動態加載(explicit linking) DLL的注意事項

─ 問題描述：

一開始測試載dll的時候，一直都載不進程式中，用depends22_86這個軟體檢查dll 檔，確認是否有我定義的函式進入點，檢查結果是有的。

DLL的程式碼如下：

VC14Dll.h

#pragma once

#include "opencv2\opencv.hpp"

#include <windows.h>

#ifdef VC14DLL_EXPORTS

#define VC14DLL_EXPORTS __declspec(dllexport)

#else

#define VC14DLL_EXPORTS __declspec(dllimport)

#endif //  DLLEXPORT

VC14DLL_EXPORTS int Test(cv::Mat input);

VC14DLL_EXPORTS void Test2();

VC14Dll.cpp

// VC14Dll.cpp : 定義 DLL 應用程式的匯出函式。

//

#include "stdafx.h"

#include "VC14Dll.h"

#include "opencv2\opencv.hpp"

VC14DLL_EXPORTS int Test(cv::Mat input)

{

        try {

               int threshval = 100;

               cv::Mat grayImg;

               cv::cvtColor(input, grayImg, cv::COLOR_BGR2GRAY);

               cv::Mat binaryImg = threshval < 128 ? (grayImg < threshval) : (grayImg > threshval);

               cv::Mat labelImage;

               int nLabels = cv::connectedComponents(binaryImg, labelImage, 8);

               return 5;

               //std::cout << nLabels << std::endl;

        }

        catch (cv::Exception err) {

               return -1;

        }

        catch (...) {

               return -1;

        }

        return -1;

}

VC14DLL_EXPORTS void Test2()

{

        try {

               std::cout << "yo~~~~~" << std::endl;

        }

        catch (...) {

        }

}

─ 想法：

之前都是找英文的資訊，這次直接用中文找，看到了一篇文章

[筆記]C#呼叫C++動態連結庫(DLL) + Debug

裡面有用extern "C"，當下我就想：「LoadLibrary是不是C的runtime library，所以少了這個就無法呼叫了」

─ 解決方法：

在標頭檔宣告時，加上extern "C"就可以呼叫成功了！

程式碼如下：

VC14Dll.h

#pragma once

#include "opencv2\opencv.hpp"

#include <windows.h>

#ifdef VC14DLL_EXPORTS

#define VC14DLL_EXPORTS __declspec(dllexport)

#else

#define VC14DLL_EXPORTS __declspec(dllimport)

#endif //  DLLEXPORT

extern "C" VC14DLL_EXPORTS int Test(cv::Mat input);

extern "C" VC14DLL_EXPORTS void Test2();

VC14Dll.cpp

// VC14Dll.cpp : 定義 DLL 應用程式的匯出函式。

//

#include "stdafx.h"

#include "VC14Dll.h"

#include "opencv2\opencv.hpp"

int Test(cv::Mat input)

{

        try {

               int threshval = 100;

               cv::Mat grayImg;

               cv::cvtColor(input, grayImg, cv::COLOR_BGR2GRAY);

               cv::Mat binaryImg = threshval < 128 ? (grayImg < threshval) : (grayImg > threshval);

               cv::Mat labelImage;

               int nLabels = cv::connectedComponents(binaryImg, labelImage, 8);

               return 5;

               //std::cout << nLabels << std::endl;

        }

        catch (cv::Exception err) {

               return -1;

        }

        catch (...) {

               return -1;

        }

        return -1;

}

void Test2()

{

        try {

               std::cout << "yo~~~~~" << std::endl;

        }

        catch (...) {

        }

}

─ 參考資料

[1]  【C++ 懶人用法】製作及使用 Dll @ 冰雨小記 :: 痞客邦 ::

[2][C++ 小學堂] 如何建立 export 的 dll 與如何動態呼叫 export 的 dll 

http://puremonkey2010.blogspot.com/2010/08/c-export-dll-export-dll.html

[3] C++ Dynamic Link Libraries : Creating a DLL and Implicit Linking

http://barngoggles.com/a-bunch-of-words-test-post-2/

[4]C++ Dynamic Link Libraries : Part 2 (Explicit Linking)

http://barngoggles.com/c-dynamic-link-libraries-part-2-explicit-linking/