BUG: CDK: VBSetVariantValue GPF with Null Pointer (154033)
The information in this article applies to:
- Microsoft Visual Basic Professional Edition, 16-bit, for Windows 4.0
- Microsoft Visual Basic Enterprise Edition, 16-bit, for Windows 4.0
This article was previously published under Q154033 SYMPTOMS
The 16-bit edition of Visual Basic 4.0 stops with a General Protect Fault
in module VB.EXE at 002F:350B.
CAUSE
Passing a NULL pointer to the lpData argument of VBSetVariantValue() in the
VBAPI library will cause this General Protection Fault. Normally, you would
not pass a NULL pointer to lpData, but the VBCreateTempHlstr() function is
documented as returning a NULL if you create a TempHLSTR on an empty (zero-
length) string. Because you can call VBSetVariantValue() using a TempHLSTR
to assign a string to a variant, this function should work with a NULL
pointer in the lpData argument, but it doesn't.
MORE INFORMATION
Visual Basic 3.0 is not affected by this problem, as the
VBCreateTempHlstr() function will not return a NULL, even for empty
strings. This is the only situation that is documented as acceptable to
pass NULL as the lpData argument.
STATUS
Microsoft has confirmed this to be a problem in the Microsoft products
listed at the beginning of this article. We are researching this problem
and will post new information here in the Microsoft Knowledge Base as it
becomes available.
WORKAROUND
There are several workarounds available to you. Choose the one that best
fits your needs:
- Avoid using a TempHLSTR to set a variant's value. Instead, use a normal
HLSTR to set the variant's value instead.
You must be sure not to call VBDestroyHlstr() on this HLSTR before you
return control to Visual Basic. Visual Basic 4.0 simply copies a
reference to the HLSTR's data in VBSetVariantValue(). Destroying the
HLSTR before Visual Basic regains control from your function prevents it
from reading the string data stored by the HLSTR. You may reuse the same
HLSTR in your function; that is, you do not have to maintain the data in
the HLSTR after your function has returned control to Visual Basic.
NOTE: TempHLSTR values work best when they return a string from a
function call or are used to retrieve property values from custom
controls (VBXs). Even though they are documented as being suitable for
setting variants, there are only a few TempHLSTR spaces available to
Visual Basic programs; you should use them sparingly.
- Instead of trying to return an empty string, pass another variant type.
For instance, VT_NULL indicates that the variant has no data in it.
Be aware that this workaround may affect the behavior of Visual Basic
programs that have already been compiled using your DLL or custom
control. If no Visual Basic programs have been compiled using your DLL
or custom control, this should not be a problem.
| Modification Type: | Minor | Last Reviewed: | 1/9/2003 |
|---|
| Keywords: | kbbug KB154033 |
|---|
|