|
The information in this article applies to:
- Standard and Professional Editions of Microsoft Visual Basic for
Windows, versions 2.0 and 3.0
SYMPTOMS
When you incorrectly call a Windows API function as described in the CAUSE
section further below, you can receive a general protection (GP) fault
Application error: VB caused a General Protection Fault in VB.EXE
at nnnn:nnnn
or one of the following error messages:
- Assertion failed
- Bad handle
- Bad heap block
CAUSE
Invoking a Windows API function in any of the following incorrect ways can
give you a GP fault or another memory violation error:
- a passed string initialized to a value that is too short to receive the
return value (See example below.)
- incorrect placement of ByVal in the Declare statement
- undefined parameters in the function declaration or invocation
- incorrect type or length of parameters in the function declaration or
invocation
Windows requires you to ensure memory integrity when calling API functions.
WORKAROUND
If you get a GP fault or another memory error when calling a Windows API
function, check that you have properly defined and passed all parameters.
STATUS
This behavior is by design.
MORE INFORMATION
String parameters passed from Visual Basic to Windows API functions must be
initialized to at least the size of the data returned in them, or else you
may get a general protection fault.
The following example causes a GP fault by invoking the GetProfileString
API function with a string initialized with a too-small value.
Steps to Reproduce Behavior
- Start Visual Basic with a new, empty project.
- Place the following correct Declare statement for GetProfileString
in the General Declarations section:
'Enter the following Declare statement as one, single line:
Declare Function GetProfileString Lib "Kernel"
(ByVal lpAppName As String, ByVal lpKeyName As String,
ByVal lpDefault As String, ByVal lpReturnedString As String,
ByVal nsize As Integer) As Integer
'NOTE: The GetProfileString function is located in the KERNAL.DLL file,
'which is usually located in the \WINDOWS\SYSTEM directory
- Add a Command button to Form1, and add the following code to the
Command1_Click event procedure:
dim size, str1 as string
str1 = "" ' To avoid GP fault, initialize with a longer string:
' str1="abcdefghijklmnopqrstuvwxyz"
size = GetProfileString("intl", "sLongDate", "-1", str1, 1024)
The above API function looks in the WIN.INI file under the [intl]
section and retrieves the string after sLongDate=. The function returns
a string in the address of its fourth argument. If you fail to define
the fourth argument or you initialize it to a string that is smaller
than the retrieved string, an assertion error occurs. If you define the
fourth argument with a string that is larger than the retrieved string,
the call will succeed. For example, change the line str1="" to
str1="abcdefghijklmnopqrstuvwxyz" and the code will work.
- Execute the code. VB.EXE will give the following error messages:
An error has occurred in your application. If you choose Ignore, you
should save your work in a new file. If you choose Close your
application will terminate. <Close> <Ignore>
followed by:
Application error: VB caused a General Protection Fault in VB.EXE
at 004A:0122
Windows API functions must be called with valid parameters. If you in
effect tell Windows to overwrite some part of Visual Basic internal memory,
this usually causes a GP fault or other memory problem. This usually ends
the Visual Basic session. In Windows version 3.1, memory of other Windows
applications should not be affected. But in Windows version 3.0, a GP fault
means you must restart Windows itself, thus ending all current applications
in memory.
REFERENCES
- "Visual Basic: Programmers Guide" for version 3.0, Chapter 24,
"Calling Procedures in DLLs."
- The correct Declare statements for API functions are in the Visual Basic
Professional Edition help file WIN31API.HLP, which is located in the
WINAPI subdirectory of your Visual Basic directory. The WIN31API.HLP
file contains function declarations, Type declarations, and the values
for global constants used in the API functions.
|