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Using CLongBinary for BLOBs

, 30 Nov 1999
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  • Download demo project - 29 Kb
  • Sample Image - UsingBlob.gif

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    At times, storing images and other types of documents like word document, or excel spread sheet or even a compressed zip file in the database is inevitable. MFC provides a supporting class to handle these BLOBs called CLongBinary. It is just a wrapper around the HGLOBAL of WIN32. But how do you use it? The wizard creates a member variable of type CLongBinary in your record set object and also creates the corresponding RFX routine (RFX_LongBinary).

    Okay, does it mean that you can use this member just like any other variable? No, That's where this document comes in handy. For regular data types, all you have to is create an instance of the recordset object, assign the values and call Update. But for BLOBs, you have to do something special. For inserting BLOBs, you have to remember 3 points.

    1. Allocate and lock the storage buffer using GlobalAlloc() and GlobalLock() calls
    2. Prepare the recordset for blob operation using SetFieldDirty() and SetFieldNull() calls
    3. Unlock the storage buffer

    By default, the destructor of the CLongBinary will delete the buffer using GlobalFree() call. So you don't have to bother about freeing the memory. Reading from the database is pretty simple. All you have to do is lock the buffer.

    Following code snippet shows how to do this in the actual code:

    // Store the Image to database table
    // 
    try
    {
        dbImages.Open();
        dbImages.AddNew();
    
        CFile	fileImage;
        CFileStatus	fileStatus;
    
        fileImage.Open("c:\\winnt\\winnt256.bmp", CFile::modeRead);
        fileImage.GetStatus(fileStatus);
    
        dbImages.m_BLOBName = fileImage.GetFileTitle();
        dbImages.m_BLOBImage.m_dwDataLength = fileStatus.m_size;
    
        HGLOBAL hGlobal		= GlobalAlloc(GPTR,fileStatus.m_size);
        dbImages.m_BLOBImage.m_hData = GlobalLock(hGlobal);
    
        fileImage.ReadHuge(dbImages.m_BLOBImage.m_hData,fileStatus.m_size);
    
        dbImages.SetFieldDirty(&dbImages.m_BLOBImage);
        dbImages.SetFieldNull(&dbImages.m_BLOBImage,FALSE);
        dbImages.Update();
    
        GlobalUnlock(hGlobal);
    
        dbImages.Close();
    
        pList->InsertItem(0,fileImage.GetFileTitle());
    }
    catch(CException* pE)
    {
        pE->ReportError();
        pE->Delete();
        return;
    }

     

    // To restore image from db table
    CdbImages   dbImages(&theApp.m_DB);
    CString     strFileName = pList->GetItemText(nIndex,0);
    dbImages.m_strFilter.Format("BLOBName = '%s'",strFileName);
    try
    {
        dbImages.Open();
        if  (dbImages.IsEOF())
            AfxMessageBox("Unable to get image from db");
        else
        {
            char    tmpPath[_MAX_PATH+1];
            GetTempPath(_MAX_PATH,tmpPath);
    
            strFileName.Insert(0,tmpPath);
            
            CFile	outFile(strFileName,CFile::modeCreate|CFile::modeWrite);
            LPSTR	buffer = (LPSTR)GlobalLock(dbImages.m_BLOBImage.m_hData);
            outFile.WriteHuge(buffer,dbImages.m_BLOBImage.m_dwDataLength);
            GlobalUnlock(dbImages.m_BLOBImage.m_hData);
            outFile.Close();
    
            theApp.OpenDocumentFile(strFileName);
        }
    
        dbImages.Close();
    
    }
    catch(CException* pE)
    {
        pE->ReportError();
        pE->Delete();
        return;
    }
    
    

    The demo project uses an Access database to store the images. This project also demonstrates the following:

    • DSN - less Connection to MS Access database
    • Dialog Bars
    • Image Display in CScrollView

    The images are displayed using the simplest technique of using BitBlt() call. It does not use any Palettes because it is beyond the scope of this article.

    License

    This article has no explicit license attached to it but may contain usage terms in the article text or the download files themselves. If in doubt please contact the author via the discussion board below.

    A list of licenses authors might use can be found here

    Share

    About the Author

    Shekar Narayanan

    United States United States
    No Biography provided

    Comments and Discussions

     
    GeneralBLOB Pinmemberpmk_820-Jun-05 1:04 
    GeneralAn annotation Pinmemberbuho_usp7-Jun-05 4:17 
    GeneralODBC Crash with CLongBinary PinsussAnonymous21-Oct-04 22:53 
    GeneralODBC Crash with CLongBinary PinsussAnonymous21-Oct-04 22:34 
    Generaldo the method work when considering oracle Pinmemberxuyang_deng21-Apr-04 21:28 
    GeneralCByteArray Pinmembermjwilliamson16-Mar-04 1:16 
    GeneralExcel files PinmemberAl Findlay17-Feb-04 7:12 
    GeneralError: Data truncated Pinmembermef5263-Jul-03 6:06 
    GeneralRe: Error: Data truncated Pinmemberwancol25-Aug-03 21:59 
    Generalproblem when call update() PinmemberCS4291-Jul-03 21:24 
    QuestionCan u give an example with ADO about how to save a CLongBinary into database? Pinmemberztliu014-Jun-03 22:22 
    GeneralStore Images with RecordsetPtr Pinmemberjsundjsund7-Apr-03 21:57 
    GeneralOut of memory error with Sybase PinmemberStan Harris20-Feb-03 16:10 
    GeneralRe: Out of memory error with Sybase PinmemberWes Jones28-Jul-06 8:06 
    QuestionHow to Delete or Modify a BLOB? PinsussMate7-Jan-03 15:48 
    AnswerRe: How to Delete or Modify a BLOB? PinsussBen Mathews13-Feb-03 10:42 
    AnswerRe: How to Delete or Modify a BLOB? Pinmemberipadilla4-Sep-05 6:31 
    GeneralProblem while updating the BLOB to ORACLE using CLongBinary PinmemberMytreya25-Mar-02 3:14 
    GeneralRe: Problem while updating the BLOB to ORACLE using CLongBinary PinmemberDiWa7-Aug-02 5:18 
    GeneralThanks for your excellent job,by the way,my specific solution Pinmemberseaguard9-Jan-02 20:54 
    GeneralI have Exception - "Data truncated." PinmemberAlexander Sirotkin23-Oct-01 1:15 
    GeneralLong Binary field !!! PinmemberHadi_Rezaie6-Oct-01 17:24 
    GeneralRe: Long Binary field !!! PinmemberAnonymous15-Jun-02 7:05 
    QuestionGlobalUnlock?? PinmemberScott Wood22-Dec-00 15:28 
    GeneralThanks! Pinsussscott wood10-Jul-00 6:58 

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