~/swig/test
USER@DMW:> ls
example.c  example.i  extconf.rb

~/swig/test
USER@DMW:> more example.c
double My_variable = 3.0;

int fact(int n) {
        if(n <= 1)
                return 1;
        else
                return n * fact(n-1);
}

int my_mod(int n, int m) {
        return n % m;
}


~/swig/test
USER@DMW:> more example.i
%module example

%{
extern  double  My_variable;
extern  int             fact(int);
extern  int             my_nod(int n, int m);
%}

extern  double  My_variable;
extern  int             fact(int);
extern  int             my_mod(int n, int m);

~/swig/test
USER@DMW:> swig -ruby example.i

~/swig/test
USER@DMW:> more extconf.rb
require 'mkmf'
create_makefile('example')

~/swig/test
USER@DMW:> ruby extconf.rb
creating Makefile

~/swig/test
USER@DMW:> make
…….

~/swig/test
USER@DMW:> ls
Makefile   example.i  example.so*     example_wrap.o
example.c  example.o  example_wrap.c  extconf.rb

~/swig/test
USER@DMW:> irb

irb(main):001:0> require 'example'
=> true
irb(main):002:0> Example::fact(5)
=> 120
irb(main):003:0> quit