Programmatically reading a web page

Question

I want to write a program in C/C++ that will dynamically read a web page and extract information from it. As an example imagine if you wanted to write an application to follow and log an ebay auction. Is there an easy way to grab the web page? A library which provides this functionality? And is there an easy way to parse the page to get the specific data?

Solution 1

Have a look at the cURL library:

 #include <stdio.h>
 #include <curl/curl.h>

 int main(void)
 {
   CURL *curl;
   CURLcode res;

   curl = curl_easy_init();
   if(curl) {
     curl_easy_setopt(curl, CURLOPT_URL, "curl.haxx.se");
     res = curl_easy_perform(curl);
      /* always cleanup */
    curl_easy_cleanup(curl);
   }
   return 0;
 }

BTW, if C++ is not strictly required. I encourage you to try C# or Java. It is much easier and there is a built-in way.

Solution 2

Windows code:

#include <winsock2.h>
#include <windows.h>
#include <iostream>
#pragma comment(lib,"ws2_32.lib")
using namespace std;
int main (){
    WSADATA wsaData;
    if (WSAStartup(MAKEWORD(2,2), &wsaData) != 0) {
        cout << "WSAStartup failed.\n";
        system("pause");
        return 1;
    }
    SOCKET Socket=socket(AF_INET,SOCK_STREAM,IPPROTO_TCP);
    struct hostent *host;
    host = gethostbyname("www.google.com");
    SOCKADDR_IN SockAddr;
    SockAddr.sin_port=htons(80);
    SockAddr.sin_family=AF_INET;
    SockAddr.sin_addr.s_addr = *((unsigned long*)host->h_addr);
    cout << "Connecting...\n";
    if(connect(Socket,(SOCKADDR*)(&SockAddr),sizeof(SockAddr)) != 0){
        cout << "Could not connect";
        system("pause");
        return 1;
    }
    cout << "Connected.\n";
    send(Socket,"GET / HTTP/1.1\r\nHost: www.google.com\r\nConnection: close\r\n\r\n", strlen("GET / HTTP/1.1\r\nHost: www.google.com\r\nConnection: close\r\n\r\n"),0);
    char buffer[10000];
    int nDataLength;
    while ((nDataLength = recv(Socket,buffer,10000,0)) > 0){        
        int i = 0;
        while (buffer[i] >= 32 || buffer[i] == '\n' || buffer[i] == '\r') {
            cout << buffer[i];
            i += 1;
        }
    }
    closesocket(Socket);
        WSACleanup();
    system("pause");
    return 0;
}

Solution 3

There is a free TCP/IP library available for Windows that supports HTTP and HTTPS - using it is very straightforward.

Ultimate TCP/IP

CUT_HTTPClient http;
http.GET("http://folder/file.htm", "c:/tmp/process_me.htm");    

You can also GET files and store them in a memory buffer (via CUT_DataSource derived classes). All the usual HTTP support is there - PUT, HEAD, etc. Support for proxy servers is a breeze, as are secure sockets.

Solution 4

Try using a library, like Qt, which can read data from across a network and get data out of an xml document. This is an example of how to read an xml feed. You could use the ebay feed for example.

Solution 5

You're not mentioning any platform, so I give you an answer for Win32.

One simple way to download anything from the Internet is the URLDownloadToFile with the IBindStatusCallback parameter set to NULL. To make the function more useful, the callback interface needs to be implemented.

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