/* file example.c */
int fact(int n) {
     if (n <= 1) return 1;
     else return n*fact(n-1);
 }

/* file example.i */
%module example
%{
extern int fact(int n);
%}
extern int fact(int n);

/* file extconf.rb */
require 'mkmf'
create_makefile('example')

/* file test.rb */
require 'example'
puts Example.fact(4)

/* sent into terminal stdin from the directory all these files are in */
$ swig -ruby example.i
$ ruby extconf.rb
creating Makefile
$ make
compiling example_wrap.c
compiling example.c
linking shared object example.so
$ ruby test.rb
/usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require': cannot load such file -- example (LoadError)
	from /usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require'
	from test.rb:1:in `<main>'

/* versions (output) */
$ ruby --version
ruby 1.9.3p286 (2012-10-12 revision 37165) [x86_64-linux]
$ swig -version

SWIG Version 2.0.8

Compiled with g++ [x86_64-unknown-linux-gnu]

Configured options: +pcre

Please see http://www.swig.org for reporting bugs and further information