Mixing FFI, Fiddle, and C Extension in Ruby

Introduction

If you're working with Ruby and need to invoke a function written in C language, there are some convenient gems available: Ruby-FFI and Fiddle.

Ruby-FFI has many features, handling most challenges you might encounter. Fiddle might seem a bit less convenient, but being an official Ruby gem, it is available from the start in most environments.

When You Want to Use Both FFI or Fiddle and C Extension

There can be situations where you want to rewrite certain parts of your Gem implemented with FFI or Fiddle into C extensions. Function calls using libffi are known to be nearly 100 times slower than that of native C extensions. If large numbers of calls need to be made with a demand for speed, you might want to consider rewriting the FFI-implemented function using C extensions.

The Method

Basic Principles

The main challenge here is determining how to handle the pointer of FFI or Fiddle's structure as an argument in a C extension function. The solution is straightforward: get the memory address from the Fiddle::Pointer or FFI::Pointer Ruby objects.

Here you will learn how to write a C Extension function that takes FFI::Pointer as an argument, referring to the rcairo gem.

Check for the existence of the constant FFI::Pointer

We begin by ensuring the constant FFI::Pointer is defined. This step verifies that require "ffi" has been executed and Fiddle::Pointer is available.

  if (NIL_P (rb_cairo__cFFIPointer))
    {
      rb_raise (rb_eNotImpError,
                "%s: FFI::Pointer is required",
                rb_id2name (rb_frame_this_func ()));
    }

The rb_cairo__cFFIPointer is pre-set in Init_cairo_private.

void
Init_cairo_private (void)
{
  // -- code omission --
  if (rb_const_defined (rb_cObject, rb_intern ("FFI")))
    {
      rb_cairo__cFFIPointer =
        rb_const_get (rb_const_get (rb_cObject, rb_intern ("FFI")),
                      rb_intern ("Pointer"));
    }
  else
    {
      rb_cairo__cFFIPointer = Qnil;
    }
}

In the case of Fiddle, execute:

rb_const_get (rb_const_get (rb_cObject, rb_intern ("Fiddle")), rb_intern ("Pointer"));

Ensure Argument Type Consistency

After confirming the constant, we ensure the argument classes are consistent. As FFI::Pointer and Fiddle::Pointer obtain addresses using relatively common names - address and to_i respectively, performing a type check helps prevent errors.

  if (!RTEST (rb_obj_is_kind_of (pointer, rb_cairo__cFFIPointer)))
    {
      rb_raise (rb_eArgError,
                "must be FFI::Pointer: %s",
                rb_cairo__inspect (pointer));
    }

Acquiring the Address

With FFI, you can get the address with the address method.

# Ruby-FFI
pt = FFI::MemoryPointer.new(:int)
p pt.address

With Fiddle, to_i method helps in getting the address.

# Fiddle
pt = Fiddle::Pointer.new(Fiddle::SIZEOF_INT)
p pt.to_i

In C extensions, these Ruby methods are invoked using rb_funcall.

rb_funcall (ffi_pointer, rb_intern ("address"), 0)
rb_funcall (fiddle_pointer, rb_intern ("to_i"), 0)

Call a C Function Using the Acquired Address as an Argument

The above Ruby code is executed within the C extension code.

VALUE rb_cr_address;
rb_cr_address = rb_funcall (pointer, rb_intern ("address"), 0);
cr = NUM2PTR (rb_cr_address);
cr_check_status (cr);

Here, the NUM2PTR macro is not provided by ruby.h so you'll need to define it yourself:

#if SIZEOF_LONG == SIZEOF_VOIDP
#  define PTR2NUM(x)   (ULONG2NUM((unsigned long)(x)))
#  define NUM2PTR(x)   ((void *)(NUM2ULONG(x)))
#else
#  define PTR2NUM(x)   (ULL2NUM((unsigned long long)(x)))
#  define NUM2PTR(x)   ((void *)(NUM2ULL(x)))
#endif

The cr_check_status function calls the native Cairo function cairo_status_to_string. It's safe to insert a function like this in the middle.

Creating a Ruby Object

To create a Ruby object from the obtained address, do as follows:

rb_cr = rb_obj_alloc (self);
cairo_reference (cr);
RTYPEDDATA_DATA (rb_cr) = cr;
rb_ivar_set (rb_cr, cr_id_surface, Qnil);

Use rb_obj_alloc to create an instance of the class (self, in this case). cairo_reference() is a Cairo function that increases the reference count, which ensures Garbage Collection won't remove your ruby-FFI object. RTYPEDDATA_DATA is used to access the data of TypedData Objects directly. Lastly, rb_ivar_set sets an instance variable.

Basic Example

Let's walk through a basic example. For this, piyo.h and piyo.c have been prepared as targets to create bindings.

piyo.h

#ifndef PIYO_H
#define PIYO_H

#include <stdio.h>

typedef struct Piyo
{
    int age;
    char *name;
} Piyo;

void displayPiyoInfo(const Piyo *piyo);

#endif

piyo.c

#include "piyo.h"

void displayPiyoInfo(const Piyo *piyo)
{
  printf("Name: %s\n", piyo->name);
  printf("Age: %d\n", piyo->age);
}

Write the C extension so that the following code functions correctly:

require 'fiddle/import'
require_relative './piyo.so'

module Piyo
  Piyo = Fiddle::Importer.struct(['int age', 'char* name'])
end

tori_name = 'piyoko'

Piyo::Piyo.malloc(Fiddle::RUBY_FREE) do |piyo|
  piyo.age = 100
  piyo.name = tori_name
  Piyo.display_info(piyo)
end

Ruby C Extension

piyo_rb.c

#include "ruby.h"
#include "piyo.h"

#if SIZEOF_LONG == SIZEOF_VOIDP
#define PTR2NUM(x) (ULONG2NUM((unsigned long)(x)))
#define NUM2PTR(x) ((void *)(NUM2ULONG(x)))
#else
#define PTR2NUM(x) (ULL2NUM((unsigned long long)(x)))
#define NUM2PTR(x) ((void *)(NUM2ULL(x)))
#endif

VALUE rb_cFiddlePointer;

VALUE rb_display_info(VALUE self, VALUE piyo)
{
  Piyo *ptr;

  VALUE rb_address = rb_funcall(piyo, rb_intern("to_i"), 0);
  ptr = NUM2PTR(rb_address);

  displayPiyoInfo(ptr);

  return Qnil;
}

void Init_piyo(void)
{
  VALUE mPiyo = rb_define_module("Piyo");
  rb_define_singleton_method(mPiyo, "display_info", rb_display_info, 1);
}

Create Makefile with:

extconf.rb

require 'mkmf'

find_header('piyo.h', __dir__)
create_makefile('piyo')

Compile with:

ruby extconf.rb
make

Execute with:

ruby test.rb

If everything runs correctly, you should see the output as:

Name: piyoko
Age: 100

While this is a simple example and doesn't include every aspect, such as class definition verification and argument type checks, you will need to add these elements to transition it into a practical gem.

That's all for this post.

---

This article was translated from Japanese to English by a collaboration of ChatGPT, DeepL, and the author. The author, despite having the weakest command of English among the three, played a crucial role in providing instructions to ChatGPT and DeepL. In Japanese, 'Piyo' represents the chirping sound of a chick and is often used as a meta-syntax variable, following 'hoge' and 'fuga'.