How to install json gem – Failed to build gem native extension(mac 10.10)

Sad, however, JSON-1.7.7 (and upto 1.8.1) is incompatible with Ruby-2.2.x. As you are using Ruby-2.2.1, it will not work for you.

There are 2 options for you:

1. Update your json gem to 1.8.2 version. — Preferable
2. Edit ruby-2.2.1/ext/json/fbuffer/fbuffer.h file and replace problematic code with VALUE result = rb_str_new(FBUFFER_PTR(fb), FBUFFER_LEN(fb));. Look here for details