Reads or writes a byte out to a PCF8574 chip on the I2C bus. This uses pure Linux C with no library dependencies.
This program uses hardcoded addresses on bus /dev/i2c-1. There's nothing on this bus on my raspberry pi, but you shouldn't run this on your own computer unless you're sure that you're writing to the right chip. Use i2cdetect from the i2c-tools package to check for devices.
pi@raspberrypi:~/rpi-pcf8574 $ ./i2c-write 255 # writes all the pins high
pi@raspberrypi:~/rpi-pcf8574 $ ./i2c-read
255
pi@raspberrypi:~/rpi-pcf8574 $ ./i2c-write 0 # writes all the pins low
pi@raspberrypi:~/rpi-pcf8574 $ ./i2c-read
0