Node-red C program communication

General
1

I am attempting to pass in and extract data from a C program from my node-red application. I have successfully been able to pass out data in my C program from stdout into node red but am unsure as to how to get data into my C program from node-red. Will I need to use some sort of inter process mechanism like linux d-bus etc?

Any suggestions would be greatly appreciated.

2

Assuming your application is long running (i.e. NOT a fire up, process args, write to STDOUT, quit), a common approach is to have your application communicate to node-red via MQTT.

There are a few C libs for MQTT.

If you are not familiar with MQTT, I can assure you it is far easier that setting up pipes or busses etc. (just ask)

3

Can you do it just using command line parameters to the program?

4

Depends how complex the data is. For simple data, Colin's suggestion of command line parameters may be sufficient. For text-based but longer data, you could make your C++ app accept piped input from std-in.

For more complex data or better performance most of what you may choose to use would need you to turn your app into a daemon/service so that it could be listening on a port or pipe. Probably not what you want. You could also write your c++ code as a library and then wrap it with a Node.js module that could be used inside Node-RED.

You may wish to avoid platform-specific IPC though since that restricts what platforms you can run on - which may or may not be an issue for you.

5

The C application will be long running and sending floats, nothing too fancy. I would also like to send data (floats, ints etc) to the C program whilst its running so I don't think I would be able to pass that data into the C program using command line parameters. Hope that paints a better picture of what I'm trying to do.

6

Thanks for the suggestion Steve!

7

In that case, Node-RED can use many different IPC methods - though I've never actually gotten round to trying UNIX pipes :slight_smile:

But if you are happy to have another service running (the MQTT broker), certainly MQTT makes this pretty straight-forwards though at the cost of some bloat in your C++ app since you will need a suitable MQTT library linked or compiled in. But truthfully, if your C++ programme can create a TCP or UDP listener and sender, that might be enough. But MQTT will give you more flexibility of course and some pre-built structure to work with.

8

I'll experiment with both the MQTT approach first as this seems more straight forward than using UNIX pipes. I will however investigate using pipes as this seems to be more efficient, are there any resources you know that would help me get started with UNIX pipes?

9

Well they seem to be one of UNIX's (actually I think they may be a POSIX standard) better kept secrets. So I've never found a lot of information that didn't need a lot of head-scratching and re-reading :cloud_with_lightning:

But I have played with them and got them working between Node.js apps in the past - my understanding is that Node-RED should also allow them in some places but I'm afraid that I've not tried.

This might help though: node-red-contrib-ipc (node) - Node-RED (nodered.org)

10

You can use the node-red-node-daemon node to run your program. Then the msg.payload is fed to STDIN. And receives stout and std err like the exec node does

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