Error: Failed to build gem native extension for ffi on Mac OS X

September 28th at 12:00am

I'm starting a new ruby project to run my LED Lightboard using this gem from github. After following the instructions to add the dream-cheeky-led gem to the gemfile I ran bundle and found this error:

/Users/rioncarter/.rvm/rubies/ruby-2.1.4/lib/ruby/2.1.0/mkmf.rb:456:in `try_do': The compiler failed to generate an executable file. (RuntimeError)
You have to install development tools first.
   from /Users/rioncarter/.rvm/rubies/ruby-2.1.4/lib/ruby/2.1.0/mkmf.rb:587:in `try_cpp'
   from /Users/rioncarter/.rvm/rubies/ruby-2.1.4/lib/ruby/2.1.0/mkmf.rb:1067:in `block in have_header'
   from /Users/rioncarter/.rvm/rubies/ruby-2.1.4/lib/ruby/2.1.0/mkmf.rb:918:in `block in checking_for'
   from /Users/rioncarter/.rvm/rubies/ruby-2.1.4/lib/ruby/2.1.0/mkmf.rb:351:in `block (2 levels) in postpone'
   from /Users/rioncarter/.rvm/rubies/ruby-2.1.4/lib/ruby/2.1.0/mkmf.rb:321:in `open'
   from /Users/rioncarter/.rvm/rubies/ruby-2.1.4/lib/ruby/2.1.0/mkmf.rb:351:in `block in postpone'
   from /Users/rioncarter/.rvm/rubies/ruby-2.1.4/lib/ruby/2.1.0/mkmf.rb:321:in `open'
   from /Users/rioncarter/.rvm/rubies/ruby-2.1.4/lib/ruby/2.1.0/mkmf.rb:347:in `postpone'
   from /Users/rioncarter/.rvm/rubies/ruby-2.1.4/lib/ruby/2.1.0/mkmf.rb:917:in `checking_for'
   from /Users/rioncarter/.rvm/rubies/ruby-2.1.4/lib/ruby/2.1.0/mkmf.rb:1066:in `have_header'
   from extconf.rb:16:in `<main>'

extconf failed, exit code 1

Gem files will remain installed in /Users/rioncarter/.rvm/gems/ruby-2.1.4/gems/ffi-1.9.10 for inspection.
Results logged to /Users/rioncarter/.rvm/gems/ruby-2.1.4/extensions/x86_64-darwin-14/2.1.0-static/ffi-1.9.10/gem_make.out
An error occurred while installing ffi (1.9.10), and Bundler cannot continue.
Make sure that `gem install ffi -v '1.9.10'` succeeds before bundling.

I was able to find the specific mkmf.log file by running this search command from my user directory:

find ~/.rvm -name mkmf.log

I found this file:

/Users/username/.rvm/gems/ruby-2.1.4/extensions/x86_64-darwin-14/2.1.0-static/ffi-1.9.10/mkmf.log

Which had this information in it:

Agreeing to the Xcode/iOS license requires admin privileges, please
re-run as root via sudo.

Apparently the solution is to open XCode and accept the license agreement, then try running bundle again.