Simple Windows C++ DLL Example with Implicit and Explicit Calls

Aim
The aim of this Windows C++ tutorial is to build a simple Dynamic Link Library or DLL using the command line Visual C++ compiler (cl.exe0. Then create two small C++ test programs that will call this DLL implicitly by linking with the lib file and explicitly using LoadLibrary and GetProcAddress. Also note that the explicit linking requires a "def" definition file with the exported funtion names.

Assumptions
This article assumes that you have a compatible version of Visual C++ installed and you have run vcvars32. The file is called MyHook because it will be used later as a windows keyboard hook dll.

Versions used in this example

Sofware/Component	Image
Windows XP SP2	N/A
Visual C++ 2008 Express Edition	N/A

Links to these files can be found here


CPPDLLExample
_|_MyHook.cpp
_|_MyHook.h
_|_MyHook.def
_|_CPPDllImplicit.cpp
_|_CPPDllExplicit.cpp

The dll requires 3 files. Source file, header file and a definitions file. This def file is required to expose the functions for explicit linking only

Create the DLL

1. Create the source file and save it as MyHook.h
   

   1. #ifndef MYHOOK_H 2. #define MYHOOK_H 3. 4. #ifdef MYHOOK_DLLEXPORT 5. #define MYHOOK_API __declspec(dllexport) 6. #else 7. #define MYHOOK_API 8. #endif 9. 10. MYHOOK_API int AddNumbers(int, int); 11. MYHOOK_API int GetMessage(char*, int); 12. 13. #endif Hide line numbers

   
   
2. Create the source file and save it as MyHook.cpp
   

   1. #include "MyHook.h" 2. #include <string.h> 3. 4. MYHOOK_API int AddNumbers(int x, int y){ 5. return x + y; 6. } 7. MYHOOK_API int GetMessage(char* x, int length){ 8. if(length>32){ 9. strcpy(x, "hello from dll"); 10. return 0; 11. } 12. else 13. return -1; 14. } Hide line numbers

   
   
3. Finally create the definitions file and save it as MyHook.def
   

   1. ; MyHook.def : Declares the module parameters for the DLL. 2. 3. LIBRARY "myhook" 4. DESCRIPTION "MyHook Windows Dynamic Link Library" 5. 6. EXPORTS 7. ; Explicit exports can go here 8. AddNumbers 9. GetMessage 10. 11. SECTIONS 12. ; Pragma sections Hide line numbers

   
   
4. Build the library on the command line
   

   ...CPPDLLExample>cl -o myhook.dll myHook.cpp /D MYHOOK_DLLEXPORT /link /DLL /DEF:"MyHook.def"

   
   

Write and Compile Implicit caller

1. Write implicit compiler and save it as CallDLLImplicit.cpp
   

   1. #include <iostream> 2. #include "MyHook.h" 3. 4. using namespace std; 5. int main(){ 6. 7. char msg[64]; 8. cout << AddNumbers(5,10) << endl; 9. GetMessage(msg, 64); 10. cout << msg << endl; 11. 12. } Hide line numbers

   
   
2. Build the program using,
   

   ...CPPDLLExample>cl CallDllImplicit.cpp MyHook.lib

   
   

Write and compile the explicit client

1. Write the explicit caller and save it as CallDllExplicit.cpp
   

   1. #include <iostream> 2. #include <windows.h> 3. 4. typedef int(*pAddNumbers)(int,int); 5. typedef int(*pGetMessage)(char*,int); 6. 7. using namespace std; 8. int main(){ 9. 10. HINSTANCE hInstance; 11. 12. if(!(hInstance=LoadLibrary("myhook.dll"))){ 13. cout << "could not load library" << endl; 14. goto FINISH; 15. } 16. 17. pAddNumbers padd = (pAddNumbers)GetProcAddress(hInstance, "AddNumbers"); 18. pGetMessage pget = (pGetMessage)GetProcAddress(hInstance, "GetMessage"); 19. 20. if(!padd || !pget){ 21. cout << "could no load functions" << endl; 22. goto FINISH; 23. } 24. 25. char msg[64]; 26. 27. cout << padd(20,35) << endl; 28. pget(msg, 64); 29. cout << msg << endl; 30. 31. FINISH: 32. cout<<"finished"<<endl; 33. } Hide line numbers

   
   
2. Now cd out to the workingdirectory build it
   

   ...CPPDLLExample>cl CallDLLExplicit.cpp

   
   

Running the executables

1. Open a command prompt into your working directory and the executables. You should be able to see the results. Delete the dll and run them again - you should get an error message

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