ILockBytes::ReadAt

Article
03/24/2011

ReadAt reads a specified number of bytes starting at a specified offset from the beginning of the byte array.

Syntax

HRESULT ReadAt(
ULARGE_INTEGER ulOffset,
   void *pv,
   ULONG cb,
   ULONG *pcbRead 
   );

Parameters

Parameter	Description
ulOffset [in]	Specifies the starting point from the beginning of the byte array for reading data
pv [in]	Pointer to the buffer into which the byte array is read.
cb [in]	Specifies the number of bytes of data to attempt to read from the byte array.
pcbRead [out]	Pointer to the location where this method will write the actual number of bytes read from the byte array. You can set this pointer to NULL to indicate that you are not interested in this value. In this case, this method does not provide the actual number of bytes that were read.

Return Value

Term	Definition
NOERROR	The method succeeded.
E_UNEXPECTED	Rowset is in a "zombied" state.
STG_E_INVALIDPOINTER	A pointer that was passed was not valid.

Remarks

The following example shows how to use the ILockBytes::ReadAt method to retrieve binary large object (BLOB) data. This example reads image data from the "Photo" column of the Employees table.

The following example has been updated for both 32-bit and 64-bit platforms. The 64-bit compatible types are from the sqlce_oledb.h native header file. You can find the sqlce_oledb.h native header file in the %ProgramFiles%\Microsoft SQL Server Compact Edition\v3.5\Include folder. For more information, see the OLE DB 64-bit Information topic in the OLE DB Programmer's Guide on the Microsoft Web site.

Example

// Declarations
HRESULT            hr;
DBBINDING          rgBindings[1];
DBPROP             dbprop[1];
DBPROPSET          dbpropset[1];
DBOBJECT           dbObject;
BYTE               pData[100];   // Buffer for holding ILockBytes pointer.
HROW               rghRows[1];   // Array of row handles obtained from rowset object.
HROW               *prghRows           = rghRows;
BYTE               pBuffer[1000];  // Buffer for holding a chunk of BLOB data.
ULARGE_INTEGER     ulOffset;
DBROWCOUNT         cRows               = 0;
IDBInitialize      *pIDBInitialize     = NULL;        
IDBProperties      *pIDBProperties     = NULL;        
IDBCreateSession   *pIDBCreateSession  = NULL;
IDBCreateCommand   *pIDBCreateCommand  = NULL;
ICommandText       *pICommandText      = NULL;
IAccessor          *pIAccessor         = NULL;
IRowset            *pIRowset           = NULL;
ILockBytes         *pILockBytes        = NULL;
HACCESSOR          hAccessor           = DB_NULL_HACCESSOR;

if (FAILED(hr = CoInitialize(NULL)))
{
    return;
}

VariantInit(&dbprop[0].vValue);        

// Create an instance of the OLE DB Provider.
hr = CoCreateInstance(CLSID_SQLSERVERCE, 0, CLSCTX_INPROC_SERVER,
                IID_IDBInitialize, (void**)&pIDBInitialize);
if(FAILED(hr))
{
    // Send error-specific message and do error handling.
    goto Exit;
}

// Initialize a property with name of the database.
// Open an exsiting database myDatabase.
dbprop[0].dwPropertyID     = DBPROP_INIT_DATASOURCE;
dbprop[0].dwOptions         = DBPROPOPTIONS_REQUIRED;
dbprop[0].vValue.vt         = VT_BSTR;
dbprop[0].vValue.bstrVal = SysAllocString(L"\\windows\\MyDB.sdf");
if(NULL == dbprop[0].vValue.bstrVal)
{
    hr = E_OUTOFMEMORY;
    goto Exit;
}

// Initialize the property set.
dbpropset[0].guidPropertySet = DBPROPSET_DBINIT;
dbpropset[0].rgProperties     = dbprop;
dbpropset[0].cProperties     = sizeof(dbprop)/sizeof(dbprop[0]);

// Set initialization properties.
hr = pIDBInitialize->QueryInterface(IID_IDBProperties, 
                (void **)&pIDBProperties);
if(FAILED(hr))
{
    goto Exit;
}

// Sets properties in data source and initialization property groups.
hr = pIDBProperties->SetProperties(1, dbpropset); 
if(FAILED(hr))
{
    goto Exit;
}

// Initializes a data source object. 
hr = pIDBInitialize->Initialize();
if(FAILED(hr))
{
    goto Exit;
}

// Get the IDBCreateSession interface.
hr = pIDBInitialize->QueryInterface(IID_IDBCreateSession,
    (void**)&pIDBCreateSession);
if (FAILED(hr))
{
    // Send error-specific message and do error handling.
    goto Exit;
}
// Get the interface to create a command.
hr = pIDBCreateSession->CreateSession(NULL, IID_IDBCreateCommand,
    (IUnknown**) &pIDBCreateCommand);
if (FAILED(hr))
{
    // Send an error-specific message and do error handling.
    goto Exit;
}


// Create the new command that uses parameters.
hr = pIDBCreateCommand->CreateCommand(NULL, IID_ICommandText, (IUnknown**) &pICommandText);
if (FAILED(hr))
{
    // Send an error-specific message and do error handling.
    goto Exit;
}

// Specify the command text using parameter markers in query syntax.
// This command will return 1 row and one image column (Photo).
hr = pICommandText->SetCommandText(DBGUID_DBSQL, L"SELECT Photo FROM Employees WHERE \"Employee ID\" = 2");
if (FAILED(hr))
{
    // Send an error-specific message and do error handling.
    goto Exit;
}

hr = pICommandText->Execute(NULL, IID_IRowset, NULL, NULL, (IUnknown**) &pIRowset);
if(FAILED(hr))
{
    // Send an error-specific message and do error handling.
    goto Exit;
}

// Create the acessor object and column specific bindings.

hr = pIRowset->QueryInterface(IID_IAccessor, (void**) &pIAccessor);
if (FAILED(hr))
{
    // Send an error-specific message and do error handling.
    goto Exit;
}

// Create the bindings for the photo (BLOB) column.
dbObject.dwFlags = STGM_READ;
dbObject.iid     = IID_ILockBytes;
rgBindings[0].iOrdinal = 1;
rgBindings[0].obStatus = 0;
rgBindings[0].obLength = rgBindings[0].obStatus + sizeof(DBSTATUS);
rgBindings[0].obValue = rgBindings[0].obLength + sizeof(DBLENGTH);
rgBindings[0].pTypeInfo = NULL;
rgBindings[0].pObject = &dbObject;
rgBindings[0].pBindExt = NULL;
rgBindings[0].dwPart = DBPART_VALUE | DBPART_LENGTH | DBPART_STATUS;
rgBindings[0].dwMemOwner = DBMEMOWNER_CLIENTOWNED;
rgBindings[0].eParamIO = DBPARAMIO_NOTPARAM;
rgBindings[0].cbMaxLen = sizeof(IUnknown*);
rgBindings[0].dwFlags = 0;
rgBindings[0].wType = DBTYPE_IUNKNOWN;
rgBindings[0].bPrecision = 0;
rgBindings[0].bScale = 0;

// Create accessor.
hr = pIAccessor->CreateAccessor(DBACCESSOR_ROWDATA, 1, rgBindings,
    0, &hAccessor, NULL);
if(FAILED(hr))
{
    // Send an error-specific message and do error handling.
    goto Exit;
}

// Get the first row.
hr = pIRowset->GetNextRows(NULL, 0, 1, &cRows, &prghRows);
if(FAILED(hr) || (0 == cRows))
{
    // Send an error-specific message and do error handling.
    goto Exit;
}

// Get the ILockBytes pointer.
hr = pIRowset->GetData(rghRows[0], hAccessor, pData);
if(FAILED(hr))
{
    // Send an error-specific message and do error handling.
    goto Exit;
}

if(DBSTATUS_S_OK != *(DBSTATUS*)(pData + rgBindings[0].obStatus))
{
    // Send an error-specific message and do error handling.
    goto Exit;
}

// Get the ILockBytes pointer from the buffer.
pILockBytes = *(ILockBytes**)(pData + rgBindings[0].obValue);

// Initialize Offset. 
ulOffset.QuadPart = 0;

// Retrieve data from the interface in chunks. 
// The infinite loop for(;;) will terminate when the ReadAt call fails 
//   or if fewer bytes are returned than were requested. 
for(;;)

{

    ULONG cbRead;

    // Read data at the specified offset.
    hr = pILockBytes->ReadAt(ulOffset, pBuffer, sizeof(pBuffer)-sizeof(WCHAR), 
    &cbRead);

///////////////////////////////////////////////////////////////////////
// Do some useful things here with the chunks of data.
///////////////////////////////////////////////////////////////////////

    if(cbRead < sizeof(pBuffer)-sizeof(WCHAR) || FAILED(hr)) break;

    ulOffset.QuadPart += cbRead;
}

// If more rows need to be read, release the ILockBytes pointer  
//   and row handle here.

Exit: 

// When finished, clear the properties arrays and release interfaces.
// Uninitialize the environment.

return;

ILockBytes::ReadAt

Syntax

Parameters

Return Value

Remarks

Example

See Also

Reference

Other Resources

Additional resources