communication between native-app and chrome-extension

Question

I have a native app written in c++ and a chrome-extension.

I am communicating between them using 'chrome native messaging'.

Native-App code:

int main(int argc, char* argv[]) {
 unsigned int a, c, i, t=0;
 std::string inp;  do {
 inp="";
 t=0;
 // Sum the first 4 chars from stdin (the length of the message passed).
  for (i = 0; i <= 3; i++) {
    t += getchar();
  }

  // Loop getchar to pull in the message until we reach the total
  //  length provided.
  for (i=0; i < t; i++) {
    c = getchar();
    inp += c;
  }

// Collect the length of the message
unsigned int len = inp.length();
//// We need to send the 4 btyes 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 << inp <<std::endl;
flushall();
}while(cnt < 2 );
return 0;  }

Here I'm reading message sent by chrome-extension on stdin. and sending the same message back by writing it on stdout.

Extension is using PostMessage()

This is working... BUT ..

When I put my program under continuous while loop, the flow executes only once!

i.e port.postMessage({'text':'hello_1'}) gets echoed back as expected but if I do

port.postMessage({'text':'hello_2'}) it doesn't get echoed back.

I'm unable to understand what the problem is. Does it require threading?

Please help!

Thanks!

Answer 1

Marc's answer contains some errors (inherited from the question) and will not work for messages with lengths that do not fit in one byte.

Chrome's protocol, when communicating with native apps is:

• requests to native app are received through stdin
• responses to Chrome are sent through stdout

• Chrome doesn't deal well with Windows style \r\n so avoid that in the messages and set stdin mode to binary (so you can correctly read the request len and \n doesn't 'turn' into \r\n):

  _setmode(_fileno(stdin),_O_BINARY);
  

The request and response messages are JSON with a 4 byte header (uint32) containing the length of the message: [length 4 byte header][message]

Reading the request header:

uint32_t reqLen = 0;
cin.read(reinterpret_cast<char*>(&reqLen) ,4);

Writing the response header:

cout.write(reinterpret_cast<char*>(&responseLen),4); 

Answer 2

This works for me:

 int main(int argc, char* argv[])
 {

 std::cout.setf( std::ios_base::unitbuf ); //instead of "<< eof" and "flushall"
 unsigned int a, c, i, t=0;
 std::string inp;  

 do {

 inp="";
 t=0;
 // Sum the first 4 chars from stdin (the length of the message passed).
  for (i = 0; i <= 3; i++) {
    t += getchar();
  }

  // Loop getchar to pull in the message until we reach the total
  //  length provided.
  for (i=0; i < t; i++) {
    c = getchar();
    inp += c;
  }

//Collect the length of the message
unsigned int len = inp.length();
//// We need to send the 4 btyes 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 << inp;
}

...

Answer 3

The string length decode algorithm is incorrect. This is the correction:

for (i = 0; i <= 3; i++) {
    c = getchar();
    l |= (c << 8*i);
}