Setting Large Data

How to Set Large Data

To pass a pointer to its own storage object, the consumer creates an accessor that binds the value of the BLOB column and then calls the IRowsetChange::SetData or IRowsetChange::InsertRow methods. To set BLOB data:

1. Create a DBOBJECT structure describing how the BLOB column should be accessed. Set the dwFlag element of the DBOBJECT structure to STGM_READ and set the iid element to IID_ISequentialStream (the interface to be exposed).

2. Set the properties in the DBPROPSET_ROWSET property group so the rowset is updatable.

3. Create a set of bindings (one of each column) by using an array of DBBINDING structures. Set the wType element in the DBBINDING structure to DBTYPE_IUNKNOWN, and the pObject element to point to the DBOBJECT structure you created.

4. Create an accessor using the binding information in the DBBINDINGS array of structures.

5. Call GetNextRows to fetch next rows into the rowset. Call GetData to read the data from the rowset.

6. Create a storage object containing the data (and also the length indicator), and then call IRowsetChange::SetData (or IRowsetChange::InsertRow) with the accessor that binds the BLOB column to set the data.

Example

This example shows how to set BLOB data. The example creates a table, adds a sample record, fetches that record in the rowset, and then sets the value of the BLOB field:

#define UNICODE
#define DBINITCONSTANTS
#define INITGID

#include <windows.h>
#include <stdio.h>
#include <stddef.h>
#include <iostream.h>

#include <oledb.h>
#include <oledberr.h>

#include <sqlncli.h>

#define SAFE_RELEASE(pIUnknown) if(pIUnknown) (pIUnknown)->Release();
HRESULT GetCommandObject(REFIID riid, IUnknown** ppIUnknown);
HRESULT CreateTable(ICommandText* pICommandText);

class CSeqStream : public ISequentialStream
{
public:
    //Constructors
    CSeqStream();
    virtual ~CSeqStream();

    virtual BOOL Seek(ULONG iPos);
    virtual BOOL Clear();
    virtual BOOL CompareData(void* pBuffer);
    virtual ULONG Length()  { return m_cBufSize; };

    virtual operator void* const() { return m_pBuffer; };

    STDMETHODIMP_(ULONG)    AddRef(void);
    STDMETHODIMP_(ULONG)    Release(void);
    STDMETHODIMP QueryInterface(REFIID riid, LPVOID *ppv);
    
    STDMETHODIMP Read( 
            /* [out] */ void __RPC_FAR *pv,
            /* [in]  */ ULONG cb,
            /* [out] */ ULONG __RPC_FAR *pcbRead);
        
    STDMETHODIMP Write( 
            /* [in] */ const void __RPC_FAR *pv,
            /* [in] */ ULONG cb,
            /* [out]*/ ULONG __RPC_FAR *pcbWritten);

protected:
    //Data

private:

    ULONG       m_cRef;         // reference count
    void*       m_pBuffer;      // buffer
    ULONG       m_cBufSize;     // buffer size
    ULONG       m_iPos;         // current index position in the buffer
};

//class implementation

CSeqStream::CSeqStream()
{
    m_iPos         = 0;
    m_cRef         = 0;
    m_pBuffer      = NULL;
    m_cBufSize     = 0;

    //The constructor AddRef's
    AddRef();
}

CSeqStream::~CSeqStream()
{
    //Shouldn't have any references left
//    ASSERT(m_cRef == 0);
    CoTaskMemFree(m_pBuffer);
}

ULONG    CSeqStream::AddRef(void)
{
    return ++m_cRef;
}

ULONG    CSeqStream::Release(void)
{
//    ASSERT(m_cRef);

    if(--m_cRef)
        return m_cRef;
    
    delete this;
    return 0;
}

HRESULT CSeqStream::QueryInterface(REFIID riid, void** ppv)
{
//    ASSERT(ppv);
    *ppv = NULL;

    if (riid == IID_IUnknown)
        *ppv = this;
    if (riid == IID_ISequentialStream)
        *ppv = this;
    
    if(*ppv)
    {
        ((IUnknown*)*ppv)->AddRef();
        return S_OK;
    }

    return E_NOINTERFACE;
}

BOOL CSeqStream::Seek(ULONG iPos)
{
    // Make sure the desired position is within the buffer.
//    ASSERT(iPos == 0 || iPos < m_cBufSize);

    // Reset the current buffer position.
    m_iPos = iPos;
    return TRUE;
}

BOOL CSeqStream::Clear()
{
    //Frees the buffer
    m_iPos         = 0;
    m_cBufSize     = 0;

    CoTaskMemFree(m_pBuffer);
    m_pBuffer = NULL;

    return TRUE;
}

BOOL CSeqStream::CompareData(void* pBuffer)
{
//    ASSERT(pBuffer);

    // Quick and easy way to compare user buffer with the stream.
    return memcmp(pBuffer, m_pBuffer, m_cBufSize)==0;
}

HRESULT CSeqStream::Read(void *pv, ULONG cb, ULONG* pcbRead)
{
    //Parameter checking
    if(pcbRead)
        *pcbRead = 0;

    if(!pv)
        return STG_E_INVALIDPOINTER;

    if(cb == 0)
        return S_OK;

    // Actual code.
    ULONG cBytesLeft = m_cBufSize - m_iPos;
    ULONG cBytesRead = cb > cBytesLeft ? cBytesLeft : cb;

    // If no more bytes to retrieve return.
    if(cBytesLeft == 0)
        return S_FALSE; 

    // Copy to users buffer the number of bytes requested or remaining.
    memcpy_s(pv, sizeof(pv), (void*)((BYTE*)m_pBuffer + m_iPos), cBytesRead);
    m_iPos += cBytesRead;

    if(pcbRead)
        *pcbRead = cBytesRead;

    if(cb != cBytesRead)
        return S_FALSE; 

    return S_OK;
}
        
HRESULT CSeqStream::Write(const void *pv, ULONG cb, ULONG* pcbWritten)
{
    // Parameter checking.
    if(!pv)
        return STG_E_INVALIDPOINTER;

    if(pcbWritten)
        *pcbWritten = 0;

    if(cb == 0)
        return S_OK;

    // Enlarge the current buffer.
    m_cBufSize += cb;

    // Need to append to the end of the stream.
    m_pBuffer = CoTaskMemRealloc(m_pBuffer, m_cBufSize);
    memcpy_s((void*)((BYTE*)m_pBuffer + m_iPos), sizeof((void*)((BYTE*)m_pBuffer + m_iPos)), pv, cb);
    // m_iPos += cb;

    if(pcbWritten)
        *pcbWritten = cb;

    return S_OK;
}
//...........................................................
void main()
{
    CoInitialize(NULL);

    DBOBJECT ObjectStruct;
    ObjectStruct.dwFlags = STGM_READ;
    ObjectStruct.iid     = IID_ISequentialStream;

    struct BLOBDATA
    {
        DBSTATUS            dwStatus;   
        DWORD               dwLength; 
        ISequentialStream*  pISeqStream;
    };

    
    BLOBDATA BLOBGetData;
    BLOBDATA BLOBSetData;

    const ULONG cBindings = 1;
    DBBINDING rgBindings[cBindings]; 
    HRESULT hr = S_OK;
    IAccessor*          pIAccessor          = NULL;
    ICommandText*       pICommandText       = NULL;
    ICommandProperties* pICommandProperties = NULL;
    IRowsetChange*      pIRowsetChange      = NULL;
    IRowset*            pIRowset            = NULL;
    CSeqStream*         pMySeqStream        = NULL;
    ULONG cRowsObtained = 0;
    HACCESSOR hAccessor = DB_NULL_HACCESSOR;
    DBBINDSTATUS rgBindStatus[cBindings];
    HROW* rghRows = NULL;
    const ULONG cPropSets = 1;
    DBPROPSET   rgPropSets[cPropSets];
    const ULONG cProperties = 1;
    DBPROP      rgProperties[cProperties];
    const ULONG cBytes = 10;
    BYTE        pBuffer[cBytes];
    ULONG       cBytesRead = 0;

    BYTE pReadData[cBytes];  //read BLOB data in this array
    memset(pReadData, 0xAA, cBytes);

    BYTE pWriteData[cBytes];  //write BLOB data from this array
    memset(pWriteData, 'D', cBytes);

    // Get the Command object.
    hr = GetCommandObject(IID_ICommandText, 
                          (IUnknown**)&pICommandText);
    if (FAILED(hr))
    {
        cout << "Failed to get ICommandText interface.\n";
        // Release any references and return.
        goto Exit;
    } //end if

    // Create table with image column and index.
    hr = CreateTable(pICommandText);
    if (FAILED(hr))
    {
        cout << "Failed to create table.\n";
        // Release any references and return.
        goto Exit;
    } //end if
    
    /*
    Set the DBPROPSET structure.  It is used to pass an array 
    of DBPROP structures to SetProperties().
    */
    rgPropSets[0].guidPropertySet = DBPROPSET_ROWSET;
    rgPropSets[0].cProperties = cProperties;
    rgPropSets[0].rgProperties = rgProperties;

    // Now set properties in the property group (DBPROPSET_ROWSET).
    rgPropSets[0].rgProperties[0].dwPropertyID = DBPROP_UPDATABILITY;
    rgPropSets[0].rgProperties[0].dwOptions = DBPROPOPTIONS_REQUIRED;
    rgPropSets[0].rgProperties[0].dwStatus = DBPROPSTATUS_OK;
    rgPropSets[0].rgProperties[0].colid = DB_NULLID;
    rgPropSets[0].rgProperties[0].vValue.vt = VT_I4;
    V_I4(&rgPropSets[0].rgProperties[0].vValue) = DBPROPVAL_UP_CHANGE;

    // Set the rowset properties.
    hr = pICommandText->QueryInterface(IID_ICommandProperties,
                            (void **)&pICommandProperties);
    if (FAILED(hr))
    {
        cout << "Failed to get ICommandProperties to set rowset properties.\n";
        // Release any references and return.
        goto Exit;
    } //end if
    hr = pICommandProperties->SetProperties(cPropSets, rgPropSets);
    if (FAILED(hr))
    {
        cout << "Execute failed to set rowset properties.\n";
        // Release any references and return.
        goto Exit;
    } //end if
    
    // Execute a command (SELECT * FROM TestISeqStream).
    hr = pICommandText->SetCommandText(DBGUID_DBSQL,
                                       L"SELECT * FROM TestISeqStream");
    if (FAILED(hr))
    {
        cout << "Failed to set command text SELECT * FROM.\n";
        // Release any references and return.
        goto Exit;
    } //end if

    hr = pICommandText->Execute(NULL, IID_IRowsetChange, NULL, NULL,
                                (IUnknown**)&pIRowsetChange);
    if (FAILED(hr))
    {
        cout << "Failed to execute the command SELECT * FROM.\n";
        // Release any references and return.
        goto Exit;
    } //end if

    // Fill the DBBINDINGS array.
    rgBindings[0].iOrdinal = 2; //ordinal position
    rgBindings[0].obValue = offsetof(BLOBDATA, pISeqStream);
    rgBindings[0].obLength = offsetof(BLOBDATA, dwLength);
    rgBindings[0].obStatus = offsetof(BLOBDATA, dwStatus);
    rgBindings[0].pTypeInfo = NULL;
    rgBindings[0].pObject = &ObjectStruct;
    rgBindings[0].pBindExt = NULL;
    rgBindings[0].dwPart =  DBPART_VALUE | DBPART_STATUS | DBPART_LENGTH;
    rgBindings[0].dwMemOwner = DBMEMOWNER_CLIENTOWNED;
    rgBindings[0].eParamIO = DBPARAMIO_NOTPARAM;
    rgBindings[0].cbMaxLen = 0; 
    rgBindings[0].dwFlags = 0;
    rgBindings[0].wType = DBTYPE_IUNKNOWN;
    rgBindings[0].bPrecision = 0;
    rgBindings[0].bScale = 0;

    // Create an accessor using the binding information.
    hr = pIRowsetChange->QueryInterface(IID_IAccessor, 
                                        (void**)&pIAccessor);
    if (FAILED(hr))
    {
        cout << "Failed to get IAccessor interface.\n";
        // Release any references and return.
        goto Exit;
    } //end if

    hr = pIAccessor->CreateAccessor(DBACCESSOR_ROWDATA,
                                    cBindings,
                                    rgBindings, 
                                    sizeof(BLOBDATA),
                                    &hAccessor,
                                    rgBindStatus);
    if (FAILED(hr))
    {
        cout << "Failed to create an accessor.\n";
        // Release any references and return.
        goto Exit;
    } //end if 

    // Now get the first row.
    hr = pIRowsetChange->QueryInterface(IID_IRowset, 
                                        (void **)&pIRowset);
    if (FAILED(hr))
    {
        cout << "Failed to get IRowset interface.\n";
        // Release any references and return.
        goto Exit;
    } //end if

    hr = pIRowset->GetNextRows(NULL, 
                               0, 
                               1, 
                               &cRowsObtained, 
                               &rghRows);

    hr = pIRowset->GetData(rghRows[0], 
                           hAccessor, 
                           &BLOBGetData);

    // Verify the retrieved data, only if data is not null.
    if (BLOBGetData.dwStatus == DBSTATUS_S_ISNULL)
    {
        // Process null data.
        cout << "Provider returned a null value.\n";
    } else if(BLOBGetData.dwStatus == DBSTATUS_S_OK) 
      // Provider returned a nonNULL value.
    {
        BLOBGetData.pISeqStream->Read(
                                    pBuffer, 
                                    cBytes, 
                                    &cBytesRead);
        if(memcmp(pBuffer, pReadData, cBytes) != 0)
        {
            //cleanup 
         }

        SAFE_RELEASE(BLOBGetData.pISeqStream);
    }

    // Set up data for SetData.
    pMySeqStream = new CSeqStream();

    /*
    Put data in to the ISequentialStream object 
    for the provider to write.
    */
    pMySeqStream->Write(pWriteData, 
                        cBytes, 
                        NULL);

    BLOBSetData.pISeqStream = (ISequentialStream*)pMySeqStream;
    BLOBSetData.dwStatus    = DBSTATUS_S_OK;
    BLOBSetData.dwLength    = pMySeqStream->Length();
      
    // Set the data.
    hr = pIRowsetChange->SetData(rghRows[0], 
                                 hAccessor, 
                                 &BLOBSetData);
        if (FAILED(hr))
    {
        cout << "Failed to set data.\n";
        // Release any references and return.
        goto Exit;
    } //end if

    hr = pIAccessor->ReleaseAccessor(hAccessor, NULL);
    if (FAILED(hr))
    {
        cout << "Failed to release accessor.\n";
        // Release any references and return.
        goto Exit;
    } //end if
    hr = pIRowset->ReleaseRows(cRowsObtained, 
                            rghRows, 
                            NULL, 
                            NULL, 
                            NULL);
    if (FAILED(hr))
    {
        cout << "Failed to release rows.\n";
        // Release any references and return.
        goto Exit;
    } //end if
Exit:
    // Free up all allocated memory and release interface pointers.

    CoUninitialize();
} //end main.
//..........................................................
HRESULT GetCommandObject(REFIID riid, IUnknown** ppIUnknown)
{
    HRESULT hr = S_OK;

    // Local interface pointers, until a connection is made.
    IDBInitialize* pIDBInitialize = NULL;
    IDBProperties*  pIDBProperties = NULL;
    IDBCreateSession* pIDBCreateSession = NULL;
    IDBCreateCommand* pIDBCreateCommand = NULL;

    const ULONG cPropSets = 1;
    DBPROPSET rgPropSets[cPropSets];

    const ULONG cProperties = 4;
    DBPROP rgProperties[cProperties];

    /*
    Initialize the property values needed to 
    establish the connection.
    */
    for(ULONG i = 0; i < 4; i++)
        VariantInit(&rgProperties[i].vValue);

    // Server name.
    rgProperties[0].dwPropertyID = DBPROP_INIT_DATASOURCE;
    rgProperties[0].vValue.vt = VT_BSTR;
    rgProperties[0].vValue.bstrVal = 
                    SysAllocString(L"server");
    rgProperties[0].dwOptions = DBPROPOPTIONS_REQUIRED;
    rgProperties[0].colid = DB_NULLID;

    // Database.
    rgProperties[1].dwPropertyID = DBPROP_INIT_CATALOG;
    rgProperties[1].vValue.vt = VT_BSTR;
    rgProperties[1].vValue.bstrVal = 
                    SysAllocString(L"pubs");
    rgProperties[1].dwOptions = DBPROPOPTIONS_REQUIRED;
    rgProperties[1].colid = DB_NULLID;


    // Username (login).
    rgProperties[2].dwPropertyID = DBPROP_AUTH_USERID;
    rgProperties[2].vValue.vt = VT_BSTR;
    rgProperties[2].vValue.bstrVal = 
                    SysAllocString(L"login");
    rgProperties[2].dwOptions = DBPROPOPTIONS_REQUIRED;
    rgProperties[2].colid = DB_NULLID;

    // Password.
    rgProperties[3].dwPropertyID = DBPROP_AUTH_PASSWORD;
    rgProperties[3].vValue.vt = VT_BSTR;
    rgProperties[3].vValue.bstrVal = 
                    SysAllocString(L"password");
    rgProperties[3].dwOptions = DBPROPOPTIONS_REQUIRED;
    rgProperties[3].colid = DB_NULLID;

    
    /*
    Now that the properties are set, construct the DBPROPSET 
    structure (rgInitPropSet).  The DBPROPSET structure is used 
    to pass an array of DBPROP structures (InitProperties) to the 
    SetProperties method.
    */
    rgPropSets[0].guidPropertySet   = DBPROPSET_DBINIT;
    rgPropSets[0].cProperties       = cProperties;
    rgPropSets[0].rgProperties      = rgProperties;

    // Get the IDBInitialize interface.
    hr = CoCreateInstance(CLSID_SQLNCLI10, 
                            NULL, 
                            CLSCTX_INPROC_SERVER,
                            IID_IDBInitialize, 
                            (void**)&pIDBInitialize);
    if(FAILED(hr))
    {
        cout << "Failed to get IDBInitialize interface.\n";
        // Release any references and return.
        goto Exit;

    } //end if

    // Set initialization properties.
    hr = pIDBInitialize->QueryInterface(IID_IDBProperties,
                                        (void **)&pIDBProperties);
    if(FAILED(hr))
    {
        cout << "Failed to get IDBProperties interface.\n";
        // Release any references and return.
        goto Exit;
    } //end if

    hr = pIDBProperties->SetProperties(cPropSets, rgPropSets);
     if(FAILED(hr))
    {
        cout << "Failed to set properties for DBPROPSET_DBINIT.\n";
        // Release any references and return.
        goto Exit;
    } //end if

     hr = pIDBInitialize->Initialize();
      if(FAILED(hr))
    {
        cout << "Failed to initialize.\n";
        // Release any references and return.
        goto Exit;
    } // end if

    //Create a session object.
      hr = pIDBInitialize->QueryInterface(
                                IID_IDBCreateSession,
                                (void **)&pIDBCreateSession);
       if(FAILED(hr))
    {
        cout << "Failed to get pIDBCreateSession interface.\n";
        // Release any references and return.
        goto Exit;
    } // end if

       hr = pIDBCreateSession->CreateSession(
                                    NULL, 
                                    IID_IDBCreateCommand,
                                    (IUnknown**)&pIDBCreateCommand);
     if(FAILED(hr))
    {
        cout << "Failed to create session object.\n";
        // Release any references and return.
        goto Exit;
    } // end if

     // Get the CommandText object.
     hr = pIDBCreateCommand->CreateCommand(
                                    NULL, 
                                    riid, 
                                    (IUnknown**)ppIUnknown);
     if(FAILED(hr))
    {
        cout << "Failed to create CommandText object.\n";
        // Release any references and return.
        goto Exit;
    } // end if

     return hr;
Exit:
    // Free up all allocated memory and release interface pointers.
     return hr;

} //end function
//...............................................................
HRESULT CreateTable(ICommandText* pICommandText)
{
    HRESULT hr = S_OK;

    // Drop the xisting table.
    hr = pICommandText->SetCommandText(
                            DBGUID_DBSQL,
                            L"DROP TABLE TestISeqStream");
    if(FAILED(hr))
    {
        cout << "Failed to set command text DROP TABLE.\n";
        // Release any references and return.
        goto Exit;

    } //end if

    hr = pICommandText->Execute(NULL, IID_NULL, NULL, NULL, NULL);
    if(FAILED(hr))
    {
        cout << "Failed to drop the table.\n";
        // Release any references and return.
        goto Exit;
    } // end if

    // Create a new table.
    hr = pICommandText->SetCommandText(DBGUID_DBSQL,
         L"CREATE TABLE TestISeqStream (col1 int,col2 image)");
    if(FAILED(hr))
    {
        cout << "Failed to set command text CREATE TABLE.\n";
        // Release any references and return.
        goto Exit;
    } // end if

    hr = pICommandText->Execute(NULL, IID_NULL, NULL, NULL, NULL);
    if(FAILED(hr))
    {
        cout << "Failed to create new table.\n";
        // Release any references and return.
        goto Exit;
    } // end if

    // Insert one row into table.
    hr = pICommandText->SetCommandText(DBGUID_DBSQL,
    L"INSERT INTO TestISeqStream(col1,col2) VALUES (1,0xAAAAAAAAAAAAAAAAA)");
    if(FAILED(hr))
    {
        cout << "Failed to set command text INSERT INTO.\n";
        // Release any references and return.
        goto Exit;
    } // end if

    hr = pICommandText->Execute(NULL, IID_NULL, NULL, NULL, NULL);
    if(FAILED(hr))
    {
        cout << "Failed to insert record in the table.\n";
        // Release any references and return.
        goto Exit;
    } // end if
   
Exit:
    // Free up all allocated memory and release interface pointers.
     return hr;

} //end function

See Also

Concepts