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I am writing two programs one in c++ and the other in Python to communicate with each other using unix domain sockets. What I am trying to do is have c++ code send a number to the python code, which in turn send another number back to c++. This goes on till c++ code runs out of numbers to send and the execution stops. Below are my codes. I can't seem to run them past the first iteration of the loop.

I run Python first:
python code.py /tmp/1 /tmp/2

Then I run the c++ code:
./code /tmp/1 /tmp/2

Here is the output:

C++ output:

sent 0
Listening
Connection successful
received 5
sent 1
Listening



Python output:

listening ...
received (0,)
>5
sent 5
listening ...

C++ Code:

static int connFd;

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

   int recv_sock, 
   send_sock;

   struct sockaddr_un server, client;

   ///////////////////////////////////////////
   //                                       
   // setup send             
   //                                       
   ///////////////////////////////////////////

   /* Create socket on which to send. */
   send_sock = socket(AF_UNIX, SOCK_STREAM, 0);
   if (send_sock < 0) 
   {
     perror("opening unix socket");
     exit(1);
   }

   /* Construct name of socket to send to. */
   client.sun_family = AF_UNIX;

   strcpy(client.sun_path, argv[1]);

   if (connect(send_sock, (struct sockaddr *) &client, sizeof(struct sockaddr_un)) < 0) 
   {
     close(send_sock);   
     perror("connecting stream socket");
     exit(1);
   }


   ///////////////////////////////////////////
   //                                       
   // setup recv 
   //                                       
   ///////////////////////////////////////////

   recv_sock = socket(AF_UNIX, SOCK_STREAM, 0);

   if(recv_sock< 0)
   {
     cerr << "Cannot open socket" << endl;
     return 0;
   }

   bzero((char*) &server, sizeof(server));

   server.sun_family = AF_UNIX; 
   strcpy(server.sun_path,  argv[2]);

  //bind socket
  if(bind(recv_sock, (struct sockaddr *)&server, sizeof(server)) < 0)
  {
    cerr << "Cannot bind" << endl;
    return 0;
  }

  listen(recv_sock, 10);

  int X;
  for (int i = 0; i < 10; i++)
  {
    write(send_sock, &i, sizeof(i));
    cout << "sent " << i << endl;
    cout << "Listening" << endl;
    connFd = accept(recv_sock, 0, 0);
    if (connFd < 0)
    {
        cerr << "Cannot accept connection" << endl;
        return 0;
    }
    else
    {
       cout << "Connection successful" << endl;
       read(connFd, &X, sizeof(X));
       cout << "received " << X << endl;
    }

    usleep(2000000);

 }


 close(send_sock);
 close(recv_sock);
 unlink(argv[2]);
 unlink(argv[1]);


 return 0;

}

Python Code:

import socket,os,struct, glob, sys
import random

send_socket = socket.socket(socket.AF_UNIX, socket.SOCK_STREAM)
recv_socket = socket.socket(socket.AF_UNIX, socket.SOCK_STREAM)
try:
    os.remove(sys.argv[1])
except OSError:
    pass

recv_socket.bind(sys.argv[1])



recv_socket.listen(10)
while 1:
   print "listening ..."
   conn, addr = recv_socket.accept()
   data = conn.recv(4)


   p = struct.unpack('i',data)
   print 'received ', p

   if p is '9':
      break


   l = int(raw_input(">"))
   a = struct.pack('i', l) 
   send_socket.connect(sys.argv[2])
   send_socket.sendall(a)
   print 'sent ', l

send_socket.close()
conn.close()
recv_socket.close()

What am I doing wrong in this approach? Do I need to use threads?

Thanks

Aliean User
  • 3
  • 1

1 Answers1

1

You handle differently the send and receive sockets in your C++ code: the send socket is bound once at the start of the program whereas the receive socket accepts a new connection at each iteration.

Your current Python implementation accepts a new connection on recv_socket and connects send_socket at each iteration which explains the issue that you are facing.

The most efficient fix would be to connect each socket once prior to the loop unless you have a good reason to open a new connection at each iteration. Here are the corresponding code listings:

C++

static int connFd;

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

  int recv_sock, send_sock;

  struct sockaddr_un server, client;

  ///////////////////////////////////////////
  //
  // setup send
  //
  ///////////////////////////////////////////

  /* Create socket on which to send. */
  send_sock = socket(AF_UNIX, SOCK_STREAM, 0);
  if (send_sock < 0) {
    perror("opening unix socket");
    exit(1);
  }

  /* Construct name of socket to send to. */
  client.sun_family = AF_UNIX;

  strcpy(client.sun_path, argv[1]);

  if (connect(send_sock, (struct sockaddr *)&client,
              sizeof(struct sockaddr_un)) < 0) {
    close(send_sock);
    perror("connecting stream socket");
    exit(1);
  }

  ///////////////////////////////////////////
  //
  // setup recv
  //
  ///////////////////////////////////////////

  recv_sock = socket(AF_UNIX, SOCK_STREAM, 0);

  if (recv_sock < 0) {
    cerr << "Cannot open socket" << endl;
    return 0;
  }

  bzero((char *)&server, sizeof(server));

  server.sun_family = AF_UNIX;
  strcpy(server.sun_path, argv[2]);

  // bind socket
  if (::bind(recv_sock, (struct sockaddr *)&server, sizeof(server)) < 0) {
    cerr << "Cannot bind" << endl;
    return 0;
  }

  listen(recv_sock, 10);
  connFd = accept(recv_sock, 0, 0);
  if (connFd < 0) {
    cerr << "Cannot accept connection" << endl;
    return 0;
  } else {
    cout << "Connection successful" << endl;
  }

  int X;
  for (int i = 0; i < 10; i++) {
    write(send_sock, &i, sizeof(i));
    cout << "sent " << i << endl;
    cout << "Listening" << endl;
    read(connFd, &X, sizeof(X));
    cout << "received " << X << endl;
    usleep(2000000);
  }

  close(send_sock);
  close(recv_sock);
  unlink(argv[2]);
  unlink(argv[1]);

  return 0;
}

Python

recv_socket = socket.socket(socket.AF_UNIX, socket.SOCK_STREAM)
send_socket = socket.socket(socket.AF_UNIX, socket.SOCK_STREAM)
try:
    os.remove(sys.argv[1])
except OSError:
    pass

recv_socket.bind(sys.argv[1])

recv_socket.listen(10)
conn, addr = recv_socket.accept()

send_socket.connect(sys.argv[2])

while 1:
   print "listening ..."
   data = conn.recv(4)
   p = struct.unpack('i',data)
   print 'received ', p

   if p is '9':
      break


   l = int(raw_input(">"))
   a = struct.pack('i', l)
   send_socket.sendall(a)
   print 'sent ', l

send_socket.close()
conn.close()
recv_socket.close()
qfiard
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