Socket - C server , Android client

Question

Hello i have made a C server and an android client made in java. , im trying to let the server send the string vibrate and let the phone vibrate for some time. i can see that the client is connected but i dont think the server actualy send any data at all , after i press enter to quit the program for getchar() this error pops up in logcat.

logcat error

01-28 14:10:50.400: W/System.err(30663): java.net.SocketException: recvfrom failed: ECONNRESET (Connection reset by peer)
01-28 14:10:50.400: W/System.err(30663):    at libcore.io.IoBridge.maybeThrowAfterRecvfrom(IoBridge.java:552)
01-28 14:10:50.400: W/System.err(30663):    at libcore.io.IoBridge.recvfrom(IoBridge.java:516)
01-28 14:10:50.400: W/System.err(30663):    at java.net.PlainSocketImpl.read(PlainSocketImpl.java:488)
01-28 14:10:50.400: W/System.err(30663):    at java.net.PlainSocketImpl.access$000(PlainSocketImpl.java:46)
01-28 14:10:50.400: W/System.err(30663):    at java.net.PlainSocketImpl$PlainSocketInputStream.read(PlainSocketImpl.java:240)
01-28 14:10:50.400: W/System.err(30663):    at java.io.InputStreamReader.read(InputStreamReader.java:244)
01-28 14:10:50.410: W/System.err(30663):    at java.io.BufferedReader.fillBuf(BufferedReader.java:130)
01-28 14:10:50.410: W/System.err(30663):    at java.io.BufferedReader.readLine(BufferedReader.java:390)
01-28 14:10:50.410: W/System.err(30663):    at com.example.rat.RatService$ClientThread.run(RatService.java:76)
01-28 14:10:50.410: W/System.err(30663):    at java.lang.Thread.run(Thread.java:864)
01-28 14:10:50.410: W/System.err(30663): Caused by: libcore.io.ErrnoException: recvfrom failed: ECONNRESET (Connection reset by peer)
01-28 14:10:50.420: W/System.err(30663):    at libcore.io.Posix.recvfromBytes(Native Method)
01-28 14:10:50.420: W/System.err(30663):    at libcore.io.Posix.recvfrom(Posix.java:131)
01-28 14:10:50.420: W/System.err(30663):    at libcore.io.BlockGuardOs.recvfrom(BlockGuardOs.java:164)
01-28 14:10:50.420: W/System.err(30663):    at libcore.io.IoBridge.recvfrom(IoBridge.java:513)
01-28 14:10:50.420: W/System.err(30663):    ... 8 more

C server code.

int main(int argc, char *argv[])
{
  WSADATA wsa;
  SOCKET s, new_socket;
  struct sockaddr_in server, client;
  int c;
  char *message;

  if (WSAStartup(MAKEWORD(2, 2), &wsa) != 0)
  {
    printf("ERROR initlizaing winsock2 dll : %d", WSAGetLastError());
    return 1; // exit program with error.
  }

  printf("INitialised.\n");

  // create a socket
  s = socket(AF_INET, SOCK_STREAM, 0);
  if (s == INVALID_SOCKET)
  {
    printf("error at creating socket :%d", WSAGetLastError());
    WSACleanup();
    return 1;
  }

  printf("socket created");

  //Sockad_addr in strucsture prepare
  memset(&server, 0, sizeof server);
  server.sin_family = AF_INET;
  server.sin_addr.s_addr = INADDR_ANY;
  server.sin_port = htons(5000);

  // bind the socket

  if (bind(s, (struct sockaddr *) &server, sizeof(server)) == SOCKET_ERROR)
  {
    printf("Bind fialed with errror code : %d", WSAGetLastError());
    closesocket(s);
    WSACleanup();
    return 1;
  }

  listen(s, 3);

  printf("waiting for incoming connections");
  // this part is what i dont understand.
  c = sizeof(struct sockaddr_in);
  new_socket = accept(s, (struct sockaddr *) &client, &c);
  if (new_socket == INVALID_SOCKET)
  {
    printf("accept fialed with error code : %d", WSAGetLastError());

    closesocket(s);
    WSACleanup();
    return 1;
  }

  printf("Connection acceptëd");

  //reply to CLIENTCREATESTRUCT
  message = "vibrate";
  send(new_socket, message, strlen(message), 0);
  getchar();
  closesocket(s);
  WSACleanup();
  return 0;
}

ANdroid java client ,

public class Service extends Service{
    private boolean isRunning = false;
    private static final  String SERVER_IP = "192.168.178.11";
    private static final int SERVERPORT = 5000;
    private Socket socket;
    Thread ClientThread = null;
    String st = null;



    public void onCreate() {

    }


    public int onStartCommand(Intent intent, int flags, int startId){
        super.onStartCommand(intent, flags, startId);

        // Annoucment about starting serivce

        // Start a Thread Called MyThread
        isRunning = true;
        this.ClientThread = new Thread(new ClientThread());
        this.ClientThread.start();

        // Keep running until it explicitly stopped.
        // stopped so returns sicky
        return START_STICKY;
    }
    // code


    public void onDestroy(){
        super.onDestroy();
        // stop background Thread
        isRunning = false;


    }

    public class ClientThread implements Runnable {
        public void run() {


            try{
                InetAddress serverAddr = InetAddress.getByName(SERVER_IP);

                socket = new Socket(serverAddr,SERVERPORT);


                BufferedReader input =  new BufferedReader(new InputStreamReader(socket.getInputStream()));


                String st = input.readLine();

                if (st.equals("vibrate")){
                    Vibrator v = (Vibrator) getSystemService(Context.VIBRATOR_SERVICE);
                    v.vibrate(3000);
                }
                st = null;

            } catch (Exception e) {
                e.printStackTrace();
            }
        } 
    }

    @Override
    public IBinder onBind(Intent intent) {
        // TODO Auto-generated method stub
        return null;
    }
}

Android Client Manifest

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.example.rat"
    android:versionCode="1"
    android:versionName="1.0" >
    <uses-permission android:name="android.permission.VIBRATE" />
    <uses-permission android:name="android.permission.INTERNET" />
    <uses-sdk
        android:minSdkVersion="8"
        android:targetSdkVersion="19" />

    <application
        android:allowBackup="true"
        android:icon="@drawable/ic_launcher"
        android:label="@string/app_name"
        android:theme="@style/AppTheme" >
        <activity
            android:name="com.example.Service.MainActivity"
            android:label="@string/app_name" >
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
        <service android:name=".Service" />
    </application>

</manifest>

Answer 1

BufferedReader.readLine() blocks until a new-line had been received.

From BufferedReader's documenation:

Reads a line of text. A line is considered to be terminated by any one of a line feed ('\n'), a carriage return ('\r'), or a carriage return followed immediately by a linefeed.

So to fix this issue just also make the server send a new-line by changing this line:

message = "vibrate";

to become:

message = "vibrate\n";

---

Update:

The error message the client logs clearly shows that the server closes the connection due to ending.

This implies send() had returned. It would be interesting to know which value send() returned.

So I propose you modify this line:

send(new_socket, message, strlen(message), 0);

To become:

{
  int result = send(new_socket, message, strlen(message), 0);

  if (SOCKET_ERROR == result)
  {
    fprintf(stderr, "send() failed with error code: %d\n", WSAGetLastError());
  }
  else
  {
    fprintf(stderr, "Sent %d bytes out of %u.\n", result, strlen(message));
  }
}

Answer 2

you can try java.util.Scanner for reading jut like java.io.BufferedReader, I mean:

java.util.Scanner sc = new java.util.Scanner(socket.getInputStream(), "US-ASCII").useDelimiter("\\n");
String st = sc.nextLine();
if (st.equals("vibrate")){
    // ...
}

In the server you must add the new line character:

message = "vibrate\n";

Note: If you wish to change separation character from "\n" to any other, just specify on useDelimiter and add it on message at server.

Note: Here is a comparision between Scanner vs BufferedReader.