some uncertain thread is getting created and not terminating - multithreading

Question

I have doubt about multithreaded application(ip scanner). When i put large ip range like 192.168.0.1 to 192.168.5.1 and thread limit as 99 .So when i run my application there should be 101 threads running at a time(99 threads(ScannerChild) + Main thread + Scannerthread) and when scanning is done 99 scannerchild and 1 scannethreads would be terminated and only 1 thread should run that time(main thread). But sometimes thread count is going to 102 and after scanning thread count is not coming to 1 it shows threadcount as 2 in task manager. Whats the problem ?
code for Scannerthread
/
/Creating constructor of scannerthread
Constructor ScannerThread.Create(CreateSuspended: Boolean );
Begin
Inherited Create(CreateSuspended);
Freeonterminate:= true; //Freeonterminate is true
End;
{ScannerThread Thread }
procedure ScannerThread.Execute;
var
I : integer;
ScannerCh : array of ScannerChild; //array of ScannerChild
IpList : TStringlist; //Iplist as tstringlist
IPs: Integer; //ipcount is count of iplist
Begin
ScannerchCount:=0; //Initialising scannerchcount as 0
IpList:=TStringList.Create;//creating stringlist
IF GetNumberOfIpsInRange(Ip_From, Ip_To, IpList) Then //Function call that returns iplist if TRUE
Begin
Try
IF Assigned(LvHosts) Then //Clearing LvHosts field
LvHosts.Clear;
IPs := IpList.Count; //Ipcount is given value of iplists count
SetLength(ScannerCh, IPs); //Setting length of scannerch as ipcount
I:=0;
Repeat
While ScannerChcount > tcount-1 do //Checking if is greater than tcount(thread input) by user
Sleep(30);
ScannerCh[I]:=ScannerChild.Create(True, IpList[i]);
ScannerCh[I].FreeOnTerminate:=True;
ScannerCh[I].OnTerminate:= ScanchildTerminated; // Event scanchildterminated occurs on termination of Scannerch thread
ScannerCh[I].LvHostname := LvHosts; //Lhostname is private listview of scannechild
ScannerCh[I].Resume;
ScannerChCount:=Scannerchcount+1; //Incrementing scannerchcounts
I:=I+1;
Sleep(20); //Sleep after each thread is created so that threads will enter critical section properly
until I = IPs;
Scannerch:=nil;
If Assigned(IpList) Then //Free iplist
FreeAndNil(IpList);
Except
On E: Exception do
Begin
ShowMessage('Invalid operation :' + E.Message); //Showexception message
If Assigned(IpList) Then //Free iplist
FreeAndNil(IpList);
end;
End;
End
Else
Begin
Ipscan.lbResult.caption:='Invalid Ip Range';
Exit;
End;
Repeat //Main Thread Waiting For Ip scan Threads to finish
Sleep(100);
until ScannerChCount = 0;
End;
Scannerchild code
Constructor ScannerChild.Create(CreateSuspended: Boolean; IP: String);
Begin
Inherited Create(CreateSuspended);
//FCriticalsection := TCriticalSection.create; //Creating critical section
IPToScan:=IP;
End;
//Execution procedure for scannerchild
procedure ScannerChild.Execute;
Var
MainOutput : TListItem;//Listitem variable for adding listitems
Hostname : String; //Hostname is declared as string
Begin
Try
MainOutput:=LvHostname.Items.Add; //Adding items to mainoutput
MainOutput.Caption:=IPToScan;
Hostname := IPAddrToName(IPToScan);
If Hostname <> EmptyStr Then
Begin
MainOutput.SubItems.Add(IPAddrToName(IPToScan)); //Displaying output
End
Else
Mainoutput.subitems.add('No host');
Finally
End;
End;
//this event get called when scannerch thread terminates
procedure Scannerthread.ScanchildTerminated( Sender : TObject );
Begin
ScannerChCount:=ScannerchCount-1; // Decrementing scannerchcount
End;

Answer 1

There are plenty of problems here. I'm going to give you some general advice as well as try to answer your question.
Your accessing of the GUI outside of the main thread is wrong, as we've said before. No need to cover that again, please go back over your previous questions.
The design of your threading is poor. If you would ask a high level question about that we could help you fix it. I'd be happy if you asked a question that allowed me to demonstrate a simple thread pool.
As well as the problems with the threading design, you've got no separation of concerns. No modularity. The threading and the tasks and the GUI code are all mixed in with each other. You need to keep the concerns separate to make the code maintainable and well factored. If you'd only ask us how to design your program rather than to fix the bugs in your weak design we could help you.
All of the calls to Sleep and the polling are symptoms of this bad design. There should be no sleeping.
Your code has way too many comments that make it hard to read. There's no need to comment a statement like i := i+1. The effect of that is self-evident.
You need to learn how to debug threaded code. The interactive debugger is not so useful. It interferes with the timing of thread execution. Use trace logging to debug such problems. Until you learn how to do this you cannot expect to make progress. I repeat, it is critical that you learn how to debug.
As to the problem you asked about, you have a data race on the ScannerChCount variable. So the threads are probably terminating correctly but you are counting them incorrectly.
Use InterlockedIncrement and InterlockedDecrement to modify it in a thread safe manner. That is both in the child termination code and the controller thread.
You might think this is not needed because ScanChildTerminated which decrements the counter is an OnTerminate event and so executed by the main thread. But the controller thread code that increments the counter is not executed in the main thread.
If you don't yet know what a data race is, then you have started multi threaded programming too soon. Rather than my explain it I would refer you to the shared data sections of any good text book on parallel programming. Or Wikipedia: http://en.m.wikipedia.org/wiki/Race_condition.