An Intro to Windows Socket (Winsock2) Programming 17






Connectionless Communication


Connectionless communication behaves differently than connection-oriented communication, so the method for sending and receiving data is substantially different. First we'll discuss the receiver (or server, if you prefer) because the connectionless receiver requires little change when compared with the connection-oriented servers. After that we'll look at the sender. In IP, connectionless communication is accomplished through UDP/IP. UDP doesn't guarantee reliable data transmission and is capable of sending data to multiple destinations and receiving it from multiple sources. For example, if a client sends data to a server, the data is transmitted immediately regardless of whether the server is ready to receive it. If the server receives data from the client, it doesn't acknowledge the receipt. Data is transmitted using datagrams, which are discrete message packets. The following Figure shows a simplified UDP communication flow between server and client.


Windows Socket (Winsock2) Programming and  C Language: the connectionless (UDP) client-server datagram connection flow




The steps in the process of receiving data on a connectionless socket are simple. First, create the socket with either socket() or WSASocket(). Next, bind the socket to the interface on which you wish to receive data. This is done with the bind() function (exactly like the session-oriented example). The difference with connectionless sockets is that you do not call listen() or accept(). Instead, you simply wait to receive the incoming data. Because there is no connection, the receiving socket can receive datagrams originating from any machine on the network. The simplest of the receive functions is recvfrom(), which is defined as:


int recvfrom(

    SOCKET s,

    char FAR* buf,

    int len,

    int flags,

    struct sockaddr FAR* from,

    int FAR* fromlen);


The first four parameters are the same as recv(), including the possible values for flags: MSG_OOB and MSG_PEEK. The same warnings for using the MSG_PEEK flag also apply to connectionless sockets. The from parameter is a SOCKADDR structure for the given protocol of the listening socket, with fromlen pointing to the size of the address structure. When the API call returns with data, the SOCKADDR structure is filled with the address of the workstation that sent the data. The Winsock 2 version of the recvfrom() function is WSARecvFrom(). The prototype for this function is:


int WSARecvFrom(

    SOCKET s,

    LPWSABUF lpBuffers,

    DWORD dwBufferCount,

    LPDWORD lpNumberOfBytesRecvd,

    LPDWORD lpFlags,

    struct sockaddr FAR * lpFrom,

    LPINT lpFromlen,

    LPWSAOVERLAPPED lpOverlapped,



The difference is the use of WSABUF structures for receiving the data. You can supply one or more WSABUF buffers to WSARecvFrom() with dwBufferCount indicating this. By supplying multiple buffers, scatter-gather I/O is possible. The total number of bytes read is returned in lpNumberOfBytesRecvd. When you call WSARecvFrom(), the lpFlags parameter can be 0 for no options, MSG_OOB, MSG_PEEK, or MSG_PARTIAL. These flags can be bitwise OR together. If MSG_PARTIAL is specified when the function is called, the provider knows to return data even if only a partial message has been received. Upon return, the flag MSG_PARTIAL is set if only a partial message was received. Upon return, WSARecvFrom() will store the address of the sending machine in the lpFrom parameter (a pointer to a SOCKADDR structure). Again, lpFromLen points to the size of the SOCKADDR structure, except that in this function it is a pointer to a DWORD. The last two parameters, lpOverlapped and lpCompletionRoutine, are used for overlapped I/O. Another method of receiving (and sending) data on a connectionless socket is to establish a connection. This might seem strange, but it's not quite what it sounds like. Once a connectionless socket is created, you can call connect() or WSAConnect() with the SOCKADDR parameter set to the address of the remote machine to communicate with. No actual connection is made, however. The socket address passed into a connect() function is associated with the socket so recv() and WSARecv() can be used instead of recvfrom() or WSARecvFrom() because the data's origin is known. The capability to connect a datagram socket is handy if you intend to communicate with only one endpoint at a time in your application. The following code sample demonstrates how to construct a simple UDP receiver application.


1.   While in the Visual C++ IDE, click File menu > Project sub menu to create a new project.

2.   Select Win32 for the Project types: and Win32 Console Application for the Templates:. Put the project and solution name. Adjust the project location if needed and click OK.


The UDP connectionless select Winsock2 and C program: creating new Win32 console application in Visual C++


3.      Click Next for the Win32 Application Wizard Overview page. We will remove all the unnecessary project items.


The UDP connectionless select Winsock2 and C program: Win32 application project overview page


4.   In the Application page, select Empty project for the Additional options:. Leave others as given and click Finish.


The UDP connectionless select Winsock2 and C program: selecting an empty project for the Win32 console mode application


5.   Next, we need to add new source file. Click Project menu > Add New Item sub menu or select the project folder in the Solution Explorer > Select Add menu > Select New Item sub menu.

6.   Select C++ File (.cpp) for the Templates:. Put the source file name and click Add. Although the extension is .cpp, Visual C++ IDE will recognize that the source code used is C based on the Compile as C Code (/TC) option which will be set in the project property page later.


The UDP receiver connectionless select Winsock2 and C program: adding the C++ source file to the existing project


7.   Before we can build this Winsock C Win32 console application project, we need to set the project to be compiled as C code and link to ws2_32.lib, the Winsock2 library. Invoke the project property page.

8.   Expand the Configuration folder > Expand the C/C++ sub folder. Select the Advanced link and for the Compile As option, select Compile as C Code (/TC).


The UDP receiver connectionless select Winsock2 and C program: setting the Visual C++ project to be compiled as C code


9.    Next, expand the Linker folder and select the Input link. For the Additional Dependencies option, click the ellipses at the end of the empty field on the right side.

10.  Manually, type the library name and click OK or you can just directly type the library name in the empty field on the right of the Additional Dependencies. Click OK.


The UDP receiver connectionless select Winsock2 and C program: adding additional dependencies, that is the Winsock2 library - ws2_32.lib to be linked together resolving the winsock2.h


11.  Now, add the source code as given below.


#include <winsock2.h>

#include <stdio.h>


// #pragma comment(lib, "ws2_32.lib")


int main(int argc, char **argv)


            WSADATA                            wsaData;

            SOCKET                                ReceivingSocket;

            SOCKADDR_IN                   ReceiverAddr;

            int                                            Port = 5150;

            char                             ReceiveBuf[1024];

            int                                            BufLength = 1024;

            SOCKADDR_IN                   SenderAddr;

            int                                            SenderAddrSize = sizeof(SenderAddr);

            int                                            ByteReceived = 5;


   // Initialize Winsock version 2.2

   if( WSAStartup(MAKEWORD(2,2), &wsaData) != 0)


               printf("Server: WSAStartup failed with error %ld\n", WSAGetLastError());

               return -1;



                        printf("Server: The Winsock DLL status is %s.\n", wsaData.szSystemStatus);


            // Create a new socket to receive datagrams on.

            ReceivingSocket = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP);


            if (ReceivingSocket == INVALID_SOCKET)


                        printf("Server: Error at socket(): %ld\n", WSAGetLastError());

                        // Clean up


                        // Exit with error

                        return -1;



                        printf("Server: socket() is OK!\n");


            // Set up a SOCKADDR_IN structure that will tell bind that we

            // want to receive datagrams from all interfaces using port 5150.


            // The IPv4 family

            ReceiverAddr.sin_family = AF_INET;

            // Port no. 5150

            ReceiverAddr.sin_port = htons(Port);

            // From all interface (

            ReceiverAddr.sin_addr.s_addr = htonl(INADDR_ANY);


   // Associate the address information with the socket using bind.

   // At this point you can receive datagrams on your bound socket.

   if (bind(ReceivingSocket, (SOCKADDR *)&ReceiverAddr, sizeof(ReceiverAddr)) == SOCKET_ERROR)


               printf("Server: bind() failed! Error: %ld.\n", WSAGetLastError());

               // Close the socket


               // Do the clean up


               // and exit with error

               return -1;



                        printf("Server: bind() is OK!\n");


   // Some info on the receiver side...

   getsockname(ReceivingSocket, (SOCKADDR *)&ReceiverAddr, (int *)sizeof(ReceiverAddr));


   printf("Server: Receiving IP(s) used: %s\n", inet_ntoa(ReceiverAddr.sin_addr));

   printf("Server: Receiving port used: %d\n", htons(ReceiverAddr.sin_port));


   printf("Server: I\'m ready to receive a datagram...\n");


   // At this point you can receive datagrams on your bound socket.

   ByteReceived = recvfrom(ReceivingSocket, ReceiveBuf, BufLength, 0, (SOCKADDR *)&SenderAddr, &SenderAddrSize);

   if ( ByteReceived > 0 )


               printf("Server: Total Bytes received: %d\n", ByteReceived);

               printf("Server: The data is \"%s\"\n", ReceiveBuf);


   else if ( ByteReceived <= 0 )

               printf("Server: Connection closed with error code: %ld\n", WSAGetLastError());


               printf("Server: recvfrom() failed with error code: %d\n", WSAGetLastError());


   // Some info on the sender side

   getpeername(ReceivingSocket, (SOCKADDR *)&SenderAddr, &SenderAddrSize);

   printf("Server: Sending IP used: %s\n", inet_ntoa(SenderAddr.sin_addr));

   printf("Server: Sending port used: %d\n", htons(SenderAddr.sin_port));


   // When your application is finished receiving datagrams close the socket.

   printf("Server: Finished receiving. Closing the listening socket...\n");

   if (closesocket(ReceivingSocket) != 0)

               printf("Server: closesocket() failed! Error code: %ld\n", WSAGetLastError());


               printf("Server: closesocket() is OK\n");


   // When your application is finished call WSACleanup.

   printf("Server: Cleaning up...\n");

   if(WSACleanup() != 0)

               printf("Server: WSACleanup() failed! Error code: %ld\n", WSAGetLastError());


               printf("Server: WSACleanup() is OK\n");

   // Back to the system

            return 0;



12.  Build the project and make sure there is no error which can be seen (if any) in the Output window normally docked at the bottom of the IDE by default. Run the project and unblock the Windows firewall if any.


The UDP receiver connectionless select Winsock2 and C program: unblocking the Windows firewall protection


13.  If there is no error, a sample output is shown below. The UDP receiver is ready to receive data, unreliably.


The UDP receiver connectionless select Winsock2 and C program: a sample UDP select server in action, waiting client connections


Now that you understand how to construct a receiver that can receive a datagram, we will describe how to construct a sender.





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