How to send message FROM native app TO Chrome extension?

In order for a native messaging host to send data back to Chrome, you must first send four bytes of length information and then send the JSON formatted message as a string/char-array.

Below are two examples for C and C++ respectively that do the same thing in slightly different ways.

C example:

#include <stdio.h>
#include <string.h>

int main(int argc, char* argv[]) {
    // Define our message
    char message[] = "{\"text\": \"This is a response message\"}";
    // Collect the length of the message
    unsigned int len = strlen(message);
    // We need to send the 4 bytes of length information
    printf("%c%c%c%c", (char) (len & 0xff),
                       (char) ((len>>8) & 0xFF),
                       (char) ((len>>16) & 0xFF),
                       (char) ((len>>24) & 0xFF));
    // Now we can output our message
    printf("%s", message);
    return 0;
}

C++ example:

#include <string.h>

int main(int argc, char* argv[]) {
    // Define our message
    std::string message = "{\"text\": \"This is a response message\"}";
    // Collect the length of the message
    unsigned int len = message.length();
    // We need to send the 4 bytes of length information
    std::cout << char(((len>>0) & 0xFF))
              << char(((len>>8) & 0xFF))
              << char(((len>>16) & 0xFF))
              << char(((len>>24) & 0xFF));
    // Now we can output our message
    std::cout << message;
    return 0;
}

(The actual message can be sent at the same time as the length information; it is merely broken out for clarity.)

So following the OP Chrome example, here is how to output the message:

port.onMessage.addListener(function(msg) {
    console.log("Received" + msg.text);
});

In reality, there is no requirement to use “text” as the key returned from your native messaging app; it could be anything. The JSON string passed to the listener from your native messaging app is converted to a JavaScript Object.

For a C++ example of a native messaging app that uses the above technique in combination with jsoncpp (C++ JSON library) and also parses the request sent to the app, see here: https://github.com/kylehuff/libwebpg/blob/22d4843f41670d4fd7c4cc7ea3cf833edf8f1baf/webpg.cc#L4501