The Arduino IDE is a thin wrapper around the avr-gcc compiler and the avr-libc run-time library. It also includes a fairly basic IDE, i.e. a text editor and conventions for managing projects, in the form of “sketches” and libraries.

I prefer to use my own programmer’s editor, because it supports multiple programming languages and has a lot more features for software development (such as Git integration, code folding, and save-on-lose-focus). The Arduino IDE supports external editors by disabling its own one – which is an option in the preferences:

Now I can simply use my own editor and switch to the Arduino IDE for compiling and uploading.

One advantage of using an external editor, is that you can look at other source code than just your own sketches. In the rest of this post, I’m going to describe how to look at one of the most interesting parts of the Arduino IDE: its run-time library, i.e. the Wiring code which adds supports for everything which makes an Arduino different from the ATmega’s on which it is based.

Note: what follows is specific for Mac OSX, but apart from the location of these files and the editor used, you should be able to transpose all of this to your own beloved computer environment.

The first task, is to figure out where the Arduino IDE’s run-time code is located. In Mac OSX, this code is located inside the Arduino application. To view this area, you can right-click on the Arduino app:

This leads to the following directory structure:

The interesting bits are inside the “hardware” folder:

Here are the first few lines of “Arduino.h”, for example (this used to be “WProgram.h”):

These source files are where everything specific to the Arduino IDE’s runtime happens. The “Serial” object, and the “millis()” code, for example. If you want to really understand what the Arduino is about, then it’s well worth going through some of these files. As you’ll find out, there’s no magic once you start looking in the right places.

Try it!