Ruby interpreter embed in C code

Question

I just try a simple example from a book: I have a sum.rb file:

class Summer
  def sum(max)
  raise "Invalid maximum #{max}" if max < 0
  (max*max + max)/2
  end
end

And a embed_sum.c file:

#include <stdio.h>
#include <ruby/ruby.h>
int main ( int argc, char ** argv) 
{
  VALUE result;
  ruby_sysinit(&argc, &argv);
  RUBY_INIT_STACK;
  ruby_init();
  ruby_init_loadpath();
  rb_require("sum");
  rb_eval_string("$summer = Summer.new");
  rb_eval_string("$result = $summer.sum(10)");
  result = rb_gv_get("result");
  printf("Result = %d\n", NUM2INT(result));
  return ruby_cleanup(0);
}

The I compile it with:

gcc -Wall -lruby -I/usr/include/ruby-1.9.1/  embed_sum.c -o embed_sum

When I launch ./embed_sum it gives me a segmentation fault from the first rb_eval_string. my version of ruby is : ruby 1.9.3p125 (2012-02-16 revision 34643) [x86_64-linux] on Archlinux.

What can be the problem with this example?

Answer 1

The short answer to your problem is to change the line rb_require("sum"); to rb_require("./sum");. This is the change introduced in Ruby 1.9.2 where the current directory is no longer on the load path.

The more general problem is the way embedded Ruby deals with exceptions. The Pickaxe book (which I think is the book you're using, it uses a similar example) has this to say:

If the Ruby code raises an exception and it isn't caught, your C program will terminate. To overcome this, you need to do what the interpreter does and protect all calls that could raise an exception. This can get messy.

You'll need to look into using the rb_protect function to wrap calls to Ruby that might cause an exception. The Pickaxe book has an example of this.