I need some help on calling DLL functions that I've written in Visual C++
5.0, from Visual Basic.  It seems some weird things are happening with the
stack on calling the DLL function.  I am able to call my DLL from a 'C'
program, but it doesn't work from Visual Basic.  I've debugged to the point
where I get entry to the DLL function (I display message boxes from the
DLL), but the passed data is corrupted.  Also, on return from the DLL, I get
a "Bad DLL calling convention" message from VB.

Here is my DLL code:

#include <windows.h>
#include <stdio.h>

__declspec (dllexport) short xftest (char *p);

BOOL WINAPI DllMain (HINSTANCE hinstDll, DWORD fdwReason, LPVOID fImpLoad )
{
    return (TRUE);

Public strbuf As String * 2000
Public Declare Function xftest Lib "SWC40FT" (ByRef x As String) As Integer

Private Sub Command1_Click()
    Dim x As Integer

    strbuf = "Before"
    x = xftest (strbuf)
End Sub

Is there a compatibility problem with the use of __declspec (new DLL
function declaration method) and VB5?  Is there a problem of calling the DLL
function from a Private event handler?  Are my VB5 declarations incorrect?

Any insight into my problem would be greatly appreciated.

Regards,

Steven Szabo

Division Softworks Inc.
601 W 16th Ave. Vancouver BC V5Z 1S5
(604) 872-3677

#include <windows.h>
#include <stdio.h>

extern "C"
void WINAPI
__declspec (dllexport) short xftest (char *p);

BOOL WINAPI DllMain (HINSTANCE hinstDll, DWORD fdwReason, LPVOID fImpLoad )
{
    return (TRUE);

Public strbuf As String * 2000
' error! Public Declare Function xftest Lib "SWC40FT" (ByRef x As String) As
Integer
Public Declare Function xftest Lib "SWC40FT" (ByVal x As String) As Integer

Private Sub Command1_Click()
    Dim x As Integer

    strbuf = "Before"
    x = xftest (strbuf)
End Sub

Andrew Kotsar'
IT specialist
Moscow
Sodexho Russia

First, the C function should be made _stdcall.  This can happen in a couple
of ways, but the simplest is just to include the _stdcall keyword in the
function definition / prototype.

Second, the VB side should use ByVal xx As String, not ByRef.  If you use
ByRef, your C DLL will appear to get char **p, not char *p, since a ByVal
string in VB is already at heart a pointer (it's a BSTR, which loosely
translates to LPSTR).

--
     Jim Mack
     MicroDexterity, Inc

     http://www.microdexterity.com

---(H file)
#ifdef __cplusplus
extern "C" {
#endif

DWORD __stdcall myFunctionName( float *pfValue )

#ifdef __cplusplus

---(C++ file)
DWORD __stdcall myFunctionName( float *pfValue )
{
   // Some code here...
   return 0;

It is exported by having an entry in the .def file:

---
EXPORTS
   myFunctionName
---

And declared in VB like:

Declare Function myFunctionName Lib "DLLNAME" (fValue As Single) As Long

I haven't tried it in VB5, but this at least works well in VB4.

Hope this helps!

-+-Ben-+-