WinSock Introduction
MCS 4613 – Computer Networks
David Brown
Overview
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Sockets are used to connect between two
processes on the network
There is a client and server process
Server process creates a socket and listens
for a connection
Client process creates a socket and
connects to server
Sockets support different network standards
Discuss use of sockets for TCP/IP protocols
Nov 3, 2011
MCS 4613 Computer Networks
Prof David Brown
2
Server
Steps
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6.
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Initialize Winsock
Create a socket
Bind the socket
Listen on the socket for a client
Accept a connection from a client
Receive and send data
Disconnect
Nov 3, 2011
MCS 4613 Computer Networks
Prof David Brown
3
Building an application
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Must call WSAStartup before using sockets
When done with sockets call WSACleanup
#include <winsock2.h>
#include <stdio.h>
int main(int argc, char **argv)
{
WSADATA
wsaData;
// Initialize Winsock version 2.2
WSAStartup(MAKEWORD(2,2), &wsaData);
printf("Winsock DLL status is %s.\n", wsaData.szSystemStatus);
// Code to handle socket
// Cleanup socket
WSACleanup();
}
Nov 3, 2011
MCS 4613 Computer Networks
Prof David Brown
4
Client/Server
Creating a Socket
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Simplest way is with a call to socket
Sockets support different protocols
SOCKET Socket;
// Create a new socket to make a client or server connection.
// AF_INET = 2, The Internet Protocol version 4 (IPv4) address family, TCP protocol
Socket = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
if(Socket == INVALID_SOCKET)
{
printf(“Socket() failed! Error code: %ld\n", WSAGetLastError());
// Do the clean up
WSACleanup();
// Exit with error
return -1;
}
Nov 3, 2011
MCS 4613 Computer Networks
Prof David Brown
5
Server
Socket Address Structure
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Server must specify what address to listen on
There is a special address for any address
SOCKADDR_IN
ServerAddr;
unsigned int
Port = 21073;
// Set up a SOCKADDR_IN structure that will be used to connect
// to a listening server on port 21073.
// IPv4
ServerAddr.sin_family = AF_INET;
// Port no.
ServerAddr.sin_port = htons(Port);
// The IP address
ServerAddr.sin_addr.s_addr = htonl (INADDR_ANY);
Nov 3, 2011
MCS 4613 Computer Networks
Prof David Brown
6
Server
Bind to socket
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The SOCKADDR_IN structure must be binded with the
socket
NOTE: SOCKADDR_IN vs. SOCKADDR
// Associate the address information with the socket using bind.
// Call the bind function, passing the created socket and the sockaddr_in
// structure as parameters. Check for general errors.
if (bind(Socket, (SOCKADDR *)&ServerAddr, sizeof(ServerAddr)) == SOCKET_ERROR)
{
printf("Server: bind() failed! Error code: %ld.\n", WSAGetLastError());
// Close the socket
closesocket(Socket);
// Do the clean up
WSACleanup();
// and exit with error
return -1;
}
else
MCS 4613 Computer Networks
Nov 3, 2011
7
printf("Server: bind() is OK!\n");
Prof David Brown
Server
Listen to Socket
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Server must listen for a connection
Backlog is number of connections to allow in
the queue
// Listen for client connections. We use a backlog of 5, which
// is normal for many applications.
if (listen(Socket, 5) == SOCKET_ERROR)
{
printf("Server: listen(): Error listening on socket %ld.\n", WSAGetLastError());
// Close the socket
closesocket(Socket);
// Do the clean up
WSACleanup();
// Exit with error
return -1;
}
Nov 3, 2011
MCS 4613 Computer Networks
Prof David Brown
8
Server
Accept connection
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Once a socket is listening you have to call
Accept to accept any connection
Note: code below assumes blocking socket
// Accept a new connection when one arrives.
NewConnection = accept(Socket, NULL, NULL);
if (NewConnection == SOCKET_ERROR) {
printf(“Server: accept() failed! Error code: %ld\n", WSAGetLastError());
// Close the socket
closesocket(Socket);
// Do the clean up
WSACleanup();
// Exit with error
return -1;
}
Nov 3, 2011
MCS 4613 Computer Networks
Prof David Brown
9
Client
Steps
1.
2.
3.
4.
5.
Initialize Winsock
Create a socket
Connect to the server
Send and receive data
Disconnect
Nov 3, 2011
MCS 4613 Computer Networks
Prof David Brown
10
Client
Socket Address Structure
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sockaddr structure is used to either connect or listen to a
socket
It specifies the protocol, the port and the IP address
SOCKADDR_IN ServerAddr;
unsigned int
Port = 21073;
// Set up a SOCKADDR_IN structure that will be used to connect
// to a listening server on port 21073. For demonstration
// purposes, let's assume our server's IP address is 127.0.0.1 or localhost
// IPv4
ServerAddr.sin_family = AF_INET;
// Port no.
ServerAddr.sin_port = htons(Port);
// The IP address
ServerAddr.sin_addr.s_addr = inet_addr("127.0.0.1");
Nov 3, 2011
MCS 4613 Computer Networks
Prof David Brown
11
Client
Connect to Socket
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Connect to Socket before transmitting data
Similar to server bind/listen but all in one call
// Make a connection to the server with socket SendingSocket.
RetCode = connect(Socket, (SOCKADDR *) &ServerAddr, sizeof(ServerAddr));
if(RetCode != 0)
{
printf("Client: connect() failed! Error code: %ld\n", WSAGetLastError());
// Close the socket
closesocket(Socket);
// Do the clean up
WSACleanup();
// Exit with error
return -1;
}
Nov 3, 2011
MCS 4613 Computer Networks
Prof David Brown
12
Client/Server
Send data
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Once connection is made you can send data
Bytes actually sent may not be the same number of
bytes requested to be sent
Server should send to the socket returned by accept
Data can be sent after one end closes connection
// Sends some data
BytesSent = send(Socket, sendbuf, strlen(sendbuf), 0);
if (BytesSent == SOCKET_ERROR)
printf("Client: send() error %ld.\n", WSAGetLastError());
Nov 3, 2011
MCS 4613 Computer Networks
Prof David Brown
13
Client/Server
Receive data
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Recv will block until data is available
Recv returns 0 bytes if connection is closed
Recv can receive data after connection is
closed
Server should use socket returned by accept
BytesReceived = recv(RecvSocket, recvbuf, sizeof(recvbuf)-1, 0);
recvbuf[BytesReceived] = ‘\0’;
if (BytesReceived == SOCKET_ERROR)
printf("Client: recv() error %ld.\n", WSAGetLastError());
Nov 3, 2011
MCS 4613 Computer Networks
Prof David Brown
14
Client/Server
Shutdown Connection
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You should shutdown the connection when done
Both sides should shut down the connection
Shutdown can be for SD_SEND, SD_RECV, or
SD_BOTH
Server should use socket returned by accept
// Shutdown sending of data
if( shutdown(Socket, SD_SEND) != 0)
printf("Client: Well, there is something wrong with the shutdown(). The error code: %ld\n",
WSAGetLastError());
else
printf("Client: shutdown() looks OK...\n\n");
Nov 3, 2011
MCS 4613 Computer Networks
Prof David Brown
15
Client/Server
Close socket
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Once a socket is completely done it should
be closed
Sockets returned from accept do not have to
be closed.
if(closesocket(Socket) != 0)
printf("Server: Cannot close socket. Error code: %ld\n", WSAGetLastError());
else
printf("Server: Closing socket...\n");
Nov 3, 2011
MCS 4613 Computer Networks
Prof David Brown
16
Client/Server
Cleanup WSA
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You should call WSACleanup when done
with all sockets
if(WSACleanup() != 0)
printf("Client: WSACleanup() failed!...\n");
Nov 3, 2011
MCS 4613 Computer Networks
Prof David Brown
17
Miscellaneous Information
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The information in this introduction describes very simple
socket based programs
Handling of asyncronous calls, multiple simultaneous
connections and other topics are beyond this
introduction
The server I provided uses the function “select” to
provide a timeout feature
For more information on sockets in windows go to
http://msdn.microsoft.com/en-us/library/ms740673(VS.85).aspx
http://docs.microsoft.com/en-us/windows/win32/winsock/gettingstarted-with-winsock
Nov 5, 2019
MCS 4613 Computer Networks
Prof David Brown
18