Download a remote file from a valid URL in C++

Introduction

Sometimes you need to download the content of a remote file into a buffer for subsequent processing.

You can do this in many ways:

• Using low level sockets functions.
• Using API functions like InternetReadFile which reads data from a handle opened by the InternetOpenUrl, FtpOpenFile, GopherOpenFile, or HttpOpenRequest function.
• Using other specialized classes from MFC like CInternetSession.

The goal of this  article is to describe the last 2 methods mentioned above.

Download a remote file from a valid URL using API

bool ReadURL(const char* szURL, char* szContent, int nMaxSize) { szContent[0]=0; HINTERNET hInternet=InternetOpen("Microsoft Internet Explorer", INTERNET_OPEN_TYPE_PRECONFIG, NULL, NULL, 0); if (!hInternet) return false; HINTERNET hInternetURL=InternetOpenUrl(hInternet, szURL, NULL, 0, INTERNET_FLAG_NO_CACHE_WRITE|INTERNET_FLAG_RELOAD|INTERNET_FLAG_HYPERLINK, 0); if (!hInternetURL) { InternetCloseHandle(hInternet); return false; } char buff[1024]; DWORD dwReadit=0; bool bReadFile=false; DWORD l=1024; if (HttpQueryInfo(hInternetURL, HTTP_QUERY_STATUS_CODE, buff, &l, 0) && atoi(buff)!=HTTP_STATUS_OK) return false; MSG msg; int pos=0; while ((bReadFile=InternetReadFile(hInternetURL, buff, 1024, &dwReadit)) && dwReadit) { memcpy(szContent+pos, buff, min((int)dwReadit, nMaxSize-pos-1)); pos+=min((int)dwReadit, nMaxSize-pos-1); szContent[pos]=0; } InternetCloseHandle(hInternetURL); InternetCloseHandle(hInternet); return bReadFile; }

API functions used above:

• InternetOpen - Initializes an application's use of the WinINet functions.
• InternetOpenUrl - Opens a resource specified by a complete FTP, Gopher, or HTTP URL.
• HttpQueryInfo - Retrieves header information associated with an HTTP request.
• InternetReadFile - Reads data from a handle opened by the InternetOpenUrl, FtpOpenFile, GopherOpenFile, or HttpOpenRequest function.
• InternetCloseHandle - Closes a single Internet handle.

Download a remote file from a valid URL using CInternetSession class

As follows I will describe a wrapper class which will incapsulate the CInternetSession functionality .To use this class you should include afxinet.h (#include <afxinet.h>)

class CInternetConn { public: CInternetConn(void) { m_pSharedInternetSession = 0; if (!m_pSharedInternetSession) m_pSharedInternetSession=new CInternetSession; } ~CInternetConn(void) { if (m_pSharedInternetSession) { m_pSharedInternetSession->Close(); delete m_pSharedInternetSession; m_pSharedInternetSession = 0; } } BOOL ReadFile(CString strUrl , CString &strFileContent) { CStdioFile* pFile=0; strFileContent=""; if(!m_pSharedInternetSession) return FALSE; try { m_pSharedInternetSession->SetOption(INTERNET_OPTION_CONNECT_TIMEOUT, 20000); pFile=m_pSharedInternetSession->OpenURL(strUrl, 1, INTERNET_FLAG_TRANSFER_ASCII|INTERNET_FLAG_DONT_CACHE); if (!pFile) return FALSE; char buff[1025]; UINT len=0; while ((len=pFile->Read(buff, 1024))) { buff[len]=0; strFileContent+=buff; } pFile->Close(); delete pFile; } catch (CInternetException* e) { e->Delete(); return FALSE; } return TRUE; } protected: CInternetSession* m_pSharedInternetSession; };

Using this code

1. Using method based on API functions
char strBuffer[MAX_BUFF]; bool isOK = ReadURL("www.google.ro", strBuffer,MAX_BUFFER);
2. Using method based on MFC classes

CString strBuffer; CinternetConn myConnection; BOOL isOK = myConnection.ReadFile("www.google.ro" , strBuffer);

License

This article, along with any associated source code and files, is licensed under The Intelliproject Open License (IPOL)

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