Hello everyone, I am a newbie in UNIX/Linux socket programming. This is a class project that I had trouble with.

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I was trying to make �Keep-Alive� HTTP connections to the server in a tiny web crawler project. Here is the problem: when I tried to recv() the first page, it succeeded. However, the 2nd consecutive recv() will receive zero bytes, for which I really have no idea. I did put a �Keep-Alive� field in the request message when I called send().

I don't quite know how to implement HTTP persistent connection by using Keep-Alive. I extracted the socket-related code into a small program as attached.

Would you please take a look at the code and explain what was wrong with it to me? That would be of great help since I have stuck on this problem for days.

By the way, the program was compiled under SunOS using g++ and -lsocket.

#include <sys/types.h>
#include <sys/socket.h>
#include <netdb.h>
#include <netinet/in.h>
#include <unistd.h>
#include <string.h>
#include <errno.h>

#include <string>
#include <iostream>

using namespace std;

// ------------------------------------------------------
const short SOCKET_ERROR = -1;

const short RECV_BUFFER_SIZE = 20;
const short REQUEST_BUFFER_SIZE = 255;

int sock;
char recv_buf [RECV_BUFFER_SIZE + 1];
char req_buf [REQUEST_BUFFER_SIZE + 1];

const static char REQUEST_TEMPLATE [] = 
{
	"GET %s HTTP/1.1\r\n"
	"Host: xxx.xxx.xxx\r\n" // should be replaced with a really host
	"Connection: Keep-Alive\r\n"
	"\r\n"
};

// ------------------------------------------------------
void create_socket ();
void download (const string& path, string& response);

// ------------------------------------------------------

int main (void)
{
	string first_addr = "/~pdu/index.html"; // should be replaced with a really URL
	string second_addr = "/~pdu/a.html"; // should be replaced with a really URL
	string response;

	create_socket ();
	download (first_addr, response);
	cout << response << endl << endl;
	response = "";
	download (second_addr, response);
	if (response.size() > 0)
	{
		cout << response << endl;
	}
	else
	{
		cout << "### The 2nd recv() failed to receive any bytes from the socket!" << endl << endl;
	}
	
	close (sock);

	return 0;
}

void create_socket ()
{
	struct sockaddr_in addr;

	// ------------------------------------------------------
	sock = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
    if(sock == SOCKET_ERROR)
    {
        perror ("Could not make a socket.\n");
		exit (-1);
    }

	cout << ">>> Socket created!" << endl;

	// ------------------------------------------------------
	struct hostent* host_info = gethostbyname("cse.unl.edu");	
	
	cout << ">>> DNS done!" << endl;

	long host_addr;

	/* copy address into long */
	memcpy(&host_addr, host_info->h_addr,
		host_info->h_length);

	/* fill address struct */
	addr.sin_addr.s_addr = host_addr;
	addr.sin_port = htons(80);
	addr.sin_family = AF_INET;

	// ------------------------------------------------------
	if( connect(sock, (struct sockaddr*)(&addr),
		sizeof(addr)) == SOCKET_ERROR )
    {
        perror("Could not connect to HTTP server.\n");
		exit (-1);
    }

	cout << ">>> Connection established!" << endl;
}

void download (const string& path, string& response) 
{
	size_t nBytes = snprintf(
		req_buf, 
		REQUEST_BUFFER_SIZE, 
		REQUEST_TEMPLATE, 
		path.c_str());

	if (nBytes >= REQUEST_BUFFER_SIZE)
	{
        cerr << "Buffer is too small for making a request message" << endl;
		exit (-1);
	}

	if (send(sock, req_buf, nBytes, 0) != nBytes)
	{
        perror("Could not send request to the HTTP server.\n");
		exit (-1);
	}
	cout << ">>> Request sent! -> " << path << endl << req_buf << endl;
	
	ssize_t size = 0;
	
	while ((size = recv(sock, recv_buf, 
		RECV_BUFFER_SIZE, 0)) > 0) 
	{
		recv_buf = '\0';
		response.append(recv_buf);
    }

	cout << ">>> Response received!" << endl;
}

Thank you!