Good doc: https://learn.sparkfun.com/tutorials/raspberry-gpio/c-wiringpi-setup

http://wiringpi.com/

There also appears to be a Pine64 port:
https://github.com/pfeerick/wiringPi-Pine64

Nice demo re. RGB LED (for toolbox) https://www.youtube.com/watch?v=b4_R1eX9K6s
