Trying to retreiving all data from a truncated string

Question

I have a generic function that reads data from tables without knowing the maximum data length.

This should be fairly streight forward but I cannot get it to work.

My application is written in C++ using SQL Native Client 10.0.

I have already established the DB connect and perform the following to get to my data:

char*	m_pszText;
char	m_szString[255];
SQLLEN	StrLen_or_Ind;

sqlRetCode = SQLAllocHandle( SQL_HANDLE_STMT, m_hdbc, &m_hstmt );
sqlRetCode = SQLPrepare( m_hstmt, (SQLTCHAR *) "SELECT StringColumn from table", SQL_NTS );
sqlRetCode = SQLBindCol( m_hstmt, (SQLUSMALLINT) 1, SQL_C_CHAR, m_szString, 255, &StrLen_or_Ind );
sqlRetCode = SQLExecute( m_hstmt );

do
{
	sqlRetCode = SQLFetch( m_hstmt );

	if( sqlRetCode == SQL_NO_DATA )
	{
		m_bEOF = TRUE;
		return;
	}

	if ( sqlRetCode == SQL_SUCCESS_WITH_INFO )
	{
		SDWORD nErr;
		TCHAR szSQLSTATE[10];
		SWORD cbms

		if( ( sqlRetCode = SQLGetDiagRec( SQL_HANDLE_STMT, m_hstmt, 1, (SQLTCHAR*) szSQLSTATE, &nErr, NULL, 0, &cbmsg ) ) == SQL_SUCCESS &&
			_tcscmp( szSQLSTATE, _T("01004") ) == 0 )
	        {

			m_pszText = (char*) malloc( StrLen_or_Ind + 1 );
			sqlRetCode = SQLBindCol( m_hstmt, (SQLUSMALLINT) 1, SQL_C_CHAR, m_pszText, StrLen_or_Ind + 1, StrLen_or_Ind );

			//  NOW HOW DO I GET m_pszText LOADED WITH THE FULL DATA STRING??????????????
		}
	}
} while( 1 );

Actually, when I try to free m_pszText, I get: 0xC0000005: Access violation reading location 0x00000000

Answer 1

hmm, I'm not sure what you're trying to do with the second SQLBindCol. According to the documentation of SQLBindCol, the value should already be available in m_szString. btw, you shoud use SQL_C_CHAR as type in the first SQLBindCol call.

Answer 2

According to the documentation, I need to rebind the column with the new increased buffer. The m_szString only contains 255 characters. The first SQLBindCol is a SQL_C_CHAR.

Answer 3

I see. You need either SQLColAttribute or SQLDescribeCol to retrieve the maximum size before binding.

Answer 4

Other alternative could be to increase the size of buffer. Instead of SQLBindCol you could have use SQLGetData as well to retreive the data

regards,

Vatsa

www.objectiveprogramming.com

Answer 5

Hi Stefan, thanks for the info. I don't actually need to perform an SQLColAttribute or SQLDescribeCol because the real length is already available in the field StrLenOrInd which is set by SQLFetch().

Hi vatsa, yes I have been comtemplating using SQLGetData instead of SQLBindCol. I was worried about system performance. Do you know which method is most efficient and/or flexible?

Answer 6

To be very frank, I never measured performance.However, I have used SQLGetData and it worked really well with my code. I have used SQLBindCol to bind SPs parameters.

below link answers your question:

http://msdn.microsoft.com/en-us/library/ms131269.aspx

regards,

Vatsa

www.objectiveprogramming.com