Socket Programming between C server and Java client

Question

I am trying to write a c language server and a java client to send messages.

I am able to successfully connect the client and server. However, the c server is not able to receive the data from the client server.

The expected result, I would like to get is.

1) Start the server

2) start the client application

3) User key in some words into the textfield and click send.

4) Then the server side receive the String I send from client.

Is there some sort of encoding errors that I missed out or data mismatch? I would like to get some help on why the data is not received on the server side. Thank You!

The essential code used in my java client code are:

private Socket socket;
private static final int SERVERPORT = 4000;
private static final String SERVER_IP = "192.168.8.112";

InetAddress serverAddr = InetAddress.getByName(SERVER_IP);
socket = new Socket(serverAddr, SERVERPORT);              

In the java application I have a text field for user to input data:

public void onClick(View view) {
    try {
        EditText et = (EditText) findViewById(R.id.EditTextT);

        String str = et.getText().toString();
        PrintWriter out = new PrintWriter(new BufferedWriter(new OutputStreamWriter(socket.getOutputStream())), true);
        out.println(str);

    } 
}

In my server code, I used the code from here

#include<stdio.h>
#include<string.h>    //strlen
#include<sys/socket.h>
#include<arpa/inet.h> //inet_addr
#include<unistd.h>    //write

int main(int argc , char *argv[])
{
int socket_desc , client_sock , c , read_size;
struct sockaddr_in server , client;
char client_message[2000];

//Create socket
socket_desc = socket(AF_INET , SOCK_STREAM , 0);
if (socket_desc == -1)
{
    printf("Could not create socket");
}
puts("Socket created");

//Prepare the sockaddr_in structure
server.sin_family = AF_INET;
server.sin_addr.s_addr = INADDR_ANY;
server.sin_port = htons( 4000 );

//Bind
if( bind(socket_desc,(struct sockaddr *)&server , sizeof(server)) < 0)
{
    //print the error message
    perror("bind failed. Error");
    return 1;
}
puts("bind done");

//Listen
listen(socket_desc , 3);

//Accept and incoming connection
puts("Waiting for incoming connections...");
c = sizeof(struct sockaddr_in);

//accept connection from an incoming client
client_sock = accept(socket_desc, (struct sockaddr *)&client, (socklen_t*)&c);
if (client_sock < 0)
{
    perror("accept failed");
    return 1;
}
puts("Connection accepted");

//Receive a message from client
while( (read_size = recv(client_sock , client_message , 2000 , 0)) > 0 )
{
    //Send the message back to client
    write(client_sock , client_message , strlen(client_message));
}
printf("Message received: %s\n",client_message);
if(read_size == 0)
{
    puts("Client disconnected");
    fflush(stdout);
}
else if(read_size == -1)
{
    perror("recv failed");
}

return 0;
}

Editing Notes: I changed 8888 to 4000.

Additional Information : There is no error on compilation for both piece of code. Connection between the client and server can be established successfully. The output on the server sides print Message received:. It is empty without the word i send from the client app.

Answer 1

Your C code is reading until end of stream before it prints 'message received'.

Your Java code is not closing the socket.

So the C code will never see end of stream.

So it will never print 'message received'.

This is not evidence that no message was received.

Furthermore, your C code is sending the message back to the client, which doesn't appear to be reading from the socket, so eventually the C code will block in send().

So the first thing you have to do is print 'message received' inside the receive loop, not when it terminates.

Secondly, this:

write(client_sock , client_message , strlen(client_message));

is wrong. There is no guarantee of a trailing null. It should be:

write(client_sock , client_message , read_size);

Similarly this:

printf("Message received: %s\n",client_message);

should be:

printf("Message received: %.*s\n",read_size,client_message);

inside the loop.