ILockBytes::ReadAt

  • [アーティクル]

ReadAt は、バイト配列の先頭から指定されたオフセット位置を起点として、指定された数のバイトを読み取ります。

構文

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

パラメータ

パラメータ 説明

ulOffset [in]

バイト配列の先頭を起点とする、データ読み取りの開始位置を指定します。

pv [in]

読み取ったバイト配列を格納するバッファへのポインタ。

cb [in]

バイト配列から読み取るデータのバイト数を指定します。

pcbRead [out]

このメソッドがバイト配列から実際に読み取るバイト数を書き込む場所へのポインタ。この値を取得する必要がないときは、NULL へのポインタを設定します。この場合、実際に読み取られたバイト数は提供されません。

戻り値

用語 定義

NOERROR

メソッドが成功しました。

E_UNEXPECTED

行セットはゾンビ状態です。

STG_E_INVALIDPOINTER

渡されたポインタが有効ではありませんでした。

解説

次の例は、ILockBytes::ReadAt メソッドを使用して BLOB (Binary Large Object) データを読み取る方法を示しています。この例では、Employees テーブルの Photo 列のイメージ データを読み取ります。

使用例

// Declarations
HRESULT            hr;
DBBINDING          rgBindings[1];
DBPROP             dbprop[1];
DBPROPSET          dbpropset[1];
DBOBJECT           dbObject;
BYTE               pData[100];    //buffer for holding the ILockBytes pointer
HROW               rghRows[1]; // Array of row handles obtained from the rowset object
HROW               *prghRows           = rghRows;
BYTE               pBuffer[1000];  //Buffer for holding a chunck of BLOB data
ULARGE_INTEGER     ulOffset;
ULONG              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_3_0, 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 the 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 the query syntax.
// We know that 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(ULONG);
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 less 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, then release the ILockBytes pointer and 
// row handle here

Exit: 

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

return;

参照

関連項目

ILockBytes::SetSize
ILockBytes::Stat
ILockBytes::WriteAt

その他の技術情報

ILockBytes (SQL Server Compact Edition)

ヘルプおよび情報

SQL Server Compact Edition のサポートについて