Calling C++ DLL from VB6 passes garbage data for arguments?

VC functions all use _stdcall
functions exported using .def file 
     (e.g.  AliasFuncName = _FuncName@NumCallArgBytes)
Existing C DLL was revised to have some new call arguments 
  (revised function names, built to new dll name)

Functions with unrevised call arguments work when calling the new DLL
Functions with revised call arguments do not work when calling the new DLL
   (all call arguments are garbage on entry)
Call arguments are several input doubles and a few return double*

Prototype.h call definition matches c source code definition
Visual Basic declarations to new DLL match in style those to the old DLL
   (several ByVal double input args and a few ByRef return args)

Arguments look good in VB Debugger before calling VC debugger where they are garbage (e.g. 1.34867e-308, 3.49732e-88, etc.).

I would appreciate any thoughts on possible causes. I have been struggling with this for a few days. By the way, I don't choose to work in legacy code!

Below are the C header prototype, the .DEF definition and the VB declaration.

Header file definition:

LONG _stdcall SYSDll_FRoulSlideXa(
    double ATest, double Hc, double Hivr,
    double Eeq, double Rx, double Rk,
    double L, double U, double SlRol,
    double R, double Wlc, double Wpc,
    double Mu, double MuOil, double Cor2AL,
    double Fs, double Ft,
    double *FRoul, double *FSlid);

.DEF file definition:

LIBRARY "SYSx32d10a"

DESCRIPTION 'SYSx Dlls'

EXPORTS
    SYSDll_FRoulSlideXa = _SYSDll_FRoulSlideXa@144

VB6 declaration:

Declare Function SYSDll_FRoulSlideXa Lib "SYSX32D10A.DLL" ( _
    ByVal ATest As Double, ByVal Hc As Double, ByVal Hivr As Double, _
    ByVal Eeq As Double, ByVal rx As Double, ByVal Rk As Double, _
    ByVal L As Double, ByVal U As Double, ByVal SlRol As Double, _
    ByVal r As Double, ByVal Wlc As Double, ByVal Wpc As Double, _
    ByVal Mu As Double, ByVal MuOil As Double, ByVal Cor2AL As Double, _
    ByVal Fs As Double, ByVal Ft As Double, _
    FRoul As Double, FSlid As Double)

Note: I have already tried explicit ByRef on the last two arguments instead of relying on default passing convention being ByRef.

Solution

You VB Declare doesn't include a return type for the function. Unless there's a DEFxxx statement that you don't show, that means VB expects a Variant. Because Variant functions return their value using a hidden parameter, the stack will be misaligned. That alone can cause what you're seeing.

The solution is to add the correct return type to the VB Declare.