Chami.com Tips

Home > Tips > Programming > Dynamically calling functions in DLLs

300+ book mentions, 95% thumbs up, millions of downloads. Get HTML-Kit Tools for $39 (limited time offer)

Web Hosting Web Hosting UK Dedicated Servers Domain Names

Categories

Internet
HTML
JavaScript
» Programming
Windows

Keywords

Delphi
Functions
Windows 95/98/ME

Dynamically calling functions in DLLs

See Also

Where did all the DLL and other system files go?

By definition DLLs are dynamically loaded libraries of functions and sometimes data. However, it's possible to either hard code the ability to "import" functions from DLLs or dynamically "bind" a DLL during the run time -- which of course means that we don't necessarily need to know the name of the DLL nor the name of the function we're about to call (to a certain extent) during the time we code. Dynamically loading and unloading DLLs could not only save memory, but also can help you write programs that are able to "adjust" itself if certain DLLs are missing.

Following "LoadAndRunDLLProcedure()" function will let you pass the name of the DLL you want to connect to and the name of the function you want to call. If everything goes well, it will load the DLL, call the function, and then unload the DLL.

function LoadAndRunDLLProcedure(
  sDLL,
  sFunc : string )
  : boolean;
type
  // define the type of "function"
  // we're calling
  TFunc_Start = procedure;
var
  Func_Start : TFunc_Start;
  
  hDll       : THandle;
  FuncPtr    : TFarProc;
  sMsg       : string;
begin
  Result := False;
  hDll   := LoadLibrary(
              PChar( sDLL ) );
  if(hDll > 32)then
  begin
    FuncPtr :=
      GetProcAddress(
        hDll, PChar( sFunc ) );
    @Func_Start := FuncPtr;
    if(nil <> @Func_Start)then
    begin
      Func_Start;
      Result := True;
    end else
    begin
      sMsg := 'DLL entry point ' +
              sFunc + ' not found';
      MessageBox(
        0, PChar( sMsg ), 'Error',
        MB_OK );
    end;
    FreeLibrary( hDll );
  end else
  begin
    sMsg := 'File ' + sDLL +
            ' not found';
    MessageBox(
      0, PChar( sMsg ), 'Error',
      MB_OK );
  end;
end;

For example, let's say you want to call a procedure called "HelloWorld()" in a DLL named "MyStuff.DLL:"

LoadAndRunDLLProcedure(
  'MyStuff.DLL',
  'HelloWorld' );

Please note that HelloWorld() must be a procedure, for example, declared as:

procedure HelloWorld;

or in C:

void HelloWorld();

Applicable Keywords : Delphi, Delphi 2.x, Functions, Win32

Previous Tip : How to retrieve environment variables
Random Tip : How to create a console mode program
Next Tip : How to get current user name