Compiling a Ruby Extension in Objective-C is rather confusing at first. In large part, this is due to mkmf. You can see an e-mail I sent to ruby-core, for some background:
My hope is to eventually create something similar to mkmf, sans the problems that have been dug into mkmf.
Before I finally get started, I'd like to note that I've attempted to keep this along the lines of "How to create a Ruby extension in C in under 5 minutes." In addition, I highly recommend reading "Writing your very own Ruby extension with C."
Mac OS X: The standard XCode should suffice. I'm using ruby-1.9.2-p0, installed via rvm.
Linux: You'll need to install objective-c (gobjc), in addition to gcc. Again, ruby-1.9.2-p0 via rvm.
More to come
This README is unfinished, at present. One special note is to read my comments in rb_fraction.m (method: fraction_initialize). You cannot call "Data_Get_Struct" as Ruby's macro performs Type checking on the datatype you want back. As you are storing an Objective-C Object, and not a C Struct, it will fail the Macro's type checking.