From: Tim Hunter <TimHunter@...>
Date: 2009-01-01T07:48:23+09:00
Subject: Re: Ruby arguments into C function are warped

Daniel Waite wrote:
> I want to code a few methods in C for speed reasons. The Pick Axe book
> (2nd edition) is a wonderful guide to writing Ruby code in C, but that's
> not what I want. The benchmarks I ran for the Ruby-in-Ruby and Ruby-in-C
> programs showed similar results. So allow me to clarify:
> 
> I do not want a Ruby array in C. I want a C array in C.
> 
> Which I have done...
> 
> int array[1000000];
> 
> static VALUE t_add(VALUE self, VALUE obj, VALUE i)
> {
>   array[i] = obj;
>   return 1;
> }
> 
> static VALUE t_include(VALUE self, VALUE obj)
> {
>   int size = sizeof(array) / sizeof(int);
>   int index;
> 
>   for(index = 0; index < size; index++) {
>     if(FIX2INT(obj) == FIX2INT(array[index])) {
>       return Qtrue;
>     }
>   }
> 
>   return Qfalse;
> }
> 
> static VALUE t_at(VALUE self, VALUE obj)
> {
>   return array[obj];
> }
> 
> The value of "obj" comes from within Ruby (IRB, actually). But unless I
> coerce it using FIX2INT, it's always a weird value. Say I put in 1,
> it'll tell me it received 31478 or something similar.
> 
> Another thing... I thought that if I passed in an 'a' or any other
> non-integer value into the add method, I'd get an error. I don't. In
> fact...
> 
> require 'my_test'
> mt = MyTest.new
> mt.add('a', 0)
> mt.at(0) # 'a'
> 
> But...
> 
> mt.include?('a') # false
> 
> So... what's happening to my arguments as they pass through Ruby and
> into C?

Notice that you've declared the arguments to t_add as VALUEs, not ints. 
This is because Ruby passes VALUE arguments to your function. A VALUE is 
a reference to a Ruby object. If you want to store the arguments in an 
int array, convert the objects to ints via FIX2INT or some other 
conversion function:

static VALUE t_add(VALUE self, VALUE obj, VALUE i)
{
    array[FIX2INT(i)] = FIX2INT(obj);
    return self;
}

A VALUE can reference any Ruby object, but the FIX2INT macro expects its 
argument to be a Ruby Fixnum. If it's not, it will raise an exception. 
You'll get an exception if you call mt.add("a", 0).

Also, the return value from t_add should be a VALUE, not an int. It's 
your choice which VALUE to return. Here I chose self.

-- 
RMagick: http://rmagick.rubyforge.org/