The bool-file interface

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The bool-file interface allows access to a specific class of file that contains boolean semantics, typically used to toggle or represent the state of binary hardware values.

This interface is primarily intended to be used with Ubuntu Core devices, it’s also restricted because it provides privileged access to hardware.

These kinds of file are located within specific directories inside the sysfs filesystem (/sys) and this interface allows a file to be read, to obtaining a current value, or written to, setting a new value.

The LED class is a good example of file type for this interface, as it potentially allows LEDs to be turned on and off, and have their brightness modified.

Use the snap interface bool-file command to see which boolean files are available on the system:

$ snap interface bool-file
name:    bool-file
summary: allows access to specific file with bool semantics
slots:
  - my-gadget:green-led
  - my-gadget:red-led
[...]

Interface documentation: See Interface management and Supported interfaces for further details on how interfaces are used.

Developer details

Auto-connect: no

Attributes:

  • path (slot): path to the file in sysfs
    Example: /sys/class/leds/green/brightness

    The path value must match one of the following regular expressions:

    • For GPIO devices: ^/sys/class/gpio/gpio[0-9]+/value$
    • For LED devices: ^/sys/class/leds/[^/]+/brightness$

The gpio interface provides another option for accessing GPIO devices.

To use a boolean file, the snap developer must add plugs: [ bool-file ] to a snap’s snapcraft.yaml. The snap user can then access a specific boolean file with an interface connection.

Unless a snap specifically expects a set of boolean files that cannot be predefined, the recommended approach is to define distinct plugs for each boolean file the snap wishes to use:

plugs:
  green-led:
    interface: bool-file
  red-led:
    interface: bool-file

Defining distinct plugs for each boolean file has the advantage of being self-documenting, and 1:1 connections like these are easier to track and setup with auto-connections, if needed.

Once connected, the consuming snap can use the boolean file via the path mentioned in the path attribute specified by the connected slot.

The slot side on a gadget snap may be declared as follows:

slots:
  green-led:
    interface: bool-file
    path: /sys/class/leds/green0/brightness
  red-led:
    interface: bool-file
    path: /sys/class/leds/red0/brightness

Code examples

The test code for this interface can be found in the snapd repository:
https://github.com/snapcore/snapd/blob/master/interfaces/builtin/bool_file_test.go

The source code for the interface is in the snapd repository:
https://github.com/snapcore/snapd/blob/master/interfaces/builtin/bool_file.go

ⓘ This is a snap interface. See Interface management and Supported interfaces for further details on how interfaces are used.

2

The above feels a bit sparse as documentation …

Here is an example for a slot definition with this interface granting access to an led (it also allows access to gpio sysfs nodes):

slots:
[...]
  led-green:
    interface: bool-file
    path: /sys/class/leds/green/brightness
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3

@degville can you review/fix my edits to this interface? thanks

4

I’ve tweaked your updates - thanks so much for adding all this detail!

1 Like