How to properly send data via sockets between java and c?

Question

I am currently trying to send strings via sockets between Java and c. I am able to either send a String to the client (c) from the server (java), or vice versa, but not BOTH, which is how I need to communicate between the two. In my c (client) code, as soon as I insert the read portion, the code haults.

Here are my two portions of code. It is safe to assume the connection between the sockets is successful.

java:

private void handshake(Socket s) throws IOException{
    this.out = new PrintStream(s.getOutputStream(), true);
    this.in = new BufferedReader(new InputStreamReader(s.getInputStream()));
    String key = in.readLine(); //get key from client
    if(!key.equals(CLIENTKEY)){
        System.out.println("Received incorrect client key: " + key);
        return;
    }

    System.out.println("received: " + key);
    System.out.println("sending key");
    out.println("serverKEY"); //send key to client
    System.out.println("sent");
}

c:

    int n;
    n = write(sockfd,"clientKEY",9);
    if (n < 0)
    {
      perror("ERROR writing to socket");
      exit(1);
    }
  n = read( sockfd,recvBuff,255 );
  if (n < 0)
    {
      perror("ERROR reading from socket");
      exit(1);
    }
  printf("Here is the message: %s\n",recvBuff);

Does it display an error message when it halts? If so, what is the message? — ajb
– ajb, Commented Nov 7, 2013 at 18:19

Robert S. Barnes · Accepted Answer · 2013-11-07 18:36:58Z

2

Modify your C send code:

char clientKey[] = "clientKEY\n"
n = write(sockfd,clientKey, strlen(clientKey));

It's better to use a variable for clientKey and then call strlen so you don't have to count char's manually. As Jiri pointed out, Java's readLine function is probably expecting a newline char that it's never getting so it hangs.

answered Nov 7, 2013 at 18:36

Robert S. Barnes

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1 Comment

kekkles4 Over a year ago

ah!! that was obvious. Thank you! I have to remember println() in java appends the \n character.

JiP · Accepted Answer · 2013-11-07 20:24:05Z

2

It seems to me that the C/C++ server sends a clientKEY message to the Java client. The Java client reads a line, i.e. waits till it receives the \n character from the C/C++ server. However, it is never sent by the C/C++ server and so the Java client waits... forever.

edited Nov 7, 2013 at 20:24

answered Nov 7, 2013 at 18:24

JiP

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