Ruby to C, a couple snippets

As you might already know, the Ruby interpreter you’re (probably) using (the one at ruby-lang.org) know as MRI, for Matz Ruby Interpreter, is written in C. The VM is entirely written in C and most of the standard libraries too. So it seems that the way to make code run faster is really to dive into C.

Now I hear you yell with your ears all red, eyes full of blood and a big vein pumping on your forehead: But it will be unmaintainable!

If everything seems under control, you’re not going fast enough

– Mario Andretti

The hard part is that the C API of MRI lacks documentation. You either have to search through the mailinglist, look at other people’s code or try to guess from Ruby’s code. I recently ported a bit of code from Ruby to C and here are a couple of Ruby to C mapping that might help you.

If you’ve never written or looked at a Ruby extension before, I suggestion reading Peter Cooper excellent and very simple tutorial on how to set things up.

Objects are VALUEs

my_var = nil this_is_true = true some_fixnum = 1

VALUE my_var = Qnil; VALUE this_is_true = Qtrue; VALUE some_fixnum = INT2FIX(1);

Strings

string = "hi"

VALUE string = rb_str_new2("hi"); /* Ruby string to C string */ char *s = RSTRING_PTR(str); int len = RSTRING_LEN(str);

Object to C struct

In C you’ll probably want to store your data in a struct. Ruby provide some things to wrap a struct inside a Ruby object. Also, since Ruby is not aware of the stuff created in the C world we have to be pretty explicit about everything.

module MyModule
  class MyClass
    def close
      @closed = true
    end
  end
end


In C
/* This is called by Ruby GC when the object is freed
 * so free all resources used here. We hook it up
 * in the Data_Wrap_Struct call down there.  */
void my_obj_free(my_struct_t *s)
{
  free(s);
}

/* Called by Ruby then and instance of your class is created
 * hooked by rb_define_alloc_func. */
VALUE my_obj_alloc(VALUE klass)
{
  my_struct_t *s = ALLOC_N(my_struct_t, 1);
  /* This stores the struct inside the Ruby object, so you
   * can get the struct back on each method call. */
  return Data_Wrap_Struct(klass, NULL, my_obj_free, s);
}

/* The actual MyClass#close method, first argument is the
 * instance object. It's hook into our class by the rb_define_method
 * call in Init_ */
VALUE my_obj_close(VALUE self)
{
  my_struct_t *s;
  /* This is where we get the struct back from the Ruby object */
  Data_Get_Struct(self, my_struct_t, s);
  
  s->closed = 1;
}

/* Init_* is magically called by Ruby to bootstrap your extension,
 * it's like a main function if you will. */
void Init_name_of_your_extension()
{
  VALUE mMyModule = rb_define_module("MyModule");
  VALUE cMyClass = rb_define_class_under(mMyModule, "MyClass", rb_cObject);  

  rb_define_alloc_func(cMyClass, my_obj_alloc);
  rb_define_method(cMyClass, "close", my_obj_close, 0);
}


Calling methods



obj.method_name

result = begin
  obj.method_name(arg)
rescue
  # …
end




rb_funcall(obj, rb_intern("method_name"), 0);
/* if an error is raised, result will be set to Qundef */
VALUE result = rb_funcall_rescue(obj, rb_intern("method_name"), 1, arg);





Blocks



my_ary.each do |i|
  puts i
end




VALUE i_each(VALUE elem, VALUE *obj)
{
  /* elem is the yielded object and obj is a pointer to obj down there v */
  printf("%d\n", FIX2INT(elem));
  return Qnil;
}
rb_iterate(rb_each, my_ary, i_each, (VALUE) obj);





Error handling



raise ArgumentError, "..."




rb_raise(rb_eArgError, "...");
/* To raise an error corresponding to the one in errno */
rb_sys_fail("oops");





Garbage Collection
When creating Ruby objects in the C world, Ruby is not aware of the scope of each, it’s impossible to track when a variable is used and when it’s not, so it’s impossible for the garbage collector to know when do leave or sweep objects. If you create a long live object, that will persist for several method calls you have to tell Ruby’s garbage collector when it’s in use and when you’re finished with it. If you don’t, Ruby GC will free your variable in the worst possible time, ending up with unthinkable tragedies around the world, leaving you homeless and poor for all eternity.
VALUE var = rb_str_new2("prout");

/* Tell Ruby GC this object is in use */
rb_gc_register_address(&var);
/* when you're finished you can unregister it and let the GC free it */
rb_gc_unregister_address(&var);


Note that you don’t need to do this if you only use the variable in one method since the control is not given back to Ruby during the method call. Also if the variable is still referenced in the Ruby world it’s already registered.
More, more, more!
This is far from a complete guide to MRI, you’ll need to dig the Ruby doxygen doc if you need more info.
Now lets port Rails to C!