Modify Ruby array in C with RubyInline

Question

I have arrays in Ruby and I would like to extend them with .normalize method. This method should modify array such that all it`s elements sum up to 1. This is way too expensive in Ruby, so I want to do it in C with RubyInline.

require "rubygems"
require "inline"

class Array
inline do |builder|
    builder.c_raw '
     static VALUE normalize(VALUE self) {
        double total_size = 0, len;
        int i;

        VALUE* array = RARRAY_PTR(self);
        len = RARRAY_LEN(self);

        for(i=0; i < len; i++){
            total_size += NUM2DBL(array[i]);
        }

        for(i=0; i < len; i++){
            array[i] = INT2NUM(NUM2DBL(array[i])/total_size);
        }

        return array;
    }'
  end
end

a = [1,2,0,0,0,0,0,3,0,4]

puts a.normalize.inspect

This results in

$ ruby tmp.rb 
tmp.rb:29: [BUG] Segmentation fault
ruby 1.8.7 (2011-06-30 patchlevel 352) [x86_64-linux]

Aborted (core dumped)

EDIT: after some debugging, crash seems to come at

VALUE* array = RARRAY_PTR(self);

Are there several inline gems ? I have RVM and did `gem install inline`. Running your script : `undefined method 'inline' for Array:Class (NoMethodError)`. Checked the RDoc for this gem, no inline method. Seems to be an editor, not a C compiler. — BernardK, Dec 31 '12 at 12:08
I did run into this too. You need to install it with `gem install RubyInline`. — user1939480, Dec 31 '12 at 13:56
I have also found that it is at the beginning. I did very few Ruby-C extension 2 years ago, the macros have completely changed (before it was `RARRAY() -> ptr`). I'm not sure if `Check_Type(self, T_ARRAY);` is still valid. If I put it at the beginning of the block ==> `in 'normalize': wrong argument type false (expected Array) (TypeError)` — BernardK, Dec 31 '12 at 17:31
Have you seen the column **Related** on the right ? Among others -> http://stackoverflow.com/questions/4523597/passing-ruby-array-values-into-a-c-array?rq=1 — BernardK, Dec 31 '12 at 17:49

Frederick Cheung · Accepted Answer · 2012-12-31T21:21:55.780

There are a few things to fix here:

When you use c_raw rubyinline doesn't try to detect arity, and instead assumes that you want to use a variable number of arguments. You can either override this (pass :arity => 0) or change your method signature to

VALUE normalize(int argc, VALUE *argv, VALUE self)

At the moment rubyinline is assuming your method has that signature, so you're probably reinterpreting the integer 0 as a pointer.

Next up, at the moment you're always filling the arrays with zero, because all the array elements are < 1, and then you're converting to an integer so you get 0 - use rb_float_new to turn a double back into a ruby Float.

Lastly, your return value is wrong, it's a VALUE * instead of a VALUE. You probably want to return self instead.

Finally it would be more ruby-like to call your method normalize!. By default ruby inline extracts the method name from the c function name, which of course doesn't let you use exclamation marks like that. You can override that with the method_name option.

Alltogether, my version of your example looks like

require "rubygems"
require "inline"

class Array
  inline do |builder|
    builder.c_raw <<-'SRC', :method_name => 'normalize!', :arity => 0
     static VALUE normalize(VALUE self) {
        double total_size = 0;
        size_t len, i;

        VALUE* array = RARRAY_PTR(self);
        len = RARRAY_LEN(self);

        for(i=0; i < len; i++){
            total_size += NUM2DBL(array[i]);
        }
        for(i=0; i < len; i++){
            array[i] = rb_float_new((NUM2DBL(array[i])/total_size));
        }

        return self;
    }
    SRC
  end
end

a = [1,2,0,0,0,0,0,1,0,4]

puts a.normalize!.inspect

Great ! I'll use this solution the next time I need to write some C glue code. Note that in order to avoid a compilation error (`error: expected identifier or ‘(’ before ‘int’`), I had to replace `size_t len, int i;` by `int len, i;`. — BernardK, Dec 31 '12 at 20:58

Modify Ruby array in C with RubyInline

1 Answers1