sdbusExamples

Example code in C using the linux sd-bus library

In needing to work with this library for a project I found the documentation sparse and the examples limited. My hope is that this repo can serve as a jumping off point for others in the future.

Please feel free to open a PR if you would like to contribute examples. I'd love for this to end up as comprehensive as the freedesktop python examples

Prerequisites

sudo apt install the following:

• libsystemd-dev
• d-feet (optional for testing)

In order to allow dbus to register a service we need to add an entry to /etc/dbus-1/system.d/. confFile.c contains the necessary bus configurations, see the comments in that file for exact instructions

How to build & run

mkdir build
cd build
camke ..
make
./dbusExample

Clean

• Remove output binary: make clean
• Remove all generated files: make clean-all

How do I know if it's working?

List running services & call methods

• Open up d-feet
• Select "Session Bus"
• Look for a service called com.example.DbusServiceC
• If it's there then yay! Your dbus service is running
• You can check the service name, object path, interface name, and method name and arguments in d-feet

You can also use d-feet to call methods

• Find & click on your service in d-feet
• Navigate to your interface->method and double click
• A new window will pop up with a "Method Input" box allowing you to send data (click "Execute"), replies will appear in th "Method output" box

Observe sent dbus messages

• Run dbus-monitor in the background or a separate terminal

• Send a dbus message

• dbus-monitor will show the traffic on the bus including message type (signal, method, method reply), time stamp, sender, destination, and contents

  Example:

  signal time=1633748547.920393 sender=org.freedesktop.DBus -> destination=(null destination) serial=880 path=/org/freedesktop/DBus; interface=org.freedesktop.DBus; member=NameOwnerChanged
    string ":1.414"
    string ":1.414"
    string ""
  
  method call time=1633748556.627750 sender=:1.415 -> destination=org.freedesktop.DBus serial=1 path=/org/freedesktop/DBus; interface=org.freedesktop.DBus; member=Hello
  
  method return time=1633748556.627837 sender=org.freedesktop.DBus -> destination=:1.415 serial=1 reply_serial=1
    string ":1.415"
  
  

General dbus info

Signals & Methods

A signal is a one part broadcast. A service sends out a message without receiving a reply.

For example: The temp monitor will send out a signal when the temp changes. The main service can subscribe to this signal and trigger internal callbacks when a new signal is received.

A method consists of two parts, a received message and a sent reply. A method is called by another service.

For example: The main service periodically sends update requests to the temp monitor (by calling the method) and the temp monitor replies with the updated temperature

Servers & Clients

Each service can act as a server or a client with regards to another service. A server may have methods that clients can call and it may send out signals that clients can subscribe to. A single service can be both a server and a client at the same time.

For example: The main service wants to receive messages about the temperature from the temp monitor and send messages to the fan controller. The temp monitor could act as a server and send out signals when the temperature changes, the main service would subscribe to that signal as a client. The main service could simultaneously act as a server and create a method which the fan controller (client) polls to give its status and receive new speed settings in the reply.