DBus Signal Questions

Hello!

 

I have been playing around and writing my own DBus bindings for IPC using C++ to communicate using DBus signals to mimic “socket like” communications on top of the libdbus C API.

 

This is the situation I am running into that I would like to see if there is some rules or exceptions I can use to work around some functionality:

 

1.       I have 2 processes attached to a session bus named “test.echo” on an object / interface named “pingpong” with a signal called “pingpong”.

2.       Each process can read and write to that signal to act as a pipe.

3.       Using synchronous reading and writing, process 2 reads the data and echo’s the data back for a ping pong demo.

4.       After process 1 sends data, process 2 then reads the data and echo’s it back, in which process 1 waits to read the data.

 

Eventually, I would like to attach multiple processes to the same signal for broadcasting.

 

Here is some sudo code

 

Process 1:

                writeSignalData(“pingpong”)

                print(“Wrote pingpong”)

                while (true) {

                                message = readSignalData()

                                if (message.size()) {

                                                print(“Echo received:” + message)

                                }

                }

 

Process 2:

                while (true) {

                                message = readSignalData()

                                if (message.size()) {

                                                print(“Echoing: “ + message)

                                                writeSignalData(message)

                                }

                }

 

The problem that I am running into is that in the echoing process 2 is receiving its own signal after process 2 calls “writeSignalData()” since it is also technically subscribed to the signal…

 

For example, the output of the sudo code would be:

 

Process 1 – Wrote pingpong

Process 2 – Echoing: pingpong

Process 1 – Echo received: pingpong

Process 2 – Echoing: pingpong

Process 2 – Echoing: pingpong

Process 2 – Echoing: pingpong

Process 2 – Echoing: pingpong

Process 2 – Echoing: pingpong

Process 2 – Echoing: pingpong

Process 2 – Echoing: pingpong

Process 2 – Echoing: pingpong

…

 

One way I can think of getting around this is to add a randomly generated serial (the serial functionality in a libdbus message) to each message sent and keep an internal list of ignored serials after echoing. If the serial from the message received is a part of the blacklisted serials, then pop the message, ignore it, and move on.

 

I didn’t know if there was a better way to do this using rules, or possibly using two signals for bi-directional communications so that I don’t get my own sent data. I found very little for examples on the net for this way of using DBus.

 

I know I may not be using the DBus the way it was intended… I just like the fact that all of the TCP packet parsing is already done and grouped, also the fact that it is typed data transactions.

 

 

 

Derrill Vezina, PE
[email protected]
207-251-9072

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