Handling Errors (Windows Internet)

The GetLastError function retrieves the last error code for all of the WinINet functions. If ERROR_INTERNET_EXTENDED_ERROR is returned, there is a string or buffer that contains a detailed error message. Call the InternetGetLastResponseInfo function to retrieve the extended error text.

To get the error text for an error, call the FormatMessage function, passing it an HMODULE handle to Wininet.dll, which can be obtained using the GetModuleHandle function.

The following is an example of an error-handling function.

#include <windows.h>
#include "strsafe.h"
#include "wininet.h"
#include <stdlib.h>
#pragma comment(lib, "wininet.lib")
#pragma comment(lib, "user32.lib")

#define  INET_ERR_OUT_MSG_BOX_BUFFER_SIZE      512
#define  INET_ERR_OUT_FORMAT_BUFFER_SIZE       256

#ifdef  _UNICODE
#define _itot _itow_s
#else
#define _itot _itoa_s
#endif

// Forward declaration of helper function
void WINAPI addLastErrorToMsg( LPTSTR szMsgBuffer, DWORD dwSize );

// Function for displaying Internet Error information in a message box
BOOL WINAPI InternetErrorOut(
                              HWND hWnd,
                              DWORD dwError,
                              LPCTSTR szFailingFunctionName )
{
  HANDLE hProcHeap;
  TCHAR  szMsgBoxBuffer[INET_ERR_OUT_MSG_BOX_BUFFER_SIZE],
         szFormatBuffer[INET_ERR_OUT_FORMAT_BUFFER_SIZE],
         szConnectiveText[]  = { TEXT( "\nAdditional Information: " )},
        *szExtErrMsg         = NULL,
        *szCombinedErrMsg    = NULL;
  DWORD  dwInetError,
         dwBaseLength,
         dwExtLength = 0;

  if( ( hProcHeap = GetProcessHeap( ) ) == NULL )
  {
    StringCchCopy( szMsgBoxBuffer, INET_ERR_OUT_MSG_BOX_BUFFER_SIZE, 
      TEXT( "Call to GetProcessHeap( ) failed..." ) ); 
    goto InetErrorOutError_1;
  }

  dwBaseLength = FormatMessage(
    FORMAT_MESSAGE_FROM_HMODULE,             // dwFlags
    GetModuleHandle( TEXT("wininet.dll") ),  // lpSource
    dwError,                                 // dwMessageId
    0,                                       // dwLanguageId
    (LPTSTR)szFormatBuffer,                  // lpBuffer
    INET_ERR_OUT_FORMAT_BUFFER_SIZE,         // nSize
    NULL );                                  // * arguments

  if( !dwBaseLength )
  {
    StringCchCopy( szMsgBoxBuffer, INET_ERR_OUT_MSG_BOX_BUFFER_SIZE, 
                   TEXT( "Call to FormatMessage( ) failed..." ) ); 
    addLastErrorToMsg( szMsgBoxBuffer, 
                       INET_ERR_OUT_MSG_BOX_BUFFER_SIZE );
    goto InetErrorOutError_1;
  }

  if( FAILED( StringCchPrintf( szMsgBoxBuffer, 
                INET_ERR_OUT_MSG_BOX_BUFFER_SIZE,
                TEXT( "%s error; code: %d\nDescription: %s\n" ),
                szFailingFunctionName, dwError, szFormatBuffer ) ) )
  {
    StringCchCopy( szMsgBoxBuffer, INET_ERR_OUT_MSG_BOX_BUFFER_SIZE, 
      TEXT( "Call to StringCchPrintf( ) failed...\n" ) ); 
    goto InetErrorOutError_1;
  }

  StringCchLength( szMsgBoxBuffer, INET_ERR_OUT_MSG_BOX_BUFFER_SIZE,
                (size_t*) &dwBaseLength );
  // Adjust base-length value to count the number of bytes:
  dwBaseLength *= sizeof( TCHAR );

  if( dwError == ERROR_INTERNET_EXTENDED_ERROR )
  {
    InternetGetLastResponseInfo( &dwInetError, NULL, &dwExtLength );
    // Adjust the extended-length value to a byte count 
    // that includes the terminating null:
    ++dwExtLength *= sizeof( TCHAR );
    if( ( szExtErrMsg = (TCHAR*)HeapAlloc( hProcHeap, 0, dwExtLength)) 
         == NULL )
    {
      StringCchCat( szMsgBoxBuffer, INET_ERR_OUT_MSG_BOX_BUFFER_SIZE, 
        TEXT("\nFailure: Could not allocate buffer for addional details.")); 
      addLastErrorToMsg( szMsgBoxBuffer, 
                         INET_ERR_OUT_MSG_BOX_BUFFER_SIZE );
      goto InetErrorOutError_1;
    }

    if( !InternetGetLastResponseInfo( &dwInetError, 
                                      szExtErrMsg, 
                                      &dwExtLength ) )
    {
      StringCchCat( szMsgBoxBuffer, INET_ERR_OUT_MSG_BOX_BUFFER_SIZE, 
        TEXT( "\nCall to InternetGetLastResponseInfo( ) failed--" ) ); 
      addLastErrorToMsg( szMsgBoxBuffer, 
                         INET_ERR_OUT_MSG_BOX_BUFFER_SIZE );
      goto InetErrorOutError_2;
    }
    dwBaseLength += dwExtLength + sizeof( szConnectiveText );
  }

  if( ( szCombinedErrMsg = (TCHAR*)HeapAlloc( hProcHeap, 0, 
                                              dwBaseLength + sizeof(TCHAR) ) ) 
                         == NULL )
  {
    StringCchCat( szMsgBoxBuffer, INET_ERR_OUT_MSG_BOX_BUFFER_SIZE, 
      TEXT( "\nFailure: Could not allocate final output buffer." ) ); 
    addLastErrorToMsg( szMsgBoxBuffer, 
                       INET_ERR_OUT_MSG_BOX_BUFFER_SIZE );
    goto InetErrorOutError_2;
  }

  if( FAILED( StringCchCopy( szCombinedErrMsg, 
                dwBaseLength, szMsgBoxBuffer ) ) ||
      ( dwExtLength && 
        ( FAILED( StringCchCat( szCombinedErrMsg, 
                    dwBaseLength, szConnectiveText ) ) ||
          FAILED( StringCchCat( szCombinedErrMsg, 
                    dwBaseLength, szExtErrMsg ) ) ) ) )
  {
    StringCchCat( szMsgBoxBuffer, INET_ERR_OUT_MSG_BOX_BUFFER_SIZE, 
      TEXT( "\nFailure: Could not assemble final message." ) ); 
    goto InetErrorOutError_3;
  }

  MessageBox( hWnd, 
    szCombinedErrMsg, 
    TEXT( "Internet Error Message" ),
    MB_OK | MB_ICONERROR );
  HeapFree( hProcHeap, 0, szExtErrMsg );
  HeapFree( hProcHeap, 0, szCombinedErrMsg );

  return( TRUE );

InetErrorOutError_3:
  HeapFree( hProcHeap, 0, szCombinedErrMsg );
InetErrorOutError_2:
  HeapFree( hProcHeap, 0, szExtErrMsg );
InetErrorOutError_1:
  MessageBox( hWnd,
    (LPCTSTR) szMsgBoxBuffer,
    TEXT( "InternetErrorOut( ) Failed" ),
    MB_OK | MB_ICONERROR );
  return( FALSE );
}


void WINAPI addLastErrorToMsg( LPTSTR szMsgBuffer, DWORD dwSize )
{
  TCHAR szNumberBuffer[32];

  _itot( GetLastError( ), szNumberBuffer, 10 );
  StringCchCat( szMsgBuffer, dwSize, 
        TEXT( "\n   System Error number: " ) ); 
  StringCchCat( szMsgBuffer, dwSize, szNumberBuffer );
  StringCchCat( szMsgBuffer, dwSize, TEXT( ".\n" ) ); 
}