Question

I am trying to use the http protocol GET to ask the server to send me a picture. It compiles and will ask for the picture but the server only will send me a small part of it.

My code is:

#include <iostream>
#include <string>
#include <Windows.h>
#include <fstream>

#include <stdlib.h>
#include <winsock.h>

#pragma comment(lib, "wsock32.lib")

using namespace std;

#define BUFFERSIZE 1024
void die_with_error(char *errorMessage);
void die_with_wserror(char *errorMessage);

int main(int argc, char *argv[])
{
string request;
string response;
int resp_leng;

char buffer[BUFFERSIZE];
struct sockaddr_in serveraddr;
int sock;

WSADATA wsaData;
char *ipaddress = "210.125.167.240";
int port = 80;

//http://cwc.ghc.ac.kr/skin/base/board/view_brit2.gif
request+="GET /skin/base/board/view_brit2.gif HTTP/1.0\r\n";
request+="Host: cwc.ghc.ac.kr\r\n";
request+="\r\n";

//init winsock
if (WSAStartup(MAKEWORD(2, 0), &wsaData) != 0)
    die_with_wserror("WSAStartup() failed");

//open socket
if ((sock = socket(PF_INET, SOCK_STREAM, IPPROTO_TCP)) < 0)
    die_with_wserror("socket() failed");

//connect
memset(&serveraddr, 0, sizeof(serveraddr));
serveraddr.sin_family      = AF_INET;
serveraddr.sin_addr.s_addr = inet_addr(ipaddress);
serveraddr.sin_port        = htons((unsigned short) port);
if (connect(sock, (struct sockaddr *) &serveraddr, sizeof(serveraddr)) < 0)
    die_with_wserror("connect() failed");

//send request
if (send(sock, request.c_str(), request.length(), 0) != request.length())
    die_with_wserror("send() sent a different number of bytes than expected");

//get response
response = "";
resp_leng= BUFFERSIZE;
while (resp_leng == BUFFERSIZE)
{
    resp_leng= recv(sock, (char*)&buffer, BUFFERSIZE, 0);
    if (resp_leng>0)
        response+= string(buffer).substr(0,resp_leng);
}

ofstream myfile;
myfile.open ("C:\\a\\example.gif");

//display response
cout << response << endl;

//get response
response = "";
resp_leng= BUFFERSIZE;
while (resp_leng == BUFFERSIZE)
{
    resp_leng= recv(sock, (char*)&buffer, BUFFERSIZE, 0);
    if (resp_leng>0)
        response+= string(buffer).substr(0,resp_leng);
}


//display response
cout << response << endl;


myfile << response;
myfile.close();

//disconnect
closesocket(sock);

//cleanup
WSACleanup();
return 0;
}

void die_with_error(char *errorMessage)
{
cerr << errorMessage << endl;
   exit(1);
}

void die_with_wserror(char *errorMessage)
{
    cerr << errorMessage << ": " << WSAGetLastError() << endl;
    exit(1);
} 

/*

It compiles and runs fine the output is: 

HTTP/1.1 200 OK
Server: Apache-Coyote/1.1
ETag: W/"60-1144808550000"
Last-Modified: Wed, 12 Apr 2006 02:22:30 GMT
Content-Type: image/gif
Content-Length: 60
Date: Wed, 22 Feb 2012 04:29:04 GMT
Connection: close

GIF89a
*/

What is wrong, why does it not download the full picture?

Answer 1

It looks like this loop is the problem:

while (resp_leng == BUFFERSIZE)
{
    resp_leng= recv(sock, (char*)&buffer, BUFFERSIZE, 0);
    if (resp_leng>0)
        response+= string(buffer).substr(0,resp_leng);
}

Calling recv() will give you some number of bytes from the socket, that may be less than what you ask for. So unlike reading from a file, asking for BUFFERSIZE bytes may return anywhere from 1 to BUFFERSIZE bytes.

Instead, loop until you get to the end of the socket:

while (true)
{
    resp_leng= recv(sock, (char*)&buffer, BUFFERSIZE, 0);
    if (resp_leng <= 0) {
        break;
    }
    response+= string(buffer,resp_leng);
}

Notice that I’ve also changed the call to the string() constructor on the last line of the loop (yours would work, but would do more effort than is necessary).