ch20.htm

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    // Continue on
}
else
    // Perform error handling here
</PRE>
<P>At this point, the connecting application is connected to the second socket on
the listening application.</P>
<P>
<H3><A NAME="Heading5"></A>Sending and Receiving Messages</H3>
<P>Sending and receiving message through a socket connection gets slightly involved.
Because you can use sockets to send any kind of data, and they don't care what the
data is, the functions to send and receive data expect to be passed a pointer to
a generic buffer. For sending data, this buffer should contain the data to be sent.
For receiving data, this buffer will have the received data copied into it. As long
as you are sending and receiving strings and text, you can use fairly simple conversions
to and from CStrings with these buffers.</P>
<P>To send a message through a socket connection, you use the Send method. This method
requires two parameters and has a third, optional parameter that can be used to control
how the message is sent. The first parameter is a pointer to the buffer that contains
the data to be sent. If your message is in a CString variable, you can use the LPCTSTR
operator to pass the CString variable as the buffer. The second parameter is the
length of the buffer. The method returns the amount of data that was sent to the
other application. If an error occurs, the Send function returns SOCKET_ERROR. You
can use the Send method as follows:</P>
<P>
<PRE>CString strMyMessage;
int iLen;
int iAmtSent;
.
.
.
iLen = strMyMessage.GetLength();
iAmtSent = m_sMySocket.Send(LPCTSTR(strMyMessage), iLen);
if (iAmtSent == SOCKET_ERROR)
{
    // Do some error handling here
}
else
{
    // Everything's fine
}
</PRE>
<P>When data is available to be received from the other application, an event is
triggered on the receiving application. This lets your application know that it can
receive and process the message. To get the message, the Receive method must be called.
This method takes the same parameters as the Send method with a slight difference.
The first parameter is a pointer to a buffer into which the message may be copied.
The second parameter is the size of the buffer. This tells the socket how much data
to copy (in case more is received than will fit into the buffer). Like the Send method,
the Receive method will return the amount that was copied into the buffer. If an
error occurs, the Receive method also returns SOCKET_ERROR. If the message your application
is receiving is a text message, it can be copied directly into a CString variable.
This allows you to use the Receive method as follows:</P>
<P>
<PRE>char *pBuf = new char[1025];
int iBufSize = 1024;
int iRcvd;
CString strRecvd;
iRcvd = m_sMySocket.Receive(pBuf, iBufSize);
if (iRcvd == SOCKET_ERROR)
{
    // Do some error handling here
}
else
{
    pBuf[iRcvd] = NULL;
    strRecvd = pBuf;
    // Continue processing the message
}
</PRE>


<BLOCKQUOTE>
	<P>
<HR>
<STRONG>TIP:</STRONG> When receiving text messages, it's always a good idea to place a NULL
	in the buffer position just after the last character received, as in the preceding
	example. There may be garbage characters in the buffer that your application might
	interpret as part of the message if you don't add the NULL to truncate the string.
<HR>

	<P>
<HR>
<STRONG>NOTE:</STRONG> The Close function is one of the few CAsyncSocket methods that does
	not return any status code. For all the previous member functions that we have examined,
	you can capture the return value to determine if an error has occurred.
<HR>


</BLOCKQUOTE>

<H3><A NAME="Heading6"></A>Closing the Connection</H3>
<P>Once your application has finished all of its communications with the other application,
it can close the connection by calling the Close method. The Close method doesn't
take any parameters, and you use it as follows:</P>
<P>
<PRE>m_sMySocket.Close();
</PRE>
<H3><A NAME="Heading7"></A>Socket Events</H3>
<P>The primary reason that you create your own descendent class of CAsyncSocket is
that you want to capture the events that are triggered when messages are received,
connections are completed, and so on. The CAsyncSocket class has a series of functions
that are called for each of these various events. These functions all use the same
definition--the function name is the only difference--and they are intended to be
overridden in descendent classes. All of these functions are declared as protected
members of the CAsyncSocket class and probably should be declared as protected in
your descendent classes. The functions all have a single integer parameter, which
is an error code that should be checked to make sure that no error has occurred.
Table 20.1 lists these event functions and the events they signal.</P>
<P>
<H4>TABLE 20.1. CAsyncSocket OVERRIDABLE EVENT-NOTIFICATION FUNCTIONS.</H4>
<P>
<TABLE BORDER="1">
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT"><I>Function</I></TD>
		<TD ALIGN="LEFT"><I>Event Description</I></TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">OnAccept		</TD>
		<TD ALIGN="LEFT">This function is called on a listening socket to signal that a connection request
			from another application is waiting to be accepted.		</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">OnClose		</TD>
		<TD ALIGN="LEFT">This function is called on a socket to signal that the application on the other end
			of the connection has closed its socket or that the connection has been lost. This
			should be followed by closing the socket that received this notification.		</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">OnConnect		</TD>
		<TD ALIGN="LEFT">This function is called on a socket to signal that the connection with another application
			has been completed and that the application can now send and receive messages through
			the socket.		</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">OnReceive		</TD>
		<TD ALIGN="LEFT">This function is called to signal that data has been received through the socket
			connection and that the data is ready to be retrieved by calling the Receive function.		</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">OnSend		</TD>
		<TD ALIGN="LEFT">This function is called to signal that the socket is ready and available for sending
			data. This function is called right after the connection has been completed. Usually,
			the other time that this function is called is when your application has passed the
			Send function more data than can be sent in a single packet. In this case, this is
			a signal that all of the data has been sent, and the application can send the next
			buffer-full of data.		</TD>
	</TR>
</TABLE>

<H3><A NAME="Heading8"></A>Detecting Errors</H3>
<P>Whenever any of the CAsyncSocket member functions return an error, either FALSE
for most functions or SOCKET_ERROR on the Send and Receive functions, you can call
the GetLastError method to get the error code. This function returns only error codes,
and you have to look up the translation yourself. All the Winsock error codes are
defined with constants, so you can use the constants in your code to determine the
error message to display for the user, if any. You can use the GetLastError function
as follows:</P>
<P>
<PRE>int iErrCode;
iErrCode = m_sMySocket.GetLastError();
switch (iErrCode)
{
case WASNOTINITIALISED:
.
.
.
}
</PRE>
<H2><A NAME="Heading9"></A>Building a Networked Application</H2>
<P>For the sample application that you will build today, you'll create a simple dialog
application that can function as either the client or server in a Winsock connection.
This will allow you to run two copies of the sample application, one for each end
of the connection, on the same computer or to copy the application to another computer
so that you can run the two copies on separate computers and see how you can pass
messages across a network. Once the application has established a connection with
another application, you will be able to enter text messages and send them to the
other application. When the message has been sent, it will be added to a list of
messages sent. Each message that is received will be copied into another list of
all messages received. This will allow you to see the complete list of what is sent
and received. It will also allow you to compare what one copy of the application
has sent and what the other has received. (The two lists should be the same.)</P>
<P>
<H3><A NAME="Heading10"></A>Creating the Application Shell</H3>
<P>For today's sample application, just to keep things simple, you'll create a dialog-style
application. Everything that you are doing in today's application can be done in
an SDI or MDI application just as easily as with a dialog-style application. By using
a dialog-style application today, we are getting everything that might distract from
the basic socket functionality (such as questions about whether the socket variable
belongs in the document or view class, how much of the application functionality
belongs in which of these two classes, and so on) away from the sample application.</P>
<P>To start today's sample application, create a new MFC AppWizard project, giving
the project a suitable name, such as Sock. On the first step of the AppWizard, specify
that the application will be a dialog-based application. On the second step of the
AppWizard, specify that the application should include support for Windows Sockets,
as in Figure 20.3. You can accept the default settings for the rest of the options
in the AppWizard.</P>
<P>
<H3><B>Window Layout and Startup Functionality</B></H3>
<P>Once you create your application shell, you can lay out the main dialog for your
application. On this dialog, you'll need a set of radio buttons to specify whether
the application is running as the client or server. You'll also need a couple of
edit boxes for the computer name and port that the server will be listening on. Next,
you'll need a command button to start the application listening on the socket or
opening the connection to the server, and a button to close the connection. You'll
also need an edit box for entering the message to be sent to the other application
and a button to send the message. Finally, you'll need a couple of list boxes into
which you can add each of the messages sent and received. Place all these controls
on the dialog, as shown in Figure 20.4, setting all of the control properties as
specified in Table 20.2.</P>
<P><A HREF="javascript:popUp('20fig03.gif')"><B>FIGURE 20.3.</B></A><B> </B><I>Including
sockets support.</I></P>

<P><A HREF="javascript:popUp('20fig04.gif')"><B>FIGURE 20.4.</B></A><B> </B><I>The
main dialog layout.</I></P>

<P><I></I>
<H4>TABLE 20.2. CONTROL PROPERTY SETTINGS.</H4>
<P>
<TABLE BORDER="1">
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT"><I>Object</I></TD>
		<TD ALIGN="LEFT"><I>Property</I></TD>
		<TD ALIGN="LEFT"><I>Setting</I></TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">Group Box		</TD>
		<TD ALIGN="LEFT">ID		</TD>
		<TD ALIGN="LEFT">IDC_STATICTYPE		</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">			<P>
		</TD>
		<TD ALIGN="LEFT">Caption		</TD>
		<TD ALIGN="LEFT">Socket Type		</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">Radio Button		</TD>
		<TD ALIGN="LEFT">ID		</TD>
		<TD ALIGN="LEFT">IDC_RCLIENT		</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">			<P>
		</TD>
		<TD ALIGN="LEFT">Caption		</TD>
		<TD ALIGN="LEFT">&amp;Client		</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">			<P>
		</TD>
		<TD ALIGN="LEFT">Group		</TD>
		<TD ALIGN="LEFT">Checked		</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">Radio Button		</TD>
		<TD ALIGN="LEFT">ID		</TD>
		<TD ALIGN="LEFT">IDC_RSERVER		</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">			<P>
		</TD>
		<TD ALIGN="LEFT">Caption		</TD>
		<TD ALIGN="LEFT">&amp;Server		</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">Static Text		</TD>
		<TD ALIGN="LEFT">ID		</TD>
		<TD ALIGN="LEFT">IDC_STATICNAME		</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">			<P>
		</TD>
		<TD ALIGN="LEFT">Caption		</TD>
		<TD ALIGN="LEFT">Server &amp;Name:		</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">Edit Box		</TD>
		<TD ALIGN="LEFT">ID		</TD>
		<TD ALIGN="LEFT">IDC_ESERVNAME		</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">Static Text		</TD>
		<TD ALIGN="LEFT">ID		</TD>
		<TD ALIGN="LEFT">IDC_STATICPORT		</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">			<P>
		</TD>
		<TD ALIGN="LEFT">Caption		</TD>
		<TD ALIGN="LEFT">Server &amp;Port:		</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">Edit Box		</TD>
		<TD ALIGN="LEFT">ID		</TD>
		<TD ALIGN="LEFT">IDC_ESERVPORT		</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">Command Button		</TD>
		<TD ALIGN="LEFT">ID		</TD>
		<TD ALIGN="LEFT">IDC_BCONNECT		</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">			<P>
		</TD>
		<TD ALIGN="LEFT">Caption		</TD>
		<TD ALIGN="LEFT">C&amp;onnect		</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">Command Button		</TD>
		<TD ALIGN="LEFT">ID		</TD>
		<TD ALIGN="LEFT">IDC_BCLOSE		</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">			<P>
		</TD>
		<TD ALIGN="LEFT">Caption		</TD>
		<TD ALIGN="LEFT">C&amp;lose		</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">			<P>
		</TD>
		<TD ALIGN="LEFT">Disabled		</TD>
		<TD ALIGN="LEFT">Checked		</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">Static Text		</TD>
		<TD ALIGN="LEFT">ID		</TD>
		<TD ALIGN="LEFT">IDC_STATICMSG		</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">			<P>
		</TD>
		<TD ALIGN="LEFT">Caption		</TD>
		<TD ALIGN="LEFT">&amp;Message:		</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">			<P>