Hello lazarus-list,
I'm working in a program that needs multiple threads and one will
synchronize with main thread to display a log. I do not wish to
continue the caller thread meanwhile the Synchronize has not been
finished, so I had added a set of events to handle this. My program
locks up just after start, and the offended method is in the
Synchronize feature. So I start to rewrite the same idea around the
multithread example code keeping only the things that I need, and to
ask to you if this is a Synchronize bug/problem or a non-wanted
programming technique, and I must replace the code with a different
approach (if possible).
The main problem is here, in the execute method:
-----------------------------------------
procedure TMyThread.Execute;
begin
fStatusText := 'TMyThread Starting...';
Synchronize(@Showstatus);
fStatusText := 'TMyThread Running...';
while (not Terminated) and (true {any condition required}) do begin
m_Event.WaitFor(1000); //It should be INFINITE...
m_Event.ResetEvent;
if NewStatus <> fStatusText then begin
m_Lock.Enter;
fStatusText := newStatus;
Synchronize(@Showstatus);
m_Lock.Leave;
end;
m_EventDone.SetEvent;
end;
end;
-----------------------------------------
The idea is to modify "NewStatus" from an outside procedure
(CriticalSection protected) and activate m_Event to let the Execute
know that it must launch the syncronize, then it will wait for
"m_EventDone" to be signaled before continue.
Attached is the Multithread example mainunit.pas modified with my
changes to raise the bug/problem, simply replace the example file with
this one.
System: WinXP SP2
Lazarus: 0.9.24 and 0.9.25 #13510 (packages from the web)
FPC: As shipped with lazarus 2.2.0 and 2.2.1
CPU: Pentium Hyperthreading (tested without hyper too).
--
Best regards,
JoshyFun{
***************************************************************************
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* This source is free software; you can redistribute it and/or modify *
* it under the terms of the GNU General Public License as published by *
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* (at your option) any later version. *
* *
* This code is distributed in the hope that it will be useful, but *
* WITHOUT ANY WARRANTY; without even the implied warranty of *
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU *
* General Public License for more details. *
* *
* A copy of the GNU General Public License is available on the World *
* Wide Web at <http://www.gnu.org/copyleft/gpl.html>. You can also *
* obtain it by writing to the Free Software Foundation, *
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***************************************************************************
Abstract:
Demo to show, how to start a thread and how synchronize with the main
thread.
Important: The cthread unint must be added to the uses section of the .lpr
file. See multithreadingexample1.lpr.
}
unit MainUnit;
{$mode objfpc}{$H+}
interface
uses
Classes, SysUtils, LResources, Forms, Controls, Graphics, Dialogs,
ExtCtrls,syncobjs;
type
{ TMyThread }
TMyThread = class(TThread)
private
fStatusText: string;
procedure ShowStatus;
protected
m_Event: TEvent;
m_EventDone: TEvent;
m_Lock: TCriticalSection;
procedure Execute; override;
public
newStatus : string;
procedure SetString(S: String);
constructor Create(CreateSuspended: boolean);
end;
{ TForm1 }
TForm1 = class(TForm)
Timer1: TTimer;
procedure FormCreate(Sender: TObject);
procedure Timer1Timer(Sender: TObject);
private
public
MyThread : TMyThread;
end;
var
Form1: TForm1;
implementation
{ TForm1 }
procedure TForm1.FormCreate(Sender: TObject);
begin
MyThread := TMyThread.Create(true); // With the True parameter it doesn't
start automatically
if Assigned(MyThread.FatalException) then
raise MyThread.FatalException;
// Here the code initialises anything required before the threads starts
executing
MyThread.Resume;
end;
procedure TForm1.Timer1Timer(Sender: TObject);
begin
// newStatus:='TMyThread Time: '+FormatDateTime('YYYY-MM-DD HH:NN:SS',Now);
MyThread.SetString('TMyThread Time: '+FormatDateTime('YYYY-MM-DD
HH:NN:SS',Now));
end;
{ TMyThread }
procedure TMyThread.ShowStatus;
// this method is only called by Synchronize(@ShowStatus) and therefore
// executed by the main thread
// The main thread can access GUI elements, for example Form1.Caption.
begin
Form1.Caption := fStatusText;
end;
procedure TMyThread.Execute;
begin
fStatusText := 'TMyThread Starting...';
Synchronize(@Showstatus);
fStatusText := 'TMyThread Running...';
while (not Terminated) and (true {any condition required}) do begin
//here goes the code of the main thread loop
m_Event.WaitFor(1000);
m_Event.ResetEvent;
if NewStatus <> fStatusText then begin
m_Lock.Enter;
fStatusText := newStatus;
Synchronize(@Showstatus);
m_Lock.Leave;
end;
m_EventDone.SetEvent;
end;
end;
procedure TMyThread.SetString(S: String);
begin
m_Lock.Enter;
newStatus:=S;
m_Lock.Leave;
m_Event.SetEvent;
//Wait for thread synchronize completation...
m_EventDone.WaitFor(5000);
end;
constructor TMyThread.Create(CreateSuspended: boolean);
begin
FreeOnTerminate := True;
m_Event:=TEvent.Create(nil,true,false,'');
m_EventDone:=TEvent.Create(nil,true,false,'');
m_Lock:=TCriticalSection.Create;
inherited Create(CreateSuspended);
end;
initialization
{$I mainunit.lrs}
end.
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