FIX: CLongBinary Causes "Invalid String or Buffer Length"

• The Microsoft Foundation Classes (MFC), included with: Microsoft Visual C++, 32-bit Edition, versions 4.0, 4.1, 4.2

SYMPTOMS

When updating CLongBinary data using CRecordset::Update(), you may receive the error "Invalid string or buffer length."

CAUSE

MFC's RFX_LongBinary function uses an obsolete method to specify the size of the field when binding the CLongBinary buffer for the update. MFC specifies that the length of the long binary field is SQL_DATA_AT_EXEC, a symbol that signifies that data is transferred via SQLPutData() without providing any information about the field's size.

The following is true according to the documentation for SQLExecDirect (Problems and Information) in the ODBC SDK 2.10 Release Notes, included with VC++ 4.1:

   SQLSTATE S1090 (Invalid string or buffer length):
   A parameter length value bound by SQLBindParameter was set to
   SQL_DATA_AT_EXEC; the SQL type was either SQL_LONGVARCHAR,
   SQL_LONGVARBINARY, or a long, data-source specific data type; and the
   SQL_NEED_LONG_DATA_LEN information type in SQLGetInfo was "Y."

RESOLUTION

To avoid this error, you can modify MFC to use the SQL_LEN_DATA_AT_EXEC() macro that provides information about the size of the data and thereby allows the driver to perform the update.

STATUS

Microsoft has confirmed this to be a bug in the Microsoft products listed at the beginning of this article. This bug was corrected in Visual C++ version 5.0.

MORE INFORMATION

The following steps detail one method of making the needed changes in RFX_LongBinary:

1. Copy the implementation of RFX_LongBinary() (Msdev\Mfc\Src\Dbrfx.cpp) into a new .cpp file, and rename the function to something like RFX_LongBinary2(). Create a header file for the .cpp file and include that in your recordset .cpp file. Be sure to #include "stdafx.h" at the top of your new .cpp file and include the .cpp file in your project.

2. Change all RFX_LongBinary() calls in your recordset's DoFieldExchange() to calls to RFX_LongBinary2().

3. In your new RFX_LongBinary2() implementation, copy the following code that appears under "case CFieldExchange::SetFieldNull":

         // Length is signed value, it's limited by LONG_MAX
         if (value.m_dwDataLength >
             (ULONG)(LONG_MAX - labs(SQL_LEN_DATA_AT_EXEC_OFFSET)))
         {
            ASSERT(FALSE);
            *plLength = LONG_MAX - labs(SQL_LEN_DATA_AT_EXEC_OFFSET);
         }
         else
            *plLength = value.m_dwDataLength;
   
         *plLength = SQL_LEN_DATA_AT_EXEC(*plLength);
   
      Paste this code over the line that contains SQL_DATA_AT_EXEC in the
      following code, which appears under "case CFieldExchange::Value":
   
         {
            // Indicate data will be sent after SQLExecute
            *plLength = SQL_DATA_AT_EXEC;       << REPLACE THIS LINE
         }
   
      This causes the macro to be used instead of the constant.
   
   

4. In the CFieldExchange::SetFieldNull case, you can simply comment out the conditional that currently causes the macro to be used only in those cases where m_bUseUpdateSQL is FALSE. Comment out a total of five lines as noted below:

        if (pFX->m_prs->m_bUseUpdateSQL)      // COMMENT OUT THIS LINE
           *plLength = SQL_DATA_AT_EXEC;      // COMMENT OUT THIS LINE
        else                                  // COMMENT OUT THIS LINE
        {                                     // COMMENT OUT THIS LINE
           // Length is signed value, it's limited by LONG_MAX
           if (value.m_dwDataLength >
               (ULONG)(LONG_MAX - labs(SQL_LEN_DATA_AT_EXEC_OFFSET)))
           {
              ASSERT(FALSE);
              *plLength = LONG_MAX - labs(SQL_LEN_DATA_AT_EXEC_OFFSET);
           }
           else
              *plLength = value.m_dwDataLength;
   
           *plLength = SQL_LEN_DATA_AT_EXEC(*plLength);
        }                                     // COMMENT OUT THIS LINE
   
   

REFERENCES

ODBC Programmer's Reference ODBC SDK 2.10 Release Notes