多线程编程(8) - 多线程同步之 CriticalSection(临界区)

先看一段程序, 代码文件:

---

unit Unit1;



interface



uses

  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,

  Dialogs, StdCtrls;



type

  TForm1 = class(TForm)

    ListBox1: TListBox;

    Button1: TButton;

    procedure FormCreate(Sender: TObject);

    procedure Button1Click(Sender: TObject);

  end;



var

  Form1: TForm1;



implementation



{$R *.dfm}



function MyThreadFun(p: Pointer): DWORD; stdcall;

var

  i: Integer;

begin

  for i := 0 to 99 do Form1.ListBox1.Items.Add(IntToStr(i));

  Result := 0;

end;



procedure TForm1.Button1Click(Sender: TObject);

var

  ID: DWORD;

begin

  CreateThread(nil, 0, @MyThreadFun, nil, 0, ID);

  CreateThread(nil, 0, @MyThreadFun, nil, 0, ID);

  CreateThread(nil, 0, @MyThreadFun, nil, 0, ID);

end;



procedure TForm1.FormCreate(Sender: TObject);

begin

  ListBox1.Align := alLeft;

end;



end.


 
   

窗体文件:

---

object Form1: TForm1

  Left = 0

  Top = 0

  Caption = 'Form1'

  ClientHeight = 154

  ClientWidth = 214

  Color = clBtnFace

  Font.Charset = DEFAULT_CHARSET

  Font.Color = clWindowText

  Font.Height = -11

  Font.Name = 'Tahoma'

  Font.Style = []

  OldCreateOrder = False

  OnCreate = FormCreate

  PixelsPerInch = 96

  TextHeight = 13

  object ListBox1: TListBox

    Left = 9

    Top = 9

    Width = 121

    Height = 97

    ItemHeight = 13

    TabOrder = 0

  end

  object Button1: TButton

    Left = 131

    Top = 112

    Width = 75

    Height = 25

    Caption = 'Button1'

    TabOrder = 1

    OnClick = Button1Click

  end

end


 
   

在这段程序中, 有三个线程几乎是同时建立, 向窗体中的 ListBox1 中写数据, 最后写出的结果是这样的:

---

能不能让它们别打架, 一个完了另一个再来? 这就要用到多线程的同步技术.
前面说过, 最简单的同步手段就是 "临界区".

先说这个 "同步"(Synchronize), 首先这个名字起的不好, 我们好像需要的是 "异步"; 其实异步也不准确...
管它叫什么名字呢, 它的目的就是保证不冲突、有次序、都发生.

"临界区"(CriticalSection): 当把一段代码放入一个临界区, 线程执行到临界区时就独占了, 让其他也要执行此代码的线程先等等; 这和前面用的 Lock 和 UnLock 差不多; 使用格式如下:

var CS: TRTLCriticalSection;   {声明一个 TRTLCriticalSection 结构类型变量; 它应该是全局的}

InitializeCriticalSection(CS); {初始化}

EnterCriticalSection(CS);      {开始: 轮到我了其他线程走开}

LeaveCriticalSection(CS);      {结束: 其他线程可以来了}

DeleteCriticalSection(CS);     {删除: 注意不能过早删除}



//也可用 TryEnterCriticalSection 替代 EnterCriticalSection.


 
   

用上临界区, 重写上面的代码, 运行效果图:

---

代码文件:

---

unit Unit1;



interface



uses

  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,

  Dialogs, StdCtrls;



type

  TForm1 = class(TForm)

    ListBox1: TListBox;

    Button1: TButton;

    procedure FormCreate(Sender: TObject);

    procedure FormDestroy(Sender: TObject);

    procedure Button1Click(Sender: TObject);

  end;



var

  Form1: TForm1;



implementation



{$R *.dfm}



var

  CS: TRTLCriticalSection;



function MyThreadFun(p: Pointer): DWORD; stdcall;

var

  i: Integer;

begin

  EnterCriticalSection(CS);

  for i := 0 to 99 do Form1.ListBox1.Items.Add(IntToStr(i));

  LeaveCriticalSection(CS);

  Result := 0;

end;



procedure TForm1.Button1Click(Sender: TObject);

var

  ID: DWORD;

begin

  CreateThread(nil, 0, @MyThreadFun, nil, 0, ID);

  CreateThread(nil, 0, @MyThreadFun, nil, 0, ID);

  CreateThread(nil, 0, @MyThreadFun, nil, 0, ID);

end;



procedure TForm1.FormCreate(Sender: TObject);

begin

  ListBox1.Align := alLeft;

  InitializeCriticalSection(CS);

end;



procedure TForm1.FormDestroy(Sender: TObject);

begin

  DeleteCriticalSection(CS);

end;



end.


 
   

Delphi 在 SyncObjs 单元给封装了一个 TCriticalSection 类, 用法差不多, 代码如下:

---

unit Unit1;



interface



uses

  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,

  Dialogs, StdCtrls;



type

  TForm1 = class(TForm)

    ListBox1: TListBox;

    Button1: TButton;

    procedure FormCreate(Sender: TObject);

    procedure FormDestroy(Sender: TObject);

    procedure Button1Click(Sender: TObject);

  end;



var

  Form1: TForm1;



implementation



{$R *.dfm}



uses SyncObjs;



var

  CS: TCriticalSection;



function MyThreadFun(p: Pointer): DWORD; stdcall;

var

  i: Integer;

begin

  CS.Enter;

  for i := 0 to 99 do Form1.ListBox1.Items.Add(IntToStr(i));

  CS.Leave;

  Result := 0;

end;



procedure TForm1.Button1Click(Sender: TObject);

var

  ID: DWORD;

begin

  CreateThread(nil, 0, @MyThreadFun, nil, 0, ID);

  CreateThread(nil, 0, @MyThreadFun, nil, 0, ID);

  CreateThread(nil, 0, @MyThreadFun, nil, 0, ID);

end;



procedure TForm1.FormCreate(Sender: TObject);

begin

  ListBox1.Align := alLeft;

  CS := TCriticalSection.Create;

end;



procedure TForm1.FormDestroy(Sender: TObject);

begin

  CS.Free;

end;



end.