C++_编写动态链接库

动态链接库简介

动态链接库（Dynamic Link Library 或者 Dynamic-link Library，缩写为 DLL），是微软公司在微软Windows操作系统中，实现共享函数库概念的一种方式。这些库函数的扩展名是 ”.dll”、”.ocx”（包含ActiveX控制的库）或者 “.drv”（旧式的系统驱动程序）。 
动态链接提供了一种方法，使进程可以调用不属于其可执行代码的函数。函数的可执行代码位于一个 DLL 文件中，该 DLL 包含一个或多个已被编译、链接并与使用它们的进程分开存储的函数。DLL 还有助于共享数据和资源。多个应用程序可同时访问内存中单个 DLL 副本的内容。 
通过使用DLL可以实现模块化，使之由相对独立的组件组成，可以更快的加载应用各个模块的功能，还可以更容易的将更新应用于各个模块。 
例如，一个程序含有很多个功能，将他们都编写成DLL文件，当软件更新时，只需更新相应的DLL文件而不需要重新安装整个程序

编写一个DLL

1、新建工程，使用VS2015编写一个实现两数相加的DLL 
文件->新建项目->win32项目->下一步->DLL->勾选预编译头文件，导出符号->完成

此时VS2015会自动生成3个头文件和3个cpp文件 

其中TestDll.h是DLL的头文件

<code class="hljs cs has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">#ifdef TESTDLL_EXPORTS</span><span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">#<span class="hljs-keyword" style="box-sizing: border-box;">define</span> TESTDLL_API __declspec(dllexport)</span><span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">#<span class="hljs-keyword" style="box-sizing: border-box;">else</span></span><span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">#<span class="hljs-keyword" style="box-sizing: border-box;">define</span> TESTDLL_API __declspec(dllimport)</span><span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">#<span class="hljs-keyword" style="box-sizing: border-box;">endif</span></span><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 此类是从 TestDll.dll 导出的</span>class TESTDLL_API CTestDll {<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span>:    <span class="hljs-title" style="box-sizing: border-box;">CTestDll</span>(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span>);    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// TODO:  在此添加您的方法。</span>};<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//导出的变量</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">extern</span> TESTDLL_API <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> nTestDll;<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//导出函数</span>TESTDLL_API <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> fnTestDll(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span>);</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li></ul>

stdafx.h包含需要的头文件

<code class="hljs vala has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">#pragma once</span><span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">#include "targetver.h"</span><span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">#define WIN32_LEAN_AND_MEAN             </span><span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">#include <windows.h></span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li></ul>

TestDll.cpp是源文件 
DllMain是动态链接库的入口点，在DllMain函数中，hModule参数是该DLL模块的句柄，代表这个文件的镜像加载到进程的地址空间使用的基地址，ul_reason_for_call参数的指标是本次调用的原因，包括

• DLL_PROCESS_ATTACH，表示动态链接库刚被某一个进程加载，映射到了某一个地址空间，可以在此进行初始化
• DLL_THREAD_ATTACH，动态链接库将被卸载，可以在这进行资源释放
• DLL_THREAD_DETACH，应用程序创建了一个新的进程
• DLL_PROCESS_DETACH，某个进程正常终止

<code class="hljs objectivec has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// dllmain.cpp : 定义 DLL 应用程序的入口点。</span><span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">#include "stdafx.h"</span><span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">BOOL</span> APIENTRY DllMain( HMODULE hModule,                       DWORD  ul_reason_for_call,                       LPVOID lpReserved                     ){    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">switch</span> (ul_reason_for_call)    {    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">case</span> DLL_PROCESS_ATTACH:    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">case</span> DLL_THREAD_ATTACH:    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">case</span> DLL_THREAD_DETACH:    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">case</span> DLL_PROCESS_DETACH:        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">break</span>;    }    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> <span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">TRUE</span>;}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li></ul>

2、将TestDll.h中的导出函数改为

<code class="hljs cs has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">TESTDLL_API <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> fnTestDll(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> a,<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> b);</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li></ul>

删除导出变量和导出类部分

3、将TestDll.cpp文件中导出函数改为

<code class="hljs cs has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">TESTDLL_API <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> fnTestDll(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> a, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> b){    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> a+b;}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li></ul>

删除导出类和导出变量部分 
注：Dll能够定义两种函数，一种是内部函数，一种是导出函数。内部函数只能被定义这个函数的模块使用，而导出函数不仅可以在本模块调用，还可以被其他模块调用。dll的主要功能就是向外导出函数供其他模块使用

此时一个简单的两数相加的DLL就写完了，接下来便是如何加载这个Dll了，加载这个dll有两种方法，一种是隐式动态链接，一种是显示动态链接

隐式动态链接

新建一个win32控制台程序，生成刚刚写的dll，将TestDll.lib,TestDll.dll,TestDll.h复制到该控制台程序的文件夹下，然后添加下面的代码

<code class="hljs cpp has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">#include "stdafx.h"</span><span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">#include <iostream></span><span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">#include "TestDll.h"</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">using</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">namespace</span> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">std</span>;<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">#pragma comment(lib,"TestDll")</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> main(){    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> a, b, result;    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">while</span> (<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">cin</span> >> a >> b)    {        result = fnTestDll(a, b);        <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">cout</span> << <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"Result : "</span> << result << endl;    }    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>;}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li></ul>

#pragma comment(lib,”TestDll”)语句表示要连接到TestDll.lib库，使用隐式动态链接需要指明库所在的位置（本例中在当前文件夹下），静态变异也会使可执行文件的体积变大 
测试结果 
 
可以看到运行成功

显式动态链接

模块使用LoadLIbrary()或者LoadLIbraryEx()函数显式加载DLL，加载后调用GetProcAddress()函数取得DLL导出函数的地址，然后调用函数。

1、在原DLL工程中建立一个模块定义文件（DEF）来指定要导入的函数。 
在TestDll中项目->添加新项->模块定义文件(.def) 
然后在其中添加

<code class="hljs delphi has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">EXPORTS</span>fnTestDll</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li></ul>

fnTestDll表示要向外导出的函数名 
重新生成该项目 
2、新建一个win32控制台程序，添加如下代码

<code class="hljs cpp has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// ExplicitTestDll.cpp : 定义控制台应用程序的入口点。</span><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//</span><span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">#include "stdafx.h"</span><span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">#include <iostream></span><span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">#include <Windows.h></span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">using</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">namespace</span> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">std</span>;<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">typedef</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span>(*PFNEXPORTFUNC)(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span>, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span>);<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> main(){    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> a, b, result;    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">while</span> (<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">cin</span> >> a >> b)    {        HMODULE hModule = LoadLibrary(_T(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"TestDll.dll"</span>));        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (hModule != NULL)        {            PFNEXPORTFUNC mDLLFuncAdd = (PFNEXPORTFUNC)GetProcAddress(hModule, <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"fnTestDll"</span>);            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (mDLLFuncAdd != NULL)            {                result = mDLLFuncAdd(a, b);                <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">cout</span> << <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"Result： "</span> << result << endl;            }            FreeLibrary(hModule);        }    }    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>;}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li><li style="box-sizing: border-box; padding: 0px 5px;">29</li></ul>

显式调用DLL分为三个步骤

1. 声明要导出的DLL函数
2. 加载目标DLL，即 LoadLibrary()函数，将DLL加载到进程的虚拟地址空间，若成功则返回该DLL模块的句柄，否则返回NULL
3. 获得导出函数的地址，即GetProcAddress()函数，成功时返回函数地址，否则返回NULL

测试结果 

参考：

1. 百度百科
2. 《小小黑客之路-黑客工具、攻防及防火墙入门》