Microsoft KB Archive/105807

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Article ID: 105807

Article Last Modified on 12/9/2003


  • Microsoft Visual Basic 2.0 Standard Edition
  • Microsoft Visual Basic 3.0 Professional Edition
  • Microsoft Visual Basic 2.0 Professional Edition
  • Microsoft Visual Basic 3.0 Professional Edition

This article was previously published under Q105807


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


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.


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.


This behavior is by design.


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

  1. Start Visual Basic with a new, empty project.
  2. 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
  3. 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.

  4. 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.


- "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.

Additional query words: 2.00 3.00 GPF

Keywords: kbprb KB105807