FIX: GP Faults from Using IIF with Temporary Strings (127069)
The information in this article applies to:
- Microsoft Visual Basic Standard Edition for Windows 3.0
- Microsoft Visual Basic Professional Edition for Windows 3.0
This article was previously published under Q127069 SYMPTOMS
Using the IIF statement may cause a general protection (GP) fault in a
range of addresses from 0023:6BB0 to 0023:7DE0 within VBRUN300.DLL or from
004A:68D0 or 004a:7B00 within VB.EXE. This is a subtle bug that may not be
encountered and will not occur on a line using the IIF statement.
Other GP faults in segment 23 of VBRUN300.DLL are more indicative of
calling a DLL incorrectly, such as forgetting to put "byval" on a DLL's
string parameter.
CAUSE
The IIF statement does not release string handles for temporary strings
used in its arguments. Under certain conditions, this can result in a GP
fault occurring at a seemingly random location later in your program. The
code below demonstrates the condition where IIF will not release its string
handles.
dim x$
x$ = "World"
y$ = "Hello World"
Label1.Caption = IIf(3 > 1000, "Hello " & x$, Mid$( y$, 1, 5 ) )
The second and third arguments of this statement result in temporary
strings being created. This does not, however, cause the GPF. One of two
other events must occur for that to happen: - If the user allocates a large string and there is not enough contiguous
space for it, Visual Basic will attempt to compact the heap, creating a
condition in which the GP fault can occur.
-or-
- Upon exit of the procedure the IIF was located in, the heap will be
compacted.
Once either condition occurs, the GP fault will not occur until the next
temp string is allocated, which just happens to use the string handle used
by the IIF statement. Then the GP fault will occur within one of the two
address ranges given above.
WORKAROUND
Avoid the use of functions that return Strings within the IIF statement.
For example, to avoid the bug, modify the code listed in the CAUSE section
of this article to this:
dim y$, z$
y$ = "Hello World"
z$ = "Small World"
Label1.Caption = IIf(3 > 1000, y$, z$ )
Here's another alternative:
if 3 > 1000 then
label1.caption = y$
else
label1.caption = z$
end if
STATUS
This bug was corrected in Microsoft Visual Basic version 4.0 for Windows.
| Modification Type: | Major | Last Reviewed: | 10/30/2003 |
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| Keywords: | kbbug kbfix KB127069 |
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