C++ 从DLL导入函数时出现错误LNK2019

我正在学习如何构建一个DLL并从另一个项目中调用它（我还希望DLL不仅可以被C/C++调用，还可以被Python调用）。以下是我构建DLL的代码：

callbacktesetDLL.h:

#ifdef CALLBACKTESTDLL_EXPORTS
#define CALLBACKTESTDLL_API __declspec(dllexport)    
#else
#define CALLBACKTESTDLL_API __declspec(dllimport)
#endif

typedef int(CALLBACK *p)(char*);   

CALLBACKTESTDLL_API int __stdcall StrToInt(char*);      

CALLBACKTESTDLL_API char* __stdcall NumCompare(p FuncP, char*, int b);      

#include "stdafx.h"
#include <stdio.h>
#include "callbacktestDLL.h"
#include <stdlib.h>

CALLBACKTESTDLL_API int __stdcall StrToInt(char* StrInput)
{
    int IntResult;
    IntResult = atoi(StrInput);
    return IntResult;
}


CALLBACKTESTDLL_API char* __stdcall NumCompare(p FuncP, char* StrInput, int b)          
{
    int a = FuncP(StrInput);
    if (a>b)
    {
        return "a is bigger than b";
    }
    else
    {
        return "b is bigger than a";
    }
}

LIBRARY

EXPORTS
StrToInt @1
NumCompare @2

#pragma comment(lib,"callbacktestDLL.lib")

#include "targetver.h"

#include <stdio.h>
#include <tchar.h>

typedef int(*p)(char*);

extern "C" _declspec(dllimport) int StrToInt(char* InpuString);
extern "C" _declspec(dllimport) char* NumCompare(p FuncP, char*, int b);

#include "stdafx.h"

int main()
{
    p FuncP_R = StrToInt;
    NumCompare(FuncP_R, "1234", 40);
    return 0;
}

typedef int(CALLBACK *p)(char*);   

extern "C" __declspec(dllexport) int __stdcall StrToInt(char* InputString);     

extern "C" __declspec(dllexport) char* __stdcall NumCompare(p FuncP, char* InputString, int b); 

#include "stdafx.h"
#include <stdio.h>
#include "callbacktestDLL.h"
#include <stdlib.h>

extern "C" CALLBACKTESTDLL_API int __stdcall StrToInt(char* InputString)        
{
    int IntResult;
    IntResult = atoi(InputString);
    return IntResult;
}

extern "C" __declspec(dllexport) char* __stdcall NumCompare(p FuncP, char* InputString, int b)  
{
    int a = FuncP(InputString);
    if (a>b)
    {
        return "a is bigger than b\n";
    }
    else
    {
        return "b is bigger than a\n";
    }
}

#pragma comment(lib,"callbacktestDLL")

#include "targetver.h"

#include <stdio.h>
#include <tchar.h>

typedef int(__stdcall *p)(char*); //same as typedef int(CALLBACK *p)(char*) in callbacktestDLL.h

extern "C" _declspec(dllimport) int __stdcall StrToInt(char* InputString); //exactly same as what's in callbacktestDLL.h apart from dllimport  
extern "C" _declspec(dllimport) char* __stdcall NumCompare(p FuncP, char* InputString, int b);  //exactly same as what's in callbacktestDLL.h apart from dllimport

#include "stdafx.h"
    int main()
    {
        p FuncP_R;
        char* a = "1234";
        FuncP_R = StrToInt;
        printf(NumCompare(FuncP_R, a, 42));
        return 0;
    }

callbacktestDLL.cpp:

#ifdef CALLBACKTESTDLL_EXPORTS
#define CALLBACKTESTDLL_API __declspec(dllexport)    
#else
#define CALLBACKTESTDLL_API __declspec(dllimport)
#endif

typedef int(CALLBACK *p)(char*);   

CALLBACKTESTDLL_API int __stdcall StrToInt(char*);      

CALLBACKTESTDLL_API char* __stdcall NumCompare(p FuncP, char*, int b);      

#include "stdafx.h"
#include <stdio.h>
#include "callbacktestDLL.h"
#include <stdlib.h>

CALLBACKTESTDLL_API int __stdcall StrToInt(char* StrInput)
{
    int IntResult;
    IntResult = atoi(StrInput);
    return IntResult;
}


CALLBACKTESTDLL_API char* __stdcall NumCompare(p FuncP, char* StrInput, int b)          
{
    int a = FuncP(StrInput);
    if (a>b)
    {
        return "a is bigger than b";
    }
    else
    {
        return "b is bigger than a";
    }
}

LIBRARY

EXPORTS
StrToInt @1
NumCompare @2

#pragma comment(lib,"callbacktestDLL.lib")

#include "targetver.h"

#include <stdio.h>
#include <tchar.h>

typedef int(*p)(char*);

extern "C" _declspec(dllimport) int StrToInt(char* InpuString);
extern "C" _declspec(dllimport) char* NumCompare(p FuncP, char*, int b);

#include "stdafx.h"

int main()
{
    p FuncP_R = StrToInt;
    NumCompare(FuncP_R, "1234", 40);
    return 0;
}

typedef int(CALLBACK *p)(char*);   

extern "C" __declspec(dllexport) int __stdcall StrToInt(char* InputString);     

extern "C" __declspec(dllexport) char* __stdcall NumCompare(p FuncP, char* InputString, int b); 

#include "stdafx.h"
#include <stdio.h>
#include "callbacktestDLL.h"
#include <stdlib.h>

extern "C" CALLBACKTESTDLL_API int __stdcall StrToInt(char* InputString)        
{
    int IntResult;
    IntResult = atoi(InputString);
    return IntResult;
}

extern "C" __declspec(dllexport) char* __stdcall NumCompare(p FuncP, char* InputString, int b)  
{
    int a = FuncP(InputString);
    if (a>b)
    {
        return "a is bigger than b\n";
    }
    else
    {
        return "b is bigger than a\n";
    }
}

#pragma comment(lib,"callbacktestDLL")

#include "targetver.h"

#include <stdio.h>
#include <tchar.h>

typedef int(__stdcall *p)(char*); //same as typedef int(CALLBACK *p)(char*) in callbacktestDLL.h

extern "C" _declspec(dllimport) int __stdcall StrToInt(char* InputString); //exactly same as what's in callbacktestDLL.h apart from dllimport  
extern "C" _declspec(dllimport) char* __stdcall NumCompare(p FuncP, char* InputString, int b);  //exactly same as what's in callbacktestDLL.h apart from dllimport

#include "stdafx.h"
    int main()
    {
        p FuncP_R;
        char* a = "1234";
        FuncP_R = StrToInt;
        printf(NumCompare(FuncP_R, a, 42));
        return 0;
    }

通过上面的代码，我得到了

callbacktestDLL.dll

和

callbacktestDLL.lib

。使用dependens，可以显示DLL中函数的名称：

现在我想从另一个项目调用DLL中的函数：

CallDLL.h:

#ifdef CALLBACKTESTDLL_EXPORTS
#define CALLBACKTESTDLL_API __declspec(dllexport)    
#else
#define CALLBACKTESTDLL_API __declspec(dllimport)
#endif

typedef int(CALLBACK *p)(char*);   

CALLBACKTESTDLL_API int __stdcall StrToInt(char*);      

CALLBACKTESTDLL_API char* __stdcall NumCompare(p FuncP, char*, int b);      

#include "stdafx.h"
#include <stdio.h>
#include "callbacktestDLL.h"
#include <stdlib.h>

CALLBACKTESTDLL_API int __stdcall StrToInt(char* StrInput)
{
    int IntResult;
    IntResult = atoi(StrInput);
    return IntResult;
}


CALLBACKTESTDLL_API char* __stdcall NumCompare(p FuncP, char* StrInput, int b)          
{
    int a = FuncP(StrInput);
    if (a>b)
    {
        return "a is bigger than b";
    }
    else
    {
        return "b is bigger than a";
    }
}

LIBRARY

EXPORTS
StrToInt @1
NumCompare @2

#pragma comment(lib,"callbacktestDLL.lib")

#include "targetver.h"

#include <stdio.h>
#include <tchar.h>

typedef int(*p)(char*);

extern "C" _declspec(dllimport) int StrToInt(char* InpuString);
extern "C" _declspec(dllimport) char* NumCompare(p FuncP, char*, int b);

#include "stdafx.h"

int main()
{
    p FuncP_R = StrToInt;
    NumCompare(FuncP_R, "1234", 40);
    return 0;
}

typedef int(CALLBACK *p)(char*);   

extern "C" __declspec(dllexport) int __stdcall StrToInt(char* InputString);     

extern "C" __declspec(dllexport) char* __stdcall NumCompare(p FuncP, char* InputString, int b); 

#include "stdafx.h"
#include <stdio.h>
#include "callbacktestDLL.h"
#include <stdlib.h>

extern "C" CALLBACKTESTDLL_API int __stdcall StrToInt(char* InputString)        
{
    int IntResult;
    IntResult = atoi(InputString);
    return IntResult;
}

extern "C" __declspec(dllexport) char* __stdcall NumCompare(p FuncP, char* InputString, int b)  
{
    int a = FuncP(InputString);
    if (a>b)
    {
        return "a is bigger than b\n";
    }
    else
    {
        return "b is bigger than a\n";
    }
}

#pragma comment(lib,"callbacktestDLL")

#include "targetver.h"

#include <stdio.h>
#include <tchar.h>

typedef int(__stdcall *p)(char*); //same as typedef int(CALLBACK *p)(char*) in callbacktestDLL.h

extern "C" _declspec(dllimport) int __stdcall StrToInt(char* InputString); //exactly same as what's in callbacktestDLL.h apart from dllimport  
extern "C" _declspec(dllimport) char* __stdcall NumCompare(p FuncP, char* InputString, int b);  //exactly same as what's in callbacktestDLL.h apart from dllimport

#include "stdafx.h"
    int main()
    {
        p FuncP_R;
        char* a = "1234";
        FuncP_R = StrToInt;
        printf(NumCompare(FuncP_R, a, 42));
        return 0;
    }

---

然而，当我运行该项目时，它告诉我：

错误LNK2019：未解析的外部符号\uuuu imp\uu strott

和

错误LNK2019：未解析的外部符号\uuu imp\uu NumCompare

。我已经在CallDLL项目的根文件夹下复制了

.lib

和

.dll

文件。为什么会发生这种情况？我怎样才能解决它？谢谢你的关注。

我终于来了。详情如下: 生成DLL的文件包括：

callbacktestDLL.h:

#ifdef CALLBACKTESTDLL_EXPORTS
#define CALLBACKTESTDLL_API __declspec(dllexport)    
#else
#define CALLBACKTESTDLL_API __declspec(dllimport)
#endif

typedef int(CALLBACK *p)(char*);   

CALLBACKTESTDLL_API int __stdcall StrToInt(char*);      

CALLBACKTESTDLL_API char* __stdcall NumCompare(p FuncP, char*, int b);      

#include "stdafx.h"
#include <stdio.h>
#include "callbacktestDLL.h"
#include <stdlib.h>

CALLBACKTESTDLL_API int __stdcall StrToInt(char* StrInput)
{
    int IntResult;
    IntResult = atoi(StrInput);
    return IntResult;
}


CALLBACKTESTDLL_API char* __stdcall NumCompare(p FuncP, char* StrInput, int b)          
{
    int a = FuncP(StrInput);
    if (a>b)
    {
        return "a is bigger than b";
    }
    else
    {
        return "b is bigger than a";
    }
}

LIBRARY

EXPORTS
StrToInt @1
NumCompare @2

#pragma comment(lib,"callbacktestDLL.lib")

#include "targetver.h"

#include <stdio.h>
#include <tchar.h>

typedef int(*p)(char*);

extern "C" _declspec(dllimport) int StrToInt(char* InpuString);
extern "C" _declspec(dllimport) char* NumCompare(p FuncP, char*, int b);

#include "stdafx.h"

int main()
{
    p FuncP_R = StrToInt;
    NumCompare(FuncP_R, "1234", 40);
    return 0;
}

typedef int(CALLBACK *p)(char*);   

extern "C" __declspec(dllexport) int __stdcall StrToInt(char* InputString);     

extern "C" __declspec(dllexport) char* __stdcall NumCompare(p FuncP, char* InputString, int b); 

#include "stdafx.h"
#include <stdio.h>
#include "callbacktestDLL.h"
#include <stdlib.h>

extern "C" CALLBACKTESTDLL_API int __stdcall StrToInt(char* InputString)        
{
    int IntResult;
    IntResult = atoi(InputString);
    return IntResult;
}

extern "C" __declspec(dllexport) char* __stdcall NumCompare(p FuncP, char* InputString, int b)  
{
    int a = FuncP(InputString);
    if (a>b)
    {
        return "a is bigger than b\n";
    }
    else
    {
        return "b is bigger than a\n";
    }
}

#pragma comment(lib,"callbacktestDLL")

#include "targetver.h"

#include <stdio.h>
#include <tchar.h>

typedef int(__stdcall *p)(char*); //same as typedef int(CALLBACK *p)(char*) in callbacktestDLL.h

extern "C" _declspec(dllimport) int __stdcall StrToInt(char* InputString); //exactly same as what's in callbacktestDLL.h apart from dllimport  
extern "C" _declspec(dllimport) char* __stdcall NumCompare(p FuncP, char* InputString, int b);  //exactly same as what's in callbacktestDLL.h apart from dllimport

#include "stdafx.h"
    int main()
    {
        p FuncP_R;
        char* a = "1234";
        FuncP_R = StrToInt;
        printf(NumCompare(FuncP_R, a, 42));
        return 0;
    }

callbacktestDLL.cpp:

#ifdef CALLBACKTESTDLL_EXPORTS
#define CALLBACKTESTDLL_API __declspec(dllexport)    
#else
#define CALLBACKTESTDLL_API __declspec(dllimport)
#endif

typedef int(CALLBACK *p)(char*);   

CALLBACKTESTDLL_API int __stdcall StrToInt(char*);      

CALLBACKTESTDLL_API char* __stdcall NumCompare(p FuncP, char*, int b);      

#include "stdafx.h"
#include <stdio.h>
#include "callbacktestDLL.h"
#include <stdlib.h>

CALLBACKTESTDLL_API int __stdcall StrToInt(char* StrInput)
{
    int IntResult;
    IntResult = atoi(StrInput);
    return IntResult;
}


CALLBACKTESTDLL_API char* __stdcall NumCompare(p FuncP, char* StrInput, int b)          
{
    int a = FuncP(StrInput);
    if (a>b)
    {
        return "a is bigger than b";
    }
    else
    {
        return "b is bigger than a";
    }
}

LIBRARY

EXPORTS
StrToInt @1
NumCompare @2

#pragma comment(lib,"callbacktestDLL.lib")

#include "targetver.h"

#include <stdio.h>
#include <tchar.h>

typedef int(*p)(char*);

extern "C" _declspec(dllimport) int StrToInt(char* InpuString);
extern "C" _declspec(dllimport) char* NumCompare(p FuncP, char*, int b);

#include "stdafx.h"

int main()
{
    p FuncP_R = StrToInt;
    NumCompare(FuncP_R, "1234", 40);
    return 0;
}

typedef int(CALLBACK *p)(char*);   

extern "C" __declspec(dllexport) int __stdcall StrToInt(char* InputString);     

extern "C" __declspec(dllexport) char* __stdcall NumCompare(p FuncP, char* InputString, int b); 

#include "stdafx.h"
#include <stdio.h>
#include "callbacktestDLL.h"
#include <stdlib.h>

extern "C" CALLBACKTESTDLL_API int __stdcall StrToInt(char* InputString)        
{
    int IntResult;
    IntResult = atoi(InputString);
    return IntResult;
}

extern "C" __declspec(dllexport) char* __stdcall NumCompare(p FuncP, char* InputString, int b)  
{
    int a = FuncP(InputString);
    if (a>b)
    {
        return "a is bigger than b\n";
    }
    else
    {
        return "b is bigger than a\n";
    }
}

#pragma comment(lib,"callbacktestDLL")

#include "targetver.h"

#include <stdio.h>
#include <tchar.h>

typedef int(__stdcall *p)(char*); //same as typedef int(CALLBACK *p)(char*) in callbacktestDLL.h

extern "C" _declspec(dllimport) int __stdcall StrToInt(char* InputString); //exactly same as what's in callbacktestDLL.h apart from dllimport  
extern "C" _declspec(dllimport) char* __stdcall NumCompare(p FuncP, char* InputString, int b);  //exactly same as what's in callbacktestDLL.h apart from dllimport

#include "stdafx.h"
    int main()
    {
        p FuncP_R;
        char* a = "1234";
        FuncP_R = StrToInt;
        printf(NumCompare(FuncP_R, a, 42));
        return 0;
    }

CallDLL.cpp:

#ifdef CALLBACKTESTDLL_EXPORTS
#define CALLBACKTESTDLL_API __declspec(dllexport)    
#else
#define CALLBACKTESTDLL_API __declspec(dllimport)
#endif

typedef int(CALLBACK *p)(char*);   

CALLBACKTESTDLL_API int __stdcall StrToInt(char*);      

CALLBACKTESTDLL_API char* __stdcall NumCompare(p FuncP, char*, int b);      

#include "stdafx.h"
#include <stdio.h>
#include "callbacktestDLL.h"
#include <stdlib.h>

CALLBACKTESTDLL_API int __stdcall StrToInt(char* StrInput)
{
    int IntResult;
    IntResult = atoi(StrInput);
    return IntResult;
}


CALLBACKTESTDLL_API char* __stdcall NumCompare(p FuncP, char* StrInput, int b)          
{
    int a = FuncP(StrInput);
    if (a>b)
    {
        return "a is bigger than b";
    }
    else
    {
        return "b is bigger than a";
    }
}

LIBRARY

EXPORTS
StrToInt @1
NumCompare @2

#pragma comment(lib,"callbacktestDLL.lib")

#include "targetver.h"

#include <stdio.h>
#include <tchar.h>

typedef int(*p)(char*);

extern "C" _declspec(dllimport) int StrToInt(char* InpuString);
extern "C" _declspec(dllimport) char* NumCompare(p FuncP, char*, int b);

#include "stdafx.h"

int main()
{
    p FuncP_R = StrToInt;
    NumCompare(FuncP_R, "1234", 40);
    return 0;
}

typedef int(CALLBACK *p)(char*);   

extern "C" __declspec(dllexport) int __stdcall StrToInt(char* InputString);     

extern "C" __declspec(dllexport) char* __stdcall NumCompare(p FuncP, char* InputString, int b); 

#include "stdafx.h"
#include <stdio.h>
#include "callbacktestDLL.h"
#include <stdlib.h>

extern "C" CALLBACKTESTDLL_API int __stdcall StrToInt(char* InputString)        
{
    int IntResult;
    IntResult = atoi(InputString);
    return IntResult;
}

extern "C" __declspec(dllexport) char* __stdcall NumCompare(p FuncP, char* InputString, int b)  
{
    int a = FuncP(InputString);
    if (a>b)
    {
        return "a is bigger than b\n";
    }
    else
    {
        return "b is bigger than a\n";
    }
}

#pragma comment(lib,"callbacktestDLL")

#include "targetver.h"

#include <stdio.h>
#include <tchar.h>

typedef int(__stdcall *p)(char*); //same as typedef int(CALLBACK *p)(char*) in callbacktestDLL.h

extern "C" _declspec(dllimport) int __stdcall StrToInt(char* InputString); //exactly same as what's in callbacktestDLL.h apart from dllimport  
extern "C" _declspec(dllimport) char* __stdcall NumCompare(p FuncP, char* InputString, int b);  //exactly same as what's in callbacktestDLL.h apart from dllimport

#include "stdafx.h"
    int main()
    {
        p FuncP_R;
        char* a = "1234";
        FuncP_R = StrToInt;
        printf(NumCompare(FuncP_R, a, 42));
        return 0;
    }

---

它像预期的那样工作。我认为我犯的一个错误是导入时缺少

\uu stdcall

。也许这个名字也有错误。我将继续测试它。

您应该始终包含相同的头文件以定义一致的接口。创建不同的头文件容易出错。对接口的下一次更改可能会使接口不兼容

实现函数的模块与使用函数的模块之间存在细微差异。区别在于

\u declspec（dllimport）

或

\u declspec（dllexport）

。这就是为什么头文件包含

# ifdef CALLBACKTESTDLL_EXPORTS

实现这些函数的模块也会导出这些函数。因此，必须在项目设置中定义符号。如果在命令行编译，则必须将

/D CALLBACKTESTDLL\u EXPORT

添加到编译器参数中

---

顺便说一句：如果您定义了

CALLBACKTESTDLL\u EXPORT

宏

CALLBACKTESTDLL\u API

包含

\u declspec（dllexport）

。这将导致链接器创建DLL导出表。

.DEF

不是必需的。您应该删除它，因为。。。。。它再次定义了接口（在不同的级别）。

是否与库链接？并且DLL应该与可执行文件位于同一目录中。您是否为DLL定义了

CALLBACKTESTDLL\u EXPORTS

？（顺便说一句，您只需要在函数声明之前设置导出声明。）

extern“C”

防止名称损坏，而库仍然导出损坏的名称。该.def文件没有用处，因为它没有列出正确（损坏）的名称。在客户端代码中不使用.h文件是一个非常糟糕的主意。重新键入函数名时，您忘记了u stdcall属性。这将更改修饰的函数名。不要这样做，请使用

#在客户端代码中也包括“callbacktestDLL.h”

。不太明显这是怎么回事，如果有必要，请仔细检查您没有在客户端项目中定义CALLBACKTESTDLL导出。该.h文件不是秘密，需要正确使用DLL。用于构建DLL的其他文件（如.cpp文件）与客户端程序员无关。因此，在极不可能的情况下，有人这样做，一定要扔掉DLL。如果你不这样做，那么你就有可能侵犯版权，这可能是相当昂贵的。这种方法可能会给你一个DLL，你可以使用。但它也有一些退步。有关详细信息，请参阅我的答案。谢谢您告诉我这些详细信息。您能告诉我什么时候使用

.DEF

？另一个问题，在我自己的回答中，虽然

CallDLL.h

中的函数名称与

中的函数名称不同，但为什么我仍然可以导入它们？