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Beginning Winsock Programming - Simple TCP client

, 28 Feb 2002 CPOL
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A simple TCP client is explained.

Introduction

This is a sequel to the article Beginning Winsock Programming - Simple TCP server and if you have not read that already I would recommend that you do that first. In this article I'll show how you can write a simple TCP client program. We'll write a program that will connect to an HTTP server and retrieve a file.

Program Flow of a simple TCP client

  1. Initialize WinSock library using WSAStartup()
  2. Create a IPPROTO_TCP SOCKET using socket()
  3. Retrieve host information using gethostbyname()/gethostbyaddr()
  4. Connect to the server using the socket we created, using connect()
  5. Send and Receive data using send()/recv() till our tcp chat is over
  6. Close the socket connection using closesocket()
  7. De-Initialize WinSock using WSACleanup()

Initialize WinSock

As with every other WinSock program we need to initialize the WinSock library. Basically it is also a kind of check to see if WinSock is available on the system in the precise version we expect it to be.

int wsaret=WSAStartup(0x101,&wsaData);
if(wsaret)	
    return;

Create the SOCKET

The socket is the entity that acts as the endpoint between the client and the server. When a client is connected to a server, there are two sockets. The socket at the client side and the corresponding socket at the server side. Lets call them CLIENTSOCK and SERVERSOCK. When the client uses send() on CLIENTSOCK the server can use recv() on the SERVERSOCK to receive what the client sends. Similarly the reverse is also true. For our purposes we create the socket using a function called socket().

SOCKET conn;
conn=socket(AF_INET,SOCK_STREAM,IPPROTO_TCP);
if(conn==INVALID_SOCKET)
    return;

Getting host information

Obviously we need to get info about the host [the server] before we can connect to it. There are two functions we can use - gethostbyname() and gethostbyaddr(). The gethostbyname() function is used when we have the DNS name of our server, something like codeproject.com or ftp.myserver.org. The gethostbyaddr() function is used when we actually have the IP address of the server to connect to, something like 192.168.1.1 or 202.54.1.100.

Obviously we would want to give our end user the option of entering either a DNS name or an IP address. Thus for making that part of it transparent to him, we do a little trick as shown below. We use the function inet_addr() on the entered string. The inet_addr() function converts an IP address into a standard network address format. Thus if it returns failure, we now know that the string cannot be an IP address, if it succeeds we assume that it was a valid IP address.

if(inet_addr(servername)==INADDR_NONE)
{
    hp=gethostbyname(servername);
}
else
{
    addr=inet_addr(servername);
    hp=gethostbyaddr((char*)&addr,sizeof(addr),AF_INET);
}
if(hp==NULL)
{
    closesocket(conn);
    return;
}

Connecting to the server

The connect() function is used to establish a connection to the destination server. We pass it the socket we created earlier as well as a sockaddr structure. We populate the sockaddr with the host address returned by gethostbyname()/gethostbyaddr(), as well as enter a valid port to connect to.

server.sin_addr.s_addr=*((unsigned long*)hp->h_addr);
server.sin_family=AF_INET;
server.sin_port=htons(80);
if(connect(conn,(struct sockaddr*)&server,sizeof(server)))
{
    closesocket(conn);
    return;	
}

Chatting

Once the socket connection is established the client and the server can send() and recv() data between themselves. This is popularly referred to as TCP chatting. In our particular case we need to HTTP chat, which is comparatively simple when you consider other slightly more complicated protocols like SMTP or POP3. The HTTP GET command is used to retrieve a file from the HTTP server. This might be an HTML file or an image file or a zip or an MP3 or whatever. It is send thus [in it's simplest form]. There are other slightly more complex ways of using this command.

GET http-path-to-file\r\n\r\n

And in our program we do something like this to send the GET command :-

sprintf(buff,"GET %s\r\n\r\n",filepath);
send(conn,buff,strlen(buff),0);

Once we have send the command we know that the server is going to start sending us the file we just requested. Just as we used send() to send our command we can use recv() to receive the data that the server is going to send us. We loop on recv() till it returns zero when we understand that the server has finished sending us the data. And in our particular case we write all this data to a file as our intention is to download and save a file.

while(y=recv(conn,buff,512,0))
{
    f.Write(buff,y);
}

Close the connection

Now that our chat is over, we must close the connection. In our case the HTTP connection is closed by the server the moment it finishes sending the file, but that doesn't matter. We need to close our socket and release the resource. In more complex chats we usually call shutdown() before we call closesocket() to ensure that the buffers are flushed. Otherwise we might encounter some data loss.

closesocket(conn);

De-Initialize WinSock

We call WSACleanup() to conclude our usage of WinSock.

WSACleanup();

Thank you.

License

This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

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About the Author

Nish Nishant

United States United States
Nish Nishant is a Software Architect/Consultant based out of Columbus, Ohio. He has over 15 years of software industry experience in various roles including Lead Software Architect, Principal Software Engineer, and Product Manager. Nish is a recipient of the annual Microsoft Visual C++ MVP Award since 2002 (13 consecutive awards as of 2014).

Nish is an industry acknowledged expert in the Microsoft technology stack. He authored
C++/CLI in Action for Manning Publications in 2005, and had previously co-authored
Extending MFC Applications with the .NET Framework for Addison Wesley in 2003. In addition, he has over 140 published technology articles on CodeProject.com and another 250+ blog articles on his
WordPress blog. Nish is vastly experienced in team management, mentoring teams, and directing all stages of software development.

Contact Nish : You can reach Nish on his google email id voidnish.

Website and Blog

Comments and Discussions

 
GeneralMy vote of 3 Pinmemberhoseinhero12-Aug-12 8:49 
Questiongethostbyaddr is unnecessary Pinmembertamas.somogyi25-Apr-12 5:54 
GeneralMy vote of 3 Pinmemberenhzflep26-Oct-11 22:34 
GeneralMy vote of 5 PinmemberRock Dai14-Aug-10 23:08 
Generalstudy Pinmemberycd7-Jun-10 17:42 
Generalnew bgs Pinmembercen_jin_long22-Feb-10 13:45 
Questionwinsock programming for ethernet communication using ADAM 6050 module Pinmembermayank17satra4-Nov-09 4:17 
GeneralA Universal C++ TCP Socket Class for Non-blocking Server/Clients PinmemberElmue22-Mar-09 10:08 
QuestionWhat 's the problem of the send? Anyone know it? PinmemberMicarpolite6-Jan-09 22:19 
QuestionConnecting with Hostname to host with Static IP Addresses Pinmemberddas774-Sep-07 2:34 
GeneralGreat Example Pinmemberisnainsd21-Aug-07 0:39 
Questionwhat if proxy server is used ? PinmemberHarviz Harrison26-Apr-07 0:43 
Generalsynchronous windows socket Pinmembermavsolos11-Apr-07 4:25 
Questionhow to implement webserver optimization project codings Pinmemberilaiah21-Mar-07 1:16 
GeneralRedefinition Error Pinmemberdeepak.cse0710-Feb-07 20:45 
Generalnew to winsock programming Pinmemberdhavall8-Sep-06 22:03 
QuestionHow to do socket and user interface therad in vc++ Pinmemberanjicafe30-Jun-06 4:25 
GeneralAdditional information for users PinmemberBrit25-Jun-06 17:26 
Generalnot connecting to server PinmemberRakesh K R11-Feb-06 2:24 
Generalcallback function with winsocks PinmemberAmir Ram Shrestha24-Jul-05 20:19 
GeneralRegarding SMTP PinmemberPoornimaS7-Apr-05 21:43 
GeneralTQConvert in .NET Pinmemberchal_adiera3-Mar-05 15:38 
Generalclient not receiving Pinmembersubhashkjain2001@hotmail.com2-Feb-05 22:14 
GeneralEthernet Programming PinmemberMiekie15-Sep-04 17:58 
GeneralWinsock Programming PinmemberL.R. Krishnan23-May-04 22:59 
GeneralVery good code! PinmemberRyan McDermott18-Mar-04 8:25 
GeneralExcellent Pinmemberhesterloli8-Jan-04 19:47 
Generalhallo PinsusspsyKadeliKa28-Nov-03 0:02 
General"debug assertion failed" problem Pinmemberdelphinidae4-Nov-03 16:33 
GeneralRe: "debug assertion failed" problem Pinmemberfsdafsdfsad15-Dec-03 20:16 
Generaltelnet client Pinmembermohammad arif22-Oct-03 22:09 
Questiondoes it work??? Pinmemberscovase18-Oct-03 5:24 
AnswerRe: does it work??? PinsussAnonymous25-Jun-04 7:44 
Generalhelp! me for proxy server Pinmemberdharani8-Aug-03 0:39 
GeneralCouple of Questions Pinmemberrjahrman25-Jul-03 9:35 
GeneralRe: Couple of Questions PinmemberShadow Mil13-Jul-06 21:22 
Generaltranslated for c console Pinmemberrenejf35-Jun-03 15:22 
GeneralHm... PinmemberRickard Andersson1821-May-03 21:57 
GeneralProblem with POP3 PinmemberMazdak4-May-03 7:01 
GeneralRe: Problem with POP3 PinmemberRickard Andersson1821-May-03 22:05 
GeneralRe: Problem with POP3 PinmemberMazdak22-May-03 3:45 
Generalgethostbyaddr() Pinmemberpeter ho1-May-03 15:46 
GeneralRe: gethostbyaddr() PinmemberDavidCrow8-Apr-04 8:22 
GeneralProgress status Pinmember2RB128-Apr-03 15:18 
GeneralRe: Progress status PinsussAnonymous4-Aug-05 1:48 
QuestionWill it work with HTTPS too? Pinmembergtho930522-Feb-03 18:11 
AnswerRe: Will it work with HTTPS too? PineditorNishant S22-Feb-03 19:31 
GeneralRe: Will it work with HTTPS too? Pinmemberpracjp18-Sep-04 17:21 
Generala question Pinmemberandypai4-Feb-03 13:17 
GeneralRe: a question PineditorNishant S22-Feb-03 19:34 

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