Fetching column data using IRow::GetColumns (OLE DB)(MSSQL) - c++

Question

In my application I am trying to read data from a VARCHAR(4000) column. Since the column is 4000 bytes, I have an application buffer which is big enough to handle it. But at the moment I have only 10 bytes of data in the column. After doing IRow->GetColumns(), not sure how to copy only 10 bytes of data. I do the following, but get all 4000 bytes, so when the data is printed, it is 10 characters of actual data padded with 3990 whitespaces.
retcode = WideCharToMultiByte(CP_UTF8, 0, (WCHAR*)pDBColumnAccess[nCol].pData, -1, (char *)pReadBuf, pDBColumnAccess[nCol].cbDataLen, NULL, NULL);
I thought pDBColumnAccess.cbDataLen would have only 10, but it has the value 4000.
How to read the exact number of bytes from the column?
Thanks.

Answer 1

Figured out a way for this, not sure if this is the actual solution or just a workaround.
If ANSI_PADDING is set to OFF before the table is created, padding of whitespaces after the actual data will not happen. You can see the property value "TrimTrailingBlanks" in sp_help of the table.
Thanks.

Answer 2

You should be setting a DBBINDING that includes DBPART_LENGTH to get the length of the field. To illustrate this, I've created some VARCHAR data in SQL Server using the following DDL:
CREATE TABLE NEWS
(
ID INT NOT NULL,
ARTICLE NVARCHAR(4000)
);
INSERT INTO NEWS (1, 'Today is a sunny day.');
Then I use the following ATL based C++ code sample to read the ARTICLE field. If you put breakpoints in the code, you will see it will come back with
data.nArticleLength = 21
data.szArticle = "Today is a sunny day."
Here's the code sample sample:
#include <stdio.h>
#include <tchar.h>
#include <windows.h>
#include <oledb.h>
#include <atlbase.h>
int _tmain(int argc, _TCHAR* argv[])
{
HRESULT hr = S_OK;
hr = CoInitializeEx(NULL, COINIT_MULTITHREADED);
// Connect to SQL Server.
CComPtr<IDBInitialize> spIDBInitialize;
hr = spIDBInitialize.CoCreateInstance(OLESTR("SQLNCLI"));
CComPtr<IDBProperties> spIDBProperties;
hr = spIDBInitialize->QueryInterface(IID_IDBProperties, (void**) &spIDBProperties);
CComVariant varDataSource(OLESTR("InsertYourSQLServer"));
CComVariant varCatalog(_T("InsertYourDatabase"));
CComVariant varUserID(_T("InsertYourUserName"));
CComVariant varPassword(_T("InsertYourPassword"));
DBPROP rgProps[4] =
{
{ DBPROP_INIT_DATASOURCE, DBPROPOPTIONS_REQUIRED, 0, DB_NULLID, varDataSource },
{ DBPROP_INIT_CATALOG, DBPROPOPTIONS_REQUIRED, 0, DB_NULLID, varCatalog },
{ DBPROP_AUTH_USERID, DBPROPOPTIONS_REQUIRED, 0, DB_NULLID, varUserID },
{ DBPROP_AUTH_PASSWORD, DBPROPOPTIONS_REQUIRED, 0, DB_NULLID, varPassword }
};
DBPROPSET propSet = {rgProps, 4, DBPROPSET_DBINIT};
hr = spIDBProperties->SetProperties(1, &propSet);
spIDBProperties = NULL;
hr = spIDBInitialize->Initialize();
// Execute the query.
CComPtr<IDBCreateSession> spIDBCreateSession;
hr = spIDBInitialize->QueryInterface(IID_IDBCreateSession, (void**) &spIDBCreateSession);
CComPtr<IDBCreateCommand> spIDBCreateCommand;
hr = spIDBCreateSession->CreateSession(NULL, IID_IDBCreateCommand, (IUnknown**) &spIDBCreateCommand);
spIDBCreateSession = NULL;
CComPtr<ICommandText> spICommandText;
hr = spIDBCreateCommand->CreateCommand(NULL, IID_ICommandText, (IUnknown**) &spICommandText);
spIDBCreateCommand = NULL;
hr = spICommandText->SetCommandText(DBGUID_SQL, OLESTR("SELECT ID, ARTICLE FROM NEWS"));
DBROWCOUNT cRowsAffected = 0;
CComPtr<IRowset> spIRowset;
hr = spICommandText->Execute(NULL, IID_IRowset, NULL, &cRowsAffected, (IUnknown**) &spIRowset);
spICommandText = NULL;
// Retrieve records.
HROW hRow = NULL;
HROW *rghRow = &hRow;
DBCOUNTITEM cRowsObtained = 0;
hr = spIRowset->GetNextRows(DB_NULL_HCHAPTER, 0, 1, &cRowsObtained, &rghRow);
while (hr == S_OK && cRowsObtained == 1)
{
// Fetch the ARTICLE field.
struct
{
DBSTATUS dbArticleStatus;
ULONG nArticleLength;
char szArticle[4000 + 4];
} data = {0};
DBBINDING Binding = {0};
Binding.iOrdinal = 2;
Binding.obValue = sizeof(DBSTATUS) + sizeof(ULONG);
Binding.obLength = sizeof(DBSTATUS);
Binding.obStatus = 0;
Binding.pTypeInfo = NULL;
Binding.pObject = NULL;
Binding.pBindExt = NULL;
Binding.dwPart = DBPART_STATUS | DBPART_LENGTH | DBPART_VALUE;
Binding.dwMemOwner = DBMEMOWNER_CLIENTOWNED;
Binding.eParamIO = DBPARAMIO_NOTPARAM;
Binding.cbMaxLen = 4000 + 1;
Binding.wType = DBTYPE_STR;
Binding.dwFlags = 0;
Binding.bPrecision = 0;
Binding.bScale = 0;
CComPtr<IAccessor> spIAccessor;
hr = spIRowset->QueryInterface(IID_IAccessor, (void**) &spIAccessor);
HACCESSOR hAccessor = NULL;
hr = spIAccessor->CreateAccessor(DBACCESSOR_ROWDATA, 1, &Binding, 0, &hAccessor, NULL);
hr = spIRowset->GetData(hRow, hAccessor, &data);
DBREFCOUNT cRefCount = 0;
hr = spIAccessor->ReleaseAccessor(hAccessor, &cRefCount);
spIAccessor = NULL;
// ##TODO: Do something with data.szArticle and data.nArticleLength
// ...
// Fetch next row of data.
hr = spIRowset->ReleaseRows(1, rghRow, NULL, NULL, NULL);
cRowsObtained = 0;
hr = spIRowset->GetNextRows(DB_NULL_HCHAPTER, 0, 1, &cRowsObtained, &rghRow);
}
// Release everything
spIRowset = NULL;
spIDBInitialize = NULL;
CoUninitialize();
return 0;
}