How to use Xcode 6 debugging for single C file
You need to have project in order to be able to debug in Xcode. Xcode has to know how to compile/link/build Your files. You can make it very quickly by creating Command line tool. Go to File -> New.. -> Project... ⌘⇧N. Choose OS X -> Application -> Command Line Tool. On the next screen choose language to be C. Then You'll have new project with main.c file where You can start coding and debugging.