HOWTO: Call C Functions That Use the _cdecl Calling Convention

• Microsoft Visual Basic Learning, Professional, and Enterprise Editions for Windows, versions 5.0, 6.0
• Microsoft Visual Basic Standard, Professional, and Enterprise Editions, 32-bit only, for Windows, version 4.0

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

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.



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: kbVBp400 kbVBp500 kbVBp600 kbVBp kbdsd kbAPI kbDSupport

Keywords : kbGrpVB
Version :
Platform : NT WINDOWS
Issue type : kbhowto