Free a TThread either automatically or manually

Question

Hello all!

I have a main thread and a separate thread in my program. If the separate thread finishes before the main thread, it should free itself automatically. If the main thread finishes first, it should free the separate thread.

I know about FreeOnTerminate, and I've read that you have to be careful using it.

My question is, is the following code correct?

procedure TMyThread.Execute;
begin
  ... Do some processing

  Synchronize(ThreadFinished);

  if Terminated then exit;

  FreeOnTerminate := true;
end;

procedure TMyThread.ThreadFinished;
begin
  MainForm.MyThreadReady := true;
end;

procedure TMainForm.Create;
begin
  MyThreadReady := false;

  MyThread := TMyThread.Create(false);
end;

procedure TMainForm.Close;
begin
  if not MyThreadReady then
  begin
    MyThread.Terminate;
    MyThread.WaitFor;
    MyThread.Free;
  end;
end;

Thanks!

Answer 1

No, your code is not good (though it probably will work in 99.99% or even 100% cases). If you are planning to terminate work thread from main thread, don't set FreeOnTerminate to True (I don't see what are you trying to gain in the above code by setting FreeOnTerminate to True, it at least makes your code less understandable).

A more important situation with terminating work threads is that you are trying to close an application while work thread is in wait state. The thread will not be awaken if you just call Terminate, generally you should use additional syncronization object (usually event) to wake up the work thread.

And one more remark - there is no need for

  begin
    MyThread.Terminate;
    MyThread.WaitFor;
    MyThread.Free;
  end;

if you look at TThread.Destroy code, it calls Terminate and WaitFor, so

    MyThread.Free;

is enough (at least in Delphi 2009, have no Delphi 7 sources at hand to check).

---

Updated

Read mghie answer. Consider the following situation (better on 1 CPU system):

main thread is executing

procedure TMainForm.Close;
begin
  if not MyThreadReady then
  begin
    MyThread.Terminate;
    MyThread.WaitFor;
    MyThread.Free;
  end;
end;

it checked MyThreadReady value (it is False) and was switched off by scheduler.

Now scheduler switches to work thread; it executes

  Synchronize(ThreadFinished);

and forces scheduler to switch back to main thread. Main thread continues execution:

    MyThread.Terminate;   // no problem
    MyThread.WaitFor;     // ???
    MyThread.Free;

can you say what will happen at WaitFor? I can't (requires a deeper look into TThread sources to answer, but at first glance looks like a deadlock).

Your real error is something different - you have written an unreliable code and trying to find out is it correct or not. That is bad practice with threads - you should learn to write a reliable code instead.

As for resources - when the TThread (with FreeOnTerminate = False) is terminated the only resources that remains allocated is Windows thread handle (it does not use substantial Windows resources after thread is terminated) and Delphi TThread object in memory. Not a big cost to be on the safe side.

Answer 2

You can simplify this to:

procedure TMyThread.Execute;
begin
  // ... Do some processing
end;

procedure TMainForm.Create;
begin
  MyThread := TMyThread.Create(false);
end;

procedure TMainForm.Close;
begin
  if Assigned(MyThread) then
    MyThread.Terminate;
  MyThread.Free;
end;

Explanation:

• Either use FreeOnTerminate or free the thread manually, but never do both. The asynchronous nature of the thread execution means that you run a risk of not freeing the thread or (much worse) doing it twice. There is no risk in keeping the thread object around after it has finished the execution, and there is no risk in calling Terminate() on a thread that has already finished either.

• There is no need to synchronize access to a boolean that is only written from one thread and read from another. In the worst case you get the wrong value, but due to the asynchronous execution that is a spurious effect anyway. Synchronization is only necessary for data that can not be read or written to atomically. And if you need to synchronize, don't use Synchronize() for it.

• There is no need to have a variable similar to MyThreadReady, as you can use WaitForSingleObject() to interrogate the state of a thread. Pass MyThread.Handle as the first and 0 as the second parameter to it, and check whether the result is WAIT_OBJECT_0 - if so your thread has finished execution.

BTW: Don't use the OnClose event, use OnDestroy instead. The former isn't necessarily called, in which case your thread would maybe continue to run and keep your process alive.

Answer 3

Have the main thread assign a handler to the worker thread's OnTerminate event. If the worker thread finishes first, then the handler can signal the main thread to free the thread. If the main thread finishes first, it can terminate the worker thread. For example:

procedure TMyThread.Execute;
begin
  ... Do some processing ...
end;

procedure TMainForm.Create;
begin
  MyThread := TMyThread.Create(True);
  MyThread.OnTerminate := ThreadFinished;
  MyThread.Resume; // or MyThread.Start; in D2010+
end;

const
  APPWM_FREE_THREAD = WM_APP+1;

procedure TMainForm.ThreadFinished(Sender: TObject);
begin
  PostMessage(Handle, APPWM_FREE_THREAD, 0, 0);
end;

procedure TMainForm.WndProc(var Message: TMessage);
begin
  if Message.Msg = APPWM_FREE_THREAD then
    StopWorkerThread
  else
    inherited;
end;

procedure TMainForm.StopWorkerThread;
begin
  if MyThread <> nil then
  begin
    MyThread.Terminate;
    MyThread.WaitFor;
    FreeAndNil(MyThread);
  end;
end;

procedure TMainForm.Close;
begin
  StopWorkerThread;
end;

Answer 4

Honestly, your

... Do some processing

Is the real problem here. Is that a loop for doing something recursively? If not and, instead, thats a huge task, you should consider split this task in small procedures / functions, and put all together in the execute body, calling one after another with conditional if's to know the thread state, like:

 

While not Terminated do
 begin

  if MyThreadReady then
    DoStepOneToTaskCompletion
  else
    clean_and_or_rollback(Something Initialized?);

  if MyThreadReady then
    DoStepTwoToTaskCompletion
  else
    clean_and_or_rollback(Something Initialized?, StepOne);

  if MyThreadReady then
    DoStepThreeToTaskCompletion
  else
    clean_and_or_rollback(Something Initialized?, StepOne, StepTwo);

  Self.DoTerminate; // Not sure what to expect from that one
 end;

It is dirty, almost a hack, but will work as expected.

About FreeOnTerminate, well... just remove the declaration and always

FreeAndNil(ThreadObject);

I'm not a fan of syncronise. I like more critical sections, for the flexibility to extend the code to handle more shared data.

On the form public section, declare:

ControlSection : TRTLCriticalSection;

On form create or somewhere else before thread.create ,

InitializeCriticalSection(ControlSection);

Then, every time you write to a shared resource (including your MyThreadReady variable), do

EnterCriticalSection ( ControlSection );
  MyThreadReady := True; //or false, or whatever else
LeaveCriticalSection ( ControlSection );

Before you go (exit), call

DeleteCriticalSection ( ControlSection );

and free your thread as you always do.

Regards Rafael

Answer 5

I would state that mixing models is simply not recommended. You either use FreeOnTerminate and never touch the thread again, or you don't. Otherwise, you need a protected way for the two to communicate.

Since you want fine control over the thread variable, then don't use FreeOnTerminate. If your thread finishes early, clear the local resources that the thread has consumed as you normally would, and then simply let the main thread free the child thread when the application is finished. You'll get the best of both worlds - resources freed by the child thread as soon as it can be, and no worries about thread synchronization. (And it's got the added bonus of being much simpler in design/code/understanding/support...)