Q153586: HOWTO: Call C Functions That Use the _cdecl Calling Convention

Article: Q153586
Product(s): Microsoft Visual Basic for Windows
Version(s): WINDOWS:4.0,5.0,6.0
Operating System(s): 
Keyword(s): kbAPI kbVBp kbVBp400 kbVBp500 kbVBp600 kbGrpDSVB kbDSupport
Last Modified: 11-JAN-2001

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The information in this article applies to:

- Microsoft Visual Basic Learning Edition for Windows, versions 5.0, 6.0 
- Microsoft Visual Basic Professional Edition for Windows, versions 5.0, 6.0 
- Microsoft Visual Basic Enterprise Edition for Windows, versions 5.0, 6.0 
- Microsoft Visual Basic Standard Edition, 32-bit, for Windows, version 4.0 
- Microsoft Visual Basic Professional Edition, 32-bit, for Windows, version 4.0 
- Microsoft Visual Basic Enterprise Edition, 32-bit, for Windows, version 4.0 
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SUMMARY
=======

It is not possible to directly call a C function in a DLL if that function uses
the _cdecl calling convention. This is because Visual Basic uses the _stdcall
calling convention for calling functions. This is a problem because if _cdecl is
used, the calling function is responsible for cleaning up the stack. However, if
_stdcall is used, the called function is responsible for cleaning up the stack.

NOTE: An .EXE file created in Visual Basic will allow you to call a DLL function
that has been declared with the _cdecl calling convention without an error. It
is only when you try to call such a function when running a program from the
Visual Basic IDE, that Visual Basic generates the following error:

  Run-time Error '49':
  Bad DLL Calling Convention

The fact that the EXE version allows you to call such functions has been
confirmed to be a bug by Microsoft. You should not rely on this behavior as this
might change in future versions of Visual Basic.


MORE INFORMATION
================

If it is necessary to call functions that use the _cdecl calling convention
(such as the MSTEST DLLs), wrap the _cdecl call inside a _stdcall call that is
exposed or exported to Visual Basic. This is demonstrated in the following
step-by-step example.

Step-by-Step Example
--------------------

1. Create a 32-bit Windows C DLL with the following functions:

        long _cdecl PassStr(LPSTR pStr)
        {
         MessageBox (NULL, pStr, "Test DLL", MB_OK);
         return 1;
        }

        long _stdcall PassStrStdCall(LPSTR pStr)
        {
         return PassStr(pStr);
        }

  Export the functions in a .DEF file as follows:

        LIBRARY   TESTDLL

        CODE      PRELOAD MOVEABLE DISCARDABLE
        DATA      PRELOAD SINGLE

        EXPORTS
        PassStr            @1
        PassStrStdCall     @2

  Name this DLL Testdll.dll, and save the file to the Windows\System directory
  in Windows 95 or Windows 98, or the Windows\System32 directory in Windows NT
  or Windows 2000.

2. Create a new project in Visual Basic. Form1 is created by default. Add the
  following code to the general declarations portion of Form1:

        Private Declare Function PassStrStdCall Lib "testdll.dll" _
                                     (ByVal s As String) As Long

        Private Sub Form_Click()
           ret& = PassStrStdCall("hello")
           Print ret&
        End Sub

3. Run the Visual Basic program, and click the form. A Msgbox will show the
  value of the passed string.

Additional query words:

======================================================================
Keywords          : kbAPI kbVBp kbVBp400 kbVBp500 kbVBp600 kbGrpDSVB kbDSupport 
Technology        : kbVBSearch kbAudDeveloper kbZNotKeyword6 kbZNotKeyword2 kbVB500Search kbVB600Search kbVBA500 kbVBA600 kbVB500 kbVB600 kbVB400Search kbVB400
Version           : WINDOWS:4.0,5.0,6.0
Issue type        : kbhowto

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