如何从C++DLL导出全局变量

How to export a global variable from a C++ DLL?

本文关键字：全局变量 C++DLL      更新时间：2023-10-16

问题是我试图从C#程序访问C++DLL中声明的全局变量。尽管我在一个重复调用的函数中将变量设置为15（本例中为Subtract），但每次调用getter（本例为Divide）时，返回值都是零。这是我的代码。头文件：

#ifdef MATHFUNCSDLL_EXPORTS
#define MATHFUNCSDLL_API __declspec(dllexport) 
#else
#define MATHFUNCSDLL_API __declspec(dllimport) 
#endif
namespace MathFuncs
{
    class MyMathFuncs
    {
    public: 
        static MATHFUNCSDLL_API double Add(double a, double b); 
        static MATHFUNCSDLL_API double Subtract(double a, double b); 
        static MATHFUNCSDLL_API double Multiply(double a, double b); 
        static MATHFUNCSDLL_API double Divide(double a, double b); 
    };
}

C++代码：

__declspec(dllexport) double signID; //this is my variable
__declspec(dllexport) double __cdecl MyMathFuncs::Subtract(double a, double b){
   //.. some code
   signID = 15; //this function is the setter
}
__declspec(dllexport) double __cdecl MyMathFuncs::Divide(double a, double b)
{
    return signID; //this function is the getter, it return zero when called from C#
}

在我的C#代码中，我使用了下面的方法：http://www.quantcode.com/modules/smartfaq/faq.php?faqid=95我一直从getter函数得到一个零返回值，为什么，以及如何解决这个问题？

编辑：C#代码：

using UnityEngine;
using System.Collections;
using System;
using System.Runtime.InteropServices;

static class NativeMethods
{
    [DllImport("kernel32.dll")]
    public static extern IntPtr LoadLibrary(string dllToLoad);
    [DllImport("kernel32.dll")]
    public static extern IntPtr GetProcAddress(IntPtr hModule, string procedureName);
    [DllImport("kernel32.dll")]
    public static extern bool FreeLibrary(IntPtr hModule);
}
public class SignDetector : MonoBehaviour
{
    //Lets make our calls from the Plugin
    [DllImport("CVDetectorDLL", CallingConvention = CallingConvention.Cdecl, EntryPoint = @"?Add@MyMathFuncs@MathFuncs@@SANNN@Z")]
    private static extern double add(double a, double b);
    [DllImport("CVDetectorDLL", CallingConvention = CallingConvention.Cdecl, EntryPoint = @"?Subtract@MyMathFuncs@MathFuncs@@SANNN@Z")]
    private static extern double subtract(double a, double b);
    [DllImport("CVDetectorDLL", CallingConvention = CallingConvention.Cdecl, EntryPoint = @"?Multiply@MyMathFuncs@MathFuncs@@SANNN@Z")]
    private static extern double multiply(double a, double b);
    [DllImport("CVDetectorDLL", CallingConvention = CallingConvention.Cdecl, EntryPoint = @"?Divide@MyMathFuncs@MathFuncs@@SANNN@Z")]
    private static extern double divide(double a, double b);
    double myvariable;
    void Start(){
        subtract(0, 0); //here we invoke the setter
        IntPtr mydll = NativeMethods.LoadLibrary("CVDetectorDLL.dll");
        //get a pointer to unmanaged heap address
        IntPtr addrUnmanagedHeap = NativeMethods.GetProcAddress(mydll, "signID");
        if (addrUnmanagedHeap != IntPtr.Zero)
        {
            //convert and read memory from unmanaged pointer 
            myvariable = Marshal.ReadInt32(addrUnmanagedHeap);
        }
    }
    void Update(){
        Debug.Log("Found = " + myvariable); //prints zero
        Debug.Log("Found = " + divide(0,0));  //getter
    }
}

我猜（在没有看到您的c#代码的情况下）问题出在您的调用约定中。__cdecl是c++的默认值，如MSDN __cdecl所述

这意味着调用函数负责堆栈。在这种情况下，你的.net程序。c#默认为__stdcall的调用约定。这对于接受或返回参数的方法非常重要，因为它决定了堆栈的行为。

我已经包含了一些代码，这些代码应该会向您展示实现这一功能的方法。

c++代码头

#define DLLEXPORT __declspec(dllexport)
#ifdef __cplusplus
extern "C" { //Used to prevent name mangling on dll export
#endif //__cplusplus
double signID;
DLLEXPORT void __stdcall SetDoubleValue();
DLLEXPORT double __stdcall ReturnDoubleValue(double dummyone, double dummytwo);
#ifdef __cplusplus
}
#endif //__cplusplus

c++

void __stdcall SetDoubleValue()
{
    signID = 15;
}
double __stdcall ReturnDoubleValue(double dummyone, double dummytwo)
{
    return signID;
}

c#导入代码

[DllImport("DllExport.dll")]
public static extern void SetDoubleValue();
[DllImport("DllExport.dll")]
public static extern double ReturnDoubleValue(double dummyone, double dummytwo);

注意，如果你不能或不想从__cdecl更改，你也可以将c#定义更改为

[DllImport("DllExport.dll", CallingConvention = CallingConvention.Cdecl)]
public static extern void SetDoubleValue();
[DllImport("DllExport.dll", CallingConvention = CallingConvention.Cdecl)]
public static extern double ReturnDoubleValue(double dummyone, double dummytwo);

看起来我的原始代码和发布的解决方案都能正常工作——我似乎错误地识别了DLL中没有的问题。

相关文章：