hello.c

/*
  PROBLEM:
  >> h = Hello.new('hi!')
  => #<Hello:0x4b730>
  >> h.greet
  hi!=> nil
  >> h.greet
  hi!=> nil
  ..
  >> h.greet
  hi!=> nil
  >> h.greet
  => nil
  >> h.greet
  => nil
*/

#include <ruby.h>

VALUE cHello;

#define HELLO(obj)	(Check_Type(obj, T_DATA), (struct Hello*)DATA_PTR(obj))

struct Hello {
  char *greeting;
};

static void hello_free(struct Hello *hello)
{
  free(hello->greeting);
}

static void hello_mark(struct Hello *hello)
{
}

static VALUE hello_new(VALUE klass)
{
  struct Hello *hello;
  return Data_Make_Struct(klass, struct Hello, hello_mark, hello_free, hello);
}

static VALUE hello_init(VALUE obj, VALUE greeting)
{
  HELLO(obj)->greeting = RSTRING(greeting)->ptr;
  return Qnil;
}

static VALUE hello_greet(VALUE obj)
{
  printf(HELLO(obj)->greeting);
  return Qnil;
}

void Init_hello()
{
  cHello = rb_define_class("Hello", rb_cObject);
  rb_define_alloc_func(cHello, hello_new);
  rb_define_method(cHello, "initialize", hello_init, 1);
  rb_define_method(cHello, "greet", hello_greet, 0);
}