1. CSI 4118 – UNIVERSITY OF OTTAWA
Part 3.1(supp)
Java Socket Details (supplemental materials)
(Client-Server Concept,
Use of Protocol Ports, Socket API)
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CSI 4118 – UNIVERSITY OF OTTAWA

2. CSI 4118 – UNIVERSITY OF OTTAWA
Creating a Socket
Application calls socket function
OS returns descriptor for socket
Descriptor valid until application closes socket or exits
Common: protofamily = PF_INET, type = SOCK_STREAM or SOCK_DGRAM
In Java, to create a client socket:
Socket socket = new Socket(string host, int port)
To create a server socket:
ServerSocket sSocket = new ServerSocket(int port)
desc = socket(protofamily,type,proto);
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3. CSI 4118 – UNIVERSITY OF OTTAWA
Socket Functionality
Socket completely general
Can be used
By client
By server
With a CO transport protocol
With a CL transport protocol
To send data, receive data, or both
Large set of operations
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CSI 4118 – UNIVERSITY OF OTTAWA

4. CSI 4118 – UNIVERSITY OF OTTAWA
Socket Operations
Close
Terminate use of socket
Permanent
In Java, use socket.close();
or sSocket.close();
close(socket);
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CSI 4118 – UNIVERSITY OF OTTAWA

5. CSI 4118 – UNIVERSITY OF OTTAWA
Socket Operations
Bind
Specify protocol port for a socket
Specify local IP address for a socket
Can use INADDR_ANY for any IP address
In Java, use socket.bind(SocketAddress endpoint) to bind the socket to a IP address and port.
bind(socket,localaddr,addrlen);
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6. Generic Address Format
struct sockaddr {
u_char sa_len; /*length of address*/
u_char sa_family; /*family of address*/
char sa_data[14]; /*address itself*/ }
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7. CSI 4118 – UNIVERSITY OF OTTAWA
TCP/IP Address Format
struct sockaddr_in {
u_char sin_len; /*length of address*/
u_char sin_family; /*family of address*/
u_short sin_port; /*protocol port number*/
struct in_addr sin_addr; /*IP address*/
char sin_zero[8]; /*not used(set to zero)*/ }
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CSI 4118 – UNIVERSITY OF OTTAWA

8. Socket Operations (continued)
Listen
Used by server
Prepares socket to accept incoming connections
Accept
Waits for next connection and returns new socket
In Java, method accept() in java.net.ServerSocket class listens for a connection and accepts it.
listen(socket,queuesize);
newsock = accept(socket,caddr,caddrlen);
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CSI 4118 – UNIVERSITY OF OTTAWA

9. Socket Operations (continued)
Connect
Used by client
Either
Performs a TCP connection
Fully specifies addresses for UDP
In Java, connect(SocketAddress server) to connect the socket to a specific server.
connect(socket,saddr,saddrlen);
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10. Two Purposes of the Connect Function
The connect function, which is called by clients, has two uses. With connection-oriented transport, connect establishes a transport connection to a specified server. With connectionless transport, connect records the server’s address in the socket, allowing the client to send many messages to the same server without specifying the destination address with each message.
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11. Socket Operations (continued)
Send, sendto, and sndmsg
Transfer outgoing data from application
In java, java.io.PrintStream is used to send data. e.g. method println()
send(socket,data,length,flags);
sendto(socket,data,length,flags,
destaddr,addrlen);
sendmsg(socket,msgstruct,flags);
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12. CSI 4118 – UNIVERSITY OF OTTAWA
Format of msgstruct
struct msgstruct {
struct sockaddr *m_saddr; /*dest address*/
struct datavec *m_dvec; /*message (vector)*/
int mdvlength; /*size of vector*/
struct access *m_rights; /*access rights*/
int m_alength; /*size of access rights*/ }
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CSI 4118 – UNIVERSITY OF OTTAWA

13. Socket Operations (continued)
Recv, recvfrom, and recvmsg
Transfer incoming data to application
In Java, java.io.BufferReader used to receive data. E.g. method readline()
recv(socket,buffer,length,flags);
recvfrom(socket,buffer,length,flags,
senderaddr,saddrlen);
recvmsg(socket,msgstruct,flags);
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14. Java Code Equivalent to Example in Chapter 30
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15. Example Server Java Code
import java.io.*;
import java.net.*;
public class server{
public static void main(String[] args) {
// variable declaration
int visits = 0;
ServerSocket srv = null;
PrintStream os;
Socket clientSocket = null;
String msg = "";
//check command-line argument ...
//(omited)
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16. CSI 4118 – UNIVERSITY OF OTTAWA
/* create socket */
try {
srv = new ServerSocket(Integer.parseInt(args[0])); }
catch (IOException e) {
System.out.println(e);
/* Main server loop - accept and handle requests */
while(true){
try {// listen for a connection from client and accept it.
clientSocket = srv.accept();
catch(IOException e) {
System.out.println("accept failed");
System.exit(1);
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17. CSI 4118 – UNIVERSITY OF OTTAWA
try{
os = new PrintStream(clientSocket.getOutputStream());
visits++;
msg = "This server has been contacted " + visits + " times";
os.println(msg); //sends msg
os.close();
clientSocket.close(); }
catch(IOException e) {
System.out.println(e);
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18. Example Client Java Code
import java.io.*;
import java.net.*;
public class client {
public static void main(String[] args)
//variable declaration
Socket socket = null;
BufferedReader in = null;
//check command-line argument...
//(omited)
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19. CSI 4118 – UNIVERSITY OF OTTAWA
try {
/* create a socket and connect to the specified server. */
socket = new Socket(args[0], Integer.parseInt(args[1]));
} catch (UnknownHostException e) {
System.exit(1);
} catch (IOException e) {
System.out.println("connect failed"); }
/* read data from socket and write to user's screen. */
try{
in = new BufferedReader(new InputStreamReader(socket.getInputStream()));
System.out.println(in.readLine());
in.close();
socket.close();
catch(IOException e) {
System.out.println(e);
}}}
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CSI 4118 – UNIVERSITY OF OTTAWA