Accessing GPIO pins in C on AVR8 involves setting up three different registers: DDRx, PORTx and PINx, each GPIO pin has a bit in each of these registers for setting direction, writing the state and reading the state, this means that if you want to define a constant for a port then you'd need up to four constants, which is not very D.R.Y., this is a particular problem when you want to define the GPIO ports one place to ease porting.

The avr8gpio.h header allows rolling two bits of information into one constant: The port and the pin, so you can finally define a GPIO pin with a single constant, like so:

#include "avr8gpio.h"

#define LED GPB0

void init() {
GPOUTPUT(LED);
}

void setLed(uint8_t on) {
GPWRITE(LED, on);
}