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(2) MRI source code structure

About this document

This document introduces the structure of the MRI source code. It also introduces the minimum required knowledge for hacking on MRI.

There are the following topics:

  • Exercise: Clone the MRI source code.
  • Exercise: Build MRI and install built binaries.
  • Exercise: Execute Ruby programs with built Ruby.
  • MRI source code structures.
  • Exercise: The 1st hack. Change the version description.


The following commands assume an Unix-like environment, such as Linux, macOS, etc. If you're using a Windows environment, you will need to refer to other resources.

NOTE: We provide an experimental docker image: docker pull koichisasada/rhc. Use rubydev account with su rubydev and enjoy hacking.

We assume the use of the following directory structure:

  • workdir/
    • ruby/ <- git cloned directory
    • build/ <- build directory (*.o files and other compilation artifacts are stored here)
    • install/ <- install directory (workdir/install/bin/ruby is the installed binary)

The commands git, ruby, autoconf, bison, gcc (or clang, etc), and make are required. Standard Ruby extensions (such as zlib, openssl, etc.) will be built if the libraries they depend on are available.

If you use apt-get (or apt) for package management in your environment, then you can get all dependencies with the following command:

$ sudo apt-get install git ruby autoconf bison gcc make zlib1g-dev libffi-dev libreadline-dev libgdbm-dev libssl-dev

Exercise: Clone the MRI source code

Use the following commands:

  1. $ mkdir workdir
  2. $ cd workdir
  3. $ git clone # The cloned source code will be available in workdir/ruby

Due to limited network bandwidth at the venue, please clone the source code at home.

Exercise: Build MRI and install built binaries

  1. Check the required commands described above.
  2. $ cd workdir/ # Move to workdir
  3. $ cd ruby # Move to workdir/ruby
  4. $ ./
  5. $ cd ..
  6. $ mkdir build
  7. $ cd build
  8. $ ../ruby/configure --prefix=$PWD/../install --enable-shared
  • the prefix option specifies an install directory. You can specify the directory of your choice by supplying the full absolute path (in this case, workdir/install is specified).
  • users of Homebrew will need to add the following options --with-openssl-dir="$(brew --prefix openssl)" --with-readline-dir="$(brew --prefix readline)" --disable-libedit
  1. $ make -j # Run build. -j specifies parallel build.
  2. $ make install # Tip: for a faster install, instead run make install-nodoc to install ruby without rdoc.
  3. $ ../install/bin/ruby -v will show the version description of your installed ruby command.

NOTE: Running make with the V=1 option (i.e. make V=1 -j, etc.) will output the full commands that are executed during the build. By default, V=0 is specified and detailed output is suppressed.

Exercise: Execute Ruby programs with the Ruby you built

There are several ways to run Ruby scripts on the Ruby you built.

The simplest way is to launch the installed Ruby directly, i.e. invoke workdir/install/bin/ruby. This is the same as invoking a pre-built Ruby binary. However, this means you will need to run make install every time you make a change to the Ruby source code, which can be rather time-consuming.

Here we introduce a few convenient ways to launch our version of Ruby without installing.

Use miniruby

After building Ruby, the miniruby command is available in workdir/build. miniruby is a limited version of Ruby for building Ruby itself. The limitations of miniruby, however, are minimal: it is unable to load extension libraries and limited encodings are available. You can try most of Ruby's syntax using miniruby.

miniruby is built during the first phase of the Ruby build process. Thus, miniruby is useful for a early verification of modifications made to MRI.

The following development loop is very efficient:

  1. Modify MRI
  2. Run make miniruby to build miniruby (this is faster than make or make all)
  3. Run a Ruby script in miniruby to test the correctness of your modifications.

To support this development loop, we provide a make run rule in the Makefile. This rule does the following:

  1. Build miniruby
  2. Run workdir/ruby/test.rb (test.rb in source directory) with the built miniruby.

Using make run, you can test your modifications with the following steps.

  1. Write a test for your modifications in ruby/test.rb. Note that you can't require gems or extension libraries in test.rb.
  2. Modify MRI.
  3. Invoke $ make run in the build directory

Use fully-featured Ruby (not miniruby)

If you want to run the "normal" Ruby, which can load extension libraries, you can use make runruby. This allows you to run Ruby without the make install step, which should save you some time.

  1. Write in ruby/test.rb what you want to check.
  2. Modify MRI.
  3. Invoke $ make runruby in the build directory.

Debug with gdb

NOTE: Running gdb on macOS can be quite difficult. The following commands assume a Linux environment.

When modifying the MRI source code, you can easily introduces critical problems that result in a SEGV. To debug such problems, we provide Makefile rules to support debugging with gdb. Of course, you can also debug with break points.

  1. Write in ruby/test.rb what you want to check. Note that you can't use gems or extension libraries in test.rb.
  2. Invoke $ make gdb to run miniruby with gdb. If there are no problems, gdb finishes silently.

make gdb uses ./miniruby. If you want to debug with ./ruby, use make gdb-ruby rule.

If you want to use break points, modify the run.gdb file generated by the make gdb command. For example, the b func_name gdb command inserts a break point at the beginning of the func_name function.

There is a similar rule for lldb, $ make lldb, for using lldb instead of gdb (but Koichi doesn't know the details because he doesn't use lldb). It may be useful if you use macOS.

Run Ruby tests

  1. $ make btest # run bootstrap tests in ruby/bootstraptest/
  2. $ make test-all # run test-unit tests in ruby/test/
  3. $ make test-spec # run tests provided in ruby/spec

These three tests have different purposes and characteristics.

MRI source code structures


At a glance, the following directory structure you can observe:

  • ruby/*.c MRI core files
    • VM cores
      • VM
        • vm*.[ch]: VM implementation
        • vm_core.h: definitions of VM data structure
        • insns.def: definitions of VM instructions
      • compile.c, iseq.[ch]: instruction sequence (bytecode)
      • gc.c: GC and memory management
      • thread*.[ch]: thread management
      • variable.c: variable management
      • dln*.c: dll management for extension libraries
      • main.c, ruby.c: the entry point of MRI
      • st.c: Hash algorithm implementation (see
    • Embedded classes
      • string.c: String class
      • array.c: Array class
      • ... (file names show class names, such as time.c for Time class)
  • ruby/*.h: internal definitions. C-extension libraries can't use them.
  • ruby/include/ruby/*: external definitions. C-extension libraries can use them.
  • ruby/enc/: encoding information.
  • ruby/defs/: various definitions.
  • ruby/tool/: tools to build MRI.
  • ruby/missing/: implementations for features that are missing in some OSes
  • ruby/cygwin/, ruby/nacl/, ruby/win32, ...: OS/system dependent code.


There are two kinds of libraries.

  • ruby/lib/: Standard libraries written in Ruby.
  • ruby/ext/: Bundled extension libraries written in C.


  • ruby/basictest/: place of old test
  • ruby/bootstraptest/: bootstrap test
  • ruby/test/: tests written in test-unit notation
  • ruby/spec/: tests written in RSpec notation


  • ruby/doc/, ruby/man/: documentation

Ruby's build process

Ruby build process is composed of several phases involving source code generation and so on. Several tools are written in Ruby, so the Ruby build process requires the Ruby interpreter. Release tarballs contain generated source code so that installing Ruby with a release tarball does not require the Ruby interpreter (and other development tools such as bison).

If you want to build MRI with source code fetched by Subversion or Git repository, you need a Ruby interpreter.

The following steps describe the build and install process:

  1. Build miniruby
    1. parse.y -> parse.c: Compile syntax rules into C code with bison
    2. insns.def -> Compile VM instructions into C code with ruby (BASERUBY)
    3. *.c -> *.o (*.obj on Windows): Compile C code into object files.
    4. link object files into miniruby
  2. Build encodings
    1. translate enc/... to appropriate C code with miniruby
    2. compile C code
  3. Build C-extension libraries
    1. Make Makefile from extconf.rb with mkmf.rb and miniruby
    2. Run make using generated Makefile.
  4. Build ruby command
  5. Generate documentation (rdoc, ri)
  6. Install MRI (to the install directory specified by the configure --prefix option)

There are actually many more steps in the process. It is difficult, however, to comprehensively list all the steps (even I don't know all of them!), so the above is an abbreviated sequence of steps. If you are curious, you can see all the rules in and related files.

Exercise: the 1st hack. Change the version description

Let's start modifying MRI. We assume that all source code is placed at workdir/ruby/.

For your first exercise, let's modify the version description which is displayed with ruby -v (or ./miniruby -v) to display it as your own Ruby (for example, show a version description with your name included).

  1. Open version.c in your editor.
  2. Briefly skim over the entirety of version.c.
  3. The function ruby_show_version() seems like what we're looking for
  4. fflush() is a C function that flushes the output buffer, so we can guess that adding some printing code just before fflush() call could work.
  5. Add the line printf("...\n"); (Replace ... with a string of your choice)
  6. $ make miniruby and build (don't forget to move to the build directory)
  7. run $ ./miniruby -v and check the result.
  8. $ make install and install build ruby.
  9. run $ ../install/bin/ruby -v and check the result with the installed ruby.

Finally, instead of just inserting a printf(...) statement, try replacing the entire ruby ... description with something else (such as perl ... and so on) would be interesting ;p