Single application instance

Limiting an application to start just once per machine is usually required when an
external resource such as a Comport is accessed. This feature is achieved by allocation
of a global variable, such as a mutex.

The original version

Be the original application MyApp.dpr generated by Delphi as dpr file.

program MyApp;
uses
  Windows,Forms,
  MyApp1 in 'MyApp1.pas' {Form1};

{$R *.RES}

begin
  Application.Initialize;
  Application.CreateForm(TForm1, Form1);
  Application.Run;
end.

The single instance version

Do the following changes ( in bold ):

program MyApp;
uses
  Windows,Forms,
  MyApp1 in 'MyApp1.pas' {Form1};


var
  Mutex : THandle;
{$R *.RES}

begin
Mutex := CreateMutex(nil, True, 'MyAppName');
if (Mutex <> 0) and (GetLastError = 0) then
  begin
  Application.Initialize;
  Application.CreateForm(TForm1, Form1);
  Application.Run;
  if Mutex <> 0 then
    CloseHandle(Mutex); 
  end;
end.

The application will only start once at a time without any further notice.
Any further attempts will be defeated.

A little addition, perhaps as modal notice, could notify the user
that only one is allowed at a time.