GPIO

General-purpose input/output (GPIO) is a generic pin on an integrated circuit or computer board whose behavior—including whether it is an input or output pin—is controllable by the user at run time. Wikipedia

Hardware Perspective

One powerful feature of the Raspberry Pi is the row of GPIO (general purpose input/output) pins along the top edge of the board. Homepage

The comprehensive GPIO Pinout guide for the Raspberry Pi. Pinout Homepage

The Raspberry Pi offers up its GPIO over a standard male header on the board. Over the years the header has expanded from 26 pins to 40 pins while maintaining the original pinout. Sparkfun Raspberry gPIo

One of the great things about the Raspberry Pi is that it has a GPIO connector to which you can attach external hardware. Adafruit GPIO Setup

The Raspberry Pi GPIOs can be controlled using many programming languages. ELinux RPi GPIO Code Samples

Operating System Perspective

• Linux Kernel GPIO Documentation

• LWN GPIO In The Kernel: An Introduction

• Wikipedia GPIO

Kernel Ring Buffer

pi@raspberrypi:~ $ dmesg | grep -i gpio
[    0.080031] uart-pl011 3f201000.serial: could not find pctldev for node /soc/gpio@7e200000/uart0_pins, deferring probe
[    3.524055] gpiomem-bcm2835 3f200000.gpiomem: Initialised: Registers at 0x3f200000
pi@raspberrypi:~ $

SysFs

GPIO Sysfs Interface for Userspace. Platforms which use the "gpiolib" implementors framework may choose to configure a sysfs user interface to GPIOs. This is different from the debugfs interface, since it provides control over GPIO direction and value instead of just showing a gpio state summary. Plus, it could be present on production systems without debugging support.

pi@raspberrypi:~ $ cd /sys/class/gpio/
pi@raspberrypi:/sys/class/gpio $ tree -L 1
.
├── export
├── gpiochip0 -> ../../devices/platform/soc/3f200000.gpio/gpio/gpiochip0
├── gpiochip100 -> ../../devices/gpiochip2/gpio/gpiochip100
├── gpiochip128 -> ../../devices/gpiochip1/gpio/gpiochip128
└── unexport

3 directories, 2 files
pi@raspberrypi:/sys/class/gpio $