While the Raspberry Pi has very good support for an I2C bus, a lot of very cool chips – including the in system programmer for just about every ATtiny and ATmega microcontroller – use an SPI bus.

The Onion SPI Library, `libonionspi` is a dynamic C library that provides functions to easily read from and write to devices communicating with the Omega over the GPIOs via the SPI protocol. The ...