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    <p>Here is a sample that adds values in list1 and list2 from your three threads.</p> <p>Each time a value is put into a list, an <code>Event</code> is triggered and the thread handling this event pulls out the last value in the list and clears the event flag.</p> <p>No new value can be put into a list until the event flag is cleared.</p> <p>The middle thread makes an intermediate storage of a new value, not to stall the first thread.</p> <p>All events are waitable and thus keeps the cpu at ease.</p> <p>The lists are thread-safe.</p> <pre><code>program Project62; {$APPTYPE CONSOLE} uses System.SysUtils, System.Classes, System.SyncObjs, System.Generics.Collections; Type TMyThread1 = Class(TThread) private fMySyncAddList : TSimpleEvent; fMyList : TThreadList&lt;Integer&gt;; fAddVal : Integer; public Constructor Create(ASyncAddList: TSimpleEvent; AList: TThreadList&lt;Integer&gt;); procedure Execute; override; End; TMyThread2 = Class(TThread) private fMySyncAddList1,fMySyncAddList2 : TSimpleEvent; fMyList1,fMyList2 : TThreadList&lt;Integer&gt;; fAddVal : Integer; public Constructor Create(ASyncAddList1,ASyncAddList2: TSimpleEvent; AList1,AList2 : TThreadList&lt;Integer&gt;); procedure Execute; override; End; TMyThread3 = Class(TThread) private fMySyncAddList2 : TSimpleEvent; fMyList2 : TThreadList&lt;Integer&gt;; fAddVal : Integer; public Constructor Create(ASyncAddList2: TSimpleEvent; AList2 : TThreadList&lt;Integer&gt;); procedure Execute; override; End; { TMyThread1 } constructor TMyThread1.Create( ASyncAddList : TSimpleEvent; AList: TThreadList&lt;Integer&gt;); begin Inherited Create(false); fMySyncAddList := AsyncAddList; fMyList := AList; end; procedure TMyThread1.Execute; var stateAcknowledged : boolean; begin stateAcknowledged := true; while (not Terminated) do begin if stateAcknowledged then begin // Do some work and adda a value to list1 fAddVal := Random(100); fMyList.Add(fAddVal); fMySyncAddList.SetEvent; // Signal a new addition stateAcknowledged := false; //ShowVal; Sleep(1000); end else begin stateAcknowledged := (fMySyncAddList.WaitFor(100) &lt;&gt; wrSignaled); end; end; end; { TMyThread2 } constructor TMyThread2.Create(ASyncAddList1, ASyncAddList2: TSimpleEvent; AList1, AList2: TThreadList&lt;Integer&gt;); begin Inherited Create(false); fMySyncAddList1 := AsyncAddList1; fMySyncAddList2 := AsyncAddList2; fMyList1 := AList1; fMyList2 := AList2; end; procedure TMyThread2.Execute; var wr : TWaitResult; list : TList&lt;Integer&gt;; pulled : Boolean; begin pulled := false; while (not Terminated) do begin if pulled then // Add a value to list2 begin wr := fMySyncAddList2.WaitFor(0); if (wr &lt;&gt; wrSignaled) then begin fMyList2.Add(fAddVal); fMySyncAddList2.SetEvent; // Signal a new addition pulled := false; end else Sleep(100); end else begin // Wait for a new value in list1 wr := fMySyncAddList1.WaitFor(INFINITE); if Terminated then Exit; if (wr = wrSignaled) then begin // Pull out the value list := fMyList1.LockList; try fAddVal := list.Last; finally fMyList1.UnlockList; end; // All clear pulled := true; fMySyncAddList1.ResetEvent; //ShowVal; end; end; end; end; { TMyThread3 } constructor TMyThread3.Create(ASyncAddList2: TSimpleEvent; AList2: TThreadList&lt;Integer&gt;); begin Inherited Create(false); fMySyncAddList2 := AsyncAddList2; fMyList2 := AList2; end; procedure TMyThread3.Execute; var wr : TWaitResult; list : TList&lt;Integer&gt;; begin while not Terminated do begin wr := fMySyncAddList2.WaitFor(INFINITE); if Terminated then Exit; if (wr = wrSignaled) then // Wait for signal begin // Pull out the value list := fMyList2.LockList; try fAddVal := list.Last; //ShowVal; finally fMyList2.UnlockList; end; // Clear event fMySyncAddList2.ResetEvent; end; end; end; var list1,list2 : TThreadList&lt;Integer&gt;; syncList1,syncList2 : TSimpleEvent; thread1 : TMyThread1; thread2 : TMyThread2; thread3 : TMyThread3; begin list1 := TThreadList&lt;Integer&gt;.Create; list2 := TThreadList&lt;Integer&gt;.Create; syncList1 := TSimpleEvent.Create(Nil,True,False,'',false); syncList2 := TSimpleEvent.Create(Nil,True,False,'',false); thread3 := TMyThread3.Create(syncList2,list2); thread2 := TMyThread2.Create(syncList1,syncList2,list1,list2); thread1 := TMyThread1.Create(syncList1,list1); Try WriteLn('Press [Enter] key to stop.'); ReadLn; Finally thread3.Terminate; syncList2.SetEvent; // Wake up call thread3.Free; thread2.Terminate; syncList1.SetEvent; // Wake up call thread2.Free; thread1.Free; syncList1.Free; syncList2.Free; list1.Free; list2.Free; End; end. </code></pre> <hr> <p>Added an example where two <code>TThreadedQueue</code>'s transfer the information between the threads. The threads keeps internal lists of the integers instead. The code is much simpler as @DavidHeffernan points out. </p> <pre><code>program Project63; {$APPTYPE CONSOLE} uses   System.SysUtils,   System.Classes,   System.SyncObjs,   System.Generics.Collections; Type   TMyThread1 = Class(TThread)     private       fMyList : TList&lt;Integer&gt;;       fMyQueue : TThreadedQueue&lt;Integer&gt;;       fAddVal : Integer;     public       Constructor Create(AQueue : TThreadedQueue&lt;Integer&gt;);       procedure Execute; override;   End;   TMyThread2 = Class(TThread)     private       fMyList1,fMyList2 : TList&lt;Integer&gt;;       fMyQueue1,fMyQueue2 : TThreadedQueue&lt;Integer&gt;;       fAddVal : Integer;     public       Constructor Create(AQueue1,AQueue2: TThreadedQueue&lt;Integer&gt;);       procedure Execute; override;   End;   TMyThread3 = Class(TThread)     private       fMyList : TList&lt;Integer&gt;;       fMyQueue : TThreadedQueue&lt;Integer&gt;;       fAddVal : Integer;     public       Constructor Create(AQueue : TThreadedQueue&lt;Integer&gt;);       procedure Execute; override;   End; constructor TMyThread1.Create( AQueue : TThreadedQueue&lt;Integer&gt;); begin   Inherited Create(false);   fMyQueue:= AQueue;   fMyList := TList&lt;Integer&gt;.Create; end; procedure TMyThread1.Execute; begin   while (not Terminated) do   begin     Sleep(1000); // Simulate some work     fAddVal := Random(100);     fMyList.Add(fAddVal);     fMyQueue.PushItem(fAddVal); // Signal a new addition   end;   fMyList.Free; end; constructor TMyThread2.Create(AQueue1,AQueue2: TThreadedQueue&lt;Integer&gt;); begin   Inherited Create(false);   fMyQueue1 := AQueue1;   fMyQueue2 := AQueue2;   fMyList1 := TList&lt;Integer&gt;.Create;   fMyList2 := TList&lt;Integer&gt;.Create; end; procedure TMyThread2.Execute; var   queueSize : Integer; begin   while (not Terminated) do   begin     if (fMyQueue1.PopItem(queueSize,fAddVal) = wrSignaled) and (not Terminated) then     begin       fMyList1.Add(fAddVal);       // Do some work and send a new value to next thread       fMyQueue2.PushItem(fAddVal); fMyList2.Add(fAddVal);           end;   end;   fMyList1.Free;   fMyList2.Free; end; constructor TMyThread3.Create(AQueue : TThreadedQueue&lt;Integer&gt;); begin   Inherited Create(false);   fMyQueue := AQueue;   fMyList := TList&lt;Integer&gt;.Create; end; procedure TMyThread3.Execute; var   queueSize : Integer; begin   while not Terminated do   begin     if (fMyQueue.PopItem(queueSize,fAddVal) = wrSignaled) and (not Terminated) then     begin       fMyList.Add(fAddVal);       // Do some work on list           end;   end; fMyList.Free; end; var   queue1,queue2 : TThreadedQueue&lt;Integer&gt;;   thread1 : TMyThread1;   thread2 : TMyThread2;   thread3 : TMyThread3; begin   queue1 := TThreadedQueue&lt;Integer&gt;.Create;   queue2 := TThreadedQueue&lt;Integer&gt;.Create;   thread3 := TMyThread3.Create(queue2);   thread2 := TMyThread2.Create(queue1,queue2);   thread1 := TMyThread1.Create(queue1);   Try     WriteLn('Press [Enter] key to stop.');     ReadLn;   Finally     thread3.Terminate;     queue2.PushItem(0); // Wake up call     thread3.Free;     thread2.Terminate;     queue1.PushItem(0); // Wake up call     thread2.Free;     thread1.Free;     queue1.Free;     queue2.Free;   End; end. </code></pre>
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    1. POAccess to data by multiple threads
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    1. POAccess to data by multiple threads
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    1. COVery low level. Don't you just build a threadsafe queue, and instantiate two of them.
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    2. COAnd waking up 10 times a second? Why?
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    3. CO@DavidHeffernan, modified code to better sleep periods. About not using queues: it seems as thread 2 wants access to both list1 and list2. Not only the last item added, but the whole list. And the same for thread 3 and list2. If you could rule that out, yes a queue would be much easier.
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