How do I reliably wait on a thread that has just been created?

Question

Consider the following program:

program TThreadBug;
{$APPTYPE CONSOLE}

uses
  SysUtils, Classes, Windows;

type
  TMyThread = class(TThread)
  protected
    procedure Execute; override;
  end;

procedure TMyThread.Execute;
var
  i: Integer;
begin
  for i := 1 to 5 do begin
    Writeln(i);
    Sleep(100);
  end;
end;

procedure UseTThread;
var
  Thread: TMyThread;
begin
  Writeln('TThread');
  Thread := TMyThread.Create;
  Thread.Terminate;
  Thread.WaitFor;
  Thread.Free;
  Writeln('Finished');
  Writeln;
end;

procedure UseTThreadWithSleep;
var
  Thread: TMyThread;
begin
  Writeln('TThreadWithSleep');
  Thread := TMyThread.Create;
  Sleep(100);
  Thread.Terminate;
  Thread.WaitFor;
  Thread.Free;
  Writeln('Finished');
  Writeln;
end;

begin
  UseTThread;
  UseTThreadWithSleep;
  Readln;
end.

The output is:

TThread
Finished

TThreadWithSleep
1
2
3
4
5
Finished

So it seems that, for some reason, the main thread has to wait some arbitrary amount of time before terminating and waiting for the worker thread. Am I right in thinking that this is a bug in TThread? Is there any way I can work around this? I expect that if I get my thread to signal that it has started (using an event), then that would work around the issue. But that makes me feel dirty.

Answer 1

You can call it a bug or a TThread design flaw, the problem was discussed many times. See for example http://sergworks.wordpress.com/2011/06/25/sleep-sort-and-tthread-corner-case/

The problem is that if TThread.Terminated flag is set too early TThread.Execute method is never called. So in your case just don't call TThread.Terminate before TThread.WaitFor.