PS. Don't do this.

libfib.go

package main

// #cgo CFLAGS: -I /usr/include/ruby-2.1.0 -I /usr/include/x86_64-linux-gnu/ruby-2.1.0
// #cgo LDFLAGS: -lruby-2.1
// #include "ruby.h"
// extern VALUE rbGoFib(VALUE, VALUE);
import "C"
import "unsafe"

//export rbGoFib
func rbGoFib(self C.VALUE, n C.VALUE) C.VALUE {
	return (C.VALUE)((fib(int(n)>>1) << 1) | 1)
}

func fib(n int) int {
	if n < 2 {
		return n
	}
	return fib(n-1) + fib(n-2)
}

//export Init_libfib
func Init_libfib() {
	fib := C.rb_define_class(C.CString("Fib"), C.rb_cObject)
	C.rb_define_method(fib, C.CString("fib"), (*[0]byte)(unsafe.Pointer(C.rbGoFib)), 1)
}

func main() {}

/*
require './libfib'
fib = Fib.new
p fib.fib(6)
# => 8
*/