MFC TCP Client Sample

Introduction

This sample demonstrates how to write a TCP client application using the MFC socket architecture 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

Access the web interface by entering the module’s IP address in a browser’s URL window.

Choose Serial Ports from the Configuration menu.

Configure the module to function as a TCP server by doing the following:

Click the TCP tab.

Check Enable TCP Server.

Specify 2101 as the Raw TCP port.

Click Save.

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

Select File > Open Workspace... from the main menu.

Change Files of type: to Projects (.dsp).

Locate and open the .dsp file for this sample.

Select Build > Rebuild All menu items to compile the sample.

Step-by-Step

1. Initialize Windows Sockets

The following initialization code should be in the main CWinApp derived object's overridable InitInstance function.
if (!AfxSocketInit())
{
   AfxMessageBox(IDP_SOCKETS_INIT_FAILED);
   return FALSE;
}
Initializing Windows Sockets is critical. Failing to initialize Windows Sockets will cause all other socket function calls to fail.

2. Create a local socket

The call to create a socket looks like this:
m_MySocket.Create();
In this sample, a socket is represented by the CAsyncSocket derived class CMySocket. An object of CMySocket type is declared as a private member variable of the main dialog class and named m_MySocket.

m_MySocket is unusable until its Create member function is called, which is done in the main dialog's OnInitDialog overridable function. Since this sample demonstrates using a TCP socket connection (socket type SOCK_STREAM) and is a client application which does not require specifying a particular well-known port, the default parameters of the Create function are used.

3. Establish a connection

Connections are established by calling the socket object's Connect member function.
m_MySocket.Connect(AddressToConnectTo,
                   PortToConnectTo);
Where AddressToConnectTo is a string that represents the IP address of the device server in dotted number format (e.g. "10.0.0.1").

And 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.

Because non-blocking sockets are being used, the call to Connect should return immediately, and the socket will receive an OnConnect notification when the operation is complete.

4. Transmit data

Transmitting data is done by calling the socket's Send member functions.
m_MySocket.Send(Data, Length);
After receiving the OnConnect notification and verifying the connection completed successfully, the sample uses the socket to transmit some data.

The sample is not concerned about sending out-of-band data or routing of the TCP packet, so the only parameters needed for the Send function are: Data which is a pointer to a buffer containing the data to transmit, and Length which is the number of bytes of data in the buffer.

5. Receive data

To retrieve data, the socket's Receive member function is called. In the function call the sample provides a buffer (Data) to store the incoming data, and the length (sizeof(Data)) of that buffer in bytes.
m_MySocket.Receive(Data, sizeof(Data));
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. The socket is notified that data has arrived for it when it receives the OnReceive notification.

6. Close the socket

Accomplish this task by calling the following two functions:
m_MySocket.ShutDown(CAsyncSocket::both);
m_MySocket.Close();
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 (Close).