LCM logger (c++)

Kumius

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Dec 11, 2016, 9:49:39 AM12/11/16

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Hello, I'm learning to use LCM (C++) to accomplish a project. I have got a problem when tyring to record the broadcast data, and then replay it. I haven't find any useful C++ cases.

lcm::LCM lcm("file://log.txt?mode=w");
if (!lcm.good())
{
    return 1;
}
Handler handler;
lcm.subscribe("EXAMPLE", &Handler::handleMessage, &handler);
 
if (0 == lcm.handle());

It seems that the code above doesn't work. Can somebody help me?

Many thanks!

Matthew Tesch

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Dec 11, 2016, 12:48:15 PM12/11/16

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I'm not on a system right now that I can compile at test this on, but my first guess would be the path you are passing to the LCM constructor needs to be a full file path, not just a local path. (Even if a local path is accepted, make sure that you are running the program with the correct environment so that the file you think is local really is local)

Looking at the documentation for the url parameter:

http://lcm-proj.github.io/group__LcmC__lcm__t.html#gaf29963ef43edadf45296d5ad82c18d4b

The examples here are of the form:

 "file:///home/albert/path/to/logfile?speed=4"

(Note the three forward slashes to give a full path of /home/albert/path/to/logfile).

If giving the full path does not solve your problem, could you provide more information about what doesn't work?

Best,

Matt

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Kumius

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Dec 11, 2016, 10:41:15 PM12/11/16

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Hi Matt,

Thanks for your response! That's very kind of you.

The program is running on a Win10 (x64) PC. If I don't set the the log path, it works and the 'receiver' can receive messages from the 'sender'.

lcm::LCM lcm("");


 if (!lcm.good())
 {
   return 1;
 }
 Handler handler;
 lcm.subscribe("EXAMPLE", &Handler::handleMessage, &handler);


 while (0 == lcm.handle());
 cout << "done..." << endl;

However, once I set the log path (local path or full file path), the function 'lcm.handle()' returns -1.

lcm::LCM lcm("file://d:/data/text/log.txt?mode=w");

Do I have to record the broadcast data by myself? I used to think tha the lcm instance will do tha for me if I set the file path of log.

Sorry to bother you again, but I'm really confused.

Best,

Kumius

Jonathan Bendes

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Dec 11, 2016, 11:01:06 PM12/11/16

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Hi Kumius,

I think there's a fundamental misunderstanding of the API here. When you create a new instance of lcm, you pass in a URL. That URL indicates the transport to be used for that instance of lcm.

The "file://" syntax is a way to get lcm to read or write from a log. So if you create a new instance of lcm with a "file://" URL in mode "w", you can "publish" events into the file. That instance of lcm will have no other traffic that it processes other than the messages you publish into the file. It seems like maybe you thought the instance would also have access to UDP lcm traffic? It would not unless initialized with a URL indicating it should set up a UDP transport.

What is the ultimate goal here? What exactly are you trying to do? Just log data coming over a transport? Take a look at lcm-logger if that's what you're trying to do. If not, feel free to provide a few more details.

Best,
Jonathan

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