WSA TCP Client Sample

Introduction

This sample demonstrates how to write a TCP client application using the Windows Socket Architecture (WSA) to communicate with a device server.

For the purpose of this demonstration, the client application initiates communication with the device server by first creating a networking socket. Then using that socket, establishes a TCP connection to a single port on the device server and attempts to transmit and receive data from the device server (similar to a loopback test).

Device Server Setup

This sample was designed to operate with the first port on the device server, and requires that the port be configured to listen for and accept raw socket connections. After the device server is configured properly, the port must also have a loopback plug attached to it.

Configure the device server

1. Access the web interface by entering the module’s IP address in a browser’s URL window.
2. Choose Serial Ports from the Configuration menu.
3. Configure the module to function as a TCP server by doing the following:
   1. Click the TCP tab.
   2. Check Enable TCP Server.
   3. Specify 2101 as the Raw TCP port.
   4. Click Save.
4. Connect the loopback plug to port 1.

How To Build

This sample has been written and tested with Microsoft Visual C++® 6.0. It contains a Developer Studio project file (.dsp), that can be opened in the development environment for editing and compiling.

To build this sample from within Microsoft Visual C++® 6.0

1. Select File > Open Workspace... from the main menu.
2. Change Files of type: to Projects (.dsp).
3. Locate and open the .dsp file for this sample.
4. Select Build > Rebuild All menu items to compile the sample.

Step-by-Step

1.  Initialize Windows Sockets

The following initialization code is found early in the program's main function.

WSABUF WSABuf = {0};
WSAStartup(WINSOCK_VERSION, &WSAData);

Initializing Windows Sockets is critical. Failing to initialize Windows Sockets will cause all other socket function calls to fail.

This sample utilizes Windows Sockets version 2.2. Therefore, the macro WINSOCK_VERSION is defined in the system header file WinSock2.h. An application that calls WSAStartup must call WSACleanup when it is done using the Windows Socket services (see step 8).

2.  Create a local socket

The call to create a socket looks like this:

SOCKET Socket = WSASocket(AF_INET, Type, Protocol,
                          NULL, 0, 0);

Where Type is SOCK_STREAM and Protocol is IPPROTO_TCP for a blocking, connection oriented TCP/IP socket.

3.  Bind to a local port

To bind the socket to a local port, call the following function:

bind(Socket, &SockAddr, sizeof(SockAddr));

Binding to a local port must be done before using the socket to calls like WSAConnect, and is used to associate the currently unnamed socket with a local name.

The SockAddr parameter is the SOCKADDR representation of the local name to bind to. This name consists of an address family (for TCP/IP, always AF_INET), a host address, and a port number.

In this sample, the calls to WSASocket and bind are both done in the function MySocketOpen.

4.  Establish a connection

Connections are established by calling the WSAConnect function.

WSAConnect(Socket, &SockAddr, sizeof(SockAddr),
           NULL, NULL, NULL, NULL);

Where SockAddr is the SOCKADDR representation of the peer name to connect to. Because blocking sockets are being used, the call to WSAConnect will not return until the operation either fails or completes successfully.

To simplify creating a connection, this sample uses a function named MySocketConnect.

MySocketConnect(MySocket,
                AddressToConnectTo,
                PortToConnectTo);

Where MySocket is the socket returned from the call to MySocketOpen.

AddressToConnectTo is a string that represents the IP address of the device server in dotted number format (e.g. "10.0.0.1").

PortToConnectTo is the port number used by the device server for raw TCP connections to port number one. On Digi device servers this port number defaults to 2101.

MySocketConnect hides the details of converting the IP address and port number to SOCKADDR representation.

5.  Transmit data

Transmitting data is done by calling WSASend.

WSASend(Socket, &WSABuf, 1, Length, 0, NULL, NULL);

After verifying the connection completed successfully, the sample uses the socket to transmit some data.

Again, to simplify transmitting data, and to hide many of the details associated with the parameters necessary to call WSASend, this sample uses its own function named MySocketSend.

MySocketSend(MySocket, SendData, &SendLength);

The only parameters needed for MySocketSend are: MySocket which is the socket returned from MySocketOpen, SendData which is a pointer to the buffer holding the data to send, and SendLength which is the number of bytes SendData points to.

6.  Receive data

To retrieve data received at the local address, call WSARecv.

WSARecv(Socket, &WSABuf, 1, Length, &Flags,
        NULL, NULL);

With the device server setup with the loopback plug, the data transmitted in the step above will be sent directly back to the sample application.

Like WSASend, WSARecv has a large number of parameters, providing great flexibility at the price of adding to its complexity. Also like WSASend, the sample provides an alternative function, MySocketRecv that is somewhat easier to use.

MySocketRecv(MySocket, RecvData, &BytesReceived);

The caller to MySocketRecv need only supply the prerequisite MySocket, a buffer RecvData to hold the received data, and BytesReceived which specifies the length of RecvData and on return exactly how many bytes were actually copied into the buffer.

7.  Close the socket

Accomplish this task by calling the following two functions:

shutdown(Socket, SD_BOTH);
closesocket(Socket);

When the sample application is done using the socket, the connection it represents must be terminated properly (shutdown) and any resources it may be using should be released (closesocket).

8.  Cleanup

Before exiting the program, call:

WSACleanup();

For every successful call to WSAStartup an application completes, the application must make one call to WSACleanup.

File List

HexDumpData.cpp

HexDumpData.h

MySocket.cpp

MySocket.h

StdAfx.cpp

StdAfx.h

TCPClient.cpp

TCPClient.dsp

Keywords

WSASocket; bind; WSAConnect; WSASend; WSARecv; shutdown; closesocket; WSACleanup