Before using the WiringPi GPIO library, you need to include its header file in your programs:

#include <wiringPi.h>

You may also need to add

-I/usr/local/include -L/usr/local/lib -lwiringPi

to the compile line of your program depending on the environment you are using. The important one is -lwiringPi

You may also need additional #include lines, depending on the modules you are using.

Reference/API Pages

These two above are the most important features of wiringPi. Below are additional functions and libraries that comprise the main wiringPi release.