Share Memory Between Applications

This example shows how to implement two separate MATLAB® processes that communicate with each other by writing and reading from a shared file. They share the file by mapping part of their memory space to a common location in the file. A write operation to the memory map belonging to the first process can be read from the map belonging to the second, and vice versa.

One MATLAB process (running send.m) writes a message to the file via its memory map. It also writes the length of the message to byte 1 in the file, which serves as a means of notifying the other process that a message is available. The second process (running answer.m) monitors byte 1 and, upon seeing it set, displays the received message, puts it into uppercase, and echoes the message back to the sender.

Prior to running the example, copy the send and answer functions to files send.m and answer.m in your current working directory.

The send Function

This function prompts you to enter a string and then, using memory-mapping, passes the string to another instance of MATLAB that is running the answer function.

function send
% Interactively send a message to ANSWER using memmapfile class.
 
filename = fullfile(tempdir, 'talk_answer.dat');
 
% Create the communications file if it is not already there.
if ~exist(filename, 'file')
    [f, msg] = fopen(filename, 'wb');
    if f ~= -1
        fwrite(f, zeros(1,256), 'uint8');
        fclose(f);
    else
        error('MATLAB:demo:send:cannotOpenFile', ...
              'Cannot open file "%s": %s.', filename, msg);
    end
end
 
% Memory map the file.
m = memmapfile(filename, 'Writable', true, 'Format', 'uint8');
 
while true
    % Set first byte to zero, indicating a message is not
    % yet ready.
    m.Data(1) = 0;
 
    str = input('Enter send string (or RETURN to end): ', 's');
 
    len = length(str);
    if (len == 0)
        disp('Terminating SEND function.')
        break;
    end
    
    % Warn if the message is longer than 255 characters.
    if len > 255
        warning('ml:ml','SEND input will be truncated to 255 characters.');
    end
    str = str(1:min(len,255));  % Limit message to 255 characters.
    len = length(str); % Update len if str has been truncated. 
    
    % Update the file via the memory map.
    m.Data(2:len+1) = str;
    m.Data(1)=len;
   
 
    % Wait until the first byte is set back to zero, 
    % indicating that a response is available.
    while (m.Data(1) ~= 0)
        pause(.25);
    end
    
    % Display the response.
    disp('response from ANSWER is:')
    disp(char(m.Data(2:len+1))')
   
end

The answer Function

The answer function starts a server that, using memory-mapping, watches for a message from send. When the message is received, answer replaces the message with an uppercase version of it, and sends this new message back to send. To use answer, call it with no inputs.

function answer
% Respond to SEND using memmapfile class.

disp('ANSWER server is awaiting message');

filename = fullfile(tempdir, 'talk_answer.dat');

% Create the communications file if it is not already there.
if ~exist(filename, 'file')
    [f, msg] = fopen(filename, 'wb');
    if f ~= -1
        fwrite(f, zeros(1,256), 'uint8');
        fclose(f);
    else
        error('MATLAB:demo:answer:cannotOpenFile', ...
              'Cannot open file "%s": %s.', filename, msg);
    end
end

% Memory map the file.
m = memmapfile(filename, 'Writable', true, 'Format', 'uint8');

while true
    % Wait until the first byte is not zero.
    while m.Data(1) == 0
        pause(.25);
    end
    
    % The first byte now contains the length of the message.
    % Get it from m.
    msg = char(m.Data(2:1+double(m.Data(1))))';

    % Display the message.
    disp('Received message from SEND:')
    disp(msg)
    
    % Transform the message to all uppercase.
    m.Data(2:1+double(m.Data(1))) = upper(msg);
   
    % Signal to SEND that the response is ready.
    m.Data(1) = 0;
end

Running the Example

To see what the example looks like when it is run, first, start two separate MATLAB sessions on the same computer system. Call the send function with no inputs in one MATLAB session. Call the answer function in the other session, to create a map in each of the processes' memory to the common file.

Run send in the first MATLAB session.

send
Enter send string (or RETURN to end):

Run answer in the second MATLAB session.

answer
ANSWER server is awaiting message

Next, enter a message at the prompt displayed by the send function. MATLAB writes the message to the shared file. The second MATLAB session, running the answer function, loops on byte 1 of the shared file and, when the byte is written by send, answer reads the message from the file via its memory map. The answer function then puts the message into uppercase and writes it back to the file, and send (waiting for a reply) reads the message and displays it.

send writes a message and reads the uppercase reply.

Hello. Is there anybody out there?
response from ANSWER is:
HELLO. IS THERE ANYBODY OUT THERE?
Enter send string (or RETURN to end): 

answer reads the message from send.

Received message from SEND:
Hello.  Is there anybody out there?

Enter a second message at the prompt display by the send function. send writes the second message to the file.

I received your reply.
response from ANSWER is:
I RECEIVED YOUR REPLY.
Enter send string (or RETURN to end):

answer reads the second message, put it into uppercase, and then writes the message to the file.

Received message from SEND:
I received your reply.

In the first instance of MATLAB, press Enter to exit the example.

Terminating SEND function.
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