TCP/IP Socket Communications in MATLAB

I often see people asking about network communications on the MATLAB Newsgroup. Often this is for the communication between instances of MATLAB.

Using the ability to call Java directly from within MATLAB, I'm going to provide a short example of a client/server written solely in MATLAB and usable from Release 14 onwards (possibly even earlier).

The example is available on the Mathworks File Exchange: Simple TCP/IP Socket Comms Example

I'm working on a little TCP/IP comms library at the moment using these techniques. It will provide a nice layer of abstraction and allow you to use Sockets as you would in other programming languages (as well as one can in a single thread). Keep an eye out for it on the File Exchange.

Interpreted Java?

Amazingly we can execute Java code, even from within the Command Window without the need to compile. For example, the traditional example:

>> import java.lang.*
>> System.out.println('Hello World')
Hello World      

To perform socket communications, we utilise the Java Socket and Input/OutputStream classes to pass data around via TCP/IP sockets.

On the server side we use (unsurprisingly) a ServerSocket, which once a client has been accepted, provides a Socket around which we wrap a DataOutputStream to which we can write data.

On the client side we use a Socket to connect to the specified host and port which provides us an InputStream which we wrap in a DataInputStream to read data from.

The code for the example server and client is outlined below.

client.m

% CLIENT connect to a server and read a message
%
% Usage - message = client(host, port, number_of_retries)
function message = client(host, port, number_of_retries)

import java.net.Socket
    import java.io.*

    if (nargin <>
        number_of_retries = 20; % set to -1 for infinite
    end
    
    retry        = 0;
    input_socket = [];
    message      = [];

    while true

        retry = retry + 1;
        if ((number_of_retries > 0) && (retry > number_of_retries))
            fprintf(1, 'Too many retries\n');
            break;
        end
        
        try
            fprintf(1, 'Retry %d connecting to %s:%d\n', ...
                    retry, host, port);

            % throws if unable to connect
            input_socket = Socket(host, port);

            % get a buffered data input stream from the socket
            input_stream   = input_socket.getInputStream;
            d_input_stream = DataInputStream(input_stream);

            fprintf(1, 'Connected to server\n');

            % read data from the socket - wait a short time first
            pause(0.5);
            bytes_available = input_stream.available;
            fprintf(1, 'Reading %d bytes\n', bytes_available);
            
            message = zeros(1, bytes_available, 'uint8');
            for i = 1:bytes_available
                message(i) = d_input_stream.readByte;
            end
            
            message = char(message);
            
            % cleanup
            input_socket.close;
            break;
            
        catch
            if ~isempty(input_socket)
                input_socket.close;
            end

            % pause before retrying
            pause(1);
   end
    end
end

server.m

% SERVER Write a message over the specified port
% 
% Usage - server(message, output_port, number_of_retries)
function server(message, output_port, number_of_retries)

    import java.net.ServerSocket
    import java.io.*

    if (nargin <>
        number_of_retries = 20; % set to -1 for infinite
    end
    retry             = 0;

    server_socket  = [];
    output_socket  = [];

    while true

        retry = retry + 1;

        try
            if ((number_of_retries > 0) && (retry > number_of_retries))
                fprintf(1, 'Too many retries\n');
                break;
            end

            fprintf(1, ['Try %d waiting for client to connect to this ' ...
                        'host on port : %d\n'], retry, output_port);

            % wait for 1 second for client to connect server socket
            server_socket = ServerSocket(output_port);
            server_socket.setSoTimeout(1000);

            output_socket = server_socket.accept;

            fprintf(1, 'Client connected\n');

            output_stream   = output_socket.getOutputStream;
            d_output_stream = DataOutputStream(output_stream);

            % output the data over the DataOutputStream
            % Convert to stream of bytes
            fprintf(1, 'Writing %d bytes\n', length(message))
            d_output_stream.writeBytes(char(message));
            d_output_stream.flush;
            
            % clean up
            server_socket.close;
            output_socket.close;
            break;
            
        catch
            if ~isempty(server_socket)
                server_socket.close
            end

            if ~isempty(output_socket)
                output_socket.close
            end

            % pause before retrying
            pause(1);
   end
end
end

Opening up two instances of Matlab:

% Instance 1
>> message = char(mod(1:1000, 255)+1);
>> server(message, 3000, 10)
Try 1 waiting for client to connect to this host on port : 3000
Try 2 waiting for client to connect to this host on port : 3000
Try 3 waiting for client to connect to this host on port : 3000
Try 4 waiting for client to connect to this host on port : 3000
Client connected
Writing 1000 bytes

% Instance 2 (simultaneously)
% NOTE: If the 'server' was runnning on a non local machine, substitute its IP address
% or host name here:
%   data = client('10.61.1.200', 2666); % To connect to server at IP 10.61.1.200:2666
>> data = client('localhost', 3000)
Retry 1 connecting to localhost:3000
Retry 2 connecting to localhost:3000
Connected to server
Reading 1000 bytes

data =



 !"#$%&'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvwxyz{|}~                                                                                                     


 !"#$%&'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvwxyz{|}~                                                                                                     


 !"#$%&'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvwxyz{|}~                                                                                                     


 !"#$%&'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvwxyz{|}~                                                                             

This code can be expanded to read/write arbitrary data types, and SHOULD be expanded to properly deal with errors (ie not getting all of the buffer on receive end), but it serves as a simple example of how to get communication between MATLAB and other applications / instances of MATLAB.