Robert Mosolgo

"Hello World" With Mruby

mruby is an implementation of Ruby that’s designed to be lightweight & integrated with C. To get started, you can run a bit of Ruby code from inside C code.

“Hello World” with mruby looks like this:

  • Set up a new project and include mruby as a submodule
  • Write some C code that loads mruby and executes some Ruby code
  • Compile the C code & run the resulting binary

You can find an example similar to this one on GitHub.

Start a project

Make a directory for your new project and enter it:

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$ mkdir ~/hello-mruby
$ cd ~/hello-mruby

Clone mruby source and compile mruby:

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$ git clone git@github.com:mruby/mruby.git
$ cd mruby
$ make
$ cd ..

(You need bison and Ruby to compile mruby, see the install guide for more information.)

You can check if compilation was successful by running mirb (interactive mruby):

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$ mruby/bin/mirb
mirb - Embeddable Interactive Ruby Shell

> 1 + 1
 => 2

Write the program

Here’s the whole of hello_world.c:

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/* include mruby VM & compiler */
#include "mruby.h"
#include "mruby/compile.h"

int main(void)
{
  /* make a mruby instance */
  mrb_state *mrb = mrb_open();

  /* write some code */
  char code[] = "p 'Hello world!'";

  /* use mruby to execute code from string */
  mrb_load_string(mrb, code);

  return 0;
}

Let’s break that down:

  • Include mruby & compiler. The mruby VM takes bytecode instructions. The compiler is used to turn a string of Ruby code into mruby bytecode. mrb_load_string handles both steps: Parse & compile Ruby code, then execute with the mruby VM.

  • Make a mruby instance. Create an instance of the mruby VM. This object contains the state of the Ruby evnironment. Besides using it to execute code, you can inject values into the Ruby environment or call Ruby code from C.

  • Use mruby to execute code from string. As described above, in this case, the string will be turned into VM instructions first, then executed by mruby.

Compile & run

Compile your C application, referencing the necessary mruby files:

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$ gcc hello_world.c -o hello_world -Imruby/include  -lmruby  -Lmruby/build/host/lib

Then, execute the resulting binary:

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$ ./hello_world
"Hello world!"

You did it!

What next?

  • Use mrbc to precompile .rb into mruby bytecode.
  • Modify mrb_state from C with things like mrb_define_class, mrb_define_method and mrb_define_const.
  • Call Ruby methods from C with mrb_funcall.

However, I don’t know of any English documentation for these things yet!