extconf.rb

require "mkmf"
create_makefile("printer")

irb

bakkdoor:~/projekte/ruby/c-ext> ruby extconf.rb 
creating Makefile
bakkdoor:~/projekte/ruby/c-ext> make
gcc -shared -o printer.so printer.o -L. -L/usr/lib -L. -Wl,-Bsymbolic-functions -rdynamic -Wl,-export-dynamic    -lruby1.8  -lpthread -lrt -ldl -lcrypt -lm   -lc
bakkdoor:~/projekte/ruby/c-ext> irb
irb(main):001:0> require "printer"
=> true
irb(main):002:0> p = Printer.new
=> #<Printer:0x7fd6227f4c70>
irb(main):003:0> p.println "fooo bar baz!"
fooo bar baz!
=> nil
irb(main):004:0> p.println [1,2,3]
1
2
3
=> nil
irb(main):005:0> p.println [1,2,3].inspect
[1, 2, 3]
=> nil

printer.c

#include <ruby.h>

static VALUE printer_init(VALUE self)
{
  return self;
}

static VALUE printer_println(VALUE self, VALUE obj)
{
  rb_funcall(self, rb_intern("puts"), 1, obj);
}

VALUE printer_class;

void Init_printer()
{
  printer_class = rb_define_class("Printer", rb_cObject);
  rb_define_method(printer_class, "initialize", printer_init, 0);
  rb_define_method(printer_class, "println", printer_println, 1);
}