sending data from Matlab to C++ over UDP

Hi all,

I'm trying to send data back and forth between a Matlab app and a C++ app. The data is comprised of a 100 char array. Both apps are on the same computer. The Matlab app in this case is the Server and the C++ is the client.

I am able to receive data properly in the direction from my C++ app to Matlab. However I am not able to receive the data in the opposite direction from Matlab to my C++ app, though I'm using exactly the same socket.

I checked my C++ app, by using a second C++ application that talks to the first one through the same UDP interface and the data was transmitted back and forth properly.

Just for testing the connection, I'm trying to send a 100 char array with 'A' in the first cell from C++ to Matlab, and return a 100 char array with 'I' in the first cell from Matlab to C++:

CPP_IP = '127.0.0.1';
CPP_Port = 817;

OpenCPP = udp(CPP_IP,CPP_Port,'LocalPort',CPP_Port);
OpenCPP.InputBufferSize = 100;
OpenCPP.OutputBufferSize = 100;
OpenCPP.ByteOrder = 'littleEndian';
OpenCPP.Timeout = 30;
fopen(OpenCPP);

CPP_Connection = 0;
while ~CPP_Connection
    fprintf('waiting for CPP to come alive...')
    msg = fscanf(OpenCPP,'%c',[100 1]);
    if OpenCPP.ValuesReceived == 100
        if msg(1) == 'A'
            fprintf('\nCPP alive and connected!\n')
            CPP_Connection = 1;
        end
    else
        fprintf('Command from CPP incomplete!')
        fclose(OpenCPP);
        delete(OpenCPP);
        clear OpenCPP
       end
    end
end
msg(1) = 'I';
fprintf(OpenCPP,'%c',msg);

When running this code and my C++ app, if I check the connection properties after receiving the letter 'A', this is what I get:

   UDP Object : UDP-127.0.0.1

   Communication Settings 
      RemotePort:         817
      RemoteHost:         127.0.0.1
      Terminator:         'LF'

   Communication State 
      Status:             open
      RecordStatus:       off

   Read/Write State  
      TransferStatus:     idle
      BytesAvailable:     100
      ValuesReceived:     100
      ValuesSent:         200

I received 100 chars as expected and sent 100 chars. It looks like the sent data is not actually read by the C++, although as I said, the interface on the C++ app was checked with another C++ app and worked fine.

It seems like an issue in the communication between Matlab and C++

Any help would be highly appreciated!

Answer 1

[Yuval Porat]

Never mind. I realized what went wrong.

Though declaring the IP and Port of the socket at the beginning, the port in which data was sent from my C++ app to Matlab was randomly selected. I fixed it by reading in the new incoming port in fscanf:

   [Buff,count,msg,datagramaddress,datagramport] = fscanf(OpenCPP,'%c',[100 1]);

Then, changing the port of my CPP connection to the new one using:

   OpenCPP.RemotePort = datagramport;

and writing my data using fprintf:

    fprintf(OpenCPP,'%c',Buff);

Answer 2

[kuan jiang]

hello, i'm trying to make communnication between c++ app and matlab like you did,could you tell me how to realize it

thanks a lot.

[Yuval Porat]

Hi, if you need general instructions I suggest reading the Matlab documentation on tcp/udp communications

https://de.mathworks.com/help/matlab/tcpip-communication.html

https://de.mathworks.com/help/instrument/tcp-ip-and-udp-interface.html

If you faced the same problem as mine, please read the answer I posted where I detailed the solution I found.

If you're having another problem, please be more specific and the community will try to help you.

Thanks.

[kuan jiang]

Ah! thank you very much.Generally, i want to make co-simulation between MATLAB and ns-3 (a network simulator), and i read some papers found they using socket to combine these two simulators to realize data transfer from matlab to ns3 and back ,question is i don't know how the matlab use socket to transfer data to external software and get back data...

Thanks a lot.