You maybe don't know or probably didn't think about this but you are using program preprocessing before compiling it. As I said before compiling I mean, that each time you are compiling your project, C compiler prepares program file to be ready for compilation. Preprocessor includes other files to main file, defines symbol constants and macros, prepares conditional compilation of code. All preprocessor tasks are marked with ampersand symbol “#”. Lets go through most of directives of preprocessor:
#include

A series of instructions that are constantly repeated. Also called an "infinite loop," it can be caused by an error in the program or be intentional. For example, a screen demo on continuous replay just keeps repeating itself.

Probably you have noticed that many programs have similar structure like:

This is as simple routine how to read 4x4 keypad keys using AVR-GCC language. The keypad is connected to AVR microcontroller 8 bit port. In this example it is B port. You can change ports depending on your needs – this is only an example ant it is not the only way to this.