Strange problem in threads, application doesn't exit

OK I'm writing a Zoom component and I wanted it to capture from screen within a secondary thread. You can pretend I just want to derive TThread in an empty component. I didn't write any codes in the thread so it is just a simple useless thread. I wrote this code: Thrd := TCaptureThread.Create(False); in the component main class. Then I wrote Thrd.Free in the main class destruction code. Now when I close the whole application, although it destroys everything, the process doesn't terminate completely. In Windows Task Manager shows that number of threads is 1 but the process remains. If I comment the thread creation line, everything becomes OK and application terminates quickly. What am I开发者_如何学Python gonna do about this? :(

Thanks in advance

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When you call Thrd.Free the following code from TThread.Destroy is run:

Terminate;
if FCreateSuspended then
  Resume;
WaitFor;

Calling Free on a thread will thus terminate the thread synchronously.

My guess is that the call to WaitFor never returns. Perhaps TCaptureThread.Execute doesn't check Terminated and exit. Perhaps TCaptureThread is waiting on the main thread and so the wait on the thread deadlocks.

It's pretty hard to do anything other than guess, based on your question, but I'd want to check whether or not your code gets past the WaitFor call in the destruction of Thrd. Enable Debug DCUs, set a breakpoint one the call to WaitFor and see for yourself.

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That can be SO many thing... From the Execute method of your thread never exiting to a deadlock.

First thing you should do to try to diagnose is run your application under Delphi. Then, since you say it's at the closing of the application it hangs, you close the application. Now that your application hanged, you pause the application and check the Thread debug panel. There, click on each thread to check their callstack. That should give a better idea of why it hangs, and exactly which thread hangs.

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Write your thread on this way:

type
  TCaptureThread = class(TThread)
  private
    //
  protected
    procedure Execute; override;
  public
    constructor Create(CreateSuspended: False);
  end;

implementation

constructor TCaptureThread.Create(CreateSuspended: Boolean);
begin
  inherited Create(CreateSuspended);
  // Set FreeOnTerminate to True
  FreeOnTerminate := True;
end;

procedure TCaptureThread.Execute;
begin
  // Write the Execute method here...
  // And don't forget to synchronise with the form
end;

end.

In the form, instantiate your thread

Thrd := TCaptureThread.Create(False);

Execute it

Thrd.Execute;

And terminate your thread

Thrd.Terminate;

I've been using threads on this way for a while, and I never had problems.