{
      Article:  Controling the number of application instances  http://delphi.about.com/library/weekly/aa100703a.htm  In this article you'll learn how to "run-once enable"  a Delphi application that can check for its previous  (running) instance. Along the process, several techniques  of implementing such a check will be discussed; as well  as how to bring your already running application to the  foreground, if a user tries to run it "one more time". By  the end of the article you'll have a copy-to-go code to  control the behavior of your application's multiple instances:  with the option to limit the number of running instances.}unit uCheckPrevious;interfaceuses Windows, SysUtils;function RestoreIfRunning( const AppHandle : THandle;  MaxInstances : integer = 1 ) : boolean;implementationtype  PInstanceInfo = ^TInstanceInfo;  TInstanceInfo = packed record    PreviousHandle : THandle;    RunCounter : integer;  end;var  MappingHandle : THandle;  InstanceInfo : PInstanceInfo;  MappingName : string;  RemoveMe : boolean = True;function RestoreIfRunning( const AppHandle : THandle;  MaxInstances : integer = 1 ) : boolean;begin  Result := True;  MappingName := StringReplace( ParamStr( 0 ), '\', '',    [ rfReplaceAll, rfIgnoreCase ] );  MappingHandle := CreateFileMapping( $FFFFFFFF, nil, PAGE_READWRITE, 0,    SizeOf( TInstanceInfo ), PChar( MappingName ) );  if MappingHandle = 0 then    RaiseLastOSError  else  begin    if GetLastError <> ERROR_ALREADY_EXISTS then    begin      InstanceInfo := MapViewOfFile( MappingHandle, FILE_MAP_ALL_ACCESS, 0, 0,        SizeOf( TInstanceInfo ) );      InstanceInfo^.PreviousHandle := AppHandle;      InstanceInfo^.RunCounter := 1;      Result := False;    end    else // already runing    begin      MappingHandle := OpenFileMapping( FILE_MAP_ALL_ACCESS, False,        PChar( MappingName ) );      if MappingHandle <> 0 then      begin        InstanceInfo := MapViewOfFile( MappingHandle, FILE_MAP_ALL_ACCESS, 0, 0,          SizeOf( TInstanceInfo ) );        if InstanceInfo^.RunCounter >= MaxInstances then        begin          RemoveMe := False;          if IsIconic( InstanceInfo^.PreviousHandle ) then            ShowWindow( InstanceInfo^.PreviousHandle, SW_RESTORE );          SetForegroundWindow( InstanceInfo^.PreviousHandle );        end        else        begin          InstanceInfo^.PreviousHandle := AppHandle;          InstanceInfo^.RunCounter := 1 + InstanceInfo^.RunCounter;          Result := False;        end      end;    end;  end;end; (* RestoreIfRunning *)initialization// nothing special here// we need this section because we have the// finalization sectionfinalization// remove this instanceif RemoveMe thenbegin  MappingHandle := OpenFileMapping( FILE_MAP_ALL_ACCESS, False,    PChar( MappingName ) );  if MappingHandle <> 0 then  begin    InstanceInfo := MapViewOfFile( MappingHandle, FILE_MAP_ALL_ACCESS, 0, 0,      SizeOf( TInstanceInfo ) );    InstanceInfo^.RunCounter := -1 + InstanceInfo^.RunCounter;  end  else    RaiseLastOSError;end;if Assigned( InstanceInfo ) then  UnmapViewOfFile( InstanceInfo );if MappingHandle <> 0 then  CloseHandle( MappingHandle );end. (* unit uCheckPrevious *)

program XXX;uses  Forms,  ...  uCheckPrevious in 'uCheckPrevious.pas';{
    $R *.RES}begin  if not uCheckPrevious.RestoreIfRunning( Application.Handle ) then  begin    Application.Initialize;    Application.Title := 'Hex Editor';    Application.CreateForm( TMain, Main );    Application.Run;  end;end.