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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2//EN"><HTML><HEAD> <META HTTP-EQUIV="Content-Type" CONTENT="text/html;CHARSET=iso-8859-1"><SCRIPT LANGUAGE="JavaScript"><!--function popUp(pPage) { var fullURL = document.location; var textURL = fullURL.toString(); var URLlen = textURL.length; var lenMinusPage = textURL.lastIndexOf("/"); lenMinusPage += 1; var fullPath = textURL.substring(0,lenMinusPage); popUpWin = window.open('','popWin','resizable=yes,scrollbars=no,width=525,height=394'); figDoc= popUpWin.document; zhtm= '<HTML><HEAD><TITLE>' + pPage + '</TITLE>'; zhtm += '</head>'; zhtm += '<BODY bgcolor="#FFFFFF">'; zhtm += '<IMG SRC="' + fullPath + pPage + '">'; zhtm += '<P><B>' + pPage + '</B>'; zhtm += '</BODY></HTML>'; window.popUpWin.document.write(zhtm); window.popUpWin.document.close(); // Johnny Jackson 4/28/98 }//--> </SCRIPT><link rel="stylesheet" href="/includes/stylesheets/ebooks.css"> <META NAME="GENERATOR" Content="Symantec Visual Page Mac 1.1.1"> <TITLE>Teach Yourself Visual C++ 6 in 21 Days -- Ch 20 -- Internet Applications and Network Communications</TITLE></HEAD><BODY TEXT="#000000" BGCOLOR="#FFFFFF"><H1 ALIGN="CENTER"><IMG SRC="../button/sams.gif" WIDTH="171" HEIGHT="66" ALIGN="BOTTOM"BORDER="0"><BR>Teach Yourself Visual C++ 6 in 21 Days</H1><CENTER><P><A HREF="../ch19/ch19.htm"><IMG SRC="../button/previous.gif" WIDTH="128" HEIGHT="28"ALIGN="BOTTOM" ALT="Previous chapter" BORDER="0"></A><A HREF="../ch21/ch21.htm"><IMGSRC="../button/next.gif" WIDTH="128" HEIGHT="28" ALIGN="BOTTOM" ALT="Next chapter"BORDER="0"></A><A HREF="../index.htm"><IMG SRC="../button/contents.gif" WIDTH="128"HEIGHT="28" ALIGN="BOTTOM" ALT="Contents" BORDER="0"></A> <HR></CENTER><H1 ALIGN="CENTER">- 20 -<BR>Internet Applications and Network Communications</H1><H1></H1><UL> <LI><A HREF="#Heading1">How Do Network Communications Work?</A> <UL> <LI><A HREF="#Heading2">Sockets, Ports, and Addresses</A> <LI><A HREF="#Heading3">Creating a Socket</A> <LI><A HREF="#Heading4">Making a Connection</A> <LI><A HREF="#Heading5">Sending and Receiving Messages</A> <LI><A HREF="#Heading6">Closing the Connection</A> <LI><A HREF="#Heading7">Socket Events</A> <LI><A HREF="#Heading8">Detecting Errors</A> </UL> <LI><A HREF="#Heading9">Building a Networked Application</A> <UL> <LI><A HREF="#Heading10">Creating the Application Shell</A> <LI><A HREF="#Heading11">Inheriting from the CAsyncSocket Class</A> <LI><A HREF="#Heading12">Connecting the Application</A> <LI><A HREF="#Heading13">Sending and Receiving</A> <LI><A HREF="#Heading14">Ending the Connection</A> </UL> <LI><A HREF="#Heading15">Summary</A> <LI><A HREF="#Heading16">Q&A</A> <LI><A HREF="#Heading17">Workshop</A> <UL> <LI><A HREF="#Heading18">Quiz</A> <LI><A HREF="#Heading19">Exercise</A> </UL></UL><P><HR SIZE="4"><BR>Thanks in part to the explosion in popularity of the Internet, more applicationshave the ability to communicate with other applications over networks, includingthe Internet. With Microsoft building networking capabilities into its operatingsystems, starting with Windows NT and Windows 95, these capabilities are becomingcommonplace in all sorts of applications.</P><P>Some applications perform simple networking tasks such as checking with a Website to see whether there are any updates to the program and giving the user theoption of updating her copy of the program. Some word processing applications willformat documents as Web pages, giving the user the option of loading the pages ontothe Web server. You've got computer games that allow the user to play against anotherperson halfway around the world instead of just competing against the game itself.</P><P>Applications can have any number of networking functions, and they all are builtaround the Winsock interface. If you know and understand how to program using theWinsock interface and the MFC Winsock classes, this entire realm of application programmingis open to you, expanding your programming options considerably. Today, you willlearn</P><P><UL> <LI>How applications use the Winsock interface to perform network communications between two or more computers. <P> <LI>The difference between a client and a server application and the role each plays in establishing a communications link. <P> <LI>How the MFC Winsock classes simplify the process of writing Internet applications. <P> <LI>How you can create your own Winsock class, descended from the MFC Winsock classes, to easily build an event-driven, networking application.</UL><H2><A NAME="Heading1"></A>How Do Network Communications Work?</H2><P>Most applications that communicate over a network, whether it's the Internet ora small office network, use the same principles and functionality to perform theircommunication. One application sits on a computer, waiting for another applicationto open a communication connection. This application is "listening" forthis connection request, much like you listen for the phone to ring if you are expectingsomeone to call.</P><P>Meanwhile, another application, most likely on another computer (but not necessarily),tries to connect to the first application. This attempt to open a connection is similarto calling someone on the telephone. You dial the number and hope that the otherperson is listening for the phone on the other end. As the person making the call,you have to know the phone number of the person you are calling. If you don't knowthe phone number, you can look it up using the person's name. Likewise, the applicationtrying to connect to the first application has to know the network location, or address,of the first application.</P><P>Once the connection is made between the two applications, messages can pass backand forth between the two applications, much like you can talk to the person on theother end of the phone. This connection is a two-way communications channel, withboth sides sending information, as shown in Figure 20.l.</P><P><A HREF="javascript:popUp('20fig01.gif')"><B>FIGURE 20.1.</B></A><B> </B><I>Thebasic socket connection process.</I></P><P>Finally, once one or both sides have finished their sides of the conversation,the connection is closed, much like you hang up the phone after you have finishedtalking to the person you called. Once the connection is closed from either side,the other side can detect it and close its side, just like you can tell if the personon the other end of the phone has hung up on you or if you've been disconnected bysome other means. This is a basic explanation of how network communications workbetween two or more applications.</P><BLOCKQUOTE> <P><HR><STRONG>NOTE:</STRONG> This is a basic description of how network communications work with the TCP/IP network protocol, which is the primary network protocol over the Internet. Many other network protocols use a subtle variation on this description. Other protocols, such as the UDP protocol, are more like radio broadcasts, where there is no connection between the two applications; one sends messages, and the other is responsible for making sure that it receives all of the messages. These protocols are more involved than we have the luxury to discuss today. If you want to learn more about network protocols and how they work, many books cover this one topic and look at the various Internet applications and how they communicate over the connections they establish.<HR></BLOCKQUOTE><H3><A NAME="Heading2"></A>Sockets, Ports, and Addresses</H3><P>The basic object used by applications to perform most network communications iscalled a socket. Sockets were first developed on UNIX at the University of Californiaat Berkley. Sockets were designed so that most network communications between applicationscould be performed in the same way that these same applications would read and writefiles. Sockets have progressed quite a bit since then, but the basics of how theywork are still the same.</P><P>During the days of Windows 3.<I>x</I>, before networking was built into the Windowsoperating system, you could buy the network protocols required for network communicationsfrom numerous different companies. Each of these companies had a slightly differentway that an application performed network communications. As a result, any applicationsthat performed network communications had a list of the different networking softwarethat the application would work with. Many application developers were not happywith this situation. As a result, all the networking companies, including Microsoft,got together and developed the Winsock (Windows Sockets) API. This provided all applicationdevelopers with a consistent API to perform all network communications, regardlessof the networking software used.</P><P>When you want to read or write a file, you must use a file object to point tothe file. Although this was hidden from you in most of the Visual C++ applicationsso far, with the ActiveX control you created yesterday, you had to work through thesteps of creating the file object for reading and writing. A socket is similar; itis an object used to read and write messages that travel between applications.</P><P>Making a socket connection to another application does require a different setof information than opening a file. To open a file, you need to know the file's nameand location. To open a socket connection, you need to know the computer on whichthe other application is running and the port on which it's listening. A port islike a phone extension, and the computer address is like the phone number. If youcall someone at a large office building, you may dial the main office number, butthen you need to specify the extension number. Likewise, ports are used to routenetwork communications (see Figure 20.2). As with the phone number, there are meansof looking up the port number, if you don't already know what it is, but this requiresyour computer to be configured with the information about which port the connectingapplication is listening on. If you specify the wrong computer address or port number,you may get a connection to a different application; with making the phone call,someone other than the person you called may answer the phone call. You also maynot get an answer at all if there is no application listening at the other end.</P><BLOCKQUOTE> <P><HR><STRONG>NOTE:</STRONG> Only one application may be listening on any specific port on a single computer. Although numerous applications may listen for connection requests on a single computer at the same time, each of these applications must listen on a different port.<HR></BLOCKQUOTE><H3><A NAME="Heading3"></A>Creating a Socket</H3><P>When you build applications with Visual C++, you can use the MFC Winsock classesto add network communications capabilities with relative ease. The base class, CAsyncSocket,provides complete, event-driven socket communications. You can create your own descendentsocket class that captures and responds to each of these events.</P><BLOCKQUOTE> <P><HR><STRONG>CAUTION:</STRONG> This discussion of socket communications assumes that you check the AppWizard option for adding support for Windows Sockets. This adds supporting functionality to the application that is not discussed here.<HR></BLOCKQUOTE><P><A HREF="javascript:popUp('20fig02.gif')"><B>FIGURE 20.2.</B></A><B> </B><I>Portsare used to route network communications to the correct application.</I></P><P>To create a socket that you can use in your application, the first thing you needto do is declare a variable of CAsyncSocket (or your descendent class) as a classmember for one of the main application classes:</P><P><PRE>class CMyDlg : public CDialog{...private: CAsyncSocket m_sMySocket;};</PRE><P>Before you can begin using the socket object, you must call its Create method.This actually creates the socket and prepares it for use. How you call the Createmethod depends on how you will be using the socket. If you will be using the socketto connect to another application, as the one placing the call (the client), thenyou do not need to pass any parameters to the Create method:</P><P><PRE>if (m_sMySocket.Create()){ // Continue on}else // Perform error handling here</PRE><P>However, if the socket is going to be listening for another application to connectto it, waiting for the call (the server), then you need to pass at least the portnumber on which the socket should be listening:</P><P><PRE>if (m_sMySocket.Create(4000)){ // Continue on}else // Perform error handling here</PRE><P>You can include other parameters in the Create method call, such as the type ofsocket to create, the events that the socket should respond to, and the address thatthe socket should listen on (in case the computer has more than one network card).All these options require a more thorough understanding of sockets than we'll beable to cover today.</P><P><H3><A NAME="Heading4"></A>Making a Connection</H3><P>Once you create a socket, you are ready to open a connection with it. Three stepsgo along with opening a single connection. Two of these steps take place on the server,the application listing for the connection, and the third step takes place on theclient, the one making the call.</P><P>For the client, opening the connection is a simple matter of calling the Connectmethod. The client has to pass two parameters to the Connect method: the computername, or network address, and the port of the application to connect to. The Connectmethod could be used in the following two ways:</P><P><PRE>if (m_sMySocket.Connect("thatcomputer.com", 4000)){ // Continue on}else // Perform error handling here</PRE><P>The second form is</P><P><PRE>if (m_sMySocket.Connect("178.1.25.82", 4000)){ // Continue on}else // Perform error handling here</PRE><P>Once the connection is made, an event is triggered to let your application knowthat it is connected or that there were problems and the connection couldn't be made.(I'll cover how these events work in the section "Socket Events," laterin this chapter.)</P><P>For the server, or listening, side of the connection, the application first musttell the socket to listen for incoming connections by calling the Listen method.The Listen method takes only a single argument, which you do not need to supply.This parameter specifies the number of pending connections that can be queued, waitingfor the connection to be completed. By default this value is 5, which is the maximum.The Listen method can be called as follows:</P><P><PRE>if (m_sMySocket.Listen()){ // Continue on}else // Perform error handling here</PRE><P>Whenever another application is trying to connect to the listening application,an event is triggered to let the application know that the connection request isthere. The listening application must accept the connection request by calling theAccept method. This method requires the use of a second CAsyncSocket variable, whichis connected to the other application. Once a socket is placed into listen mode,it stays in listen mode. Whenever connection requests are received, the listeningsocket creates another socket, which is connected to the other application. Thissecond socket should not have the Create method called for it because the Acceptmethod creates the socket. You call the Accept method as follows:</P><P><PRE>if (m_sMySocket.Accept(m_sMySecondSocket)){?? 快捷鍵說明
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