Advice needed on a suitable method to transmit ROS msgs, with unknown msg types. In cpp

asked 2013-11-01 05:51:25 -0500

kurisu
11 ●2 ●2 ●4

updated 2013-11-01 06:04:11 -0500

Hey guys,

So I have been contracted to write a ROS driver to facilitate the transmission (and reception) of currently undefined ROS msgs underwater with an acoustic modem.

My plan is to create a node that listens to all ROS msg traffic, then to discriminate the msgs to be transmitted across the acoustic link (with aid of topic names in a conf file), then serialise those selected msgs (one at a time), and then transmit. On the receiving side the date will need to be de serialised and re-published into a msg.

Due to not knowing the msgs before compilation, and the requirement of a configuration file to discriminate, this obviously means that I need a way to serialize and de-serialize without knowing the msg type (unless I have this information in the config file aswell). I have also been asked that it is programmed in CPP.

I realise that similar questions have been asked before , mostly stating that it is impossible (?) in cpp due to introspection... but these were asked several years ago.

So I wonder if there is any new advice, possibly from changes in ROS.

Many thanks,

Chris

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Hi, I've a similar problem but since m new to ros coding i'm unable to deal with the issue . can u lead through the basics ?it would b a great help.

thnx rythima

rpxz ( 2016-05-31 00:58:43 -0500 )edit

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answered 2013-11-01 12:46:22 -0500

sedwards
1601 ●28 ●43 ●61 http://rosindustrial.swri.edu/

The obvious non-answer is to use python. It's almost trivial to do what you ask in python. Assuming that python won't work, you might want to look at the rosbag code. It is written in C++ and uses a shapeShifter message type to generically read and write messages in binary format. Here is a link to the shape shifter documentation ( http://docs.ros.org/hydro/api/topic_tools/html/classtopic__tools_1_1ShapeShifter.html ).

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Thanks, I'll have a look into the rosbag code, but I'm starting to think that the non-answer might be an easier and better option, I might suggest that the serialization / deserialization stuff is coded into it's own ROS node and the rest is done in C++. Do you by chance know of any python examples?

kurisu ( 2013-11-03 21:18:28 -0500 )edit

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answered 2013-11-01 06:31:23 -0500

teddybouch
320 ●19 ●26 ●38

If your node is just listening in on topics, you're going to have to know the type of message to expect on that topic, which we will generically call Type. Then you could add some parsing to the .msg file that would tell you the format of the messages. However, with ROS's support for nested types in messages, that could get really complex, and with the bandwidth limitations of acoustic modems the additional information that you would have to encode in every message to handle this generically would be a killer. Which modems are you using? Are you writing the driver, or do you have a lower-level driver you are using?

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Evologics 18/34 (ethernet), best-case 13.9 kbps so I'd anticipated that the need to encode more information could be a problem, the data throughput apparently shouldn't be too 'heavy' though. No driver, so it is my remit to code both the low-level interface, and the high level ROS 'transport' stuff

kurisu ( 2013-11-03 21:15:28 -0500 )edit

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Advice needed on a suitable method to transmit ROS msgs, with unknown msg types. In cpp

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