Writing C Unit Tests in Ruby

Let’s say you usually code in Ruby, and your company and its build systems are built around Rakefiles and the like. Today you’ve written some C code, and you want to add unit tests. In this blog post, I present a method of writing those C unit tests in Ruby using FFI and RSpec.

/**
 * This is a very silly function that clearly requires some unit tests.
 */
int foo_count_letters(const char *source, size_t *count)
{
  if (!source || !count) return 0;

  for (*count = 0; *source; (*count)++, source++)
    ;

  return 1;
}

First, compile your C code as position independent and symbols exported. This allows you to dlopen() the executable:

$ gcc -pie -rdynamic -o foo foo.c

Next, add the FFI gem to your Gemset, in Gemfile:

source :rubygems

gem 'ffi'

Then write your rspec tests spec/foo_spec.rb:

#!/usr/bin/env ruby
require 'ffi'

# This module is your bridge from Ruby to C and back
module FOO
  extend FFI::Library

  # Use an absolute path to the executable under test, otherwise ffi will search LD_LIBRARY_PATH.
  ffi_lib File.absolute_path(File.join(File.dirname(__FILE__), "..", "foo"))

  # Function signatures for each function to be tested
  attach_function :foo_count_letters, [:string, :pointer], :int
end

describe "unit tests for foo.c" do
  before(:each) do
  end

  it "should really foo" do
    # This function takes a pointer-to-uint32 out-param
    out = FFI::MemoryPointer.new :uint32

    res = FOO.foo_count_letters("Hello", out)

    # Read back the pointers to Ruby data types, then use rspec's verification functions
    out.get_uint32(0).should == 5
    res.should == 1
  end
end

I’m excited about this approach because the tests run under rspec along with the rest of your spec tests.