DLLs are Simple! Part 3

Mahmoud Komeily

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Sep 30, 2004

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This article describes how to create a DLL with a DEF file and use it.

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 statement	statement identifies the DEF file as belonging to a DLL
EXPORTS statement	lists the names of the functions exported by the DLL
DESCRIPTION statement	describes the purpose of the DLL (optional)

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

How to Build a DLL Using a DEF File

Run VC++.
Choose File>New.
In the dialog box, choose "MFC AppWizard (DLL)" and name it, e.g. DefExported.

Declare a member function:

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

Then define it:

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

In the FileView tab, click "Source Files" and double click on DefExported.def.

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

Click the Build Button.
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.

Run VC++.
Choose from the menu File>New.
In the dialog box, choose "MFC AppWizard (EXE)" and name it, e.g. DynamicLoad.
Select "Dialog Based" and click the Finish button.
Place a button control on the dialog and double click on it to create its click event.
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: Q117428. 
```

Enter the code for the button click event:

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

Now we can use the Multiply function by calling lpfnMultiply and storing the return value.
```
m_Rslt=lpfnMuliply(m_PartOne,m_PartTwo);
```
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