Is it possible to use Ruby FFI to send a Ruby (C API) VALUE to a function?

Question

I'm currently trying to bridge Numo::NArray and FFI, so that an FFI Pointer can access the raw data in a Numo::NArray without having to copy data through String like I do now (thus causing GC churn and a wasted extra copy).

There is a C function exposed by Numo::NArray called nary_get_pointer_for_read_write, and this is used already by Numo::FFTW. However, the parameter to this function is CRuby's internal VALUE type, and I haven't found a way to pass that VALUE using FFI.

Here's a non-working rough draft of what I'd like to try. If this isn't possible, my next step might be writing another C extension gem to do the bridging, and after that maybe trying to PR a function to get an FFI pointer into the numo-narray project.

require 'numo/narray'
require 'ffi'

module NArrayFFI
  ffi_lib FFI::CURRENT_PROCESS
  attach_function :nary_get_pointer_for_read, [:RAW_RUBY_VALUE_IF_FFI_CAN_DO_THIS], :pointer
  attach_function :nary_get_pointer_for_write, [:RAW_RUBY_VALUE_IF_FFI_CAN_DO_THIS], :pointer
  attach_function :nary_get_pointer_for_read_write, [:RAW_RUBY_VALUE_IF_FFI_CAN_DO_THIS], :pointer
end

data = Numo::SFloat[1,2,3,4,5]
data_ptr = NArrayFFI.nary_get_pointer_for_read_write(data)
SomeOtherFFI.function_that_takes_pointer(data_ptr)

So, is it possible to pass the raw VALUE of a Ruby value to a function attached using FFI?

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Here are references I've checked so far, that didn't have an answer, but might be useful to others with similar questions:

• Ruby's C extension docs
• An open issue on Numo::NArray asking the same question
• The header definitions from Numo::NArray
• The source code of Numo::FFTW
• The Ruby FFI types documentation