An editor is the program that you use to edit source code. Lots of different editors are available for Linux, but the most popular and full-featured editor is probably GNU Emacs.
Emacs is much more than an editor. It is an incredibly powerful program, so much so that at CodeSourcery, it is affectionately known as the One True Program, or just the OTP for short. You can read and send email from within Emacs, and you can customize and extend Emacs in ways far too numerous to discuss here. You can even browse the Web from within Emacs!
If you're familiar with another editor, you can certainly use it instead. Nothing in the rest of this book depends on using Emacs. If you don't already have a favorite Linux editor, then you should follow along with the mini-tutorial given here.
If you like Emacs and want to learn about its advanced features, you might consider reading one of the many Emacs books available. One excellent tutorial, Learning GNU Emacs, is written by Debra Cameron, Bill Rosenblatt, and Eric S. Raymond (O'Reilly, 1996).
You can start Emacs by typing emacs in your terminal window and pressing the Return key. When Emacs has been started, you can use the menus at the top to create a new source file. Click the Files menu, choose Open Files, and then type the name of the file that you want to open in the "minibuffer" at the bottom of the screen.  If you want to create a C source file, use a filename that ends in .c or .h. If you want to create a C++ source file, use a filename that ends in .cpp, .hpp, .cxx, .hxx, .C, or .H. When the file is open, you can type as you would in any ordinary word-processing program. To save the file, choose the Save Buffer entry on the Files menu. When you're finished using Emacs, you can choose the Exit Emacs option on the Files menu.
 If you're not running in an X Window system, you'll have to press F10 to access the menus.
If you don't like to point and click, you can use keyboard shortcuts to automatically open files, save files, and exit Emacs. To open a file, type C-x C-f. (The C-x means to hold down the Control key and then press the x key.) To save a file, type C-x C-s. To exit Emacs, just type C-x C-c. If you want to get a little better acquainted with Emacs, choose the Emacs Tutorial entry on the Help menu. The tutorial provides you with lots of tips on how to use Emacs effectively.
If you're accustomed to programming in an Integrated Development Environment (IDE), you'll also be accustomed to having the editor help you format your code. Emacs can provide the same kind of functionality. If you open a C or C++ source file, Emacs automatically figures out that the file contains source code, not just ordinary text. If you hit the Tab key on a blank line, Emacs moves the cursor to an appropriately indented point. If you hit the Tab key on a line that already contains some text, Emacs indents the text. So, for example, suppose that you have typed in the following:
int main ()
printf ("Hello, world\n");
If you press the Tab key on the line with the call to printf, Emacs will reformat your code to look like this:
int main ()
printf ("Hello, world\n");
Notice how the line has been appropriately indented.
As you use Emacs more, you'll see how it can help you perform all kinds of complicated formatting tasks. If you're ambitious, you can program Emacs to perform literally any kind of automatic formatting you can imagine. People have used this facility to implement Emacs modes for editing just about every kind of document, to implement games  , and to implement database front ends.
 Try running the command M-x dunnet if you want to play an old-fashioned text adventure game.
In addition to formatting your code, Emacs can make it easier to read C and C++ code by coloring different syntax elements. For example, Emacs can turn keywords one color, built-in types such as int another color, and comments another color. Using color makes it a lot easier to spot some common syntax errors.
The easiest way to turn on colorization is to edit the file ~/.emacs and insert the following string:
Save the file, exit Emacs, and restart. Now open a C or C++ source file and enjoy!
You might have noticed that the string you inserted into your .emacs looks like code from the LISP programming language. That's because it is LISP code! Much of Emacs is actually written in LISP. You can add functionality to Emacs by writing more LISP code.