How to read Ruby global variable using internal C API?

Question

In the README.EXT documentation (which comes with the Ruby platform source), it mentions rb_define_variable, which takes a C pointer to VALUE and "connects" it to a Ruby global variable, so reading/writing the variable from Ruby-land will access the given location in memory.

But I need something a little different: from C-land, I want to access global variables which were defined and initialized from within Ruby. I don't need to list all the global variables which are defined -- I just want to access the value of a global variable given its name, and I want to do it from the C side.

If all else fails, I know I can use rb_eval_string (which is the same as eval in Ruby), but there should be a better way!

Frederick Cheung · Accepted Answer · 2012-10-11 00:07:15Z

5

The method you want is rb_gv_get (itself a wrapper around some of the global variable stuff). With rubyinline you could write

require 'inline'
class Foo
  inline :C  do |builder|
    builder.c_raw_singleton <<SRC
      VALUE read_global(VALUE self, VALUE *name){
        return rb_gv_get(rb_string_value_cstr(name));
      }
SRC
  end
end

and then

$bar = 1
Foo.read_global('bar') #=> 1

(rubyinline doesn't work in irb).

answered Oct 11, 2012 at 0:07

Frederick Cheung

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1 Comment

Alex D Over a year ago

Nice! Thanks! I tried grepping for "global" and "variable" but didn't find anything...

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