=gnu99 option to enable C99 support.

Reduce library dependencies to a minimum

The dependencies to libraries should be kept to an absolute minimum. This increases the portability but makes it also easier to cut away parts of mruby on-demand.

Insert a break after the function return value:


Ruby code

Parts of the standard library of mruby are written in the Ruby programming language itself. Please note the following hints for your Ruby code:

Comply with the Ruby standard (ISO/IEC 30170:2012)

mruby is currently targeting to execute Ruby code which complies to ISO/IEC 30170:2012 (, unless there's a clear reason, e.g. the latest Ruby has changed behavior from ISO.