How to spawn C code using exec

General
1

Hello,
I am having trouble launching a C program in spawn mode using an exec node.

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I created a simplified example where a button starts the exec node. Unfortunately when I run it, I always get an error with return code of -2.

I have this little example committed here: GitHub - hdlguy/node-red_experiments

The C code is just a loop that generates some text output and then returns 0.

#include <stdio.h>
#include <unistd.h>
int main()
{
    for(int i=0; i<=100; i+=20) {
        fprintf(stdout, "%d\n", i);
        fflush(stdout);
        usleep(500000);
    }    
    return(0);
}
2

Add debug nodes to all outputs of the exec node, set them to Output Complete Message and check all properties of the outputs. There will probably be a helpful error message there.

Have you specified the full path to the executable file?

I don't see the flow in your repository.

3

You've not checked in the flow file so we have no idea how you've configured the exec node.

Also you probably want to swap the dashboard start button for a inject node to get started and set the debug node to whole message not just payload to see more.

You should also check the Node-RED logs for more error messages.

As it is there isn't really anything we can help with so far.

And I assume you have tested running the compiles c program from the command line first?

The code when complied and run with a full path works just fine on my Ubuntu box with NR 4.1.0

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4

Please don't be offended by the question, but your exec node is trying to run the compiled executable, not the C source code? Edit: I think that would give rc -13

Does it make a difference if you specify the full pathname of the executable eg /home/pedro/.node-red/bin/gen_percent (or wherever the file is)?
One cause of rc -2 is that the OS cannot find the file to execute it.

5

The flow is checked in here, node-red_experiments/c2progress/.node-red at main · hdlguy/node-red_experiments · GitHub, but it is a hidden folder so sometimes I also miss seeing it.

I will try the an inject node but I actually want have the code started by a user hitting a button. I think I have tried several forms of the path. I will try to hard code the full path again. That should be ok since the application is an embedded system. I don't really need relative path names.

I will also change the debug node to whole messages.

I was about to give up on this but since you guys are so helpful, I will keep trying.

6

The flow is not in that directory

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7

The code which @pedro_uno included in his original post is at https://github.com/hdlguy/node-red_experiments/blob/main/c2progress/c_code/gen_percent.c

8

@jbudd I'm not interested in the C code, I have that, I want to see the Node-RED flow so I can actually see how the exec node is configured (e.g. is the full path to the executable provided)

9

OK, thanks again for the attention to my problem. I have now committed flows.json to the repo. I also got the got the flow to run with the exec node in spawn mode. The trick was to use an absolute path to the compiled C command.

My purpose was to use the exec code in spawn mode so that it could drive a progress bar during flash memory programming stages. In addition to the absolute path, I needed to put a fflush(stdout) command after each printf of the C code. I also needed to use a change block to convert the string to a number. All these changes were the result of suggestions on this forum.

Thanks for all the support and thanks for node-red itself. I do embedded design involving FPGAs and I need a way to create web UIs without being a web developer. Node-Red is really working out for me.

2 Likes
10

I uploaded a video of the DB1 chart node displaying a rapidly updating waveform. It is 600KB but it shows what is possible.

https://github.com/hdlguy/node-red_experiments/blob/main/db2_chart/db1_waveform_performance.webm