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Winsock Programming : Page 2

Most of you might have worked with Internet Transfer Control which is very handy control when it comes to Internet Programming but there is another control which even more robust and helps programmers creating more flexible applications. Winsock control comes with VB6 and is used to create applications that access the low-level functions of the Transmission Control Protocol/Internet Protocol (TCP/IP). This article shows how to use the Winsock control in a client server environment: we will create two separate applications, one of which will be a server and the other will be a client. Both client and server will interact with each other to exchange data.


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Accept Method

It accepts the request for connection from the client system. For this method to be used, the control must be in the listening state.



Close Method

The Close method terminates a TCP connection from either the client or server applications.

GetData Method

GetData is the method that retrieves the current block of data from the buffer and then stores it in a variable of the variant type.

PeekData Method

The PeekData method operates in a fashion similar to the GetData method. However, it does not remove data from the input queue.

Listen Method

This is invoked on the server application to have the server application wait for a TCP request for connection from a client system.

SendData Method

This method dispatches data to the remote computer. It is used for both the client and server systems.

Connect Method

The Connect method requests a connection to a remote computer.

I am not going to discuss events here. You can find the complete details of events on the Microsoft site (http://www.microsoft.com).

In the sample provided with this article, we are going to create two applications, one server and client. This is a real world example, where the clients requests some information from the server and the server retrieves some specific information from the database and sends the retrieved information back to the client. The database used in the sample is also provided with the code. The database name is Prices.mdb. This is a small database comprising of a single table containing two fields. The fields are item number and price. The clients sends the item number to the server and the server retrieves the price against that item number from the database and sends it back to the client. One of the current trends in software development today is the issue of thick clients versus thin clients. A thick client is basically an application that performs the bulk of the processing on the individual client PC, whereas a thin client performs the processing on the server.

Creating the Client

Follow the steps shown below:

  1. Start a new EXE project.

  2. Add a Winsock control to your application.

  3. Add all the controls to the form (See the application for details).

Here is the complete code:

Option Explicit

Private Sub cmdClose_Click()

Winsock1.Close

shpGo.Visible = False

shpWait.Visible = False

shpError.Visible = True

End Sub

Private Sub cmdConnect_Click()

Winsock1.RemoteHost = "11.0.0.1" 'Change this to your host ip

Winsock1.RemotePort = 1007

Winsock1.Connect

shpGo.Visible = True

txtItem.SetFocus

End Sub

Private Sub cmdSend_Click()

If Winsock1.State = sckConnected Then

Winsock1.SendData txtItem.Text

shpGo.Visible = True

Label3.Caption = "Sending Data"

Else

shpGo.Visible = False

shpWait.Visible = False

shpError.Visible = True

Label3.Caption = "Not currently connected to host"

End If

End Sub

Private Sub Winsock1_DataArrival(ByVal bytesTotal As Long)

Dim sData As String

Winsock1.GetData sData, vbString

'Label1.Caption = sData

txtPrice.Text = sData

Label3.Caption = "Received Data"

shpGo.Visible = True

shpWait.Visible = False

shpError.Visible = False

End Sub

Private Sub Winsock1_SendComplete()

Label3.Caption = "Completed Data Transmission"

End Sub

Creating the Server

The server portion of the price lookup example is designed to accept the item number sent from the client and look up the associated price in a database. The server than sends the information back to the client. There is file named as "path.txt" in the folder called as "server". Locate that file and change the database path in the file to the location where the database is located on your machine. The connection to the database is made in the DataArrival event of the Winsock control. The following code segment opens the database and finds the first occurrence of the value in sItemData. When the record is found, the value contained in the price field is sent back to the client.

' Get clients request from database

strData = "Item = '" & sItemData & "'"

rs.Open "select * from prices", strConnect, adOpenKeyset, adLockOptimistic

rs.Find strData

strOutData = rs.Fields("Price")

Follow the steps shown below to create the server:

  1. Start a new Standard EXE in VB.

  2. Add the Winsock control to your application.

  3. Add the controls to the form as shown in the accompanying code (See the folder named as "server").

Here is the complete code:

Option Explicit

Dim iSockets As Integer

Dim sServerMsg As String

Dim sRequestID As String

 

Private Sub Form_Load()

Form1.Show

lblHostID.Caption = Socket(0).LocalHostName

lblAddress.Caption = Socket(0).LocalIP

Socket(0).LocalPort = 1007

sServerMsg = "Listening to port: " & Socket(0).LocalPort

List1.AddItem (sServerMsg)

Socket(0).Listen

End Sub

Private Sub socket_Close(Index As Integer)

sServerMsg = "Connection closed: " & Socket(Index).RemoteHostIP

List1.AddItem (sServerMsg)

Socket(Index).Close

Unload Socket(Index)

iSockets = iSockets - 1

lblConnections.Caption = iSockets

 

End Sub

Private Sub socket_ConnectionRequest(Index As Integer, ByVal requestID As Long)

sServerMsg = "Connection request id " & requestID & " from " & Socket(Index).RemoteHostIP

If Index = 0 Then

List1.AddItem (sServerMsg)

sRequestID = requestID

iSockets = iSockets + 1

lblConnections.Caption = iSockets

Load Socket(iSockets)

Socket(iSockets).LocalPort = 1007

Socket(iSockets).Accept requestID

End If

End Sub

Private Sub socket_DataArrival(Index As Integer, ByVal bytesTotal As Long)

 

Dim sItemData As String

Dim strData As String

Dim strOutData As String

Dim strConnect As String

 

 

' get data from client

Socket(Index).GetData sItemData, vbString

sServerMsg = "Received: " & sItemData & " from " & Socket(Index).RemoteHostIP & "(" & sRequestID & ")"

List1.AddItem (sServerMsg)

 

'strConnect = "Provider=Microsoft.Jet.OLEDB.4.0;Data Source=G:.mdb;Persist Security Info=False"

Dim strPath As String

 

'Change the database path in the text file

 

Dim fso As New FileSystemObject, txtfile, _

fil1 As File, ts As TextStream

Set fil1 = fso.GetFile("path.txt")

' Read the contents of the file.

Set ts = fil1.OpenAsTextStream(ForReading)

strPath = ts.ReadLine

ts.Close

Set fso = Nothing

strConnect = "Provider=Microsoft.Jet.OLEDB.4.0;" & _

"Persist Security Info=False;Data Source=" & strPath & _

"; Mode=Read|Write"

 

Dim rs As New ADODB.Recordset

 

' Get clients request from database

strData = "Item = '" & sItemData & "'"

 

rs.Open "select * from prices", strConnect, adOpenKeyset, adLockOptimistic

rs.Find strData

strOutData = rs.Fields("Price")

 

'send data to client

sServerMsg = "Sending: " & strOutData & " to " & Socket(Index).RemoteHostIP

List1.AddItem (sServerMsg)

Socket(Index).SendData strOutData

 

End Sub

Running the example

  1. Create executable for both the applications.

  2. Launch both the applications.

  3. Click the Connect button.

  4. Enter a value from 0 to 6 (currently the database contains only six records, error handling is not done in this code, you can add the error handling yourself) and click the Lookup button. The associated price will be displayed in the price field.


Client


Server



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