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DBUS Example

In this example, we will create two processes for communicating each other over system bus with DBUS. As "root" user, to be able to send data over system bus, we need to add a new DBUS policy. Policy file can be found in new_dbus_policy.conf. Copying process is being done in post build script.

  • The first process is dbusserver which is being started right after the boot and starts to listen session bus. When message received, it both prints and logs.
  • The second process, client process, should be run manually and sends the message to server over the created DBUS session.

How to Run:

  • The server process will start automatically and listens the system bus. Check the /tmp folder and /tmp/common-dbus-session-addr.txt file.
  • We need to start client application manually. The application is under /usr/bin/ folder. To check the incoming messages at the server side, we can see the log results which dbusserver prints them in /var/log/messages log. Or we can start a new dbusserver and dbusclient processes in two different terminals and observe the prints there.
/usr/bin/dbusclient Hello server!
cat /var/log/messages

How to Configure DBUS for Session Bus:

Session bus does not need a policy to be able to run this example. But we need to create a session first and run our applications. To do this, we need to change our server.c and client.c code.

    conn = dbus_bus_get(DBUS_BUS_SYSTEM, &err);  // Change this
    conn = dbus_bus_get(DBUS_BUS_SESSION, &err); // to this
  • The server process will not start automatically since it won't find a session bus. The daemon can be configured to create the dbus session at the start and save the dbus session id to /tmp/common-dbus-session-addr.txt. But for the simplicity, we will create the session run-time manually. Below command will start a session and save it to /tmp/common-dbus-session-addr.txt. After this, we can run our server application.
dbus-daemon --session --fork --print-address 1 > /tmp/common-dbus-session-addr
export DBUS_SESSION_BUS_ADDRESS=$(cat /tmp/common-dbus-session-addr)
/usr/bin/dbusserver
  • We need to start client application manually with another terminal. The application is under /usr/bin/ folder. Before starting the client, we should set DBUS_SESSION_BUS_ADDRESS variable to be able to communicate with the server process. This bus address value must be the same with the server side. Otherwise communication will not be possible.
export DBUS_SESSION_BUS_ADDRESS=$(cat /tmp/common-dbus-session-addr)
/usr/bin/dbusclient Hello server!

To check the incoming messages at the server side, we can see the log results which dbusserver prints them in /var/log/messages log. Or we can observe the prints in terminals.

SSH Connection:

Once the board is powered up and connected to the network, you can use SSH to log in remotely:

ssh root@192.168.222.222
password: root

Supported boards:

  • Raspberry Pi Zero W
  • Raspberry Pi 3B+