Allow only one instance of the program to be started on network.

Hi,

My application is installed on a network-drive. End-users can startup this application from their local computer using a shortcut. ** I don't want to allow multiple instances of my application to be started.

After user-A has started the application on local computer-1, it is not sufficient to check for a running instance of this application on local computer-2, because that instance will not be recognized on local computer-2 (as it is running locally on computer-1).
I've thought about creating a controle-file the moment the application is started by any user/local computer and delete this file after the application is closed again. But this might give some timing-problems when two users start the application at the same time.

How can I solve this problem in an easy but certain/secure way?

Regards, Stef

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Stef MerlijnDeveloperAsked:
Who is Participating?
 
SteveBayConnect With a Mentor Commented:
This will do what you want but you will need to give some thought to where you create the InUse.Fil. This code give no consideration to the posibilty that the application is in more than one place.

procedure TForm1.FormCreate(Sender: TObject);
begin
	try
	_gFileStream := TFileStream.Create( ExtractFilePath(Application.ExeName) + 'InUse.fil',fmCreate or fmShareExclusive);
	except
		ShowMessage('Denied Access');
		timerClose.Enabled := true;
	end;
end;
 
procedure TForm1.FormDestroy(Sender: TObject);
begin
	_gFileStream.Free;
end;
 
procedure TForm1.timerCloseTimer(Sender: TObject);
begin
	Close;
end;

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MerijnBSr. Software EngineerCommented:
your idea of a 'lock' file is probably the simplest idea.
If you create the file with a TFileStream, with mode fmShareExclusive, you are sure only one user can open the file.

from the help:

fmShareExclusive Other applications can not open the file for any reason.
 
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Stef MerlijnDeveloperAuthor Commented:
It would be nice to have a bit more of a solution (code example).
In other words: how to handle checking for an existing instance of the application?
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Shanmuga SundaramDirector of Software EngineeringCommented:
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Stef MerlijnDeveloperAuthor Commented:
SteveBay: What is the use of the timer, I don't see where it is actually used in your code?
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Stef MerlijnDeveloperAuthor Commented:
Another question.
What if the application crashes because of an error, will 'InUse.fil' be freed?
If not: how to free it?
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SteveBayConnect With a Mentor Commented:
The timer will close your application.  . When you are unable to create the file an exception is raised and the close timer is enabled.

It is a graceful way to close an applicaiton from the MainForm OnShow or OnCreate event. Set the Timer to a short interval (50 or so) and diabled in the designer and enable when you wish to force the applicaiton to close.
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SteveBayConnect With a Mentor Commented:
If the applicaiton crashes it should no longer holds the file open exclusively. You should be able to restart the applicaiotn with no problem.
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JohnjcesCommented:
Just a question, but if your application crashes there is no graceful shutdown, no other events in the app are processed and I believe that the InUse.fil will still be there and Windows will still show the file as locked.

No other app will run since it is still locked or until Windows does a reboot.

I am not certain if Windows releases this kind of locking after a crash.

Just a comment for my further understanding if anyone knows!

John

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Stef MerlijnDeveloperAuthor Commented:
Thank you very much for your help.
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SteveBayCommented:
That is difficult to test, however, I crashed the program with an access violation and Task manager and the file was no longer in use. I believe that there are circumstances where the OS will continue to hold the file after the application crashes. Then the machine that was running the application may have to be logged off and back on.
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JohnjcesCommented:
Thanks Steve for the reply. I believe that to be true also and was wondering about that. I have had the same results; sometimes still locked and other times no problem.

Gotta love Windows! :)

John
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