DLL导出全局变量在多个DLL中调用

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本文讲解了从一个DLL中导出的全局变量，在程序的多个不同DLL中使用时，该变量的行表现。

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Something、UserA、UserB是三个DLL，UserC是一个EXE。

Something在UserA、UserB中被调用，UserC调用了UserA和UserB。

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Something.h

 #pragma once

 #ifdef SOMETHING_EXPORTS
 #define SOMETHING_API __declspec(dllexport)
 #else
 #define SOMETHING_API __declspec(dllimport)
 #endif

 extern SOMETHING_API int nGlobalVarInDLL;

Something.cpp

 #include "stdafx.h"
 #include "Something.h"

 SOMETHING_API int nGlobalVarInDLL = ;

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UserA.h

 #pragma once

 #ifdef USERA_EXPORTS
 #define USERA_API __declspec(dllexport)
 #else
 #define USERA_API __declspec(dllimport)
 #endif

 class USERA_API CUserA
 {
 public:
     void DoSet();
     void DoGet();
 };

UserA.cpp

 #include "stdafx.h"
 #include "UserA.h"

 #include <iostream>

 #include "Something.h"
 #pragma comment(lib,"Something.lib")

 void CUserA::DoSet()
 {
     std::cout <<"In UserA,nGlobalVarInDLL=    "<< nGlobalVarInDLL <<"        addr=    0x"<<(long)&nGlobalVarInDLL << std::endl;
     nGlobalVarInDLL = ;
     std::cout << "In UserA,We set nGlobalVarInDLL=    " << nGlobalVarInDLL <<std::endl;
 }

 void CUserA::DoGet()
 {
     std::cout << "In UserA,nGlobalVarInDLL=    " << nGlobalVarInDLL << "    addr=    0x" <<(long)&nGlobalVarInDLL << std::endl;
 }

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UserB.h

 #pragma once

 #ifdef USERB_EXPORTS
 #define USERB_API __declspec(dllexport)
 #else
 #define USERB_API __declspec(dllimport)
 #endif

 class USERB_API CUserB
 {
 public:
     void DoSet();
     void DoGet();
 };

UserB.cpp

 #include "stdafx.h"
 #include "UserB.h"

 #include <iostream>

 #include "Something.h"
 #pragma comment(lib,"Something.lib")

 void CUserB::DoSet()
 {
     std::cout << "In UserB,nGlobalVarInDLL=    " << nGlobalVarInDLL << "    addr=    0x" << (long)&nGlobalVarInDLL << std::endl;
     nGlobalVarInDLL = ;
     std::cout << "In UserB,We set nGlobalVarInDLL=    " << nGlobalVarInDLL << std::endl;
 }

 void CUserB::DoGet()
 {
     std::cout << "In UserB,nGlobalVarInDLL=    " << nGlobalVarInDLL << "    addr=    0x" << (long)&nGlobalVarInDLL << std::endl;
 }

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UserC.cpp

 #include "stdafx.h"

 #include "UserA.h"
 #pragma comment(lib,"UserA.lib")

 #include "UserB.h"
 #pragma comment(lib,"UserB.lib")

 int main()
 {
     CUserA oUserA;
     CUserB oUserB;

     oUserA.DoSet();
     oUserB.DoGet();

     oUserB.DoSet();
     oUserA.DoGet();

     return ;
 }

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调用结果

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结论就是：使用的是同一个地址空间，加载的是同一个变量。

（这篇就是给那些不想学操作系统知识的人看的。）