When I start a program I want to make sure that there only can be
started one instance of this program. When somebody tries to start
this program a second time while the first instance is still running
the second instance should notice the user.
How can I do this?

If anybody could help me with this?

Regards

program OnlyOne;

uses Wintypes, Winprocs, WObjects;

type

NoMultiApplication= object(TApplication)
  FirstInstance:boolean;
  constructor Init(AName: PChar);
  procedure InitMainWindow; virtual;
  procedure InitApplication; virtual;
end;

constructor NoMultiApplication.Init(AName: PChar);
begin
  FirstInstance:=false;
  TApplication.Init(AName)
end;

procedure NoMultiApplication.InitApplication;
begin
  FirstInstance:=true
end;

procedure NoMultiApplication.InitMainWindow;
begin
  if FirstInstance
     then begin
            MainWindow:=New(PWindow, Init(nil,
                         'First (and only) instance'))  
          end
     else begin
            Messagebox(0, 'Multiple instances not allowed',
                   'Application Error',
                    mb_TaskModal or mb_IconExclamation);
            Halt(0) {only this unwanted instance}
          end          
end; {NoMultiApplication.InitMainWindow}

var  NoMultiApp:NoMultiApplication

begin
  NoMultiApp.Init('NoMulti');
  NoMultiApp.Run;
  NoMultiApp.Done
end.

Meaning:
Since InitApplication is called once prior to creating
the first instance and InitMainWindow is only called to
set up a new adress space for every new instance,
you can modify a field (boolean variable) called
FirstInstance trough the above two virtual methods.

The system variable HPrevInst seems to have the same effect.
(HPrevInst<>0 => this is not the first instance)

I hope this is not a homework-assignment.

Hej again!

Thank you very much for your help. I found what I needed.

Best regards

When the first user closes the files, the flag "CLOS" is stored in the
flag file, to indicate that the files are closed and may be opened for
access. (I often have to go to my boss's PC and close an inventory file
so I can access it from my own.)

For the inventories accessed through Windows programs, the flag file is
used to limit read/write access to the first user to open the file and
allow read-only access to anyone else. (DOS based inventories are being
phased out slowly because they work so well that it isn't worth the
effort to transfer the data.)

Using Databases (like Paradox, Excell, etc.) provides built in access
controls but I have found that the extra work to write and maintain code
needed for databases is far more trouble to me than it's worth.

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