Mixing Ruby code and C extension code in a single file

inline_cext.rb

require 'tmpdir'
require 'rbconfig'

def inline_c_extension(c_code)
  Dir.mktmpdir('inline_c_extension') do |dir|
    File.write("#{dir}/cext.c", c_code)
    File.write("#{dir}/extconf.rb", <<~RUBY)
    require 'mkmf'
    create_makefile('cext')
    RUBY

    out = IO.popen([RbConfig.ruby, 'extconf.rb'], chdir: dir, &:read)
    raise "ruby extconf.rb failed: #{$?.inspect}\n#{out}" unless $?.success?

    out = IO.popen(['make'], chdir: dir, &:read)
    raise "make failed: #{$?.inspect}\n#{out}" unless $?.success?

    require "#{dir}/cext.#{RbConfig::CONFIG['DLEXT']}"
  end
end

inline_c_extension <<~C
#include "ruby.h"

static VALUE fibonacci(VALUE self, VALUE n) {
  long prev = 0, cur = 1;
  for (int i = 0; i < FIX2INT(n); i++) {
    long next = prev + cur;
    prev = cur;
    cur = next;
  }
  return LONG2FIX(prev);
}

void Init_cext(void) {
  VALUE c = rb_define_class("Fib", rb_cObject);
  rb_define_singleton_method(c, "fibonacci", fibonacci, 1);
}
C

(0..20).each { |i| p [i, Fib.fibonacci(i)] }