PRB: DAO 3.6 Overruns Buffers in GetRowsEx Method

ID: Q236056

The information in this article applies to:

Microsoft Visual C++, 32-bit Editions, version 6.0, used with:
- The DAO SDK, version 3.6

SYMPTOMS

The character buffer allocated for string data is overrun when using the DAO SDK and the GetRowsEx method in non-unicode builds. There are two ways character data can be fetched as described in the GetRows DAO SDK sample.

When using the dbBindSTRING method the buffer is allocated dynamically and the error message is:

DAMAGE: after Normal block(#89) at <hex address>

-OR

DAMAGE: after Normal block(#99) at <hex address>

When using the dbBindLPSTRING method some data can return truncated, not terminated by a null character, or garbled.

CAUSE

DAO version 3.6 incorrectly overwrites the character buffer supplied to it by the application when it fetches text data. It first copies the UNICODE string into the buffer which is twice as long as the length of the buffer and then converts the string to an ANSI string.

RESOLUTION

Allocate twice the space necessary if the direct binding method is used, or allocate enough space for an extra record if the indirect method is used as described in the More Information section.

STATUS

Microsoft is aware of the problem and will post more information here as it becomes available.

MORE INFORMATION

Steps to Reproduce Behavior

Open the GetRows sample from the DAOSDK, which comes with Windows platform SDK.

Go to Project Settings under the Link tab and change the .lib file used from Ddao35d.lib to Ddao36d.lib to utilize DAO version 3.6 instead of DAO version 3.5.

NOTE: In order to get Ddao36d.lib and Ddao36d.dll, you will have to compile and create them by using DAO workspace for DAO 3.6. This workspace comes with Visual Studio Service Pack 3. Information on how to get Visual Studio SP3 is given at the end of this article.

In the Getrdlg.h file change the number of records fetched from ten to four:
```
#define MAX_EMP_REC		4 // Arbitrary maximum 
```

Go to Getrdlg.cpp file. The EMP typedef is located just after the definition of the DoGetRows() function. In EMP typedef structure, change the size of array strFirstName from twenty to eight. This is the correct code:
```
typedef struct
	{
	LONG	lEmpId;
	TCHAR	*strLastName;
	TCHAR	strFirstName[8];
	} EMP, *LPEMP ; 
```

Also, make a change in the Bindings structure, which is located right after EMP structure. Change the size of strFirstName from twenty to eight. This is the correct code:


// Employee table binding 
DAORSETBINDING	Bindings[] = 
{
//Index Type		Column		Type			Offset						Size
{dbBindIndexINT,	EMP_ID,		dbBindI4,		offsetof(EMP,lEmpId),		sizeof(LONG)},
{dbBindIndexINT,	EMP_LNAME,	dbBindLPSTRING,	offsetof(EMP,strLastName),	sizeof(TCHAR *)},
{dbBindIndexINT,	EMP_FNAME,	dbBindSTRING,	offsetof(EMP,strFirstName),	sizeof(TCHAR) * 8}
};

In the Getrdlg.cpp file change the size of the character buffer that will hold the variable size data from MAX_EMP_REC * 15 to 33. The buffer is declared in the beginning of the DoGetRowsEx() function.
```
TCHAR			pBuf[33];		// allow average of 15 chars/name 
```

Compile and run the code. Select the Employee.mdb file (located in \mssdk\samples\DBMsg\DataAccess\DAO) after clicking Execute. A Debug error dialog will appear with the above described message.

To avoid the debug error, comment out the line that deletes the pEmpRows array at the end of the DoGetRowsEx function:
```
//delete [] pEmpRows; 
```

Recompile the code and run it again. The fourth name will be truncated in the dialog box.

Workaround

When retrieving the character data directly with the dbBindSTRING type, allocate twice as much space than the maximum size of the field in the custom structure. Change the size of strFirstName to twenty in both structures as follows:


typedef struct
	{
	LONG	lEmpId;
	TCHAR	*strLastName;
	TCHAR	strFirstName[20];
	} EMP, *LPEMP ;

// Employee table binding 
DAORSETBINDING	Bindings[] = 
{
//Index Type		Column		Type			Offset						Size
{dbBindIndexINT,	EMP_ID,		dbBindI4,		offsetof(EMP,lEmpId),		sizeof(LONG)},
{dbBindIndexINT,	EMP_LNAME,	dbBindLPSTRING,	offsetof(EMP,strLastName),	sizeof(TCHAR *)},
{dbBindIndexINT,	EMP_FNAME,	dbBindSTRING,	offsetof(EMP,strFirstName),	sizeof(TCHAR) * 20}
};

When retrieving the character data indirectly into a buffer for variable length data with dbBindLPSTRING, allocate the buffer big enough to hold an extra record and two extra Unicode null characters (4 bytes). So make this change to the size of pBuf character buffer:


TCHAR			pBuf[(MAX_EMP_REC + 1) * 20 + 4];// allow average of 15 chars/name

Recompile and run the code.

REFERENCES

Q235507 PRB: DAO 3.6 causes Debug errors in MFC DAO non-unicode builds

Q194022 INFO: Visual Studio 6.0 Service Packs, What, Where, Why

Additional query words:

Keywords : kbDatabase kbSDKDAO kbGrpVCDB kbDAO360 kbDAO360bug
Version : winnt:3.6,6.0
Platform : winnt
Issue type : kbprb