从Visual Basic 6调用C DLL：双重数据类型不工作

Question

我将一个简单的用户定义类型（UDT）从Visual Basic 6传递给一个C DLL。能正常工作，除了双数据类型，它表现为0。

C DLL：

#define WIN32_LEAN_AND_MEAN 

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

typedef struct _UserDefinedType 
{ 
    signed int  Integer; 
    unsigned char Byte; 
    float   Float; 
    double   Double; 
} UserDefinedType; 

int __stdcall Initialize (void); 
int __stdcall SetUDT (UserDefinedType * UDT); 

BOOL WINAPI DllMain (HINSTANCE Instance, DWORD Reason, LPVOID Reserved) 
{ 
    return TRUE; 
} 

int __stdcall Initialize (void) 
{ 
    return 1; 
} 

int __stdcall SetUDT (UserDefinedType * UDT) 
{ 
    UDT->Byte = 255; 
    UDT->Double = 25; 
    UDT->Float = 12345.12; 
    UDT->Integer = 1; 

    return 1; 
} 

的Visual Basic 6代码：

Option Explicit 

Private Type UserDefinedType 
    lonInteger As Long 
    bytByte As Byte 
    sinFloat As Single 
    dblDouble As Double 
End Type 

Private Declare Function Initialize Lib "C:\VBCDLL.dll"() As Long 
Private Declare Function SetUDT Lib "C:\VBCDLL.dll" (ByRef UDT As UserDefinedType) As Long 

Private Sub Form_Load() 

    Dim lonReturn As Long, UDT As UserDefinedType 

    lonReturn = SetUDT(UDT) 

    Debug.Print "VBCDLL.SetUDT() = " & CStr(lonReturn) 

    With UDT 
     Debug.Print , "Integer:", CStr(.lonInteger) 
     Debug.Print , "Byte:", CStr(.bytByte) 
     Debug.Print , "Float:", CStr(.sinFloat) 
     Debug.Print , "Double:", CStr(.dblDouble) 
    End With 

End Sub 

Visual Basic的输出：

VBCDLL.SetUDT() = 1 
       Integer:  1 
       Byte:   255 
       Float:  12345.12 
       Double:  0 

正如你可以看到，双被显示为，当它应该是。

Answer 1

VB6的UDT的那个是4的倍数。下面介绍如何用在C抗衡地址对齐双打：

#pragma pack(push,4) 
typedef struct _UserDefinedType 
{ 
    signed int  Integer; 
    unsigned char Byte; 
    float   Float; 
    double   Double; 
} UserDefinedType; 
#pragma pack(pop)