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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 *
* the Free Software Foundation; either version 2 of the License, or *
* (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, *
* Inc., 51 Franklin Street - Fifth Floor, Boston, MA 02110-1335, USA. *
* *
***************************************************************************
Abstract:
Demo to show how 5 threads increases a counter.
With and without critical sections.
With critical sections you will always get 50000.
Without you will see different results on each run and depending on your
system.
}
unit CriticalSectionUnit1;
{$mode objfpc}{$H+}
interface
uses
Classes, SysUtils, Forms, Controls, Graphics, Dialogs, Buttons,
StdCtrls, LCLProc, LCLType, LCLIntf;
type
{ TMyThread }
TMyThread = class(TThread)
private
FAFinished: boolean;
public
procedure Execute; override;
property AFinished: boolean read FAFinished write FAFinished;
end;
{ TForm1 }
TForm1 = class(TForm)
CountWithoutCritSecButton: TButton;
CountWithCritSecButton: TButton;
Label1: TLabel;
procedure CountWithCritSecButtonClick(Sender: TObject);
procedure CountWithoutCritSecButtonClick(Sender: TObject);
private
public
CriticalSection: TCriticalSection;
Counter: integer;
UseCriticalSection: boolean;
procedure DoCounting;
end;
var
Form1: TForm1;
implementation
{$R *.lfm}
{ TForm1 }
procedure TForm1.CountWithCritSecButtonClick(Sender: TObject);
begin
UseCriticalSection:=true;
DoCounting;
end;
procedure TForm1.CountWithoutCritSecButtonClick(Sender: TObject);
begin
UseCriticalSection:=false;
DoCounting;
end;
procedure TForm1.DoCounting;
var
i: Integer;
Threads: array[1..5] of TMyThread;
AllFinished: Boolean;
begin
Counter:=0;
// create the CriticalSection
InitializeCriticalSection(CriticalSection);
// start 5 threads
for i:=Low(Threads) to High(Threads) do
Threads[i]:=TMyThread.Create(false);
// wait till all threads finished
repeat
AllFinished:=true;
for i:=Low(Threads) to High(Threads) do
if not Threads[i].AFinished then AllFinished:=false;
until AllFinished;
// free the threads
for i:=Low(Threads) to High(Threads) do
Threads[i].Free;
// free the CriticalSection
DeleteCriticalSection(CriticalSection);
// show the Counter
Label1.Caption:='Counter='+IntToStr(Counter);
end;
{ TMyThread }
procedure TMyThread.Execute;
var
i: Integer;
CurCounter: LongInt;
j: Integer;
begin
FAFinished:=false;
// increment the counter many times
// Because the other threads are doing the same, it will frequently happen,
// that 2 (or more) threads read the same number, increment it by one and
// write the result back, overwriting the result of the other threads.
for i:=1 to 100000 do begin
if Form1.UseCriticalSection then
EnterCriticalSection(Form1.CriticalSection);
try
CurCounter:=Form1.Counter;
for j:=1 to 1000 do ;
inc(CurCounter);
Form1.Counter:=CurCounter;
finally
if Form1.UseCriticalSection then
LeaveCriticalSection(Form1.CriticalSection);
end;
end;
FAFinished:=true;
end;
end.
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