PRB: GP Fault if Uninitialized String Passed to API Function

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

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.