Can't install json with gem - ruby-on-rails

Question

I have tried following command in terminal (OSX 10.6.6) and end up with an error. Does anyone know how to solve this?
sudo gem install json
Building native extensions. This could take a while...
ERROR: Error installing json:
ERROR: Failed to build gem native extension.
/usr/local/bin/ruby extconf.rb
checking for ruby/re.h... *** extconf.rb failed ***
Could not create Makefile due to some reason, probably lack of
necessary libraries and/or headers. Check the mkmf.log file for more
details. You may need configuration options.
Provided configuration options:
--with-opt-dir
--without-opt-dir
--with-opt-include
--without-opt-include=${opt-dir}/include
--with-opt-lib
--without-opt-lib=${opt-dir}/lib
--with-make-prog
--without-make-prog
--srcdir=.
--curdir
--ruby=/usr/local/bin/ruby
/usr/local/lib/ruby/1.9.1/mkmf.rb:368:in `try_do': The complier failed to generate an executable file. (RuntimeError)
You have to install development tools first.
from /usr/local/lib/ruby/1.9.1/mkmf.rb:452:in `try_cpp'
from /usr/local/lib/ruby/1.9.1/mkmf.rb:834:in `block in have_header'
from /usr/local/lib/ruby/1.9.1/mkmf.rb:693:in `block in checking_for'
from /usr/local/lib/ruby/1.9.1/mkmf.rb:280:in `block (2 levels) in postpone'
from /usr/local/lib/ruby/1.9.1/mkmf.rb:254:in `open'
from /usr/local/lib/ruby/1.9.1/mkmf.rb:280:in `block in postpone'
from /usr/local/lib/ruby/1.9.1/mkmf.rb:254:in `open'
from /usr/local/lib/ruby/1.9.1/mkmf.rb:276:in `postpone'
from /usr/local/lib/ruby/1.9.1/mkmf.rb:692:in `checking_for'
from /usr/local/lib/ruby/1.9.1/mkmf.rb:833:in `have_header'
from extconf.rb:17:in `'
Gem files will remain installed in /usr/local/lib/ruby/gems/1.9.1/gems/json-1.5.1 for inspection.
Results logged to /usr/local/lib/ruby/gems/1.9.1/gems/json-1.5.1/ext/json/ext/generator/gem_make.out

Answer 1

You will need to install XCode. It is available on the AppStore
If XCode is installed and you are still getting this error, maybe you should try using rvm

Answer 2

The reason you're getting this error is because the json gem includes C/C++ extensions that need to be compiled on your machine before they can run.
Just installing XCode won't fix it, you need to specifically install the XCode Command Line Tools. It's a compiler/linker toolchain including C/C++ compilers, make, etc.
There's an option to install them within a menu inside the XCode GUI, see this thread for details:
Xcode 4.4 and later install Command Line Tools

Answer 3

You should be able to get the necessary tools installed by running:
> xcode-select --install