howto: d & ruby ffi

Makefile

test: test_c.o test_d.o
    cc -shared -m32 test_c.o test_d.o -o test.so -lphobos2

test_c.o: test.c
    gcc -c -m32 test.c -o test_c.o

test_d.o: test.d
    dmd -c -m32 test.d -oftest_d.o

test.c

extern void attach(void);
extern void detach(void);

void __attach(void) __attribute__((constructor));
void __detach(void) __attribute__((destructor));

void __attach(void) { attach(); }
void __detach(void) { detach(); }

test.d

// the following two links are very useful:
// 1) http://stackoverflow.com/questions/9956994/return-a-class-instance-as-a-pointer-in-d
// 2) http://stackoverflow.com/questions/9759880/automatically-executed-functions-when-loading-shared-libraries

module test;

import std.conv;
import core.memory : GC; 
import std.c.stdlib : malloc, free;

import core.runtime : Runtime;
extern (C) export void attach() { Runtime.initialize(); }
extern (C) export void detach() { Runtime.terminate(); }

public struct Foo {
    public string hello = "hello, world!";
}

extern (C) size_t foo_size = Foo.sizeof; 

import std.stdio : printf;

extern (C) void* foo_new() {
    Foo* p = cast(Foo*)malloc(Foo.sizeof);
    std.conv.emplace(p);
         
    GC.addRange(p, Foo.sizeof);
    return p;
}

extern (C) void foo_free(Foo* p) {
    GC.removeRange(p);
    free(cast(void*)p);
}

extern (C) immutable(char)* foo_hello(Foo* p) {
    return p.hello.ptr;
}

void main() {}

test.rb

#!/usr/bin/env ruby
require 'ffi'

module MyLibrary
    extend FFI::Library
    ffi_lib './test.so'
    attach_function :foo_hello, [:pointer], :string
    attach_function :foo_new, [], :pointer
    attach_function :foo_free, [:pointer], :void
end

class Foo 
    def initialize
        @ptr = MyLibrary.foo_new
        # using FFI::AutoPointer causes too much overhead.
        ObjectSpace.define_finalizer @ptr, Foo.finalize(@ptr)
    end 

    def hello
        @hello ||= MyLibrary.foo_hello @ptr
    end 

    def self.finalize ptr 
        proc { MyLibrary.foo_free ptr }
    end 
end

require 'benchmark'
10000.times { Foo.new } # warming up
GC.start

# timings on my machine:
#  
# 0.154811253             ruby 1.9.3p125 (2012-02-16 revision 34643) [i686-linux]
# 2.536235094             rubinius 1.2.4
#     --                  jruby 1.6.6 (ruby-1.8.7-p357) (2012-01-30 5673572) (Java HotSpot(TM) Client VM 1.7.0_03) [linux-i386-java]
#
# JRuby didn't manage to load D library:
#     LoadError: Could not open library './test.so' : ./test.so: undefined symbol: __data_start

p Benchmark.realtime { 10000.times { Foo.new } }