java socket 与linux c通信问题

如题，用java做客户端，linux的c程序做服务器端。目前客户端向服务器端发送信息已经没问题，但是在添加服务器端向客户端发送信息的功能时却出现了死循环。在Eclipse调试了一下客户端发现是在in.readLine()卡住了，具体代码如下：
java 客户端：
Java code:

package linux;

import java.net.*;
import java.*;
import java.util.*;
import java.io.OutputStream;
import java.io.InputStream;
import java.io.*;
import java.io.BufferedReader;
import java.net.InetSocketAddress;
public class java_c_client{
public static void main(String[] argv){
Socket socket = null;
InputStream in = null;
BufferedReader input = null;
byte [] b = new byte[5];
String url = "219.216.101.210";
int PORT = 9999;
try{

int num = -1;
socket = new Socket();
socket.setReuseAddress(true);
InetSocketAddress socketAddr = new InetSocketAddress(url, PORT);
System.out.println("ip==="+socket.getInetAddress()+"\t port=="+socket.getLocalPort());

socket.connect(socketAddr);
DataOutputStream dos = new DataOutputStream(socket.getOutputStream());
System.out.println("ip==="+socket.getInetA