DLLs are Simple! Part 3

• Download source files - 46.3 KB
• Download demo project - 7.0 KB

Introduction

In the third part of the assortment of articles "DLLs are Simple!" I describe how to create a DLL using a DEF file. What is a DEF file? It is a module-definition (*.def) file that is a text file containing one or more module statements that describe various attributes of a DLL, including:

LIBRARY      "DefExported"
DESCRIPTION  'DefExported For Present in CodeProject'
EXPORTS
   Multiply @1

How to Build a DLL Using a DEF File

1. Run VC++.
2. Choose File>New.
3. In the dialog box, choose "MFC AppWizard (DLL)" and name it, e.g. DefExported.
4. Declare a member function:

   public:
    int Multiply(int PartOne,int PartTwo);
    CDefExportedApp();

5. Then define it:

   int CDefExportedApp::Multiply(int PartOne, int PartTwo)
   {
    return PartOne*PartTwo;
   }

6. In the FileView tab, click "Source Files" and double click on DefExported.def.
7. After the EXPORT statement, enter [function name] @[number] like this:

   LIBRARY      "DefExported"
   DESCRIPTION  'DefExported For Present in CodeProject'
   EXPORTS
    ; Explicit exports can go here
    Multiply @1

8. Click the Build Button.
9. Bring the DLL out of the oven!!

How to Use a DLL

To use a DLL dynamically, there are three simple API functions:

• LoadLibrary ( [path of DLL] ) Loads a DLL into the process address, returning a handle to the DLL.
• GetProcAddress ( [loaded library] , [function name] )
  Returns a handle of a function so that it can be used in your application.
• FreeLibrary( [handle of loaded DLL] )
  Releases the memory allocated when the DLL was loaded.

1. Run VC++.
2. Choose from the menu File>New.
3. In the dialog box, choose "MFC AppWizard (EXE)" and name it, e.g. DynamicLoad.
4. Select "Dialog Based" and click the Finish button.
5. Place a button control on the dialog and double click on it to create its click event.
6. Before typing the code for the button click (BN_CLICKED) event, we must define a new function pointer with the correct number of parameters. This is done according to the parameters of the function we exported above.

   typedef int (CALLBACK* LPFNMLTPLY)(int, int);
   Sometimes you have to convert some variable types. For more information about this conversion, see Microsoft Support Article ID: "http://support.microsoft.com/default.aspx?scid=kb;en-us;117428">Q117428. 
   

7. Enter the code for the button click event:

   HINSTANCE hClcltr=LoadLibrary("DefExported.dll");
   LPFNMLTPLY lpfnMuliply;
   lpfnMuliply = (LPFNMLTPLY)GetProcAddress(hClcltr,"Multiply");

8. Now we can use the Multiply function by calling lpfnMultiply and storing the return value.

   m_Rslt=lpfnMuliply(m_PartOne,m_PartTwo);

9. When you are finished using the library, you must call the FreeLibrary API to release the memory allocated from the LoadLibrary method.

   FreeLibrary( hClcltr );

History

• 30 September, 2004 -- Original version posted