gpio allows access to a specific GPIO pin. The interface is restricted because it provides privileged access to GPIO hardware.
snap interface gpio to see which gpio devices are available on the system:
$ snap interface gpio
summary: allows access to a specific GPIO pin
The pi-fancontrol snap provides simple fan control on a Raspberry Pi with a fan connected to GPIO 14 (pin 8). With the snap installed, the following command will connect the interface to the pin:
snap connect pi-fancontrol:gpio pi:bcm-gpio-14
number(slot): GPIO pin number to export and expose to consuming snaps
Hardware IO interfaces covers some general considerations common to these kinds of devices.
Unless the snap is expected to actually use a set of gpio pins that is not predefined, it is recommended to define distinct plugs for each used gpio pin, like:
This has the advantage of being self-documenting and 1-1 connections like these are easier to track and setup with auto-connections, if the latter is needed.
When the interface is connected,
"echo (pin number) > /sys/class/gpio/export" is run internally to enable access to the GPIO pin.
Once connected, the consuming snap can use the device via
N is the pin number specified by the connected slot.
Finally, when the interface is disconnected,
"echo (pin number) > /sys/class/gpio/unexport" is run internally to disable access to the GPIO pin.
The hook and control scripts for pi-fancontrol can be found in the project’s GitHub repository: https://github.com/ogra1/pi-fancontrol-snap
The source code for the GPIO interface is in the snapd repository: https://github.com/snapcore/snapd/blob/master/interfaces/builtin/gpio.go.