How to pass keywords to a Ruby method from a C extension?

Question

I'm working on a C extension for Ruby and I want to call a method which has required keyword arguments, like this:

class Word 
  def initialize(line:, col:, value:)
  end 
end

In C, I'm familiar, with calling Ruby methods via rb_funcall and rb_funcallv, but I can't figure out how to pass keyword arguments!

Here are a few things I've tried:

Pass a hash as the last positional argument with rb_funcall:

VALUE kwargs = rb_hash_new();
rb_hash_aset(kwargs, rb_intern("name"), rb_str_new2(name));
// ... 
rb_funcall(Word_Class, rb_intern("new"), 1, kwargs);
// result: ArgumentError: wrong number of arguments (given 1, expected 0)

Pass it as the last member of the argv array with rb_funcallv:

// initialize `kwargs` as above
VALUE argv = rb_ary_new();
rb_ary_push(argv, kwargs);
rb_funcallv(Word_Class, rb_intern("new"), 1, &argv);
// result: ArgumentError: wrong number of arguments (given 1, expected 0)

Pass 0 as argc, even though argv is length 1:

// initialize `argv` as above
rb_funcallv(Word_Class, rb_intern("new"), 0, &argv);
// ArgumentError: missing keywords: line, col, value

Is it possible? How is it done? Is there anything else I can try?

I *think* you should be able to just pass in a hash (`rb_hash_new` for creating it, `rb_hash_aset` for setting values). Let me know if that works, than I can turn it into an answer. — Michael Kohl, Oct 29 '16 at 03:01
Thanks for your suggestion! I updated the question with a few things I've tried ... is there another way I could try passing the hash as keywords? — rmosolgo, Oct 29 '16 at 11:14
You’re on the right lines with your first attempt (passing a hash as the last arg), but the keys need to be symbols, not IDs. Try `ID2SYM(rb_intern("name"))` rather than just `rb_intern("name")`. I don’t know if there’s a way of converting a `char *` in C directly into a ruby symbol in one step, it looks like you need to create an ID then convert to symbol. — matt, Oct 29 '16 at 14:48
@matt it works! Could you add that suggestion as an "Answer" so that I can accept it and give you magical stack overflow points? — rmosolgo, Oct 29 '16 at 20:37
I think @MichaelKohl gets priority in claiming those points. — matt, Oct 30 '16 at 14:13

score 2 · Accepted Answer · answered Oct 31 '16 at 01:35

You can pass in a hash. Note that to create symbol keys you need a call of the form ID2SYM(rb_intern(char*)) since rb_intern returns an ID, which ID2SYM turns into an actual Ruby symbol.

VALUE kwargs = rb_hash_new();
rb_hash_aset(kwargs, ID2SYM(rb_intern("name")), rb_str_new2(name));
// ... 
rb_funcall(Word_Class, ID2SYM(rb_intern("new")), 1, kwargs);

How to pass keywords to a Ruby method from a C extension?

1 Answers1