Q157071: FIX: GetFieldValue Returns Empty String for SQL_LONGVARCHAR

Article: Q157071
Product(s): Microsoft C Compiler
Version(s): 4.2
Operating System(s): 
Keyword(s): kbtshoot
Last Modified: 30-JUL-2001

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The information in this article applies to:

- The Microsoft Foundation Classes (MFC), included with:
   - Microsoft Visual C++, 32-bit Enterprise Edition, version 4.2 
   - Microsoft Visual C++, 32-bit Professional Edition, version 4.2 
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SYMPTOMS
========

CRecordset::GetFieldValue() incorrectly returns an empty string for
SQL_LONGVARCHAR data that is 1 character long.

CAUSE
=====

GetFieldValue() allocates a buffer and then calls CRecordset::GetData() to
retrieve data from the field into this buffer. For long binary and character
data, the length of the data is not known in advance, so GetFieldValue() does
not know what size of buffer to allocate.

Therefore, for SQL_LONGVARCHAR and SQL_LONGVARBINARY data, GetFieldValue() calls
GetData() with a buffer length of 1. GetData() tries to retrieve the data and
will likely fail because the buffer is too small. Whether GetData() fails or
not, it returns the actual length of the data in the field excluding the NULL
termination byte. GetFieldValue() then calls the function
CRecordset::GetLongCharDataAndCleanup() to reallocate the buffer and retrieve
the rest of the data, if necessary.

GetLongCharDataAndCleanup() must determine whether GetData() was able to retrieve
the data by checking whether the actual data length is larger than the buffer
length:

     // If long data, may need to call SQLGetData again
     if (nLen < nActualSize &&
         (nSQLType == SQL_LONGVARCHAR || nSQLType == SQL_LONGVARBINARY))

For 1-character SQL_LONGVARCHAR data, GetData() will fail because the buffer must
be large enough to hold the data plus the NULL termination byte. Then, GetData()
returns an actual size of 1, which does not include the NULL byte.

GetLongCharDataAndCleanup() fails to consider the space required for the NULL
termination byte. If the data length is 1, then both nLen and nActualSize are 1,
which means that GetLongCharDataAndCleanup() will not attempt to get the
additional data.

RESOLUTION
==========

Override the GetLongCharDataAndCleanup() function and change the length
comparison above from "<" to "<=". See the MORE INFORMATION section below
about how to implement this workaround.

STATUS
======

This problem was corrected in Microsoft Visual C++, version 5.0, for Windows.

MORE INFORMATION
================

The following steps detail one method of making the needed changes to
GetFieldValue():

1. If you do not currently have a class derived from CRecordset, create one. If
  you have multiple classes derived from CRecordset, you may want to create a
  new base class derived from CRecordset and modify your existing recordset
  classes to be derived from this new base class so that you do not have to
  make the changes to every recordset class.

  For example, create GetSet.H:

        class CGetSet : public CRecordset
        {
        public:
            CGetSet(CDatabase* pDatabase = NULL);
        };

        and GetSet.CPP:

        #include "stdafx.h"
        #include "GetSet.h"

        CGetSet::CGetSet(CDatabase* pdb)
            : CRecordset(pdb)
        {
        }

2. Copy the prototypes of the CRecordset GetFieldValue() and
  GetLongCharDataAndCleanup() functions from MSDEV\MFC\SRC\AFXDB.H to your
  recordset .H file as public member functions. Note that all four of the
  GetFieldValue() prototypes must be copied.

3. Copy the implementations of the CRecordset::GetFieldValue() and
  CRecordset::GetLongCharDataAndCleanup() functions from
  MSDEV\MFC\SRC\DBCORE.CPP to your recordset .CPP file. Change the
  "CRecordset::" prefix to your new class name. Note that all four of the
  GetFieldValue() implementations must be copied.

4. In your implementation of GetLongCharDataAndCleanup(), replace "<" with
  "<=".

  Change:

        // If long data, may need to call SQLGetData again
        if (nLen < nActualSize &&
            (nSQLType == SQL_LONGVARCHAR || nSQLType == SQL_LONGVARBINARY))

  to:

        // If long data, may need to call SQLGetData again
        if (nLen <= nActualSize &&
            (nSQLType == SQL_LONGVARCHAR || nSQLType == SQL_LONGVARBINARY))

5. If you were using the CRecordset class directly, you will need to change your
  application to use the new class instead.

Additional query words: kbDatabase kbMFC kbVC420bug kbVC500fix kbDSupport

======================================================================
Keywords          : kbtshoot 
Technology        : kbAudDeveloper kbMFC
Version           : 4.2
Issue type        : kbbug
Solution Type     : kbfix

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